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Full text of "The Marin County breeding bird atlas : a distributional and natural history of coastal California birds"

THE MARIN COUNTY 
BREEDING BIRD ATLAS 

A Distributional and Natural History of Coastal California Birds 




W. DAVID SHUFORD 

A Project of 
Point Reyes Bird Observatory 



Bushtit Books 



Digitized by the Internet Archive 

in 2013 



http://archive.org/details/marincountybreedOOshuf 



THE MARIN COUNTY 
BREEDING BIRD ATLAS 



THE MARIN COUNTY 
BREEDING BIRD ATLAS 

A Distributional and Natural History 
of Coastal California Birds 



W. David Shuford 



Illustrations by Keith Hansen and Ane Rovetta 

Maps by Dewey Livingston 
Photographs by Ian Tait 



California Avifauna Series 1 



Bushtit 
Books 




A Project of Point Reyes Bird Observatory 




i 
y / / 



Copyright © 1993 by Point Reyes Bird Observatory. 

All rights reserved. No part of this book may be reproduced 
in any manner whatsoever without written permission from 
rite publisher except by a reviewer who wishes to quote brief 
passages in a review written for inclusion in a magazine, 
newspaper, or any electronic broadcast media. 



Publisher's Cataloging-in-Publication Data 

Shuford, W. David, 1949- 

The Marin County breeding bird atlas: a distributional and 
natural history of coastal California birds, 
p. cm. 

Includes bibliographic references and index (p. ) 
1. Birds— California— Marin County. 2. Bird populations- 
California— Marin County— Geographical distribution. 
3. Marin County (Calif.)— Natural history. I. Title. 
QL684.C2.S58 1993 598.2'9794'62 92-81834 

ISBN 0-9633050-O-X 

Library of Congress Catalog Card Number: 92-81834. 

Published by BUSHTIT BOOKS 
P.O. Box 233 
Bolinas, CA 94924 

Printed in the United States of America by Braun-Brumfield, Inc., Ann Arbor, Michigan. 

Designed and typeset by Susan Goldhaber Murray. 
Cover design by Susan Claire Peaslee. 

Printed with soy-based inks on acid-free, recycled paper. 



The Marin County Breeding Bird Adas is a project of: 

POINT REYES BIRD OBSERVATORY, 4990 Shoreline Highway, Stinson Beach, CA 94970. Founded in 
1965, Point Reyes Bird Observatory is a nonprofit membership organization dedicated to conducting 
ecological research, interpreting research results to the public, and providing a scientific basis for conservation 
of wildlife and their habitats. Funding is supplied by research grants, contracts, and individual contributions. 
Skilled volunteer work is the backbone of many PRBO projects, including die one upon which this book is 
based. PRBO provides credible, fact-based information and guidelines for policy issues and public and private 
environmental stewardship. Our studies of birds, marine mammals, and their habitats often involve issues 
of national and international significance, such as oil spill impacts, wedands conservation, wildlife/fisheries 
conflicts, and population threats to neotropical migrants. 

Suggested citations (whole book or individual species accounts): 

Shuford, W. D. 1993. The Marin County Breeding Bird Atlas: A Distributional and Natural History of 
Coastal California Birds. California Avifauna Series 1 . Bushtit Books, Bolinas, Calif. 

Peake, H. 1993. Hooded Oriole. In W. D. Shuford. The Marin County Breeding Bird Atlas: A 
Distributional and Natural History of Coastal California Birds, 405-408. California Avifauna Series 
1 . Bushtit Books, Bolinas, Calif. 



To the late David Gaines, 

my first bird mentor and an inspirational teacher, naturalist, and conservationist, 



To Stuart Johnston, 

a born naturalist who knows the birds so well and who lives as wild and free 

as any of them, or us, 



To Bob Stewart, 

who not only started the Marin adas project 

but as a teacher has probably opened the eyes of more budding naturalists in 

Marin County dian anyone else, 



and, of course, 
To my Family. 



Contents 



Contents vii 

List of Marin Breeding Bird Atlas Contributors . . . xi 
List of Marin Breeding Bird Atlas Participants . . . . xii 

Acknowledgments xiii 

Illustrations xiv 

Preface xv 

INTRODUCTION 1 

Historical Background of Breeding Bird Adases . . 1 
A Perspective on the History of Avian Distribution 

Studies in California 2 

History of Breeding Bird Studies in Marin County, 

California 3 

Origin of the Marin County Breeding Bird Adas 
Project 6 

UNDERSTANDING BIRD DISTRIBUTION 7 

Marin County Topography 7 

Geology and Soils 10 

Climate 11 

Seasonality 11 

Temperatures 11 

Precipitation 11 

Pacific Ocean Air and Current Cycles 12 

Climatic Extremes 16 

Coastal Summer Fog 16 

MARIN COUNTY 

BREEDING BIRD HABITATS 19 

Marin County Plant Communities 19 

Mixed Evergreen Forest 19 

Coast Live Oak-California Bay- 

Madrone Forest 19 

Tanbark Oak-Madrone- 

Live Oak-Douglas Fir Forest 19 

Douglas Fir Forest 21 

Oak Woodland and Oak Savannah 21 

Bishop Pine Forest 21 

Coast Redwood Forest 22 

Grassland 22 

Coastal Prairie 22 

Valley Grassland 23 

Coastal Beach-Dune Vegetation 23 

Northern Beach Association 23 



Northern Dune Scrub 24 

Northern Coastal Scrub 24 

Coyote Brush-Sword Fern Scrub 24 

Coastal Sage-Coyote Brush Scrub 24 

Chaparral 24 

Chamise Chaparral 25 

Manzanita Chaparral 25 

Mixed Chaparral 25 

Serpentine Chaparral 25 

Coastal Salt Marsh 25 

Coastal Riparian Forest 26 

Freshwater Marsh 26 

Bulrush-Cattail Marsh 26 

Coastal Swale 27 

Exotic Plants 27 

Additional Breeding Bird Habitats 28 

HISTORY OF LAND USE 

IN MARIN COUNTY 31 

TIMING OF BREEDING 37 

METHODS EMPLOYED IN THE 

MARIN ATLAS 41 

Grid System 41 

Participant Instruction and Block Assignments . . 42 

Gathering Additional Information 45 

Determining Adequacy of Coverage 46 

Other Adases 46 

The Marin Adas 47 

Data Summary 48 

Quantitative Data on Abundance 48 

RESULTS AND DISCUSSION 51 

Adas Coverage 51 

Patterns of Species Richness of the 

Breeding Avifauna 51 

Distributional Highlights of Adas Work 55 

Composition of the Breeding Avifauna 56 

Marin County Breeding Bird 

Communities 61 

Factors Limiting Species Richness of the Avifauna 61 

CONSERVATION APPLICATIONS 69 

How to Use This Book as a Conservation Tool . 69 



vi I 



CONTENTS 



Identification of Breeding Bird Species of 

Special Concern 69 

CONTENT OF SPECIES ACCOUNTS 73 

Adas Breeding Distribution Maps 73 

Key to Abundance and Distribution Data 

Accompanying Atlas Maps 73 

Seasonal Status 73 

Breeding Status 73 

Blocks Recorded 75 

Fine-Scale Abundance Rating (FSAR) 75 

Relative Distribution Index (RDl) 75 

Overall Population Index (OPI) 75 

Breeding Criteria Categories 75 

Confirmation Index (CI) 75 

Content of Species Account Text 76 

Ecological Requirements 76 

Marin Breeding Distribution 76 

Historical Trends/Population Threats 76 

Remarks 77 

Observers 77 

Abbreviations 77 

SPECIES ACCOUNTS 79 

GREBES 
Pied-billed Grebe 79 

STORM-PETRELS 
Ashy Storm-Petrel 81 

CORMORANTS 

Double-crested Cormorant 83 

Brandt's Cormorant 85 

Pelagic Cormorant 89 

BITTERNS AND HERONS 

American Bittern 91 

Great Blue Heron 92 

Great Egret 96 

Snowy Egret 98 

Green-backed Heron 100 

Black-crowned Night-Heron 101 

Recent Population Trends of Marin County 

Heron and Egret Colonies 103 

WATERFOWL 

Canada Goose 104 

Wood Duck 106 

Mallard 109 

Northern Pintail Ill 

Blue-winged Teal 113 

Cinnamon Teal 115 

Northern Shoveler 116 

Gadwall 118 

Common Merganser 120 

Ruddy Duck 122 



NEW WORLD VULTURES 

Turkey Vulture 125 

HAWKS AND EAGLES 

Osprey 129 

Black-shouldered Kite 133 

Northern Harrier 136 

Sharp-shinned Hawk 1 39 

Cooper's Hawk 141 

Red-shouldered Hawk 144 

Red-tailed Hawk 146 

Golden Eagle 148 

FALCONS 

American Kestrel 151 

Peregrine Falcon 154 

PHEASANTS AND QUAIL 

Ring-necked Pheasant 158 

California Quail 161 

RAILS, GALLINULES, AND COOTS 

Black Rail 164 

Clapper Rail 166 

Virginia Rail 169 

Sora 171 

Common Moorhen 172 

American Coot 1 74 

PLOVERS 

Snowy Plover 176 

Killdeer 179 

OYSTERCATCHERS 

Black Oystercatcher 181 

STILTS AND AVOCETS 

Black-necked Stilt 1 84 

American Avocet 187 

SANDPIPERS 

Spotted Sandpiper 189 

GULLS 

Western Gull 191 

AUKS, MURRES, AND PUFFINS 

Common Murre 194 

Pigeon Guillemot 197 

Rhinoceros Auklet 199 

Tufted Puffin 201 

PIGEONS AND DOVES 

Rock Dove 203 

Band-tailed Pigeon 205 

Mourning Dove 207 

ROADRUNNERS 

Greater Roadrunner 209 

BARN OWLS 

Barn Owl 210 

TYPICAL OWLS 

Western Screech-Owl 213 

Great Horned Owl 215 

Northern Pygmy-Owl 217 



vui 



CONTENTS 



Burrowing Owl 219 

Spotted Owl 222 

Long-cared Owl 226 

Short-eared Owl 229 

Northern Saw-whet Owl 231 

POORWILLS 

Common Poorwill 233 

SWIFTS 

Vaux's Swift 234 

White-throated Swift 236 

HUMMINGBIRDS 

Anna's Hummingbird 237 

Allen's Hummingbird 240 

KINGFISHERS 

Belted Kingfisher 241 

WOODPECKERS 

Acorn Woodpecker 243 

Red-breasted Sapsucker 245 

Nuttall's Woodpecker 246 

Downy Woodpecker 248 

Hairy Woodpecker 250 

Northern Flicker 252 

Pileated Woodpecker 253 

TYRANT FLYCATCHERS 

Olive-sided Flycatcher 255 

Western Wood-Pewee 256 

Pacific-slope Flycatcher 258 

Black Phoebe 261 

Say's Phoebe 262 

Ash-throated Flycatcher 264 

Cassin's Kingbird 265 

Western Kingbird 266 

LARKS 

Horned Lark 268 

SWALLOWS 

Purple Martin 269 

Tree Swallow 272 

Violet-green Swallow 273 

Northern Rough-winged Swallow 275 

Cliff Swallow 276 

Barn Swallow 278 

JAYS AND CROWS 

Steller'sjay 280 

Scrub Jay 282 

American Crow 284 

Common Raven 286 

TITMICE 

Chestnut-backed Chickadee 288 

Plain Titmouse 290 

BUSHTITS 

Bushtit 292 

NUTHATCHES 

Red-breasted Nuthatch 294 



White-breasted Nuthatch 296 

Pygmy Nuthatch 298 

CREEPERS 

Brown Creeper 301 

WRENS 

Rock Wren 303 

Bewick's Wren 305 

House Wren 307 

Winter Wren 309 

Marsh Wren 310 

DIPPERS 

American Dipper 312 

KINGLETS AND GNATCATCHERS 

Golden-crowned Kinglet 313 

Blue-gray Gnatcatcher 314 

THRUSHES 

Western Bluebird 316 

Swainson's Thrush 318 

Hermit Thrush 320 

American Robin 322 

WRENTITS 

Wrentit 324 

MOCKINGBIRDS AND THRASHERS 

Northern Mockingbird 327 

California Thrasher 329 

SHRIKES 

Loggerhead Shrike 330 

STARLINGS 

European Starling 333 

VIREOS 

Solitary Vireo 337 

Hutton's Vireo 338 

Warbling Vireo 340 

WOOD-WARBLERS 

Orange-crowned Warbler 342 

Northern Parula 343 

Yellow Warbler 346 

Yellow-rumped Warbler 348 

Black-throated Gray Warbler 350 

Hermit Warbler 352 

MacGillivray's Warbler 353 

Common Yellowthroat 355 

Wilson's Warbler 358 

Yellow-breasted Chat 360 

TANAGERS 

Western Tanager 362 

CARDINALINE GROSBEAKS AND BUNTINGS 

Black-headed Grosbeak 364 

Lazuli Bunting 367 

EMBERIZINE SPARROWS 

Rufous-sided Towhee 369 

California Towhee 371 

Rufous-crowned Sparrow 372 



IX 



CONTENTS 



Chipping Sparrow 374 

Black-chinned Sparrow 376 

Lark Sparrow 378 

Sage Sparrow 379 

Savannah Sparrow 380 

Grasshopper Sparrow 382 

Song Sparrow 385 

White-crowned Sparrow 388 

Dark-eyed Junco 390 

NEW WORLD BLACKBIRDS AND ORIOLES 

Red-winged Blackbird 392 

Tricolored Blackbird 394 

Western Meadowlark 397 

Brewer's Blackbird 398 

Brown-headed Cowbird 401 

Hooded Oriole 405 

Northern Oriole 409 

CARDUELINE FINCHES 

Purple Finch 411 

House Finch 412 

Red Crossbill 414 

Pine Siskin 417 

Lesser Goldfinch 418 

Lawrence's Goldfinch 420 

American Goldfinch 422 

OLD WORLD SPARROWS 

House Sparrow 424 

SPECIES OF UNCLEAR BREEDING STATUS 

OR POTENTIAL BREEDERS 429 

Eared Grebe 429 

Western Grebe/Clark's Grebe 429 

Fork-tailed Storm-Petrel 429 

Leach's Storm-Petrel 429 

Little Blue Heron 429 

Cattle Egret 430 

Fulvous Whisding-Duck 430 

Waterfowl 430 

Green-winged Teal 430 



American Wigeon 430 

Canvasback 430 

Redhead 430 

Lesser Scaup 430 

California Condor 430 

Bald Eagle 430 

Prairie Falcon 431 

Wild Turkey 431 

Wilson's Phalarope 431 

Heermann's Gull 432 

California Gull 432 

Terns 432 

Caspian Tern 432 

Forster's Tern 432 

Least Tern 432 

Marbled Murrelet 432 

Yellow-billed Cuckoo 432 

Chimney Swift 433 

Black-chinned Hummingbird 433 

Willow Flycatcher 433 

Bank Swallow 433 

Cedar Waxwing 433 

American Redstart 433 

Bobolink 434 

Yellow-headed Blackbird 434 

Great-tailed Grackle 434 

APPENDLXES 435 

A. Data from three Spring Bird Counts conducted 

in Marin County from 1977 to 1987 435 

B. Numbers of birds tallied on two USFWS 
Breeding Bird Survey routes conducted in Marin 
County from 1972 to 1986 443 

C. A list of Breeding Bird Censuses conducted in 
Marin County from 1951 to 1990 444 

LITERATURE CITED 445 

INDEX 477 



Marin Breeding Bird Adas 
Contributors 



Cosponsors— Point Reyes Bird Observatory and Marin Audubon Society 

Overall Coordinator 1982, Compiler, and Editor— W. David Sbuford 

Overall Coordinator 1976 to 1978— Robert M. Stewart 

Area Coordinators 1982— Betty Burridge (Tomales Area), Scott Carey (Novato Area), Bill 
Lenarz (South Marin), Dave Shuford (West Marin) 

Computer Entry and Summary— Bill Lenarz 

Species Account Authors— John R. Arnold (Nordiern Mockingbird), Edward C. Beedy 
(Tricolored Blackbird), A. Sidney England (Black-chinned Sparrow, Sage Sparrow), Geoftrey R. 
Geupel (Wrentit), Walter D. Koenig (Acorn Woodpecker), Holly Peake (Hooded Oriole), Helen 
M. Pratt (History of Marin County heron and egret colonies), Stephen I. Rothstein (Brown- 
headed Cowbird), W. David Shuford (153 species), Robert M. Stewart (Wilson's Warbler), and 
Pamela L Williams (Northern Oriole). 

Reviewers— Edward C. Beedy (landbirds, main text), Pete H. Bloom (diurnal raptors), Seth 
Bunnell (Spotted Owl), Scott Carey (landbirds), Harry R. Carter (seabirds), David F. DeSante 
(main text), Richard A. Erickson (short species accounts), Jules G. Evens (rails, Osprey), Sam 
Fitton (Hooded Oriole), Gordon I. Gould, Jr. (Spotted Owl), Stephen L. Granholm (landbirds), 
Roger D. Harris (Pileated Woodpecker), Paul R. Kelly (Clapper Rail), Bill Lenarz (landbirds, 
main text), M. Robert McLandress (waterfowl), Joseph Morlan (short species accounts), Gary 
W. Page (shorebirds), Helen M. Pratt (egrets and herons), Steve Simmons (Wood Duck), 
William J. Sydeman (seabirds, Pygmy Nuthatch), Irene C. Timossi (landbirds), Brian J. Walton 
(Peregrine Falcon), Jon Winter (owls). 



XI 



Marin Breeding Bird Atlas 
Participants 



Debbie Ablin, Julia Allen, Sarab Allen, Jane Anderson, Philip Ashman, Audubon Canyon 
Ranch Research Associates, Stephen F. Bailey, Nancy Barbour, Steve Barbour, Brenda Barten, 
Jim Bartholomew, Joan Basore, Dennis BeaU^-Max Beckwith, Gordon Beebe, Ted Beedy, Lori 
Belton, Irene Biagi, Steve Bobzien, Betty Boyd, Warren Bray, Joan Breece, Tony Briggs, Patty 
Briggs, Betty Burridge, Leanne Bynum, Kurt Campbell, Jean Canepa, Scott Carey, Barbara 
Chase, Frank ck Carolyn Christian, Peter Colasanti, Carolyn Corey, Robin Dager, Rosamond 
Day, Dave DeSante, John Dillon, Peter & Louise Dolcini, Doug Ellis, Michael Ellis, Jules Evens, 
Carter Faust, Mike Fennell, Mary Fishman, Barbara Ford, Carol Fraker, Mary Gillman, Helen 
ck Richard Classman, Terry Goldblatt, Jon Goodchild, Steve Granholm, Nancy Hanson, Tony 
Harrow, Kristi Hein, Luanna Helfman, Emmy Hill, Bob Hogan, Craig Hohenberger, David 
Holway, Joan Howard, Ken Howard, George Hugenberg, Doug Judell, Bill Keener, John 
Kipping, Kathy Kipping, Gerry Kleynenberg, Elsa Konig, Bob Lampee, Robert H. Laws, Jr., Rick 
LeBaudour, Bill Lenarz, R. A. Lewis, Stephen M. Long, Tom Love, Shirley McArdell, Flora 
Maclise, Gary McCurdy, Grace McMichael, Bill Manolis, Marie Mans, Buck Marcussen, Gloria 
Markowitz, Leah Marks, Mary Mayer, Sarah Mayer, Audrey Miller, Stephen H. Morrell, Brenda 
Myron, Patricia ck Anthony Napolitan, Adeene Nelligan, Dan Nelson, Don Neubacher, 
La Verne Nickel, Marcia Nute, Gary Page, Carmen Patterson, Holly Peake, Susan Claire Peaslee, 
Treet Pellitier, PRBO's Palomarin Staff and Volunteers, Charlotte Poulsen, Lina Jane Prairie, 
Helen Pratt, Alton ("Bob") Raible, Elaine ck Tom Reale, Liza Riddle, Inez Riney, Mary Louise 
Rosegay, Ane Rovetta, Allen Royer, Corinne Ryan, Mary Ann Sadler, Barbara Salzman, Susan 
Sanders, Phil ck Margaret Schaeffer, Bob ck Ruth Scott, Bob Seely, Sid ck Nancy Shadle, 
Marianne Shepard, Dave Shuford, Dianne Sierra, Sue Smith, Vernon Smith, Eric Sorenson, 
Bruce Sorrie, Barry Spitz, Spring Bird Count Participants (Even Cheaper Thrills, Pt. Reyes 
Peninsula, ck South Marin— Appendix A), John A. Sproul, Jr., Rich Stallcup, Jean Starkweather, 
Lynne Stenzel, Robert M. Stewart, Nick Story, Helen Strong, Tim Sullivan, Ian Tait, Lynn 
Tennefoss, Gil Thomson, Carol Thoney, Noel Thoney, Irene Timossi, Dorothy Tobkin, 
Beverly Treffinger, Ed Vine, Pat Welsh, Bette Wentzel, Janet Wessel, Jack Whetstone, Jim 
White, Diane Williams, Pam Williams, Summer Wilson, Claire Wolfe, Michael Wolfe, Keiko 
Yamane, Vincent S. Yoder, Florence Youngberg, Bob ck Carol Yutzy, Mark Zumsteg. Many 
other people contributed additional observations via the above participants or direcdy to the 
coordinators. 



xu 



Acknowledgments 



Financial support for the atlas project was provided by generous contributions from an 
anonymous donor, Golden Gate Audubon Society, Marin Audubon Society, Marin County 
Fish and Game Fund, Marin Municipal Water District, Andrea Meyer, Sequoia Audubon 
Society, Lynne Stenzel, and, especially, the general membership of Point Reyes Bird Observatory. 
The board, administration, and staff of Point Reyes Bird Observatory provided tremendous 
logistical and moral support throughout the evolution of the project from the initial stages of field 
work through the completion of the book. Special thanks to Gary Page for granting me an 
extended leave from my duties in PRBO's Coastal and Estuarine Program to work on this book. 
Successive Executive Directors— Jane Church, Burr Heneman, and Don McCrimmon— lent their 
full support to the project, and Laurie Wayburn's commitment at a crucial stage enabled the 
completion of the final product you hold in your hands. 

Janet Kjelmyr, Lisle Lee, Michelle Morris, Meg Sanders, Meg Simonds, Janice Tweedy, and, 
particularly, Susan Goldhaber Murray and Liz Tuomi were invaluable in crafting my handwrit- 
ten or hastily typed text and tables into a polished manuscript . . . bless their souls. Susan Claire 
Peaslee was a godsend in rising well above the call of duty to deftly manage the early and middle 
stages of book production and copyediting ... I can't thank her enough. Liz Tuomi contributed 
additional copyediting skills and along widi Pam Williams and Susan Goldhaber Murray 
proofread all of the manuscript. Mary Anne Stewart skillfully copyedited the entire final version 
of the manuscript. Edris Cole, Dianne Sierra, and Meryl Sundove proofread the spellings of 
observer names. Thanks to Bertha Rains for an initial literature search, and to Karen Hamilton 
for locating and obtaining many hard to find references and for providing cataloging data for 
the finished book. Many thanks to Julia Gennert for pasting up the illustrations on the typeset 
manuscript. Helen and Paul Green generously provided a true home away from home when I 
needed to spend innumerable days and nights researching literature at the Biology Library at 
U.C. Berkeley. Special thanks to Scott Carey, Doug Judell, Bill Lenarz, Bob Stewart, Irene 
Timossi, and Jules Evens for spending enormous amounts of time in die field and sharing their 
vast knowledge of Marin County breeding birds. Many dianks to the numerous Marin County 
landowners whp provided access to their lands without which our field work would have suffered 
gready. 

Jules Evens (rails and Osprey), Allen Fish (diurnal raptors), Roger Hothem (herons and 
egrets), John Kelly (herons and egrets), Gary Page (Snowy Plovers), Helen Pratt (herons and 
egrets), and Dave DeSante, Geoff Geupel, and Bruce Sorrie (Palomarin landbirds) kindly 
provided data from their studies. L Richard Mewaldt provided much of die material in the 
White-crowned Sparrow account. Sheila Hershon searched the files of die California Center for 
Wildlife for breeding records of Western Screech-Owl in Marin County during the adas period. 
Numerous odiers supplied essential facts and figures. Dennis Beall drew the base map used to 
construct the adas map for each species. Many dianks to Keidi Hansen, Dewey Livingston, Ane 
Rovetta, and Ian Tait for their patience in waiting for my plodding writing to catch up with their 
inspirational artwork which graces these pages. When all of the above was said and done, Susan 
Goldhaber Murray used her remarkable blend of computer skills, problem solving abilities, 
artistic talents, and great patience to craft the text into a well organized and aesthetically pleasing 
book, for which she should be duly proud. Speaking for die birds as well, I give final and 
heartfelt thanks to all diose listed above, or not, who in one capacity or another volunteered their 
time because of a deep concern for birds and their environment. 

xiii 



Illustrations 



Bird Drawings — Keith Hansen 

Pages iii, xvi, 90, 124, 128, 160, 208, 221, 239, 259, 274, 321, 366, 384, 387, and 



Marin County Maps — Dewey Livingston 
Pages 8, 9, 14, 20, 34 

Landscape and Habitat Drawings — Ane Rovetta 

Pages 6, 18, 21, 22, 23, 24, 25, 26, 27, 28, 30, 36, 50, 71, 78, back cover, and oak 
spray dingbats/snippets throughout 

Bird Photographs — Ian Tait 

Pages 102, 150, 186, 190, 196, 242, 249, 291, 300, 308, 326, 336, 339, 341, 361, 
375, and back cover 



xiv 



Preface 



The contents of this book evolved over a long period, at first expanding in scope, only later 
to contract. Beyond the grid-based distribution maps, there is no set formula (nor should 
there be) as to what warrants inclusion in a breeding bird adas, or as to whether it even need be 
a book: some county adas projects have been published as short papers in local ornithological 
journals (e.g., Klimkiewicz 6k Solem 1978). State or provincial adas books published in North 
America have ranged from compilations of computer data printouts (Adamus 1987) to full-scale 
books for Vermont (Laughlin 6k Kibbe 1985), Ontario (Cadman et al. 1987), and New York 
(Andrle 6k Carroll 1988). These three books and their counterparts from other countries include 
extensive introductory and interpretive material along with species accounts accompanying the 
maps that provide detail on such topics as habitat preferences, various aspects of the species' 
breeding biology, and historical trends of populations. The increasing inclination of writers to 
use natural history information to provide a framework for understanding distributional patterns 
is followed by this author as well. 

Although some would argue that the maps should be the highlight of an adas book— after all, 
they are usually the main data generated by the field work— they are lifeless abstractions without 
an understandng of the intricate web of niche requirements that each species must meet for 
survival, and without survival there is no distribution or map. Hence, the reader will encounter 
a strong ecological bent in the material contained in this adas. The stage is set for interpreting 
the maps, the species accounts, and the facts and concepts elucidated by the adas project by first 
describing Marin County's geographic and geologic setting, the seasonal oceanic and climatic 
cycles affecting birds here, and the county's principal bird habitats— the realm in which the 
mundane, dramatic, and poignant events of the lives of our breeding birds unfold. Also, a 
historical perspective is emphasized in describing changing land use practices, bird population 
trends, and how the concept of the bird adas has transformed the way we approach distributional 
studies. 

A great deal of additional material written for this book had to be left out because of time, 
space, and financial considerations. The original intent was to broaden the discussion of bird 
distribution patterns to include all of coastal northern California, and to provide additional 
species accounts for other species of birds that breed elsewhere in this region but not in Marin 
County. Much of this material was contributed by others and I lament its loss from die present 
publication and the diversion of many peoples' talents from other projects. Their mark was left 
on this book nonetheless. 

Ralph Hoffmann (1927) in his enchanting, but now underappreciated, field guide, Birds of 
the Pacific States, remarked diat "one cannot have too many good bird books." The author will 
feel the long effort of writing was well wordi it if but a few readers deem the present book to be 
in that category. True satisfaction, though, will come only if some acquire, in part dirough 
reading, a deeper appreciation and fascination with our winged companions that motivates them 
to be better stewards of the Eardi. May we be lucky enough to meet in nature's heardand and 
share its many pleasures together. 

Dave Shuford 
Bolinas, California 
March 1 993 



xv 




6itf-) Manser) 



A tiny fuss-budget of a Bushtit scolds a prowling Scrub fay while its partner warily peers from the 
nest hole. Drawing by Keith Hansen, I 990. 



INTRODUCTION 



Lest the uninitiated reader be led to believe that the problems of distribution of the birds of California are in the main solved 
and fully presented in this work, may we quickly disillusion him. 

— Joseph Grinnell and Alden H. Miller, 
The Distribution of the Birds of California 



UNTIL THE LATE 1960s, bird distributions were tradi- 
tionally mapped using random observations from 
scattered sources, often collected over lengthy periods of 
time. In addition, breeding distribution maps usually did 
not distinguish between records of a species based solely 
on presence during the breeding season and those backed 
with positive proof of breeding. Even when aided by 
knowledge of species' habitat needs, this manner of map- 
ping was largely a subjective process. Numerous judgments 
had to be made when encountering the inevitable gaps in 
the record in seemingly suitable habitat or isolated occur- 
rences in habitats of questionable suitability. Usually a 
great deal of uncertainty remained over whether the pattern 
of distribution plotted was pardy an artifact of uneven 
knowledge of the area in question, whether breeding 
records were sufficiendy documented, or whether the 
actual distribution had changed over the course of the 
extended period of data collection. These problems were 
especially acute where observers were few but could not be 
overcome even in areas, such as Great Britain, with a very 
high proportion of observers in the population and a 
history of ornithological exploration stretching back for 
centuries. The lack of an adequately documented record of 
changes in the distribution and abundance of highly visi- 
ble species such as birds has been particularly frustrating 
for conservationists and managers. With subjective map- 
ping methods using data from the entire historical record, 
only the most dramatic changes in distribution and abun- 
dance were noticeable. Often an awareness of a reduced 
distribution or population decline was apparent only dur- 
ing the later stages, when conservation efforts were the 
most difficult to implement. 

All this changed in the late 1960s when avian distribu- 
tion studies were revolutionized by the simple concept of 
the breeding bird adas— a compilation of accurate distribu- 
tion maps for all the bird species in a particular geographi- 
cal area under study. At the outset, the area is divided with 
a uniform grid of equal-sized adas blocks (rectangles or 
squares). These blocks are initially the basic units of field 
study and ultimately the mapping units for bird distribu- 



tion. In addition to noting presence or absence, field 
workers record for each species the highest category of 
breeding evidence they observe, based on well-defined 
criteria for possible, probable, and confirmed breeding. By 
conducting thorough field work in each of these blocks 
during a several-year period (usually, five years), the current 
details of avian distribution can effectively be frozen in 
time. Complete coverage of all blocks over a short time 
span avoids the main weaknesses of earlier mapping stud- 
ies and enables changes in distribution to be easily docu- 
mented by replicating the work in future years. 

Historical Background of 
Breeding Bird Atlases 

Ferguson-Lees (1976), Robbins (1982), and Laughlin et al. 
(1982) have summarized the history of mapping bird 
distributions. Phillips' (1922-1926) A Natural History of 
the Ducks was the first serious attempt to map the distribu- 
tion of a large number of birds. In North America, the first 
books to map selected species were Birds of New Mexico 
(Bailey 1928), Florida Bird Life (Howell 1932), and The 
Distribution of the Birds of California (Grinnell 6k Miller 
1944). In Europe, the first attempts to map bird distribu- 
tion over broad areas were the Birds of the Soviet Union 
(Dement'ev & Gladkov 1951-1954), A Field Guide to the 
Birds of Britain and Europe (Peterson et al. 1954) and the 
Atlas of European Birds (Voous 1960). In North America, 
The Birds of Canada (Godfrey 1966) and Birds of North 
America (Robbins et al. 1966) were the pioneer works in 
this vein. 

The concept of mapping distribution with the aid of a 
grid had its origin with a German botanist, Heermann 
Hoffmann, who in 1860 published the first grid-derived 
maps of certain plants in central Europe. Although orni- 
thologists began on a subjective basis to use grids to plot 
the distribution of certain bird species in Britain in the 
1950s (Norris 1960, Prestt ck Bell 1966), the main 
impetus for the current adas movement was the publica- 
tion by the Botanical Society of the British Isles of the Atlas 



1 



Historical Background 



MARIN COUNTY BREEDING BIRD ATLAS 



Historical Studies -California 



of the British Flora (Perring &. Walters 1962). This work 
systematically mapped the distribution of the British flora 
by 10-km squares. Things have never been quite the same 
since, as British bird students jumped into die objective 
comprehensive adasing of breeding birds with a passion. 
The pilot project covering three counties in England began 
in 1966 and was published as the Atlas of Breeding Birds of 
the West Midlands (Lord &. Munns 1970). Following close 
on its heels was the awe-inspiring effort of The Atlas of 
Breeding Birds of Britain and Ireland (Sharrock 1976), 
where over 10,000 observers completed field work from 
1968 to 1972 in each of the 3682 10-km adas squares 
covering all of Britain and Ireland! While the British field 
work was still in progress, other European atlas projects 
were initiated and subsequendy multiplied profusely until 
by 1981 at least 16 European countries had completed or 
started atlas projects (Robbins 1982). Avian adasing soon 
spread to other continents as well, and adasing already had 
been applied to map the distribution of other life forms 
ranging from marine algae to a host of marine and terres- 
trial invertebrates. Not ones to rest on their laurels, the 
British soon began and completed an adas of winter bird 
distribution (Lack 1986). They are also currendy working 
toward the completion of dieir second breeding bird adas 
diat will resurvey all of Britain and Ireland. 

In North America the first recognition of the impor- 
tance of grid-based mapping of bird distribution came in 
the 1960s (Skarr 1967, 1969) and resulted in the publica- 
tion of Montana Bird Distribution: Preliminary Mapping by 
Latilong (Skarr 1975). Montana was divided into 47 
1 °-blocks of longitude and latitude, and maps were con- 
structed for each species. Although different categories of 
breeding evidence were presented where applicable, the 
initial latilong study differed in several important ways 
from most other atlas projects. First, the size of the blocks 
was very large compared with breeding bird adas blocks— a 
latilong in Montana is nearly 100 times as large as a 10-km 
square. Secondly, species were initially mapped in a lati- 
long if diey occurred in any season, not just the breeding 
season (see Skarr 1 980). Lasdy, and perhaps most impor- 
tandy, observations were used from the entire historical 
record of Montana ornithology rather than from a fixed 
and limited number of years. Preliminary latilong projects 
have also been published for Colorado (Kingery & Graul 
1978), Wyoming (Oakleaf et al. 1979), and Utah (Walters 
1983). 

The first North American adas work (patterned closely 
on the European models) was initiated in 1971 on a 
county-by-county basis by the Maryland Ornithological 
Society, and the first work was published as the Breeding 
Bird Atlas of Montgomery and Howard Counties, Maryland 
(Klimkiewicz ck Solem 1978). As in Europe, the idea 
quickly spread. The number of full-state or provincial adas 
projects underway or completed in North America swelled 



from 10 in 1981 (Laughlin et al. 1982), to 26 in 1986 
(Sutcliffe et al. 1986), to 33 in 1990 (Smith 1990). The 
accuracy and conservation value of distribution studies has 
increased dramatically with the ascendancy of avian adas 
projects. 

A Perspective on the History of Avian 
Distribution Studies in California 

The bulk of the data on California's avifauna— especially in 
regard to breeding birds— was collected in the early to 
mid-1 900s by professional ornidiologists from the Muse- 
um of Vertebrate Zoology at the University of California, 
Berkeley, under the direction and inspiration of Joseph 
Grinnell, his students, and his associates in the Cooper 
Ornidiological Society. A large part of the data collected 
came from field work organized to document the distribu- 
tion of the vertebrate fauna in less well known areas of 
California, particularly in areas in danger of rapid change 
caused by human encroachment, but also in protected 
parks. Most of the important distributional studies were 
published as monographs either in the University of Cali- 
fornia Publications in Zoology or the Pacific Coast Avi- 
fauna series. The culmination of this work resulted in the 
publication of the landmark TKe Distribution of the Birds of 
California (Grinnell &. Miller 1944), supplemented by An 
Analysis of the Distribution of the Birds of California (Miller 
1951). All subsequent California workers have owed an 
enormous debt to the 1 944 publication. Though now out 
of date in many respects, it still stands as the single most 
important reference on the distribution of California birds. 

In the last 40 years, professional field ornithology has 
shifted away from an emphasis on distributional and 
taxonomic studies toward ecological and experimental 
work, often on single species. Although some professional 
ornithologists in California still contribute important dis- 
tributional studies, a host of amateur field ornithologists 
are presendy in the forefront of updating and expanding 
our knowledge of California's avifauna. Much of the recent 
distributional work has been published in the seasonal 
reports of both the Middle and Southern Pacific Coast 
regions of American Birds, in articles in Western Birds 
(formerly California Birds), or in regional distribution 
books or annotated checklists. Much information is scat- 
tered in numerous papers in a variety of scientific journals; 
and a vast store of unpublished knowledge is on file with 
the regional editors of American Birds, university muse- 
ums, government agencies, and in individual field workers' 
notebooks. 

As with past avifaunal studies in California, most recent 
work has suffered from concerted effort in certain areas at 
the expense of others. First, the distribution of observers 
in California, as elsewhere, is very clumped— most are 
concentrated close to population centers on the coast and 



Historical Studies-California 



INTRODUCTION 



Historical Studies -Marin 



in the Central Valley. Secondly, recent amateur enthusi- 
asm for searching for migrants, hence increasing one's 
chances for sighting rare birds, has further concentrated 
observers. The discovery in the 1960s that rarities can be 
found relatively frequendy in isolated habitat patches on 
the coast or in desert oases is the prime example of this 
phenomenon. Though amateur ornithologists in Califor- 
nia have added an enormous amount in recent years to our 
knowledge of vagrants and to migrational phenomena in 
general, until very recendy there has been a notable lack of 
interest in breeding birds. 

There have been few attempts to map bird distributions 
throughout California. Grinnell and Miller (1944) were 
the first to map a selected number of the state's breeding 
species to elucidate subspecific ranges. Subsequendy, Gar- 
rett and Dunn (1981) mapped selected breeding species in 
southern California, and Grenfell and Laudenslayer 
(1983) mapped the summer and/or winter distribution of 
340 species of birds in all of California. Unitt (1984) and 
Roberson (1985) mapped breeding distributions in Cali- 
fornia in San Diego and Monterey counties respectively. 
All these authors relied on subjective methods to map 
approximate breeding ranges, using largely presumptive 
evidence of breeding. The Marin adas project is the first to 
objectively plot the distribution of all breeding species in 
any area of California based on systematically collected 
data. 

The fine-scale distribution data and supplemental natu- 
ral history information of die Marin County Breeding Bird 
Atlas can be used by local conservationists as an aid to 
preserving and protecting our remaining valuable wildlife 
habitats. On a larger scale, perhaps this beginning will 
stimulate others to start adas projects in other counties and 
eventually all of California. Indications are that the Marin 
adas may already have played that role— as of 1991 there 
are adas projects underway (or completed) in 12 other 
California counties (Table 1, Figure 1; Manolis 1991). It 
is to be hoped that more bird students will seize diis 
opportunity to conduct field work with conservation rami- 
fications foremost in their minds. The human horde 
presses heavily on our remaining wildlands, and a basic 
understanding of the distribution, abundance, and habitat 
requirements of all our native fauna is essential for protect- 
ing our heritage of biological diversity. 

History of Breeding Bird Studies in 
Marin County, California 

In the late 1870s and early 1880s, C.A. Allen, living dien 
in Nicasio or San Geronimo, published several short notes 
on breeding birds in Marin County (e.g., Allen 1881). The 
Mailliard brothers, Joseph and John W., contributed the 
most to the early knowledge of Marin's avifauna, primarily 
from field work near their ranch in the San Geronimo 



Valley. Their work bore fruit in numerous papers pub- 
lished from 1881 to at least 1938 (see Grinnell 1909, 
1924, 1939) and the accumulation of a large specimen and 
egg collection eventually housed at the California Academy 
of Sciences (Mailliard 1924b). Records of Allen's and 
earlier ones of the Mailliards' attributed to Nicasio may in 
fact refer to specimens collected some miles away (Mail- 
liard 1924b). J. Mailliard's 1900 paper first summarized 
knowledge of the status of landbirds in Marin County. The 
first publication to report the status of all species of the 
county's avifauna was Stephens and Pringle's (1933) Birds 
of Marin County. They compiled information primarily 
from records of the Audubon Association of the Pacific 
(now Golden Gate Audubon), derived mosdy from obser- 
vations from 56 field trips to various places in southern 
Marin from 1919 to 1933. They also used information 
from Grinnell and Wythe's (1927) Director} to the Bird-life 
of the San Francisco Bay Region and unpublished observa- 
tions of several active observers. Additions and corrections 
to the Marin list were published in 1936 (Gull 18, No. 6). 
Limited additional information on Marin's breeding birds 
has been published in avifaunal works of broader geo- 
graphic scope (e.g., Grinnell &. Miller 1944, Miller 1951), 
as occasional notes (e.g., Ralph & Ralph 1958), as part of 
single-species studies (e.g., Page & Stenzel 1981), or as part 
of seasonal field note summaries of local or national 
Audubon Society publications (e.g., The Gull, Audubon 
Field Notes, American Birds). Even as the number of observ- 
ers in the area grew gready from the 1950s to the early 
1970s, and access improved with better roads and the 
establishment of numerous parks, little effort was focused 
on breeding birds. At the time, observers focused much of 
their field work on the coast, particularly on Point Reyes, 
and mosdy on migrational phenomena, seasonal abun- 
dance patterns, Christmas Bird Counts, and single-species 
studies. Breeding birds took a back seat. 

The picture of Marin County's breeding avifauna 
painted by work prior to the adas project was a sketchy one. 
Initial observer coverage was focused on central and south- 
ern Marin, with minimal field work from Point Reyes (see 
Shuford 1986). Early records from the drier portions of the 
county around Novato were almost unheard of. The small 
geographic focus, few observers, and difficulty of travel are 
reflected in the earlier lists of Marin's breeding birds. 
Interpretation of Mailliard's (1900) summary of the status 
of landbirds in Marin County indicated he had knowledge 
at the time of about 89 species of landbirds breeding here. 
Similar interpretation of Stephens and Pringle's (1933) list 
suggests they knew then of 96 species of breeding land- 
birds. Even including 4 species (Purple Martin, Violet- 
green Swallow, Bank Swallow, and Western Bluebird) 
considered as breeders in Marin by Mailliard (1900) and 
3 species (Northern Harrier, American Robin, and Tricol- 
ored Blackbird) by Grinnell and Wythe (1927), as of 1933 



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INTRODUCTION 




Figure I . Map of California highlighting tke 1 3 counties with breeding bird atlas projects that are completed or in progress (see 
Table I). 



Historical Studies-Mann 



MARIN COUNTY BREEDING BIRD ATIAS 



Origin of Project 



there was knowledge of only 103 species of breeding 
landbirds in Marin County. Adding the 22 species of 
waterbirds that Stephens and Pringle indicated were breed- 
ing in the county gives a total of 1 25 species of confirmed 
or suspected breeding birds in Marin County in 1933. For 
some species, breeding evidence was based largely on 
assumptions and limited data. For example, the evidence 
of California Thrasher in the county was based solely on a 
single aural record (Mailliard 1900, Stephens & Pringle 
1933). By 1951 there were indications of at least 8 addi- 
tional breeders in Marin (Grinnell 6k Miller 1944, Miller 
1951) for a total of 133 breeding species in the county. 
This total is about 82% of the number of breeding species 
known from Marin County at the time of this writing (see 
Results and Discussion p. 51). These numerical compari- 
sons tell only part of the story. Initially many species were 
considered breeders without sufficient documentation, 
and knowledge of the countywide distribution and abun- 
dance of most species was fragmentary at best. 



Origin of the Marin County 
Breeding Bird Atlas Project 

In 1976, Bob Stewart, then the landbird biologist at Point 
Reyes Bird Observatory, was inspired by the publication of 
the seminal Atlas of ike Breeding Birds of Britain and Ireland 
(Sharrock 1976). Encouraged by Chandler Robbins and 
the progress of Maryland's adas work, Bob initiated a 
proposed three-year project to map the breeding distribu- 
tion of all bird species in 221 adas blocks (2.5-km square 
equivalents) in Marin County, California. At that time, 
small-scale breeding bird adas work was in its formative 
stages. The only other projects underway in North Amer- 
ica were in Maryland and Massachusetts. There was no 
precedent whatsoever in California or the West The 
initiation of adas work in California, even on this tiny 
scale, can be viewed as an important landmark in light of 
the history of previous avifaunal work in the state. 




Uninterrupted chaparral, redwood forest, mixed evergreen forest, and marshland graced Mount Tamalpais and die 
Corte Madera shoreline in ^resettlement times. Drawing £>} Ane Rovetta, 1 989. 



UNDERSTANDING BIRD 
DISTRIBUTION 



Efforts to develop broad distributional principles and categories commonly run beyond the facts and violate the essentially 
statistical character of distributional data. There is an urge to create simplified concepts, perhaps unwittingly as paths of least 
intellectual resistance. These become lines of escape from exhaustive factual comprehension. 

- Alden H. Miller, 
An Analysis of the Distribution of the Birds of California 



SINCE THE LOCAL AVIFAUNA is a product of thousands of 
years of evolution, it stands to reason that any study of 
bird distribution must start with a solid understanding of 
each species' biology and the environment to which the 
birds have adapted. A host of climatic, topographic, and 
geologic factors interact on a local scale to provide a suite 
of habitats available for birds. The trick to unraveling the 
puzzle of bird distribution is to grasp the factors that 
influence the habitat selection of each species. This is not 
an easy task. All bird distributions are constandy changing, 
at least on a local scale, whether in response to a varying 
environment or in response to varying competition and/or 
predation influences from other species. In addition, a bird 
may reach the limit of its distribution though seemingly 
suitable habitat continues uninterrupted. Today biologists 
believe that landbirds generally select habitats according to 
the structure of plant communities, rather than selecting 
particular species of plants (e.g., Verner &. Larson 1989) 
though exceptions exist and many factors are at play. 
Foraging seabirds are generally distributed with respect to 
various water masses with characteristic ranges of tempera- 
ture and/or salinity, with the added constraint of the need 
for protected, isolated terrestrial habitat for breeding. For 
these reasons, the overview below emphasizes the seasonal 
cycles of weather and ocean currents that interact with the 
local topography and geology, which in concert shape the 
breeding habitats to which Marin County's avifauna has 
adapted. We will see that Marin County's geographic 
position and evolutionary history place it in an area of 
exceptional oceanic productivity and varied terrestrial plant 
communities. Consequently, it is home to a large and 
varied breeding avifauna. 



Marin County Topography 

Marin County's setting and topography are important 
determinants of local weather patterns and plant distribu- 
tion. Marin County lies at 38°N along the California coast 
just north of the Golden Gate at the mouth of San 
Francisco Bay. The county is roughly diamond shaped and 
covers 588 square miles— it is the fourth smallest of Califor- 
nia's 58 counties. Its long axis runs northwest to southeast, 
and it is bounded by the Pacific Ocean on the west, the 
Golden Gate on the south, San Pablo and San Francisco 
bays on the east, and the low rolling hills of the Sonoma 
County "borderlands" on the north (Figures 2 and 3). 

Though seemingly uniform from the surface, the Pacific 
Ocean off our shores can be divided into several zones 
useful for describing the distribution of sea-going birds (see 
Shuford et al. 1989). Neritic describes waters over the 
continental shelf, which off Marin varies from about 20 to 
25 miles in width, extending just seaward of the Farallon 
Islands and Cordell Bank. The neritic zone can be subdi- 
vided into inshore and offshore zones. The inshore zone 
reaches from the shoreline to a line beyond which the 
bottom is too deep for a diving seabird to exploit— a depth 
of about 230 feet. The offshore zone extends from that 
depth to the seaward edge of the continental shelf. Oceanic 
describes waters of the deep ocean from the continental 
slopes beyond the continental or insular shelves— the true 
home of pelagic seabirds. 

Marin's shoreline is dissected by several bays, lagoons, 
and estuaries: Tomales Bay, Abbott's Lagoon, Drake's and 
Limantour esteros, and Bolinas Lagoon on the outer coast; 
and several tongues of San Francisco Bay, most notably 
Richardson Bay, on the east. On the outer coast most of 
the shoreline rises abrupdy to steep cliffs, except for occa- 



MARIN COUNTY BREEDING BIRD ATLAS 



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Tenneaee Point V^^^ 

^^LaQoon 



Figure 2. Place name map of Marin County Map by Dewey Livingston, I 990. 



UNDERSTANDING BIRD DISTRIBUTION 




Marin County 

TOPOGRAPHY 

SOURCE: U. S. TOPOGRAPHIC SURVEYS 
© 1991 by Dewey Livingston 



Figure 3. Topographic map of Marin County. Dark solid lines show 500 foot topographic contours; thin dotted lines mark stream 
drainages. Map by Dewey Livingston, 1991. 



Tolwgraphy 



MARIN COUNTY BREEDING BIRD ATIAS 



Geology and Soils 



sional small pocket beaches and the long beaches fronting 
Drake's Bay and the west side of the Point Reyes peninsula. 
Along the eastern bayshore, marshlands and reclaimed 
former marshlands cover alluvial plains, particularly near 
Novato. Otherwise Marin's uplands consist predominant- 
ly of hilly and low mountainous terrain. The most promi- 
nent peak is Mount Tamalpais, a sacred Miwok refuge 
with the famed profile of the Sleeping Lady, which reaches 
a height of 2571 feet at its East Peak. Other high peaks are 
Big Rock Ridge (1887 ft.), Pine Mountain (1758 ft.), Loma 
Alta (1592 ft.), Mount Burdell (1558 ft.), Hicks Mountain 
(1532 ft.), Mount Wittenberg (1407 ft), and Black Moun- 
tain (1280 ft.). Although diese peaks are not impressive as 
mountains go, diey are rather steep; the flank of Mount 
Tamalpais rises 2000 feet in a little over a mile starting at 
sea level at Stinson Beach. 

In the pattern typical of the Coast Range mountains, 
Marin County's major ridges run northwest to southeast 
roughly paralleling the coasdine. Toward the coast these 
ridges are clodied primarily widi conifer forests, mixed 
hardwood forests, and coastal scrub. Inverness Ridge 
stretches the length of the Point Reyes peninsula, reaching 
a height of 1407 feet at Mount Wittenberg. On the east, 
the ridge rises rather steeply from Tomales Bay and the 
Olema Valley. On the west, the soudiern portion of the 
ridge descends rapidly to steep cliffs and rocky shoreline. 
The northern portion of the ridge, around Drake's and 
Limantour esteros, descends to low rolling hills and pas- 
tureland, flanked by a long beach and dune system set off 
by the steep cliffs and rocky shoreline of the Point Reyes 
headlands and Tomales Point. From the Golden Gate low 
grass- and brush-covered hills of the Marin Peninsula 
ascend to the north to Mount Tamalpais. The rest of the 
south central part of the county is dominated by several 
roughly parallel ridges, the most prominent being Bolinas 
Ridge, Carson Ridge, Loma Alta, and Big Rock Ridge. 
Smaller parallel ridges emanating from those larger ridges 
and from Mount Tamalpais separate Marin's larger towns 
and cities lying in small valleys along the San Francisco 
and San Pablo bayshores. The heavily forested southern 
ridges grade to the north into much smaller, less well 
defined ridges covered largely widi grasslands and a patch- 
work of brush and hardwood forest. To the extreme 
northwest along the drainages of Estero de San Antonio 
and Estero del Americano, the low rolling hills are clothed 
almost entirely in grasslands, as are lowlands on outer 
Point Reyes. 

Unlike most of the northern California coast, Marin 
has no major rivers draining its landmass. Instead the 
numerous canyons give rise to many small permanent and 
intermittent streams that flow into small valleys between 
the hills and then into die ocean. Natural freshwater ponds 
or lakes are very rare in Marin, as in most of die Coast 
Range. The largest of these is Laguna Lake, a seasonal lake, 



on the Marin-Sonoma border in Chileno Valley. The only 
other notable natural freshwater ponds are several near 
Double Point north of Palomarin on the Point Reyes 
peninsula. Marin Municipal Water District, however, has 
impounded water in seven sizable reservoirs: Alpine Lake, 
Bon Tempe Lake, Kent Lake, Lake Lagunitas, Nicasio 
Reservoir, Phoenix Lake, and Soulajoule Reservoir. The 
only other large impoundment is North Marin Water 
District's Stafford Lake. Numerous small diked stock 
ponds dot the grassy hills of the cattle and sheep ranches, 
mosdy in the central, northern, and western parts of the 
county. 

The topography of the coastal ridges exerts a profound 
effect on local weather. Varied coastal relief influences local 
rainfall patterns countywide, as discussed in greater detail 
below. The only major gaps in the ridge system are in the 
Estero lowlands near Tomales (which connects with the 
Petaluma Valley of southern Sonoma County), the Nicasio 
gap near the reservoir of the same name, and the Muir 
Woods gap. These gaps funnel winds and coastal fog 
eastward, moderating the summer climate of adjoining 
inland areas relative to other areas blocked from major air 
movements by high ridges. Evidence of the strong winds 
in these gaps can be readily seen in the wavelike top of the 
wind-sculptured bay laurel forest below Nicasio Reservoir. 
The influence of Marin's varied relief on microclimate 
reaches beyond its effect on large-scale air movement and 
rainfall. Local differences in slope, exposure, temperature 
inversions, cold air drainage, and ground water levels also 
have marked effects on vegetation. 

Geology and Soils 

Marin County's most infamous geologic feature, the San 
Andreas Fault, slices die earth's crust under Bolinas La- 
goon, the Olema Valley, and Tomales Bay. This fault- 
known chiefly as the source of the great 1906 San 
Francisco earthquake— separates two areas of strikingly dif- 
ferent geologic history, now juxtaposed by displacement 
along the fault of several hundred miles or more. To the 
west on the northward moving Pacific Plate, the Point 
Reyes peninsula has a base of Cretaceous (at least 84 
million years old) granitic rock overlain with relatively 
young (4-26 million years old) marine sedimentary depos- 
its of the Cenozoic age. To the east on the North American 
Plate lie the intensely folded and faulted rocks of the 
Franciscan complex, including Mesozoic (1 36 million 
years old) marine sandstones and shales, chert, sea floor 
volcanic rocks (mainly greenstone), serpentine, and 
unusual metamorphic rocks. 

As a result of its complex geologic setting and history, 
Marin County hosts many soil types (Kashiwagi 1985). 
Contrasts in soil types are most apparent on opposite sides 
of die San Andreas Fault since their respective origins and 



10 



Geology and Soils 



UNDERSTANDING BIRD DISTRIBUTION 



Climate 



histories are so different. The distribution of distinctive soil 
types appears to explain much of the overall distribution of 
Marin County's conifer forests. A thin, relatively barren 
soil derived from serpentine rock is one of the exceptional 
types found locally east of the San Andreas Fault. Although 
serpentine soils are extremely harsh and support few spe- 
cies, they harbor a number of endemic species and geneti- 
cally distinct populations of plants (Kruckeberg 1984). 
Serpentine soil also supports a unique chaparral commu- 
nity, with generally sparser and stunted shrubs favored by 
certain chaparral birds. Although soils may be important 
determinants locally, topography and microclimate gener- 
ally play a greater role in influencing broad patterns of 
plant distribution and hence bird distribution. 

Climate 

Seasonality 

Many transplanted Easterners bemoan the "lack of sea- 
sons" in coastal lowland California. Despite the relatively 
low variation between summer and winter temperatures 
along the coast, there are marked seasonal changes in 
weather, though these contrast gready with patterns typical 
of the rest of temperate North America. Marin County and 
much of lowland California enjoy what is termed a Medi- 
terranean climate because of its similarity to that of the 
eastern Mediterranean region— a climate found in only a 
handful of areas in the world. Seasonally we experience "a 
desert in summer, a sodden, dripping landscape in winter, 
and a glory of wildflowers in spring" (Major 1977). Zonally 
this is a subtropical climate combining some of the worst 
features of arid and humid climates. The basic features of 
this climate— tempered significandy along the coasdine by 
cool ocean waters— are (1) hot and arid summers and cool 
and humid winters, so that (2) the supply of water and the 
need of water for plant growth are exacdy out of phase, (3) 
the growing season is limited by cool winter temperatures 
and summer drought, and (4) native vegetation is lush in 
the spring when higher temperatures occur temporarily 
with an adequate water supply and either is desiccated or 
fails to grow in summer (Major 1977). In other words we 
have two major seasons: a distinct cool, rainy season when 
the grassy hills turn green and a dry, hot summer when the 
hills turn golden brown. Spring is characterized by increas- 
ing warmdi at the end of the rainy season and a profusion 
of wildflowers that begin to bloom in earnest in February 
and March. Many people also remember spring, not so 
fondly, by the long stretches of intensely windy days. 
Summers on the outer coast are generally characterized by 
cool ocean breezes and recurring overcast or fog, and 
inland by clear skies and hot days. Fall is a period of 
relatively calm and prolonged warmth grading into the 



cooler rainy winter. Here flowers bloom at almost any 
season and the limited fall color, from the few deciduous 
trees and shrubs, lingers into November and December. 

Temperatures 

Winter temperatures in Marin County are generally mild 
because warm air masses usually accompany the frequent 
winter rainstorms. Nevertheless, the pervasive dampness 
gives the impression of cooler temperatures than those 
recorded. Frosts may be frequent locally, but snow dusts 
the higher ridges only every few years. Summer tempera- 
tures can exceed 100° F at interior sites. Mean monthly 
temperatures vary gready between sites in Marin County, 
particularly among coastal and interior stations (Table 2). 
The narrow zone along the shore west of the low coastal 
ridges has an equitable maritime climate— that is, average 
temperatures vary little from month to month. For exam- 
ple, on outer Point Reyes, mean January and July tempera- 
tures vary by only about 4° F (Table 2). Winter 
temperatures on the outer coast remain warmer than those 
inland because of the proximity of heat-retaining ocean 
waters, whereas summer temperatures are depressed by 
cool ocean breezes and to a lesser degree by reduced solar 
radiation during frequent episodes of persistent fog. Sum- 
mer temperatures on the outer coast may occasionally 
reach those of the interior, but hot days are few and cool 
days abound. For perspective, summer temperatures on 
the coast in the San Francisco Bay Area "are among the 
coldest within the continental limits of the U.S., yet air 
temperatures rise so rapidly inland that within 60 miles of 
the ocean the maximum temperatures are comparable with 
any in the U.S. outside the Sonoran-Mohave Desert 
region" (Patton 1956). With every 10-mile increment from 
the coast, mean monthly temperature increases 3° F in July 
and August, and over the same distance the daily maxi- 
mum increases about 4-5° F from June through August. In 
contrast to the outer coast, January and July mean tempera- 
tures in the interior of Marin County vary by almost 20° F 
(Table 2). Although summer temperatures in the interior 
of Marin often hover over 90° F, they too are moderated 
to an extent, relative to inland lowland regions of the state, 
by the proximity of the ocean and San Francisco and San 
Pablo bays. 

Precipitation 

On the central California coast, precipitation falls primar- 
ily as rain, with about 95% of the yearly total compressed 
into the seven-month period from October through April 
(Table 3). Yearly rainfall on the coast generally decreases 
from north to south. Because the moisture-laden air of 
ocean-generated storms must rise when encountering 
Marin's broken, low mountainous terrain, rainfall varies 
gready over the short distances between recording stations 
(Table 3, Figure 4). Although rainfall is relatively high on 
the coastal slope, much moisture passes inland. In fact, 



11 



Climate 



MARIN COUNTY BREEDING BIRD ATLAS 



Climate 



Table 2. Air temperature (degrees F) at selected Marin County sites. 





MEAN 
Jan 


MEAN 
Jul 


MIN-MAX 

FOR ALL 

YEARS 


YEARS OF 
DATA 


Hamilton Field 


46.5 


65.5 


23-106 


25 


Point Reyes 


49.5 


53.6 


30-98 


42 


San Rafael 


49.5 


67.8 


26-110 


30 


Ml Tamalpais 


43.7 


69.0 


19-100 


25 


Kentfield 3 


46.7 


67.0 


17-112 


30 


Cone Madera 


47 


62 


22-108 


20 



Data from U.S. Air Force/Air Weather Service 



Data from U.S. Weather Bureau (1934). 



Data from National Oceanic and Atmospheric Adminisnarion (1982). 
4 Data from Marin Municipal Water District 



some of the wettest areas of Marin are on the east side of 
the first or second coastal ridges, as exemplified by the 
county's highest average precipitation at Kent Lake. While 
nearly rain-free summers are expected, winter rainfall may 
vary dramatically over the course of one rainy season or 
among years. Even in good rainfall years, a very dry early 
winter can be followed by an extremely wet late winter or 
vice versa. The period of atlas field work fortuitously 
encompassed dramatic extremes of yearly rainfall. The adas 
project began with (up to that time) the state's worst 
recorded drought in 1975-76 and 1976-77 and culmi- 
nated with the deluge of 1981-82, highlighted by the 
now-legendary flood of 4 January 1982, when almost the 
whole county was afloat (Table 3). Rainfall in California 
tends to peak and dip on approximately a five- to six-year 
cycle, though not usually reaching these extremes (Michael- 
son 1977). The amount of rainfall in California is a 
function of anomalies in sea surface temperature in the 
North Pacific Ocean as discussed in greater detail below. 

Pacific Ocean Air and Current Cycles 
Publications by Patton (1956), Gilliam (1962), Major 
(1977), and Ainley (1990) portray a dynamic interaction 
between the forces of air, sea, and land. Seasonal (and 
long-term) changes in the ocean currents and air masses of 
the Pacific Ocean drive the weather cycles in coastal Cali- 
fornia. Weather systems move across the Pacific from west 
to east fueled and modified by the direction of the rotation 
of the earth, the overall clockwise circulation of water in the 
North Pacific (Gyre), and the presence of the North Pacific 
High about halfway between California and Hawaii. This 
large high pressure system, with clockwise circulating 
winds, moves northwest in the spring and summer as the 



Northern Hemisphere warms up and southeast in fall and 
winter as it cools. The clockwise circulation of water in the 
North Pacific Gyre sends cool surface waters of the Califor- 
nia Current south along our coasdine year round. This 
boundary current is one of the most productive stretches 
of ocean in the world. It appears to be responsible for the 
largely temperate character of the local breeding marine 
avifauna (in an otherwise subtropical region) and the large 
variety and number of breeding seabirds (Ainley 6k Boekel- 
heide 1990). The main flow of the California Current is 
about 125 to 310 miles offshore; another more variable 
zone occurs close to shore where flow of the current is 
altered by bottom and coastal topography. Also a deep, 
warmer countercurrent (below 650 feet in depth) flows 
northwest along our coast. On the surface west of the 
California Current are found warm subtropical waters of 
relatively high salinity and rather depleted nutrients. 

Bolin and Abbott (1963) described three phases of the 
annual cycle of ocean circulation direcdy off northern 
California: the Davidson Period (Nov-Feb), the Upwelling 
Period (Feb-Sep), and die Oceanic Period (Sep-Oct). Tim- 
ing, intensity, and duration of these three phases varies 
from year to year, as do weather patterns and ocean 
productivity. The Davidson Period commences with the 
cessation of northwest winds and upwelling in the fall (see 
below). At this time the deep countercurrent reaches the 
surface and flows northward along the immediate coast- 
landward of the soudiward-moving California Current— in 
a band about 50 miles wide. Because the North Pacific 
High has dropped southward at this time, the rainy season 
commences as storms are no longer deflected northward 
and now reach our coast. These counterclockwise-circulat- 
ing low-pressure storm systems bring prevailing winds 



12 



UNDERSTANDING BIRD DISTRIBUTION 

Table 3. Average yearly rainfall during the California water-year (Oct-Sep) at selected Marin County stations. 
Seasonality and annual variation of rainfall depicted by patterns at Kentfield. Data primarily from California 
Department of Water Resources (1980). 



Location 


Elev. (ft.) 


Data Period 


Average (in.) 


Range (in.) 


Pt. Reyes Lighthouse 


510 


62 yrs; 1879-1944 


19.57 


9.56-47.45 


Inverness 


150 


14 yrs; 1951-1968 


36.65 


23.80-48.15 


Palomarin* 


240 


17 yrs; 1967-1983 


35.93 


15.82-61.15 


Nicasio 


205 


16 yrs; 1960-1975 


37.16 


21.50-57.85 


Novato Fire House 


18 


17 yrs; 1957-1979 


25.08 


10.19-42.20 


Hamilton Field 





24 yrs; 1934-1963 


25.90 


12.37-47.84 


Kent Lake 


360 


16 yrs; 1960-1975 


66.00 


36.14-116.20 


Woodacre* 


430 


31 yrs; 1951-1983 


45.45 


17.02-79.12 


San Rafael Nad. Bank 


25 


105 yrs; 1872-1976 


36.85 


15.01-67.43 


Kentfield* 


80 


92 yrs; 1896-1983 


47.86 


21.41-88.63 


Mt. Tarn 1 mi. S 


950 


39 yrs; 1898-1958 


35.26 


12.81-74.50 


Muir Woods* 


170 


15 yrs; 1966-1983 


40.71 


18.26-66.21 


Mill Valley 


10 


11 yrs; 1957-1975 


33.69 


18.48-58.02 


Tiburon 


400 


13 yrs; 1958-1979 


29.31 


12.81-47.26 



* Supplemental data direcdy from recording station; all data from PRBO's Palomarin Field Station courtesy of Dave DeSante and Geoff 
Geupel. 



AVERAGE MONTHLY AND YEARLY RAINFALL AT KENTFIELD 



Oct 


Nov 


Dec 


Jan 


Feb 


Mar 


Apr 


May 


jun 


Jul 


Aug 


Sep 


Total 


2.70 


5.58 


8.77 


10.63 


8.37 


6.44 


2.88 


1.33 


0.30 


0.04 


0.04 


0.58 


47.86 



YEARLY RAINFALL TOTALS AT KENTFIELD 
DURING YEARS OF THE MARIN COUNTY BREEDING BIRD ATLAS 



1975-76 


1976-77 


1977-78 


1978-79 


1979-80 


1980-81 


1981-82 


22.54 


23.40 


62.58 


38.42 


56.83 


30.14 


81.75 



13 



MARIN COUNTY BREEDING BIRD ATLAS 




.24" 



V 



£^ 



Marin County 

RAINFALL 

SOURCE: MARIN MUNICIPAL 

WATER DISTRICT 

© 1991 by Dewey Livingston 




Figure 4. Rainfall map of Marin County, Isohyetal lines connect areas of similar average annual rainfall (inches per year). Map 
by Dewey Livingston, 1991. 



14 



Climate 



UNDERSTANDING BIRD DISTRIBUTION 



Climate 



from the south as they approach the coast. The Davidson 
Current is reinforced by these southerly winds (and other 
factors) until prevailing winds shift to the northwest in 
February and March. Because flowing water (or air) tends 
to move to the right in the Northern Hemisphere (reacting 
to the Coriolis force caused by the rotation of the earth), 
water flowing northward in the Davidson Current tends to 
pile up along the coast. This causes water to sink— a 
phenomena called downwelling— and to be replaced by 
water from offshore. Although surface temperatures are 
relatively warm for winter time, they decline through the 
period because the heavy storms of the season mix waters 
to a considerable depth and partially because solar radia- 
tion decreases seasonally. The frequent winter storms often 
hit land with high, gusty winds. Between storms, weather 
can vary from cool, clear days, to variable overcast, to 
occasional periods of ground fog— particularly inland. Win- 
ter also occasionally blesses us with very warm springlike 
days. 

The onset of the Upwelling Period coincides with the 
northward movement in spring of the North Pacific High, 
which again deflects storms to the north. Exceptionally, 
summer rain reaches us from rare fragments of tropical 
storms that move north from the vicinity of Baja California 
or from ocean storms that pass by an infrequently weak- 
ened North Pacific High. Warming of the interior at this 
time sets up a low-pressure system inland. The resulting 
high- to low-pressure gradient causes winds to intensify. 
Prevailing winds are from the northwest due to their origin 
from the clockwise-rotating North Pacific High and deflec- 
tion to the right by the Coriolis force. Periods of intense 
winds will alternate with calm spells, but high winds 
dominate the weather of this period. Winds pick up in the 
morning to reach late afternoon maxima; high velocities 
often extend into predawn hours. From March to August, 
winds on nearby Southeast Farallon Island on average 
attain speeds greater than 16 mph on about one-third of 
all days and on almost one-half of all days from April to 
June. Velocities typically reach 35-40 mph and maxima 
exceed 55 mph (Ainley 1990). The mean wind velocities 
from April through June of 25.7 mph (n = 18 yr.) at Point 
Reyes and 18.8 mph (n = 4 yr.) at Southeast Farallon 
Island are the highest values of any three-month period at 
these stations (Calif. Dept. Water Resources 1978). 
Another important characteristic of these winds is their 
steadiness. Winds flow continuously from the west for 
every hour from May to September except in die early 
mornings of June to August when winds flow from the 
south or west-southwest (Patton 1956). 

These strong northwesterly spring winds increase the 
flow of the cool California Current and move water along 
the immediate coast south and offshore, again because of 
the Coriolis force causing movement to the right. The 
water moving offshore is replaced from below by cool 



nutrient rich waters. This process, called upwelling, is 
restricted to within 1 2 to 30 miles of the coast, along our 
narrow continental shelf. Surface temperatures reach the 
low of the annual cycle during peak winds and upwelling 
from April through June. Moist ocean air passing over the 
cool upwelled waters gives rise to periods of intense coastal 
summer fog described in more detail below. Although 
upwelling continues through the spring and summer, solar 
warming causes sea surface temperatures to rise. The 
nutrient-rich surface waters— supplied by both the strength- 
ened California Current and local upwelling— stimulate 
high productivity of the food chain of algae, zooplankton, 
fish, and ultimately seabirds (Ainley 1990). Marin County 
lies within the region of maximum upwelling along the 
West Coast (stretching from Cape Blanco, Oregon, to 
Point Conception, California), and Point Reyes is a partic- 
ularly important upwelling center. Upwelling occurs pro- 
gressively later in the season from south to north along the 
California coast (Bakun 1973), and ocean productivity and 
weather patterns lag as well. Breeding seabirds time their 
nesting, as do arriving migrant seabirds, to take advantage 
of this seasonal peak of food abundance in late spring and 
summer. Although upwelling occurs during the same gen- 
eral period each year, there is considerable year-to-year 
variation in timing and intensity (Ainley 1990). Conse- 
quendy, the timing and success of seabird breeding can 
vary gready— some years many species do not breed at all. 
The processes affecting upwelling are complex and not well 
understood (D.G. Ainley & W.J. Sydeman pers. comm.). 
Although years classified as cold-water years generally sig- 
nify high ocean productivity/high breeding success (vice 
versa for warm-water years), this is not always the case. The 
timing of spring winds can be as important as their 
intensity— prolonged periods of intense winds and upwell- 
ing can sometimes be too much of a good thing, disrupting 
the productivity at the base of the food chain. Even some 
years widi strong upwelling can be warm-water years. 

During July and August, northerly winds lessen as we 
grade into the relatively calm, relatively fog-free Oceanic 
Period of September and October. With the cessation of 
strong winds, upwelling slackens and the California Cur- 
rent slows down. This is a period of rather passive onshore 
movement of warm nutrient-depleted waters that raise sea 
surface temperatures sharply to their annual high. Skies 
vary from clear to overcast during this period, with rela- 
tively infrequent coastal fog, while temperatures remain 
warm throughout the county and are the warmest of the 
year on the immediate coast. This lag in the occurrence of 
seasonal high temperatures is another characteristic of the 
equitable coastal climate. This period is the West Coast 
equivalent of Indian Summer. As fall progresses and air 
temperatures drop, the North Pacific High moves farther 
south, setting the stage for the return of winter storms and 
die start of another yearly cycle. 



15 



Climate 



MARIN COUNTY BREEDING BIRD ATIAS 



Climate 



Climatic Extremes 

Although die preceding paragraphs describe the typical 
annual weather cycle, extreme conditions arise when shifts 
in global air and water circulation cause a breakdown in 
the normal pattern (see Ainley 1 990). A classic example of 
this was the now famous El Nino event of 1982-83. The 
term El Nino ("the child") was first applied to the warm 
countercurrent that normally occurs off Peru around 
Christmastime, heralding the end of the fishing season. El 
Nino now generally denotes the unusually persistent 
warm-water conditions that occur every two to seven years 
in the Peru Current brought about by atypical circulation 
patterns in the tropical South Pacific. For reasons not 
completely understood, the normally persistent easterly 
trade winds near the equator slacken or reverse. Instead of 
warm water piling up along the coast of Asia, as it usually 
does, it flows back in a long period wave toward the east, 
bringing unusually warm water to the coast of South 
America. In very strong El Nino events, the warm water 
moves up the California coast with a much strengthened 
countercurrent. Usually during such events upwelling 
winds subside, summer fog is infrequent, and winter 
rainfall is extremely high. An El Nino event is usually 
followed by "anti-El Nino" weather that is unusually 
windy, dry, and cold. Other atypical meteorological events 
can transport warm nutrient-poor waters northward along 
the California Coast, strengthening the Davidson Current 
and simulating El Nino-like conditions including high 
rainfall. 

Coastal Summer Fog 

Besides stimulating the food chain, upwelling plays an 
important part in the cycle of summer fog. Although rain 
is virtually nonexistent here in the summer, humidity 
along the shore is die highest of the year. Most people refer 
to the condensation clouds that dominate coastal summer 
weather as "fog," though in actuality they are low stratus 
clouds. Moisture in die air moving in off the open ocean 
condenses when it cools as it passes over the upwelled 
nearshore waters. Although this fog can occur at the 
surface of the ocean or land, moving landward it usually 
forms predominandy in a belt 500 to 2300 feet above sea 
level. Although summer fog can blanket virtually all of 
Marin County, most of it is blocked from penetrating 
inland by the higher coastal ridges, except where gaps or 
low-lying areas occur. Summer fog is a dominant seasonal 
feature of the seaward side of the Marin Peninsula, the west 
slope of Bolinas Ridge in the Mount Tamalpais area, and 
particularly the Point Reyes peninsula west of Inverness 
Ridge; to the north, fog penetrates through the low-lying 
hills near Tomales inland to about Hicks Valley. Except 
near Sausalito and Tiburon, summer fog is infrequent 
along the Marin shoreline of San Francisco and San Pablo 
bays, though "tule fog"— formed by the cooling of humid 
air over chilled land— often envelopes this area in winter. 



A characteristic of the coastal air column that also gready 
affects weather is an inversion layer usually lying at an 
altitude of about 2300 to 4400 feet. Both above and below 
this zone air gets cooler with increasing altitude. In con- 
trast, within the inversion layer air gets warmer with 
increasing altitude. There is also an abrupt transition from 
moist to dry air low in the inversion layer. As moist ocean 
air moves onto land it rises, cools off, and moisture 
condenses as stratus clouds up to the height of the base of 
the inversion layer, where the air is too warm to allow for 
condensation. This phenomenon explains why summer 
views from the top of Mount Tamalpais frequendy show 
just a few ridges peeking through a sea of fog enveloping 
the rest of the county. As a result, in summer the tops of 
Mount Tamalpais and other high ridges experience 
warmer temperatures than nearby lowlands because they 
are within the tempered inversion layer and bathed in 
sunny skies above the stratus layer. 

Although upwelling is thought of as the driving force 
behind summer fog, the origin of the air approaching us is 
also critical. Normally cool air moving in from the north- 
west over the ocean is ideal for fog production, but occa- 
sionally a tongue of high pressure moves over land, and 
warm air off die continent reaches us from the north or 
northeast. In this situation, despite upwelling, the ocean 
cannot cool the warm air sufficiendy to produce fog, and 
the inversion layer may come down to ground level, further 
hindering the process. The inversion layer also explains 
why we rarely have summer thunderstorms, even with a 
copious supply of moist air. This layer blocks air move- 
ment to the altitudes necessary for the production of 
diunderheads, except under the conditions mentioned 
above when die inversion comes down to ground level. 

Summer fog ebbs and flows on a daily cycle. It is most 
intense at night, dissipates normally in the morning or 
early afternoon as the air warms, and increases again in the 
late afternoon as the air cools. It typically ebbs and flows 
on a several-day cycle as well. While the immediate coast is 
enjoying cool temperatures, the interior of Marin may be 
baking in 90-plus-degree weather, with temperatures fre- 
quendy soaring over 100 degrees further inland in the 
Central Valley. As temperatures rise in the interior, the 
low-pressure system there intensifies, causing cool coastal 
air to flow inland, bringing moist air off the ocean and 
increasing fog along the coast. As the interior cools over 
several days, the inflow of air slackens, as does the intensity 
of coastal fog, until the interior heats up again, renewing 
the cycle. Although summer fog is often credited with 
keeping temperatures on the immediate coast low, there is 
evidence that advection of cool air off the ocean is five to 
six times more important in lowering temperatures than is 
loss of solar radiation blocked by the stratus layer (Patton 
1956). 



16 



Climate 



UNDERSTANDING BIRD DISTRIBUTION 



Climate 



Like rainfall, the intensity of summer fog increases from 
south to north along the California coast, with a corre' 
sponding increase in the extent of coast redwood forest. It 
has been stated or implied that coastal summer fog is the 
crucial element that maintains coast redwood forests by 
supplying large amounts of water in the form of "fog drip" 
when moisture condenses as the air collides with the tall, 
massive trees and falls to the ground like rain. In actuality, 
redwoods grow primarily in protected coastal valleys where 
ground fog, wind, and fog drip are not particularly com- 
mon. Fog drip is heaviest along ridge crests at the level of 
maximum stratus layer and where trees are exposed to 
moisture-laden winds. Fog drip per se is not the limiting 
factor in the occurrence of redwoods. Rather, the combina- 
tion of high summer humidity, year-round cool tempera- 
tures, and low evapotranspiration, along with a high water 
table and the alluvial soils of coastal valleys, all provide a 
nourishing environment for these awesome trees. 

Locally, however, fog drip does provide significant addi- 
tional moisture. Oberlander (1956) measured 2 to 60 
inches of precipitation from fog drip at various exposures 



on the San Francisco Peninsula. The highest measure- 
ments, under an exposed tanbark oak, showed more pre- 
cipitation from fog than is normally recorded in an entire 
rainy season. Parsons (1960) recorded 9.8 inches beneath 
a Monterey pine in the Berkeley Hills, and Azevedo and 
Morgan (1974) recorded 1.4 to 16.7 inches at several sites 
in the low mountains south of the Eel River Valley, 
Humboldt County. Much of the precipitation fell during a 
few heavy fog drip periods. Since rainfall is next to nil in 
summer, fog drip and humid air are important determi- 
nants of the types of plant communities growing within the 
coastal zone. There the importance of summer moisture is 
reflected in the dense, rank ground cover beneath Marin 
County's Douglas fir, Bishop pine, and bay laurel-domi- 
nated mixed evergreen forests. Forests in the interior of 
Marin, away from the consistent penetration of summer 
fog, usually have very sparse understories or ground cover 
because of the lack of ground moisture during the summer. 
The types and distribution of plant communities found in 
Marin County are described in the pages that follow. 



17 



MARIN COUNTY BREEDING BIRD ATEAS 












"*> 



.'", 



% 






sdf*3 



H 



^ 










The wind-sculpted California bay forest leaning inland at the Nicasio gap. Drawing by Ane Rovetta, 1989. 



18 



MARIN COUNTY 
BREEDING BIRD HABITATS 



Strip tKe world of its blossoms, and the higher /orms of life must come to a speedy tem\ination. Thus we see the flower playing 
a wonderfully important part in the cosmos around us . . . the instrument b} which Nature brings about the fullness of her 
perfection in her own good season. 

— Mary Elizabeth Parsons, 
The Wild Flowers of California 



MARIN COUNTY hosts a diverse array of habitats for 
breeding birds. Most of these habitats equate with 
the county's plant communities described below. Others 
do not, and these additional habitats are described briefly 
in a section following the plant community descriptions. 

Marin County Plant Communities 

Evolutionary history, varied topography, unusual soils and 
geology, and wide differences in local climate over short 
distances have combined to proyide Marin County with a 
diverse flora and a large number of plant communities 
arranged in a patchy mosaic over the landscape (Figure 5). 
Marin County hosts eleven major plant communities, of 
which six can be subdivided into fifteen associations. 
Consequendy, the county is endowed with habitat that 
supports a wide variety of breeding birds. Because birds 
generally base their habitat choice on the structure of plant 
communities rather than on particular plant species (e.g., 
Verner & Larson 1989), the following descriptions 
emphasize structure over floristics. These descriptions are 
condensed from Shuford and Timossi's (1989) Plant Com- 
munities of Marin County, California, to which the reader 
is referred for greater detail. The communities and associ- 
ations described can, of course, grade into one another to 
varying degrees. The edges of plant communities (eco- 
tones) often support a high diversity of bird species. 

Mixed Evergreen Forest 

This is the predominant forest type in Marin County and 
is characterized by closed-canopy stands of several species 
of broadleaved evergreen hardwoods. Conifers may occur 
in varying numbers and in some cases may dominate. 
Because it occupies a broad range of slope, moisture, and 
elevational gradients, this community is quite variable, 



occurring in three main associations that may grade into 
one another. Mixed evergreen forest grows throughout 
most of the hilly and mountainous terrain of Marin 
County. 

Coast Live Oak-California Bay-Madrone Forest. This 
association is dominated by one or more of these evergreen 
hardwood species: coast live oak (Quercus agrifolia), Cali- 
fornia bay (Umbellularia califomica), and madrone (Arbu- 
tus menziesii). California buckeye (Aesculus califomica) and 
black oak (Quercus kellogii) may be important locally. 
Dominant trees at maturity average 30 to 80 feet in height. 
In drier conditions, a true understory is reduced or lacking 
entirely, with scattered saplings of the dominant trees 
forming the understory where it exists. In most intermedi- 
ate conditions, poison oak (Toxicodendron diversilobum) 
and toyon (Heteromeles arbutijolia) are important under- 
story components. In moister conditions, especially toward 
the immediate coast, this forest association can have a 
well-developed understory of sword fern (Polystichum muni- 
tum), huckleberry (Vaccinium ovatum), California hazelnut 
(Corylus califomica), poison oak, and currant (Ribes spp.) 
about 3 to 6 feet in height. This association occurs at low 
to moderate elevations throughout most of the county. 

Tanbark Oak-Madrone-Live Oak-Douglas Fir Forest. 
This mixed evergreen association is dominated, in varying 
proportions according to site, by tanbark oak (Lithocarpus 
densiflorus), madrone, Douglas fir (Pseudotsuga menziesii), 
coast live oak, and canyon live oak (Quercus chrysolepis) . 
California bay, coast redwood (Sequoia sempervirens) , Cali- 
fornia nutmeg (Torreya califomica), and chinquapin (Casta- 
nopsis chrysophylla var. minor) occur locally. At maturity, 
dominant trees average 30 to 80 feet and occasionally reach 
120 feet. This forest is generally rather open under the 
canopy as the understory consists of scattered saplings of 

19 



Plant Communities 



MARIN COUNTY BREEDING BIRD ATLAS 



Plant Communities 




Figure 5. Preliminary vegetation map of Marin County. Map by Dave Shuford and Deu/ej Livingston, 1991. 



20 



Plant Communities 



MARIN COUNTY BREEDING BIRD HABITATS 



Plant Communities 







mwm 

Oak woodland and oak savannah stand watch on Mount Burdell, Novate Drawing b> Ane Rovetta, I 989. 



the dominant trees. This association occupies mid to high 
elevations on Mount Tamalpais and surrounding ridges 
and, to a limited degree, Inverness Ridge. 

Douglas Fir Forest. This closed-canopy forest is domi- 
nated by Douglas fir, which in mature stands averages 100 
to 1 60 feet in height. There may or may not be a secondary 
canopy of coast live oak, California bay, and blue blossom 
(Ceanothus thyrsiflorus) averaging 25 to 65 feet in height. 
In most cases there is a dense understory about 4 to 8 feet 
in height, consisting of huckleberry, salal (Gaukheria shal- 
lon), sword fern, California hazelnut, poison oak, red 
elderberry (Sambucus callicarpa), and thimbleberry (Rubus 
parviflorus) . In Marin County, Douglas fir forest grows 
mosdy on the southern and central portions of Inverness 
Ridge and locally throughout the Mount Tamalpais and 
Lagunitus Creek watersheds. In these latter areas, Douglas 
fir most frequendy mixes with coast redwood or with trees 
of the tanbark oak-madrone-live oak-Douglas fir forest 
described above. 

Oak Woodland and Oak Savannah 

In contrast to the mixed evergreen forest, typical oak 
woodland and oak savannah have open canopies, grassy 
ground cover below and between the trees, and a predom- 
inance of deciduous, rather than live oaks. Oak woodland 
is distinguished by tree cover greater than 30%, whereas 
oak savannah consists of isolated trees. The characteristic 
tree of Marin's oak woodlands and oak savannah is valley 
oak (Quercus lobata). Although there is no true understory, 
scattered shrubs such as manzanita (Arctosta^k^Ios spp.), 
ceanothus (Ceanothus spp.), poison oak, and several spe- 
cies of herbaceous thisdes may occur, especially on the 
edges. The grassy ground cover consists of species charac- 
teristic of valley grassland, described below. On deeper 



soils on valley floors, valley oaks at maturity vary in height 
from 30 to 100 feet, whereas smaller oaks grow on shal- 
lower soils on steeper slopes. Blue oak (Quercus douglasii), 
a characteristic tree of oak woodland and oak savannah in 
hills of the interior Coast Range, grows locally in Marin 
only in Novato on Mount Burdell and near Black Point. 
Oak woodland and oak savannah generally occupy rela- 
tively dry areas in the interior of Marin County, especially 
around Novato north of Big Rock Ridge and east of Hicks 
Valley. A lack of recruitment of sapling oaks threatens the 
long-term survival of California's oak woodlands and oak 
savannahs. 

Bishop Pine Forest 

This forest is one of a number of relict, fire-adapted, 
closed-cone pine communities that grow in disjunct stands 
along the California coast. Bishop pine (Pinus muricata) is 
the dominant tree, usually thriving in pure, even-aged 
stands that reach 60 to 70 feet in height at maturity. 
Bordering the pines are limited stands of live oak, Califor- 
nia bay, tanbark oak, madrone, California buckeye, and 
wax-myrtle (Myrica calif ornica). The understory of the 
pines is usually a dense shrub layer about 4 to 8 feet high 
consisting of huckleberry, salal, coffeeberry (Rhamnus cali- 
jomica), chinquapin, and two species each of manzanita 
and ceanothus. On deeper soils where the pines reach their 
greatest stature, the shrub layer is taller and often parklike, 
with many grassy openings between the shrubs and pines. 
On steeper slopes and rockier soils, the shrub layer grows 
as continuous low dense cover. In Marin County, bishop 
pine forest dirives primarily on granitic soils on the north 
end of Inverness Ridge on the Point Reyes peninsula. Five 

21 



Plant Communities 



MARIN COUNTY BREEDING BIRD ATIAS 



Plant Communities 




Gnarly bishop pine forest at Tomales Bay State Park. 
Drawing b} Ane Rovetta, 1 985. 



small stands grow on gravelly, sandstone-derived soils east 
of the San Andreas Fault on Bolinas Ridge and in the 
vicinity of Carson Ridge (Millar 1986). 

Coast Redwood forest 

The essence of the coast redwood forest is a towering 
canopy of coast redwoods averaging 100 to 130 feet in 
height, widi exceptional trees reaching 250 feet. California 
bay and tanbark oak may form a subcanopy 50 to 65 feet 
high. California bay is consistendy found along moist 
drainages, while tanbark oak is found on die edges or in 
occasional sunny openings in the forest. The understory 
consists primarily of California hazelnut, huckleberry, 
western azalea (Rhododendron occidentale), wood rose (Rosa 
californica), thimbleberry, and patches of sword fern, and 
is generally open except where it is locally dense along 
streams. This forest occurs primarily east of the San 
Andreas Fault in areas of high year-round humidity, hence 
mosdy in the zone of persistent summer fog. Redwoods are 
widespread on the Mount Tamalpais and Lagunitas Creek 
watersheds, and are local from there north to the north 
slopes of Big Rock Ridge. 

Grassland 

California's grasslands were formerly dominated by peren- 
nial bunch grasses, interspersed with numerous annuals. 
Today these grasslands are dominated by introduced Euro- 
pean annuals whose spread was aided and abetted by stock 
grazing and dry-land farming. Although overall introduced 
annual grasses now dominate our grasslands, native peren- 
nial bunch grasses still persist locally on the immediate 
coast. Grasslands are widespread in Marin, particularly in 
the northwestern region of the county. There are two major 
types of grassland in California and in Marin County: 
coastal prairie and valley grassland. 

Coastal Prairie. Coastal prairie has also been called the 
Festuca-Danthonia grassland after the dominant genera of 
grasses in diis community in California. The dominant 
species in ungrazed sites on Point Reyes are the perennial 
bunch grass hairgrass (Deschampsia holciformis), the low- 
growing form of coyote brush {Baccharis pilularis ssp. 
pilularis), the native biennial grass California brome 
(Bromus carinatus), sheep sorrel (Rumex acetosella), and 
bracken fern (Pteridium aquilinum var. pubescens). Scattered 
bushes of the low-growing coyote brush and bracken ferns 
are characteristic of grassland on the immediate coast but 
become scarcer inland. The scattered brush and ferns and 
the mix of perennial and annual grasses give the coastal 
prairie a more varied structure than that of interior valley 
grasslands. Typical stands of coastal prairie are less than 
about one and one-half feet high. There are marked differ- 
ences between grazed and ungrazed sites. Grazing 
decreases the average height of plants threefold, reduces 
the percent cover of perennial and biennial species, and 
reduces the percentage of native species (Elliott <Sl 



22 



Plant Communities 



MARIN COUNTY BREEDING BIRD HABITATS 



Plant Communities 



Wehausen 1974). Coastal prairie flourishes in the moist 
coastal zone shrouded by persistent summer fog, and its 
distribution seems to parallel that of breeding Grasshopper 
Sparrows (see species account). 

Valley Grassland. The perennial bunch grass that 
originally dominated the valley grassland was needlegrass 
(Stipa pulchra). Among others, two major associates were 
the rye grasses Elymus glaucus and E. triticoides. Stipa 
^ulchra-dominated grassland now occurs very locally on 
Mount Tamalpais. Valley grassland over most of the rest 
of the drier interior of Marin County is dominated by 
introduced annual grasses and forbs such as wild oats 
(Avena fatua and A. barbata), soft chess (Bromus mollis), 
ripgut grass (B. diandrus), fescues (Festuca spp.), and filaree 
(Erodtum spp.). Today valley grassland appears to have 
fewer species and a less varied structure than coastal 
prairie. Valley grassland has one or two often dense layers 
up to about three feet high. On very disturbed or over- 
grazed sites, one or a few species may predominate, and 
local patches of noxious introduced thisdes often thrive. 
Valley grassland predominates in die drier portions of die 
county. 



Coastal Beach-Dune Vegetation 

Dune communities here reside in a narrow zone above the 
wave-washed beaches, primarily on Point Reyes. There is a 
noticeable zonation of plants from the beach inland as a 
function of both changing physical gradients— exposure to 
salt spray and sandblasting by persistent onshore winds— 
and of the length of successional history on stabilized 
dunes. 

Northern Beach Association. Close to the beach the 
dunes are covered mosdy with perennial grasses, usually 
less than two feet tall, and a number of low-growing 
perennial herbs. The latter are generally prostrate, ever- 
green, and succulent as adaptations to the salty air, strong 
winds, and shifting sands. The prominent grasses are the 
perennial American dune grass (Elymus mollis) and the 
introduced European beach grass, or marram grass (Ammo- 
phila arenaria). Associated low-growing herbs include sea 
rocket (Cakile maritima), sandA'erbena (Abronia latifolia), 
silver beach weed (Ambrosia chamissonis), Atriplex leuco- 
phylla, beach morning glory (Calystegia soldanella), ice 
plant {Carpobrotus chilense and C. edulis), and lupines 
(Lupinus spp.), particularly moving landward. The amount 






^C^& 



'■^^^J^^ 













^wK, 






m T7 p - ;■■ ^m-,y^y 




•4Zi. 



Beach and dunes at Limantour Estero strand lapped this day by the gentle surf of Drake's Bay. Drawing fry Ane Rovetta, 1984. 



23 



Plant Communities 



MARIN COUNTY BREEDING BIRD ATLAS 



Plant Communities 







A mosaic o/ grassland, mixed evergreen forest, and chaparral clothing Big Rock Ridge just east of the Big Rock. 

Drawing by Ane Rovetta, 1 989. 



of plant cover can reach 100% but generally averages about 
10%-25%. The introduction for "dune stabilization" of 
the European beach grass has caused the development of 
a steep-sided foredune parallel to the beach and abutting a 
series of wind-molded dunes and coastal swales oriented 
perpendicular to the beach and coast. Formerly foredunes 
rose gradually to the landward perpendicular dunes that 
had many openings among them connected to the beach. 

Northern Dune Scrub. Landward, a dune scrub associa- 
tion about three to five feet high occupies the older, more 
stable dunes. This association is characterized by a number 
of perennial lupines {Lupinus albi}rons, L arboreus, L 
rivularis, and L. chamissonis), mock heather (Haplopappus 
ericoides), and the low-growing, small-leaved form of coyote 
brush (Baccharis pilularis). These shrubs, mixed with other 
subshrubs and perennial and annual herbs, usually form 
an open canopy. 

Northern Coastal Scrub 

Northern coastal scrub or "soft chaparral" is a two-layered, 
herb-rich, evergreen shrub community that grows on the 
lower slopes of hills in the summer fog zone along the 
immediate coast. It consists of two major associations: 

Coyote Brush-Sword Fern Scrub. Coyote brush (Bacch- 
aris pilularis ssp. consanguinea) dominates this association, 
which has a closed or open overstory about three to seven 
feet tall. Other important overstory shrubs depending on 
site and exposure are poison oak, California hazelnut, blue 
blossom, coffeeberry, thimbleberry, and, in the spring and 
summer, cow parsnip (Heracleum lanatum). The under- 
story varies from a dense, tangled interwoven thicket of 
ferns along with low woody and herbaceous perennials 

24 



and annuals to a more open one devoid of ferns. Western 
sword fern usually dominates the understory at denser and 
moister sites, but California blackberry (Rubus ursinus 
and/or R. vitifolius), salal, western bracken fern, huckle- 
berry, bush monkey-flower (Mimulus aurantiacus), and 
Douglasiris (Iris douglasiana) may be important compo- 
nents along with grasses, sedges (Carex spp.), rushes ()un- 
cus spp.), and other forbs. This association is widespread 
on the lower ocean-fronting hills the length of the county, 
particularly on north-facing slopes. 

Coastal Sage-Coyote BrusK Scrub. This is a one-layered 
coastal scrub association dominated by coastal sage (Arte- 
misia californica) about two to four feet high with lesser 
amounts of coyote brush, poison oak, bush monkey- 
flower, California blackberry, western bracken fern, 
grasses and forbs, and, in some areas, lupines. Open areas 
among the bushes in many areas are either bare or rocky 
soil or, more frequently, are covered with grasses and 
forbs. Spanning the length of the county on south-facing 
slopes, this association is most widespread on the southern 
end of the Point Reyes peninsula and east of the San 
Andreas Fault from Bolinas Lagoon south to the Golden 
Gate. 

Chaparral 

Dense chaparral scrub arises from poor rocky soils on drier 
inland hills. Dominant chaparral species are evergreen, 
densely branched, woody summer-dormant shrubs with 
small thick stiff leaves. Chaparral is highly adapted to fire 
and regenerates quickly. The shrubs in this community 
generally form a single dense, intertwining, almost impene- 
trable overstory layer with a sparse ground cover below. 



Plant Communities 



MARIN COUNTY BREEDING BIRD HABITATS 



Plant Communities 



Chaparral associations vary with slope, sun exposure, 
elevation, soil, and fire history. Chaparral grows here only 
east of the San Andreas Fault, primarily on Mount Tamal- 
pais, Pine Mountain/Carson Ridge, and Big Rock Ridge. 
Marin's hills support four chaparral associations: 

Chamise Chaparral. Chamise (Adenostoma fascicula- 
turn) dominates this association, forming almost unbroken 
stands on hot xeric sites, usually on south- or west-facing 
slopes and ridges. Chamise here reaches a height of three 
to six feet at maturity. Manzanita and ceanothus occur 
infrequently in this association. 

Manzanita Chaparral. Manzanita shrubs three to six 
feet high dominate this association. Typical manzanita 
species of the chaparral are Cushing manzanita (Arcto- 
staphylos cushingiana), hoary manzanita (A. canescens), and 
Marin manzanita (A. virgata). Manzanita and chamise 
chaparral often alternate on east- and west-facing slopes— 
for example, along the Old Railroad Grade near the West 
Point Inn on Mount Tamalpais. 

Mixed Chaparral. Mixed chaparral consists of an 
almost even mix of manzanita, chamise, buck brush (Cea- 
nothus ramulosus), and interior live oak (Quercus wislizenii 
var. frutescens) ranging from diree to ten feet high. It 
abounds on mesic sites, where it usually grades into mixed 
evergreen forest on shady slopes or in draws. Other shrubs 
of this association are chaparral pea (Pickeringia montana), 
coffeeberry, and ceanothus (Ceanothus sorediatus and C. 
foliosus). 

Serpentine Chaparral. This association is restricted to 
biologically harsh serpentine soils. There the shrub canopy 
is broken with bare ground and rock outcrops, and shrubs 



generally are dwarfed or stunted, often reaching only about 
one and one-half to three feet in height. Characteristic 
shrubs are leather oak (Quercus durata), Jepson's cea- 
nothus (Ceanothus jepsonii), Tamalpais manzanita (Arc- 
tostaphylos montana), and Sargent cypress (Cupressus 
sargentii). Elsewhere it may grow as a fairly large tree; but 
on Carson Ridge, Sargent cypress grows amid the chapar- 
ral as a striking dwarf forest ten to fifteen feet high. 
Serpentine chaparral occupies extensive areas along the 
Pine Mountain Fire Road on Carson Ridge and on Mount 
Tamalpais on Serpentine Knoll and on the Benstein Trail 
above Potrero Meadows. 

Coastal Salt Marsh 

Salt marsh is restricted to the upper intertidal zone of 
protected shallow bays, estuaries, and lagoons. Vertical 
zonation of saltmarsh plants reflects elevational gradients 
that affect the frequency and duration of tidal flooding. 
Bordering the mudflats are pure open stands of cordgrass 
(Spartina foliosa) about one and one-half to three feet tall. 
Landward, cordgrass is replaced at the mean high water 
level by thick mats of low-growing salt marsh dominated 
by pickleweed (Salicornia virginica), generally about four to 
eighteen inches in height. Other characteristic plants of the 
upper pickleweed zone are alkali heath (Frankenia grandi- 
folia), marsh rosemary (Limonium calif ornicum), jaumea 
(faumea carnosa), plantain (Plantago maritima), and salt- 
grass (Distichlis spicata). On isolated mounds or along 
natural levees of tidal sloughs not subject to frequent 
flooding grow clumps of gumplant {Grindelia humilis or G. 
stricta) and dock (Rumex occidentalis) up to about three feet 
tall. In the grasslandlike upper border of the salt marsh, 














mmmmi 

A toe hold of coastal salt marsh on the shores of San Pablo Bay abutting the mixed evergreen forest, 

China Camp State Park. Drawing by Ane Rovetta, 1 989. 



grassland-blended hills of 



25 



Plant Communities 



MARIN COUNTY BREEDING BIRD ATI AS 



Plant Communities 



saltgrass and spergularia (Styergularia spp.) mix with other 
salt-tolerant natives and introduced species. Where salt 
marshes historically graded primarily into brackish marsh 
and then into freshwater marsh, grassland, or shnjb com- 
munities, today most salt marshes abruptly abut dikes and 
roadsides. In brackish situations the marsh is dominated 
by various forms of bulrush (Scir|)us spp.) and cattails 
(Typha spp.). 

An estimated 60%-95% of the marshland in the San 
Francisco Bay system has been lost to filling and diking 
(Nichols ck Wright 1971, Josselyn 1983). Remnant stands 
of salt marsh still persist in Marin County at a number of 
sites along the shores of San Francisco and San Pablo bays 
and on the outer coast in the upper reaches of Tomales 
Bay, Limantour and Drake's esteros, and at Bolinas La- 
goon. 

Coastal Riparian Forest 

In Marin County, willow- and alder-dominated riparian 
groves border small streams and the edges of ponds and 
freshwater marshes, where the trees merge with marsh 
vegetation. Typical overstory trees of our riparian forests 
are red alder (Alnus oregona), white alder (A. rhombi folia), 
arroyo willow (Salix lasiolepis), yellow willow (S. lasiandra), 
big-leaf maple (Acer macrophyllum), and box elder (A. 
negundo ssp. califomicum). Near stream and marsh edges 
willows colonize recendy deposited soils and are tolerant 
of some flooding. Because they reproduce vegetatively, 
willows often form pure stands with overstory height 
averaging 10 to 15 feet. Landward, willows usually inter- 
grade widi alders, which may provide an overstory canopy 
30 to 40 feet high or may grow in pure stands of similar 
height. 



The riparian undcrstory may include saplings of die 
overstory trees and thickets of California blackberry or 
Flimalaya-berry (Rubus procerus) interspersed with a thick 
herbaceous ground cover. Under natural conditions, alder 
groves may sometimes have little understory or ground 
cover, but cattle often eliminate the low vegetation under 
both alders and willows by grazing and trampling. Ripar- 
ian forests may grade into a number of other communities. 
In stream canyons, moisture-loving, shade-tolerant Califor- 
nia bay trees may mix with or replace die typical riparian 
dominants. With the spread of human influence, riparian 
communities in Marin County, as elsewhere, have been 
lost or degraded at an alarming rate. Alder and willow 
riparian thickets are still widespread in drainages on the 
outer coast, but few remain in the urbanized corridor near 
the bayshore of eastern Marin. 

Freshwater Marsh 

Bulrush-Cattail Marsh. Typical freshwater marsh 
thrives in shallow standing or slow-moving water on the 
edges of ponds, lakes, or streams. Cattail and California 
bulrush (Scirpus californicus) border open water in mixed 
association with each other, or in pure stands, averaging 
five to eight feet high. Assemblages of other marsh species, 
usually about one and one-half to five feet high, grow in 
shallower water or damp soil. These include rushes (Juncus 
spp.), sedges (Cyperus eragrostis and Carex spp.), spike rush 
(Eleocharis spp.), curly dock (Rumex crispus), sheep sorrel, 
water parsley (Oenanthe sarmentosa), and the bur-reed Spar- 
ganium eurycarpum. The county's largest freshwater marsh 
and willow riparian stand is located at Olema Marsh. 




Riparian forest of Olema Valley flanked by Douglas fir forest, mixed evergreen forest, and grassland on Inverness Ridge. 

Drawing b> Ane Rovetta, 1989. 



26 



Plant Communities 



MARIN COUNTY BREEDING BIRD HABITATS 



Plant Communities 




Riparian growth Hemming in the view of pond turtles at Five Brooks Pond. Drawing by Ane Rovetta, I 985. 



Coastal Swale. Another type of freshwater marsh 
grows primarily along the outer coast where water reaches 
the surface in depressions in coastal prairie or among 
dunes. Water channels are choked with water cress (Na- 
sturtium officinale), water parsley, and marsh pennywort 
(Hydrocotyle ranunculacea and H. verticillata) ranging up to 
1 V2 feet in height. In shallower water or on saturated mud, 
the swale may be dominated by pure stands up to 4 feet 
high of the bulrush Scirpus microcarpus or slough sedge 
(Carex obnupta). On drier ground grow clumps of various 
grasses, interspersed with plants such as marsh checker- 
bloom (Sidalcea rhizomata), bog lupine (Lupinus polyphyllus 
var. grandifolius), Siberian montia (Montia sibirica), the 
monkeyflower Mimulus guttatus, and poison hemlock 
(Conium maculatum). Riparian-like patches of wax-myrde, 
6 to 12 feet high, sometimes border swales. Coastal swales 
in damp meadow soil may be dominated by clumps of 
rushes and sedges mixed with grasses. 

Exotic Plants 

Exotic plants have been introduced widely in California 
and Marin County. In urban and suburban settings, 
ornamental plants, whether alone or mixed with native 
species, provide shade and beauty for human inhabitants 
and food and shelter for wildlife. Many exotics have 
escaped and become naturalized in native communities 
with varying effects. Some introduced plants are inconspic- 
uous immigrants sharing the resources with dominant 



native species, whereas other aggressive exotics have 
pushed out and replaced the rightful heirs of our plant 
communities. As noted above, Mediterranean annual 
grasses have entirely changed the character of our native 
grasslands, and European beach grass has altered both the 
structure and flora of dune communities. The range and 
extent of effects that introduced plants have had on the 
native flora and on the birds and other native wildlife that 
depend on them are very incompletely known. 

Of the many introduced species naturalized in the 
county, only the most conspicuous, widespread, or offen- 
sive ones are mentioned here. Many species of Eucalyptus 
have been introduced to California— the most common 
and widespread is bluegum eucalyptus (E. globulus). Exten- 
sive stands, planted originally as windbreaks and woodlots, 
now grow as small patches of forest in what formerly were 
almost treeless expanses of grasslands in Marin County. 
Mature eucalyptus groves may form towering canopies 
reaching over 100 feet skyward. The volatile oils produced 
by eucalypts preclude the establishment of a rich under- 
story flora. The most frequent understory plants are sap- 
ling eucalypts, blackberries, and in some areas, the exotic 
and blanketing German ivy (Senecio mikanioides). Eucalyp- 
tus groves have shown only a limited ability to invade forest 
edges and are most successful in penetrating grasslands 
and brush communities. Planted Monterey cypress 

27 



Plant Communities 



MARIN COUNTY BREEDING BIRD ATLAS 



Additional Habitats 



(Cupressus macrocarpa) forms a similar (though infre- 
quently self-generating) community of lesser stature around 
farmyards, mosdy on the outer coast. 

Scotch broom (Census scoparius) and French broom (C. 
monspessulanus) are widely naturalized along the county's 
disturbed roadsides. French broom has been especially 
successful in invading native communities of brush, open 
woodland, and grassy hillsides. Other conspicuous invad- 
ers are the showy white-plumed jubata (Andean) grass— a 
close relative of the less invasive pampas grass— and gorse 
(Ulex europaeus), a dense diorny shrub that is difficult to 
eradicate. 

Disturbed fields and roadsides usually support diickets 
of introduced annual weeds, some of which may reach 6 
to 10 feet in height. Conspicuous in such areas are sweet 
fennel (Foeniculum vulgare), poison hemlock, wild oat 
(Avena fatua), teasel (Dipsacus sativus), and other weedy 
herbs and exotic grasses of disturbed valley grassland. 

While many exotics are here to stay, extensive monitor- 
ing and eradication efforts are needed to ensure the future 
integrity of our distinctive native flora and plant communi- 
ties. When known, the extent of use by breeding birds of 
exotic plant species and communities is described in the 
species accounts. 



Additional Breeding Bird Habitats 

Breeding birds may use a number of habitats beyond the 
standard plant communities, but these can usually be 
described verbally without resort to a formal classification 
scheme. For example, rocky cliffs may provide nest sites for 
White-throated Swifts, Cliff Swallows, Common Ravens, 
and Rock Wrens. Rocky sea stacks, wave-battered cliffs, 
and offshore islands are home to busding colonies of 
storm-petrels, cormorants, gulls, alcids, and scattered pairs 
of oystercatchers. Human structures may supply nesting 
shelter for a variety of birds, including American Kestrels, 
Pacific-slope Flycatchers, Black Phoebes, several species of 
swallows, American Robins, European Starlings, House 
Finches, and House Sparrows, among others. Although a 
pair of Killdeer may select for their nest site the worn 
pebbles along a stream margin, they seem equally at home 
incubating their eggs in similar substrate in driveways or 
on gravel roofs. Ponds may furnish the requisites for 
species such as Pied-billed Grebes or American Coots that 
build floating nests. 

For many species of landbirds the nesting habitat and 
die foraging habitat are one and the same. Other species 
may conceal their nests near the edge in one plant commu- 
nity and forage in an adjoining community or in the 




Bishop pines lean outward /rom Mount Vision toward Tomales Bay. Drawing by Ane Rovetta, 1 989. 



28 



Additional Habitats 



MARIN COUNTY BREEDING BIRD HABITATS 



Additional Habitats 



surrounding air space. Different species of swallows may 
have distinct and faidy easily described structural require- 
ments for nest sites, as well as presumably distinct, but not 
so easily described, air space requirements for foraging. 
Most seabirds breed on islands or steep mainland cliffs 
and forage considerable distances at sea. Ducks often nest 
in upland areas but forage in aquatic habitats and soon 
lead their young there as well. Similarly, herons and egrets 
select relatively predator-proof nesting sites high in trees, 
on islands, or in marshes over water and may forage in 
nearby or distant wedands. Hence it is not possible to 
classify each species by preference for one or several habi- 



tats, any of which will satisfy all their needs at a given time. 
Not only may a single species be dependent on more than 
one habitat while nesting, but its habitat needs also may 
change during the course of the breeding season. Habitat 
descriptions and preferences beyond those portrayed here 
can be found in the individual species accounts; discussion 
of species membership in various bird communities can be 
found in the Results and Discussion section (p. 61). 
Changes in land use that may have affected the suitability 
of various habitats in Marin County for breeding birds are 
discussed in the section that follows. 




29 



MARIN COUNTY BREEDING BIRD ATLAS 




Grass-covered Kills with mixed evergreen forest filling draws above Nicasio Reservoir. Drawing by Ane Rovetta, 1 989. 



30 



HISTORY OF LAND USE 
IN MARIN COUNTY 



How can you expect the birds to sing when their groves are cut down? 



— Henry David Thoreau, 
Walden 



FOR THOUSANDS of years people we now call Coast 
Miwoks lived lightly on the land in Marin and part of 
adjacent Sonoma County. In aboriginal times their entire 
population numbered about 2000 persons (Kelly 1978). 
These Native Americans subsisted by harvesting the abun- 
dant sea life, stream-dwelling fish, upland game, and a 
variety of fruits, berries, seeds, and roots that supple- 
mented their staple of acorns. Although relatively little is 
known of their history, by all accounts they lived in 
harmony with nature, preserving the bounty that greeted 
them when they first occupied these lands. 

This way of life was destined to pass as the seeds of 
enormous change were sown in the late 1 500s and early 
1600s by the arrival of the earliest European explorers- 
Drake, Cermeno, and Vizcaino— on Marin's shores. In 
1 776 the Spanish established a mission and presidio in San 
Francisco, and in the same year traveled north to explore 
parts of what we now call Marin County (Munro-Fraser 
1880). The next wave of expansionism broke with the 
founding of the San Rafael mission in 1817. Forced 
evangelization of the native population soon led, via 
demoralization and disease, to the disintegration of their 
culture (Kelly 1978). By 1851 or 1852 only about 250 
Coast Miwoks remained. 

The demise of native wildlife populations— and even 
whole ecosystems— at the hands of the invading Europeans 
was equally swift. Exploitation of die forests began almost 
as soon as Europeans visited these shores. The first com- 
mercial logging was established in Larkspur in 1816 to cut 
cordwood for Spanish troops at the presidio (Fairley 1987). 
In the 1820s and 1830s Yankee whalers and trading ships 
visiting San Francisco Bay laid anchor at Sausalito for 
wood and water. Wood was needed in quantity to fuel the 
whaler's trypots, and the mission in San Rafael undoubt- 
edly used wood extensively for various activities during its 
tenure from 1817 to 1834. 



With the secularization of the missions in 1834, timber 
was a big attraction on the new land grants at Rancho 
Corte Madera del Presidio (1834) and Rancho Sausalito 
(1836). Much of Marin's shoreline along San Francisco 
Bay was heavily forested, and the wood was quickly har- 
vested. Early logging concentrated on the lower slopes of 
Mount Tamalpais, in the bottoms of canyons where giant 
redwoods grew and where timber could be easily trans- 
ported via ships on the bay. Marin's first sawmill was built 
in Cascade Canyon on die Tamalpais slope in about 1836 
(Munro-Fraser 1880, Mason 6k Park 1975, Fairley 1987). 
Although some of the timber supplied local needs, such as 
construction of ranch buildings and fuel for brick kilns, 
most was shipped to San Francisco— redwood for wharf 
pilings and warehouses, other trees for cordwood to heat 
city buildings. Cordwood was also cut in the Novato area, 
where oak and bay were the dominant trees. Attesting to 
the rapidity of exploitation, all the choice redwoods were 
felled in Mill Valley by 1852, when a steam mill was 
moved to Bolinas, diough at the time Corte Madera was 
still being actively logged (Fairley 1987). 

Fueled by the boom of the Gold Rush, the 1850s to 
1 870s were the era of greatest timber exploitation. Lumber- 
ing concentrated then near Bolinas and on the north 
slopes and ridges of Mount Tamalpais. Dogtown became 
a major logging and lumber center, beginning with its first 
mill in 1851 (Fairley 1987). By 1880 about 15 million 
board feet of lumber had been cut near Bolinas (Munro- 
Fraser 1880, Fairley 1987). Logging continued in the 
Bolinas area throughout the nineteendi century. Much of 
it was to supply cordwood for San Francisco houses and, 
after 1875, to fuel steam locomotives. Large amounts of 
cordwood also came from the north slopes of Mount 
Tamalpais, from which it was shipped to Ross Landing 
(Corte Madera); some was burned at San Quentin prison, 
but most was sent to San Francisco. Tanbark oak was cut 
for its bark, used to tan hides, and the remainder was sold 

31 



History of Land Use 



MARIN COUNTY BREEDING BIRD ATIAS 



/ hstory of Land Use 



for cordwood (Rothwcll 1959). Extensive woodcutting in 
this area also supplied railroad ties and heavy studding for 
the White's Hill tunnel of the North Pacific Coast Rail- 
road, fence posts for big ranches being subdivided for dairy 
farms, and cordwood for the steam engines of the second 
Pioneer Paper Mill on Papermill Creek (Rothwell 1959, 
Fairley 1987). 

Logging continued in the lower drainage of Lagunitas 
Creek until 1903, when the supply of old'growth timber 
was just about exhausted (Fairley 1987). Except at Muir 
Woods, ultimately all the old-growth timber on Mount 
Tamalpais fell to the woodsman's ax and saw. The intro- 
duction of oil (1902), gas, and electricity ended the suprem- 
acy of cordwood and relaxed, somewhat, the intense 
pressure on Marin's forests. Around 1918, a second round 
of cutting occurred in the area to be flooded by Alpine 
Dam. A mill operated in the lower Lagunitas Creek drain- 
age until 1951 ; the site was flooded with the completion of 
the Kent Lake dam in 1953. Between 1946 to 1951 this 
mill sawed over 21 million board feet of lumber (Fairley 
1987). Much timber was cut on Inverness Ridge in die late 
1950s and 1960s, and die last logging in the county, on 
Bolinas Ridge above Dogtown, was shut down by court 
order in 1969 (Mason 1981, D. Livingston pers. comm.). 

We'll never know die full effect of all this logging on the 
county's birdlife, but it must have been tremendous. The 
loss of most of the old-growth forest on the slopes of 
Mount Tamalpais, largely in a period of fifty years, must 
have displaced great numbers of birds breeding in these 
habitats. One can only speculate, but it seems very likely 
that the populations of largely old growth-dependent spe- 
cies such as the Spotted Owl must have plummeted during 
this period. Great fires (usually human caused), such as 
those in 1929 and 1945, were similarly destructive (Fairley 
1987), but the return of nutrients to the soil in these cases 
undoubtedly speeded recovery. 

Logging also filled the creeks and estuaries in down- 
stream drainages with silt, altering these habitats pro- 
foundly. Boat traffic was restricted in Bolinas Lagoon, 
Tomales Bay, Corte Madera Creek, and Richardson Bay 
by the silt from logging and to a lesser degree, in most 
cases, from plowing of fields (Munro-Fraser 1880, Rodi- 
well 1959, Melbostad 1969, Mason & Park 1975, Fairley 
1987). Sedimentation from these sources, dredging of 
channels and harbors, leveeing of tidal marshes, and, 
particularly, the transport into San Pablo Bay of debris 
from the massive hydraulic mining in the Sierra Nevada 
from 1853 to 1884 all increased the amount of tidal marsh 
in Marin County (Atwater et al. 1979, Josselyn 6k Buch- 
holz 1984). Nevertheless, the extent of historic expansion 
of tidal marsh habitat has been far outweighed by losses. 
In fact, tidal marsh habitat in the greater San Francisco Bay 
estuary has decreased historically by 60% to 95% (Nichols 
6k Wright 1971, Atwater et al. 1979). As of 1984 only 



about 32% of the tidal marsh habitat diat existed in Marin 
County in 1850 remained (Josselyn 6k Buchholz 1984). 
Large tracts of tidal marsh were first diked off around 
Novato and San Rafael in the late 1800s, with diking 
accelerating in the 1900s, particularly after 1940 (Atwater 
et al. 1979, Josselyn 6k Buchholz 1984). Since 1974, 
several projects have restored some of these marshes to 
tidal action, though the total acreage is small compared to 
habitat lost. 

The effect on bird populations of these losses of tidal 
marsh habitat has been great, though little documented 
except for certain species. Loss of salt marsh is the main 
reason for the decline of the endangered California Clap- 
per Rail, and marsh loss and fragmentation currendy 
threaten populations of salt marsh-breeding Black Rails 
and Song Sparrows (see accounts). Many other birds that 
use diese habitats for breeding, foraging, or roosting have 
likewise been affected. 

Fortunately, many birds reside in the seasonal wedands 
formed by the alteration of tidal marshes. But these wet- 
lands are also being lost rapidly to urban encroachment 
(Granholm 1989). Between 1956 and 1988, 61% of the 
seasonal wedands in south San Francisco Bay were lost. 
From 1975 to 1988, 35% of the remaining seasonal 
wedands in San Francisco Bay were lost and 10% of those 
in San Pablo Bay. During the latter period, Marin County 
lost 9% of its seasonal wedands, and in the foreseeable 
future it will lose an additional 1 3% if all currendy planned 
projects are implemented. The impact on birds inhabiting 
seasonal wedands is obvious. 

The damming of Marin's streams for municipal water 
supplies beginning in 1873 (Fairley 1987) may have 
doomed breeding American Dippers in the Lagunitas 
Creek watershed (see account). But on the whole, the loss 
of streamside and upland habitat to inundation has been 
balanced by the expansion of aquatic habitat and the 
accretion of some marshland. Female Common Mergan- 
sers and their broods now ply the waters of Kent Lake 
while Pied-billed Grebes, American Coots, Marsh Wrens, 
Red-winged Blackbirds, and Song Sparrows suspend their 
nests in marshy fringes of many of the county's eight major 
reservoirs. In contrast to these upstream benefits to birds, 
it seems likely diat loss of fresh water downstream must 
have degraded some of Marin's important wedands. 

Agricultural uses have also taken their toll on the land. 
Although various crops have been grown in Marin, catde 
and, particularly, dairy ranching have dominated the agri- 
cultural economy since the early days of white setdement. 
With the establishment of the San Rafael mission in 1817, 
large herds of Mexican longhorn catde ranged freely on the 
land, to be annually slaughtered for their hides and tallow 
(Mason 6k Park 1975, Fairley 1987). In 1834 the San 
Rafael mission owned 4500 catde (Mason 6k Park 1971). 
With the secularization of the missions that year, thou- 



32 



History of Land Use 



HISTORY OF LAND USE IN MARIN COUNTY 



History of Land Use 



sands of cattle soon roamed the large land grants through- 
out the county. In response to the boom of the Gold Rush 
the dairy industry prospered, particularly on the lush 
grasslands of Point Reyes. In 1870 Point Reyes boasted the 
largest dairy operation in California (Mason &. Park 
1971); the assessor's rolls reported 25,390 cows— the high- 
est number for any county in the state (Fairley 1987). 
Overgrazing was noticed as early as the 1850s in coastal 
areas of California (Heady 1977). The introduction of alien 
grasses, dry-land farming practices, and year-round concen- 
trated grazing all combined to drastically alter native grass- 
lands from ones dominated by perennial bunch grasses to 
ones dominated by exotic annual grasses (see Bird Habitats 
section p. 22). The effects of these changes on bird popu- 
lations are undocumented but must have been great 

Agricultural practices have also inadvertendy fostered 
the pervasive expansion of species such as the European 
Starling (introduced) and Brown-headed Cowbird (native) 
that have adversely affected many native hole-nesting and 
cup-nesting landbirds, respectively (see accounts). Grazing 
and land clearing (for various purposes) have reduced and 
degraded Marin County's riparian habitat, though to an 
unknown degree as no inventories have been taken. 

Direct exploitation of the region's wildlife also exacted a 
heavy toll. In fact, Stine (MS) concluded that the California 
game trade "is the foremost example of rapid commercial 
plunder of a region's wildlife to be found on this conti- 
nent." The demise or decline of coastal populations of 
whales; sea otters, fur seals, and other pinnepeds; anad- 
romous fish; shellfish; and upland game such as tule elk, 
grizzly and black bears, and various furbearers has been 
relatively well documented (Grinnell, Dixon, ck Linsdale 
1937; Skinner 1962; Stine MS). Less is known of impacts 
on bird populations. Nevertheless, Grinnell et al. (1918) 
concluded that in California "beyond question waterfowl 
and upland game birds have both on the average decreased 
by fully one-half within the past forty years." 

Perhaps the first extensive exploitation of the region's 
bird populations was at the Farallon Islands, where Rus- 
sian sealers harvested the meat and eggs of breeding 
seabirds. Between 1812 and 1827 they annually killed 
5000 to 10,000 seabirds, peaking at 50,000 in 1828 (Stine 
MS). The Russians skinned the birds and shipped the 
dried meat to Fort Ross, where it was a highly prized food 
item. Fort Ross also served as a supply center for fur 
operations in Alaska and Kamchatka. The Russians at Fort 
Ross in 1827 and 1828 shipped nine sea lion bladders 
containing hundreds of pounds of insulating feathers of 
Farallon seabirds to Nova Arkangelsk (Sitka) in Russian 
America (Stine MS). 

The intensity of exploitation of wildlife resources accel- 
erated with the rapid increase of the human population at 
the time of the Gold Rush. The commercial harvest of 
Common Murre eggs on the Farallon Islands from 1848 



to the early 1900s had a devastating effect on populations 
of murres and most other species of seabirds breeding on 
those islands (Ainley & Lewis 1974). There appears to be 
no record of exploitation of seabird colonies on the Marin 
County coasdine, but it seems unlikely that any large 
rookery went unmolested at a time of unrestrained harvest- 
ing practices. Market hunting rapidly depleted populations 
of waterfowl, shorebirds, and Clapper Rails around San 
Francisco Bay (Grinnell et al. 1918). One observer thus 
described the decline in duck numbers in the Marin 
County area: "In 1876 ducks were very plentiful in all the 
marshes from Sausalito north to Petaluma, Napa and 
Vallejo. In those days it was easy for a boy to kill from 
twenty to thirty ducks in a day s shooting and very much 
larger bags were obtained by experienced hunters. Today 
[1913], in the region between Sausalito and Novato, I 
think it is safe to say there is not one duck in the marsh 
now where there were a hundred then" (Grinnell et al. 
1918). Egrets were also shot for their feathers, in demand 
by the millinery trade, leading to their near extinction in 
the Bay Area at the turn of the century (see accounts). 
Although measures to protect wildlife were passed in 
California as early as 1852, it was not until 1913 with the 
prohibition on the sale of game in the state and the passage 
of the Federal Migratory Bird Treaty Act that wildlife began 
to be given a semblance of the protection we see today 
(Grinnell et al. 1918). 

As enlightenment spread regarding the need to conserve 
our exploited wildlife resources, the "indirect" impacts of 
an expanding human population continued to negatively 
affect the county's birdlife. These impacts fall into two 
broad categories: direct conversion of wildlife habitat to 
industrial, agricultural, and residential uses; and indirect 
contamination or degradation of habitat from human 
activities. Marin County's population is now concentrated 
in the eastern urban corridor along Highway 101, domi- 
nated by light industry, service-oriented businesses, resi- 
dential neighborhoods, and their attendant impacts. Rural 
West Marin has a ranching- and tourist-based economy, 
wid^i much of the land there set aside in federal or state 
parks or protected by agricultural zoning (Figure 6). 
Throughout most of its history the county's population 
and development have concentrated along the shores of 
San Francisco and San Pablo bays because of the easy 
transportation links to nearby population centers. From 
323 inhabitants at the time of the first census in 1850, the 
county's population has grown exponentially to 230,096 
people in 1990 (U.S. Bureau of the Census). The postwar 
boom saw the population expand dramatically from 
52,907 people in 1940 to 206,038 in 1970. The impacts 
on the land have followed a similar pattern, as detailed 
above, with regard to the loss of tidal marshes and seasonal 
wedands. 



33 



J iistory oj iMnd Use 



MARIN COUNTY BRHHDING BIRD ATIAS 



History of Land Use 




Marin County 

LAND USE 

SOURCE: MARIN COUNTY 

PLANNING DEPARTMENT 

© 1991 by Dewey Livingston 



Figure 6. Map of land use patterns in Marin County. Map by Dewey Livingston, 1991. 



34 



History of Land Use 



HISTORY OF LAND USE IN MARIN COUNTY 



History of Land Use 



The impact of a human population is a function not 
only of population size, but also of the affluence of that 
population and the disruptiveness of the technologies 
providing the goods consumed (Ehrlich 6k Ehrlich 1990). 
The impacts of technology were observed and decried early 
in Marin County's history with respect to logging (see 
Munro-Fraser 1880). More subde and insidious impacts 
soon began to be noted. Beginning in 1884, the second 
Pioneer Paper Mill on Lagunitas Creek dumped the waste 
water, laced with acid and dyes, from its pulp vats into a 
brick sewer and then direcdy into the creek below. These 
wastes caused heavy silt to form in the creekbed from 
Taylorsville to Tomales Bay. The mill owners expressed 
concern over this problem but had found no solution 
when the mill was forced to close its doors in the financial 
crisis of 1893 (Rothwell 1959). 

More recendy, our high consumption rates were ulti- 
mately responsible for major oil spills in 1971, 1984, and 
1986 that despoiled Marin's coasdine and killed or debili- 
tated thousands of birds (Smail et al. 1972, PRBO 1985, 
Page et al. 1990). Chemical contaminants from urban, 
agricultural, and industrial activities have been detected in 
the tissues of many species of waterbirds in San Francisco 
Bay, often at levels known to impair reproductive success 



(Ohlendorf et al. 1988, Ohlendorf 6k Fleming 1988). The 
demise of Peregrine Falcon and Osprey populations, here 
and throughout the country, are among the foremost 
indicators of pesticide pollution in our environment, warn- 
ing of the direct threats to humans as well from our misuse 
of technology. These are but a few examples of advanced 
technologies gone awry as detected in birds. 

Fortunately a strong environmental movement 
coalesced in Marin County in the 1960s and 1970s to fight 
unrestricted development and to preserve large (and small) 
tracts of land such as Point Reyes National Seashore and 
the Golden Gate National Recreation Area. While our 
local environmental victories are impressive, and should 
be duly lauded, much remains to be done. Even though 
the county's population has begun to stabilize through 
restrictive zoning, traffic continues to increase from the 
relendess population expansion of nearby counties, and 
our as-yet-unchecked affluent lifestyle keeps on affecting 
wildlife. Protecting land and wildlife here in Marin is not 
enough as the effects of our lifestyles range way beyond 
county borders. What new habitat changes will our breed- 
ing birds face as nesting time approaches yet again widi 
each revolution of the Earth around the sun? 




35 



MARIN COUNTY BREEDING BIRD ATIAS 




Turkey Vultures lazily soar ov ^ r grassland and mixed evergreen forest on the hills surrounding Soulajoule Reservoir. 

Drawing by Ane Rovetta, 1 985. 



36 



TIMING OF BREEDING 



Yet the coming and going of the birds is more or less a mystery and a surprise. We go out in the morning, and no thrush or 
vireo is to be heard; we go out again, and every tree and grove is musical; yet again, and all is silent. Who saw them come? 
Who saw them depart? 

— John Burroughs, 
Wake-Robin 



THE BREEDING SEASONS of birds are typically timed to 
take advantage of periodic (often mild) conditions so 
that the young hatch out when appropriate foods are 
abundandy available (Welty & Baptista 1988). The inher- 
ited rhythms of breeding roughly match the seasonal 
rhythms of the environment as adaptions not only to food 
supply or mild weather, but in some cases to availability of 
vegetative cover, nest sites, nest materials, or avoidance of 
predation or competition. As ultimate factors driving adap- 
tation, these necessities do not always proximally trigger 
the unfolding of events in the breeding cycle. Some of the 
more important proximate factors that actually trigger the 
initiation or termination of breeding include day length, 
temperature, rainfall, and food availability. In some cases 
environmental conditions may act as both ultimate and 
proximate factors influencing the timing of breeding. Only 
rarely will a single factor determine the annual breeding 
schedule of a species. 

The timing of breeding for a single species can vary with 
latitude or altitude, between local populations breeding in 
different habitats, and from year to year. Moreover, nesting 
phenology can vary gready among many species in the 
same area. Some species may not breed at all in a given 
year unless certain environmental conditions are met. An 
understanding of variability in the timing of breeding, 
though fascinating in its own right, has practical value in 
aiding the planning of a strategy for a breeding bird adas 
project and in interpreting its results. Widiout a knowl- 
edge of the timing of local breeding events it is difficult to 
know when to concentrate field work to best advantage or 
how to interpret the significance of observations collected 
during adas field work. 

Patterns of variation in the timing of breeding of birds 
are generally attributable to variation in the natural envi- 
ronment, though the linkage between timing and particu- 
lar causative factors is not often clear. In one regard, the 
climate on the central California coast seems to exhibit 
little seasonal variability as we experience relatively mild 
temperatures year round. On the other hand, we do have 



distinct rainy and dry seasons. Moreover, rainfall patterns 
here can vary gready both within the rainy season and 
among years, while ocean conditions can also vary tremen- 
dously from year to year. Taken as a whole, our climate 
influences not only the timing and length of the breeding 
season but also the annual variation in these parameters. 

From his studies of the timing of breeding in the 
Sacramento Valley, Davis (1933) concluded that on the 
whole, flesh-eating species tended to start breeding before 
omnivorous and insectivorous species, which generally 
preceded vegetable and seed-eating birds. These patterns 
seem to hold for the coast as well, though for either region 
there are numerous exceptions. Another pattern that 
seems to apply in most areas is that year-round resident 
species tend to breed before summer residents, though 
again there are exceptions. For example, California Quail 
tend to initiate egg laying here in May (PRBO files, D. 
Shuford pers. obs.), well after most other year-round resi- 
dents and after some summer residents such as Allen's 
Hummingbirds, Orange-crowned Warblers, and Wilson's 
Warblers do (see below). 

Because Marin County's climate is so mild, the breed- 
ing season here is lengthy. Two of the earliest breeders are 
hummingbirds. Anna's Hummingbirds (year-round resi- 
dents) come into breeding condition in late November and 
early December before the winter solstice, when day length 
approaches the shortest of the year (Pitelka 1951a, Wil- 
liamson 1956). Nesting itself commences in December 
and probably reaches a peak in January and February, the 
coldest and generally the wettest months of the year. 
Rainfall seems to be the main climatic factor influencing 
the inherent rhythm of die breeding cycle. Once males 
begin to come into breeding condition, a period of consec- 
utive days of rainfall, rather than the actual amount, seems 
to abrupdy increase territorial establishment and the com- 
mencement of other breeding activities. These adaptations 
seemingly ensure breeding in a period when food plants 
are most numerous. Allen s Hummingbirds arrive in the 
San Francisco Bay area in mid- to late January and begin 

37 



Timing of Breeding 



MARIN COUNTY BREEDING BIRD ATI AS 



Timing of Breeding 



to lay eggs in early to mid-March (rarely by mid-Feb; Pitelka 
1951a). Thus this hummingbird also begins nesting in the 
rainy season, and its arrival here seems to be timed to 
coincide with the initiation of blooming of particular 
nectar-producing plants, presumably stimulated by winter 
rains and early spring warmth (see account). 

Great Horned Owls are also very early nesters and begin 
laying eggs here in February (M. Cohen in litt.). Great Blue 
Herons commence egg laying at Audubon Canyon ranch 
from early to mid-February, and their initiation of first 
clutches may peak from early to late March (Pratt 1 974). 
Killdeer may lay eggs here in early March (D. Shuford pers. 
obs.). Spotted Owls, Red-tailed Hawks, and Red-shoul- 
dered Hawks all are incubating eggs at least by mid- to late 
March (PRBO files). Clapper Rails begin laying in Marin 
County by early March (Evens 6k Page 1983), Scrub Jays 
at least by mid-March (PRBO files), and salt marsh-breeding 
Song Sparrows by late February or early March (J ohnston 
1956a, D. Shuford pers. obs.). Many resident landbirds 
begin egg laying in late March or early April, including 
Chestnut-backed Chickadees, Plain Titmice, Bushtits, 
Bewick's Wrens, Wrentits, Savannah Sparrows, upland- 
breeding Song Sparrows, and White-crowned Sparrows 
(PRBO files, Johnston 1956a, Geupel 6k DeSante 1990). 
Several resident waterbirds— Double-crested Cormorants, 
Mallards, and American Coots— begin egg laying in late 
March to early April (Ainley 6k Boekelheide 1990, D. 
Shuford pers. obs.). Overall, April and May appear to be 
the peak months for egg laying here for both landbirds and 
waterbirds. 

For summer residents (migrants), die timing of breeding 
in Marin County corresponds roughly to die timing of 
spring arrival here (Table 4). For most species diere is 
about a four- to five-week lag between first arrival of males 
and egg laying of females. For example, Orange-crowned 
Warblers arrive here in late February to early March and 
begin egg laying by at least early April (PRBO files), and 
Wilson's Warblers arrive in late March and begin egg 
laying in late April (Stewart 6k Darling 1972, Stewart 
1973). There are of course a number of exceptions to this 
rule. American Goldfinches, though resident in the 
county, are largely absent from Point Reyes until late 
March. Despite this relatively early arrival, American Gold- 
finches do not begin to nest on Point Reyes until the diird 
to fourth week of May (PRBO files), presumably because 
nesting and hatching of young is timed to coincide with the 
maturation of abundant seed crops. In the Sacramento 
Valley, American Goldfinches can start laying eggs in late 
April (Davis 1933), presumably because seed maturation 
is early in that region's dry, hot climate. Most species that 
glean insects from foliage can start breeding earlier than 
seed eaters because of the relatively early bloom of insect 
populations. The relationship of the arrival of aerial insec- 
tivores such as swallows to timing of breeding is deceptive. 

38 



Tree Swallows start to arrive in Marin County in numbers 
in mid- to late February (Table 4), but do not begin laying 
eggs on Point Reyes until early May (PRBO files). Although 
adults can survive during rainy weather early in the spring, 
by subsisting on berries or perhaps traveling some distance 
to find insects (see account), young can not; total failure of 
nests has been documented here during unseasonal rains 
in June (Stewart 1972). 

Also because of our mild climate, many species here can 
raise two or even three broods a year, particularly resident 
landbirds (DeSante 6k Baptista 1989, Geupel 6k DeSante 
1990, G.R. Geupel 6k D.F. DeSante pers. comm.). 
Although most landbird young have fledged by late July, 
nests of some species such as Barn Swallows may be active 
until late August or, rarely, early September (B. Baez 6k D. 
Shuford pers. obs.), and American Goldfinches, rarely, are 
still feeding fledged young in mid-September (J .G. Evens 
pers. obs.). Young of many of our species of breeding 
seabirds fledge in August and September or even later 
(Ainley 6k Boekelheide 1990). The Ashy Storm-Petrel 
provides an extreme example of an extended breeding 
season. At the Farallon Islands, Ashies lay eggs mosdy 
from early May to late August (sometimes later), and young 
fledge from early September to mid-November and, rarely, 
through December. Although data are lacking for this 
region, Red Crossbills elsewhere in their breeding range 
are known to breed in any month of the year (see account). 

Given this great variability in timing of nesting, what are 
the factors that initiate or terminate breeding? Of the 
proximate factors, day length seems to have the greatest 
influence through its effect on the waxing and waning of 
gonadal development (Welty 6k Baptista 1 988). As noted 
above for the Anna's Hummingbird, rainfall is a contrib- 
uting factor to the initiation of breeding. Mewaldt and 
King (1977) concluded diat warm temperatures and dry 
weather in the prenesting period advanced breeding in 
White-crowned Sparrows and cool rainy weather delayed 
it. Also in White-crowned Sparrows the timing of termina- 
tion of breeding is direcdy related to the amount of winter 
rainfall during the previous year, such that breeding 
extends later into the summer after winters of heavy rainfall 
(DeSante 6k Baptista 1989). Presumably increased rainfall 
prolongs the growing season of green plants upon which 
grazing insects depend and hence the availability of these 
insects for the sparrows to feed their young. In arid parts 
of dieir range, California Quail breed irregularly depend- 
ing on the amount of winter rainfall preceding the spring 
nesting season (Leopold et al. 1976). In dry years, quail are 
inhibited by chemicals (phytoestrogens) in stunted forbs 
and grasses, and few or no young are produced during the 
short breeding season. In contrast, in wet years lush forb 
growdi supplies large amounts of seeds for quail consump- 
tion, stimulating vigorous and extended breeding (young 
hatched as late as September). Although the initiation of 



TIMING OF BREEDING 

Table 4. Arrival dates of Marin County landbirds with comparisons to other regions of California. Data reported as average 
arrival date (x), number of years with data (n), and the span of first arrival dates (range). Lack of data for particular species may 
reflect infrequent records of the species, poor coverage of appropriate habitat, or difficulty of distinguishing individual migrants 
from birds of smaller resident populations. 





Palomarin , Point Reyes 

1967-1989 

x (n) range 


Marin County 

-1900-1980 

x (n) range 


Berkeley Area 

1911-1947 

x (n) range 


Northern California 

1972-1984 

x (n) range 


Southern California 5 

1972-1984 

x (n) range 


Vaux's Swift 


- 


- 


- 


4/6 (6) 3/26-4/19 


4/12 (15) 4/4-4/19 


Black-chinned Hummingbird 


- 


- 


- 


3/26 (5) 3/11-4/9 


3/25 (10) 2/25-4/11 


Allen's Hummingbird 


2/5 (19) 1/24-2/27 


2/5 (27) 1/16-2/28 


2/13 (30) 1/29-2/24 


- 


- 


Olive-sided Flycatcher 


4/17 (18) 4/13-4/26 


4/18 (23) 4/7-4/30 


4/19 (30) 3/28-5/5 


4/14 (6) 4/9-4/22 


4/12 (14) 3/19-4/29 


Western Wood-Pewee 


5/15 (11) 4/24-6/7 


(4) 4/14-4/26 


5/1 (13) 4/18-5/8 


4/15 (6) 4/9-4/21 


4/13 (14) 4/3-4/24 


Pacific-slope Flycatcher 


3/27 (16) 3/18-4/8 


3/25 (16) 3/11-4/5 


3/26 (32) 3/12-4/9 


3/22 (5) 3/12-4/4 


3/14 (5) 3/2-3/22 


Ash-throated Flycatcher 


4/26 (13) 4/10-5/9 


- 


- 


4/9 (5) 4/2-4/15 


3/31 (15) 3/22-4/9 


Western Kingbird 


- 


4/5 (5) 4/1-4/11 


- 


3/23 (5) 3/2-3/31 


3/14 (14) 3/5-4/3 


Purple Martin 


- 


4/7 (13) 3/6-4/30 


- 


- 


- 


Tree Swallow 


2/13 (19) 1/20-3/8 


2/22 (12) 2/3-3/5 


- 


- 


- 


Violet-green Swallow 


2/25 (18) 2/9-3/19 


2/21 (11) 2/6-3/13 


- 


- 


- 


N. Rough-winged Swallow 


3/19 (15) 3/8-3/31 


3/7 (7) 2/29-3/15 


- 


- 


- 


Cliff Swallow 


3/24 (18) 3/12-4/10 


3/18 (16) 3/9-3/29 


3/22 (11) 3/5-4/7 


- 


- 


Barn Swallow 


3/15 (14) 2/24-4/8 


3/11 (14) 3/5-3/19 


- 


- 


- 


House Wren 


- 


- 


3/21 (21) 3/4-4/6 


- 


- 


Swainson's Thrush 


4/27 (19)4/19-5/4 


4/26 (20) 4/15-5/4 


4/24 (32) 4/14-5/5 


4/21 (5) 4/18-4/28 


4/21 (15) 4/6-5/1 


Solitary Vireo 


- 


- (4) 3/20-4/5 


- 


4/1 (6) 3/22-4/9 


3/27 (15) 3/14-4/10 


Warbling Vireo 


3/26 (21) 3/15-4/3 


3/25 (20) 3/13-4/6 


3/25 (30) 3/9-4/6 


3/21 (6) 3/13-3/31 


3/11 (15) 3/1-3/20 


Orange-crowned Warbler 


3/6 (22) 2/27-3/16 


3/4 (15) 2/18-3/16 


3/3 (29) 2/21-3/14 


(4) 2/26-3/8 


- 


Yellow Warbler 


- 


4/18 (5) 4/8-4/23 


4/16 (30) 4/7-5/2 


4/6 (6) 3/21-4/17 


4/2 (15) 3/24-4/12 


Black-throated Gray Warbler 


- 


4/14 (8) 3/31-4/27 


- 


4/4 (5) 3/30-4/14 


3/24 (15) 3/11-4/7 


Hermit Warbler 


_ 


_ 


_ 


4/16 (5) 4/13-4/20 


4/16 (15) 4/7-4/24 


MacGillivray's Warbler 


4/20 (12) 4/9-4/30 


4/18 (9) 4/3-4/30 


4/12 (18) 4/3-4/26 


4/11 (6) 4/8-4/14 


4/1 (15) 3/18-4/17 


Wilson's Warbler 


3/25 (23) 3/16-4/5 


3/24 (27) 3/10-4/8 


3/22 (32) 3/11-4/3 


3/19 (6) 3/17-3/20 


3/13 (15) 3/3-3/23 


Yellow-breasted Chat 


- 


- 


- 


- 


4/13 (15) 4/4-4/24 


Western Tanager 


- 


- 


- 


4/13 (5) 4/3-4/22 


4/13 (15) 4/7-4/17 


Black-headed Grosbeak 


4/14 (18) 4/5-4/21 


4/13 (34) 4/4-4/26 


4/13 (37) 4/4-4/21 


4/3 (6) 3/28-4/8 


3/26 (15) 3/22-4/1 


Lazuli Bunting 


- 


4/28 (6) 4/21-5/2 


4/22 (30) 3/30-5/7 


4/18 (5) 4/14-4/22 


4/5 (15) 4/1-4/15 


Chipping Sparrow 


- 


4/14 (7) 4/2-4/24 


4/15 (10) 3/29-4/26 


- (4) 3/20-4/12 


- 


Black-chinned Sparrow 


- 


- 


- 


- 


3/28 (15)3/10-4/24 


Grasshopper Sparrow 


- 


4/21 (5) 4/2-4/30 


- 


• (3) 4/1-4/18 


- 


Brown-headed Cowbird 


3/28 (13) 3/3-4/14 


3/30 (6) 3/17-4/14 


- 


- 


- 


Hooded Oriole 


- 


3/29 (13) 3/16-4/13 


- 


3/19 (6) 3/5-3/30 


3/10 (15)2/27-3/24 


Northern Oriole 


- 


4/3 (6) 3/24-4/7 


- 


3/16 (6) 3/10-3/20 


3/14 (15) 3/5-3/21 


American Goldfinch 


3/30 (13) 3/2 3-4/6 


3/24 (8) 3/2-4/6 


- 


- 


- 



Data from PRBO's Palomarin Field Station courtesy of Dave DeSante and Geoff Geupel. 
Data compiled by the author from various sources and personal field notes. 
3 Data from Weston (1948). 
Data from summaries in seasonal reports of the Middle Pacific Coast Region of American Birds 
Data from summaries in seasonal reports of the Southern Pacific Coast Region of American Birds 



39 



Timing of Breeding 



MARIN COUNTY BRFFDING BIRD ATIAS 



Timing of Breeding 



breeding in Tricolored Blackbirds usually coincides witb 
rainfall or flooding of rice fields in the Central Valley, 
nesting appears to be triggered by an abundance of food 
(see account). 

Most Farallon seabirds seem to be primed by photo- 
period to both initiate and terminate egg laying (Ainley 6k 
Boekelheide 1990). Food abundance still appears to have 
some effect on timing of breeding though. In years when 
initiation of egg laying is late, laying by seabirds may begin 
en masse when prey appear. Also second and replacement 
clutches of seabirds are most frequent in years when a high 
level of breeding success indicates abundant food. The fact 
that species with the most similar diets breed at the same 
time also suggests prey availability strongly affects the 
timing of breeding of seabirds, though the complexities of 
the pathways linking upwelling, prey availability, and tim- 
ing of breeding are still poorly understood. 

Year-to-year variability in the timing of initiation or 
termination of breeding differs greatly among species. At 
Palomarin, timing of initiation of breeding in Wrentits, as 
measured by mean clutch completion dates of first nesting 
attempts, varied only from 19 to 30 April over six years 
(Geupel ck DeSante 1990). The timing there of termina- 
tion of breeding by White-crowned Sparrows, measured as 
the mean clutch completion date for the latest 10% of 
nests, ranged from 25 June to 20 July over seven years 
(DeSante 6k Baptista 1989). Timing of breeding of Faral- 
lon seabirds can vary gready from year to year, and under 
extreme conditions virtually all females of species such as 
Brandt's Cormorants, Pelagic Cormorants, and Pigeon 



Guillemots fail to lay any eggs (Ainley 6k Boekelheide 
1990). The variation in the timing of commencement of 
breeding for various Farallon seabirds is indicated by the 
range among years of mean clutch initiation dates: 
Brandt's Cormorant (28 April-6 June), Pelagic Cormorant 
(22 May-12 June), Western Gull (only 3-14 May), Com- 
mon Murre (9 May-9 June), and Pigeon Guillemot (20 
May- 17 June). In addition to the seabirds mentioned, 
Marin County hosts a number of odier species that will 
not breed unless certain food supplies are available. Long- 
eared Owls, Short-eared Owls, and Black-shouldered Kites 
will not remain to breed unless certain rodents occur in 
abundance; Red Crossbills will only breed in years of 
plentiful conifer seeds; and various dabbling ducks may fail 
to breed locally when small wedands dry up during 
droughts. 

These patterns of variation in the timing of breeding of 
a wide range of species further demonstrate that most if not 
all species have each adopted a different strategy to exploit 
their environment. It is clear from the length of the 
breeding season on the California coast, the great year-to- 
year variation in timing of breeding in certain species, and 
the lack of breeding by some species in particular years that 
efforts to document patterns of breeding distribution of 
our avifauna are best spread each year over many months 
and over enough years to sample a broad range of environ- 
mental conditions. The Marin adas project was fortunate 
enough to span some of the wettest and driest years in the 
county's history and consequendy provided information 
on how both extremes affected bird distribution here. 



40 



METHODS EMPLOYED IN THE 
MARIN ATLAS 



The detection of a pattern or test of a hypothesis can be no better than the data on which it is built. 



— John A. Wiens, 
The Ecology of Bird Communities 



Grid System 

THE GRID SYSTEM chosen in 1976 for the Marin County 
Breeding Bird Atlas was roughly comparable to the 
metric grids used in Europe at that time. Following the lead 
of North America's first adas project in Maryland (Klim- 
kiewicz &. Solem 1978), a grid system was overlain on 
7.5-minute U.S. Geological Survey topographic maps of 
Marin County. Each of the 1 7 topo maps covering Marin 
County (Figure 7) were divided into 24 equal-sized blocks. 
Because some of these topo maps included large portions 
of the ocean, San Pablo or San Francisco bays, or land in 
adjacent Sonoma County, a total of 221 blocks formed the 
basic adas grid of Marin County (Figure 8). Each of these 
block was assigned a specific numerical code. Though 
slighdy rectangular in shape (about 1.4 X 1.7 miles on a 
side), each one of our basic blocks is roughly equivalent in 
area to a metric block 2.5 km on a side. The basic blocks 
were also lumped together for later data analysis into 
groups of four and again into groups of sixteen to facilitate 
direct comparisons of the Marin adas data with data from 
other adas projects with larger basic block sizes. At the 
latitude of Marin County (38° N) our basic block, 4-block 
units, and 16-block units are slighdy larger in area (1.02 
times) than 2.5-km, 5-km, and 10-km squares, respectively. 
For all practical purposes, though, our block units are 
direcdy comparable in size to their respective metric equiva- 
lents. Although comparability with other adas projects is 
desirable, the comparison of die Marin adas that will be of 
most benefit will be that with itself when repeated at a 
future date. 

Blocks along the outer coasdine, the shorelines of San 
Francisco and San Pablo bays, and the Sonoma County 
border did not conform to the basic grid system. Those 
blocks were slighdy larger or smaller than a basic block and 
were of necessity irregular in shape. Parts of blocks were 
merged with adjacent blocks to facilitate future data com- 
parisons among blocks of roughly equivalent size. This 




Figure 1 . Overlay of U.S. Geological Survey topographic map 
grid on a Marin County map, forming tKe basis for tke Marin 
County Breeding Bird Atlas grid (Figure 8). 

avoided, for example, a comparison of a block comprised 
of 90% land and 10% ocean with a block that has 20% 
land and 80% ocean. Merging of blocks in this manner 
has precedence in the first adas project, the Atlas of British 
Flora (Perring & Walters 1962). Besides bringing odd 
blocks into closer conformity of size and composition, this 
method had the practical application, in some cases, of 
providing direct access to all of a block from one place. For 

41 



Grid System 



MARIN COUNTY BRFFDING BIRD ATIAS 



Participant Instruction 




Figure 8. The basic Mflrin County Breeding Bird Atlas grid with 221 numbered blocks. Blocks were created by dividing each of 
I 7 7-5-minute USGS topo maps (see Figure 7) into 24 equal-sized blocks; parts of some irregular-sized blocks were merged with 
adjacent ones to bring blocks into closer conformity of size- A basic Marin block is roughly equivalent in area to a metric block 2.5 
km on a side. 



example, if the grid system had been applied rigidly along 
Tomales Bay, either an observer would have had to drive 
long distances around the bay to get access to parts of a 
block on the opposite side of the bay, or else two different 
observers would have had to cover the separate parts of the 
block. With our convoluted coasdine, either method- 
using a rigid grid system or one that merged parts of 
blocks— would have created blocks of different sizes or 
shapes. The latter method was chosen as a matter of 
practicality and should pose no problem if the same exact 
grid is used when the adas project is repeated in the future. 

Participant Instruction and 
Block Assignments 

From 1976 through 1978, Bob Stewart was the sole 
coordinator of the Marin County Breeding Bird Adas 
Project. The adas was advertised in the Point Reyes Bird 
Observatory Newsletter and widely in local Audubon Society 
and conservation newsletters. In 1976 and 1977 several 
organizational workshops were held. Participants were 
instructed on how to conduct field work in their blocks 
and were provided with refresher sessions on bird songs 
and nest-finding strategies. One or more blocks were 
assigned to each participant based on his or her available 

42 



time and ability. From 1979 through 1981, there was a 
hiatus in adas work. This author became the overall 
coordinator in 1982 for the final field season of the adas 
project. In that year workshops and advertisements were 
conducted in the same manner as in previous years. 
Likewise, the vast majority of participants that year were 
solicited through personal contact. In 1982, regional coor- 
dinators were solicited to organize participants in four 
areas encompassing all of Marin County. Betty Burridge 
(who now organizes the Sonoma County Breeding Bird 
Adas Project) was coordinator for 1 7 blocks in the Tomales 
area, Scott Carey for 43 blocks in the Novato area, Bill 
Lenarz for 61 blocks in southern/eastern Marin, and Dave 
Shuford for 100 blocks in the West Marin area. An effort 
was made to maintain contact with participants throughout 
each field season. 

In all years each participant was provided with the 
following: 

1 . Instruction sheets detailing the objects of the adas 
project, how and when to conduct field work, and how to 
record the required data. 

2. A topo map (or photocopy) of his or her adas block(s) 
and adjacent blocks; the location of the blocks(s) was 
oudined on an attached map of Marin County. 



METHODS EMPLOYED IN THE MARIN ATLAS 



PRBO MARIN COUNTY BREEDING BIRD ATLAS PROJECT 



NAME L)OUG 


tuis 






19 


%Z 


ADDRESS P 6 , i^C 


jl ISS tUlviJ 


:<j Co . 


ZIP 


?iY3» 


BLOCK* $~*~I3 


Name 


A.O.U.* 


Po 


Pr 


Co 


Name 


A.O.U.» 


Po 


Pr 


Co 


Name 


A.O.U.» 


Po 


Pr 


Co 


Pied-billed Grebe 


006 








American Coot 


221 








Hairy Woodpecker 


393 








Ashy Petrel . 


108 








Black Oystercatcher 


287 








Downy Woodpecker 


394 






*i 


Double-crested Cormorant 120 








Snowy Plover 


278 








Ash-throated Flycatcher 


454 






FY 


Brandt's Cormorant 


122 








Killdeer 


273 




P 




Black Phoebe 


458 






pr 


Pelagic Cormorant 


123 








Western Gull 


049 








Western Flycatcher 


464 




5 




Great Blue Heron 


194 








Common Murre 


030 








Western Wood Pewee 


462 




S 




Green Heron 


201 








Pigeon Guillemot 


029 








Olive-sided Flycatcher 


459 








Great Egret 


196 








Band-tailed Pigeon 


312 








Horned Lark 


474 




T 


9 


Snowy Egret 


197 








Rock Dove 


313.1 








Violet-green Swallow 


615 








Mallard 


132 








Mourning Dove 


316 




r 




Tree Swallow 


614 






ON 


Pintail 


143 








Barn Owl 


365 








Rough-winged Swallow 


617 








Cinnamon Teal 


141 








Screech Owl 


373 




-r 




Barn Swallow 


613 






qki 


Ruddy Duck 


167 








Great Horned Owl 


375 








Cliff Swallow 


612 


^ 






Turkey Vulture 


325 


• 






Pygmy Owl 


379 








Purple Martin 


611 








Sharp-shinned Hawk 


332 








Burrowing Owl 


3 78 








Steller's Jay 


478 




J> 




Cooper's Hawk 


333 








Spotted Owl 


369 








Scrub Jay 


481 






*V 


Red-tailed Hawk 


337 


X 






Saw-whet Owl 


372 








Common Raven 


486 






FY 


Red shouldered Hawk 


339 


s 






Poor-will 


418 








Common Crow 


488 


•' 






Marsh Hawk 


331 








White-throated Swift 


425 








Chestnut-backed Chickadee 


741 






a/v 


Osprey 


364 








Anna's Hummingbird 


431 




T 




Plain Titmouse 


733 




P 




Sparrow Hawk 


360 


J 






Allen's Hummingbird 


434 




*f 




Common Bushtit 


743 






fir 


California Quail 


294 




r 




Belted Kingfisher 


390 


j£ 






White-breasted Nuthatch 


727 








Ring necked Pheasant 


309.1 








Red-shafted Flicker 


413 








Red-breasted Nuthatch 


728 








Virginia Rail 


212 








Pileated Woodpecker 


405 








Pygmy Nuthatch 


730 








Sora 


214 








Acorn Woodpecker 


407 








Brown Creeper 


726 








Black Rail 


216 

























Po=Possit>le ; Pr = Probable; Co=Confirmed Enter Criteria Code in Correct Column 



Name 


A.O.U.* 


Po 


Pr 


Co 


Name 


A.O.U.# 


Po 


Pr 


Co 


Name A.o 


;j.» 


Po 


P', 


Co 


Wrentit 


742 




5 




Purple Finch 


517 




5 




Rare Possibilities 








Dipper 


701 








House Finch 


519 






ft 


Black-crowned Night Heron 


202 








House Wren 


721 








Pine Siskin 


533 






n 


American Bittern 


190 








Winter Wren 


722 








American Goldfinch 


529 






rL 


Wood Duck 


144 








Bewick's Wren 


719 






rT 


Lesser Goldfinch 


530 








White-tailed Kite 


328 








Long-billed Marsh Wren 


725 








Red Crossbill 


521 








Swainson's Hawk 


342 








Rock Wren 


715 








Rufous-sided Towhee 


588 






f) 


Golden Eagle 


349 








Mockingbird 


703 








Brown Towhee 


591 




P 




Prarie Falcon 


355 








California Thrasher 


710 








Savannah Sparrow 


542 




*> 




Peregrine Falcon 


356 








Robin 


761 




f> 




Grasshopper Sparrow 


546 




S 




Clapper Rail 


210 








Hermit Thrush 


759 








Lark Sparrow 


552 




?> 




Common Gallinule 


219 








Swainson's Thrush 


758 




T 




Rufous-crowned Sparrow 


580 








Tufted Puffin 


12 








Western Bluebird 


767 




T 




Oregon Junco 


567.9 






9P 


Long-eared Owl 


286 








Golden-crowned Kinglet 


748 








Chipping Sparrow 


560 






Ft 


Short-eared Owl 


287 








Loggerhead Shrike 


622 








White-crowned Sparrow 


554 








Vaux Swift 


298 








Starling 


493 






n 


Song Sparrow 


581 






fr 


Black-chinned Hummingbird 


429 








Hutton's Vireo 


632 




s 












Nuttall's Woodpecker 


328 






Orf 


Warbling Vireo 


627 




s 












Western Kingbird 


447 




T 




Orange-crowned Warbler 


646 






tJ\ 










Cassin's Kingbird 


448 








Yellow Warbler 


652 
















Willow Flycatcher 


466 








Black-throated Gray War 


bier 665 
















Bank Swallow 


616 








Hermit Warbler 


669 
















Yellow-breasted Chat 


683 








MacGillivray's Warbler 


680 
















Hooded Oriole 


505 








Yellowthroat 


681 
















Lawrences Goldfinch 


531 








Wilson's Warbler 


665 




s 












Black-chinned Sparrow 


565 








House Sparrow 


688.2 
















Sage Sparrow 


574 








Western Meadowlark 


501.1 






fr 


















Red-winged Blackbird 


498 






fr 


















Bullock's Oriole 


508 






ft 


















Brewer's Blackbird 


510 






pp 


















Brown-headed Cowbird 


495 
























Black-headed Grosbeak 


596 
























Lazuli Bunting 


599 




> 





















Figure 9. A representative field recording card from the Marin County Breeding Bird Atlas. See Table 5 for various codes used to 
denote Possible (Po), Probable (Pr), and Confirmed (Co) breeding evidence. 



43 



MARIN COUNTY BREEDING BIRD ATLAS 

Table 5. CRITERIA FOR POSSIBLE, PROBABLE, AND CONFIRMED BREEDING CODES entered on Marin 
County Breeding Bird Adas field cards (Figure 9). 

POSSIBLE BREEDING - this code should be entered in die first column (PO) of the Atlas Card. 

Bird recorded in the breeding season in possible nesting habitat but no other indication of breeding noted. Take 1 
May through 31 July as the breeding season for most species. Summering, nonbreeding adults such as gulls in a 
dump when you know diere is no gullery in your block, migrant shorebirds and warblers, should NOT be included. 

PROBABLE BREEDING - codes entered in second column (PR). 

S Singing male present (or breeding calls heard) on more than one date in the same place. It is a good indication that 

a bird has taken up residence if the dates are a week or more apart. 

T Bird (or pair) apparendy holding territory. In addition to singing, chasing of others of the same species often marks 

territory. 

D Courtship and display; or agitated behavior or anxiety calls from adults, suggesting probable presence of nest or 

young nearby; brood-patch on trapped female or cloacal protuberance on trapped male. 

N Visiting probable nest-site. 

B Nest building by wrens and woodpeckers. Wrens may build many nests and woodpeckers, although they usually 

drill only one nesting cavity, may also drill roosting holes. 

CONFIRMED BREEDING - codes entered in diird column (CO). 

DD Distraction display or injury feigning, coition. Agitated behavior and/or anxiety calls are "D" only. 

NB Nest building by any species except wrens and woodpeckers. 

UN Used nest found. These must be carefully identified if they are to be used. Some nests (like those of Northern Oriole) 
are persistent and very characteristic. Others are more difficult to identify correcdy. 

FE Female with egg in the oviduct. 

FL Recendy fledged young (including downy young of waterfowl etc.). This code should be used with caution for species 
such as Starlings and swallows that may move some distance soon after fledging. Recently fledged passerines are still 
dependent on parents and being fed by diem. 

FS Adult carrying fecal sac. 

FY Adult(s) widi food for young. Some birds (gulls, terns, and birds of prey) continue to feed their young long after 
they've fledged and may move considerable distances. Also some birds (like terns) may carry food long distances to 
young in a neighboring block. Be careful especially on the edge of a block. Care should be taken to avoid confusion 
with courtship feeding (D). 

ON Adult(s) entering or leaving nest-site in circumstances indicating occupied nest. Not generally used for open nesting 
birds. The correct code would be "N" if you simply see a bird fly into or out of a bush or tree and do not find the 
nest. It should be used for hole nesters as when a bird enters a hole and remains inside, changes over at a hole, or 
bird leaves hole after having been inside for some time. 

NE Nest and eggs or bird setting and not disturbed or egg shells found below the nest. If you find a cowbird egg in a 
nest, it's NE for cowbird and NE for the host nest 

NY Nest widi young or downy young or downy young of waterfowl, quail, waders, etc. If you find a young cowbird with 
the other young, it's NY for the cowbird and NY for the host species. Since parents often lead downy young for 
considerable distances, care should be taken if such records are close to the edge of the block. 

44 



Participant Instruction 



METHODS EMPLOYED IN THE MARIN ATLAS 



Additional Information 



3. Atlas recording card(s) to be filled out in the field or 
immediately afterwards (Figure 9). 

4. A breeding category sheet (Table 5)— a slighdy modi- 
fied form of the one used in the Maryland county adas 
project (Klimkiewicz & Solem 1978), originally derived 
from the British categories (Sharrock 1976). 

5. The assurance that the locations of all sightings of 
rare breeding birds would, at the discretion of die observer, 
be kept confidential and protected on maps (see Data 
Summary p.48). 

In 1982— the final year of fieldwork— participants were 
provided with photocopies of the previous years' adas 
card(s) for their block(s) if there had been any prior 
coverage. They were further instructed on how to obtain 
adequate coverage of their blocks (see below). In addition, 
observers were asked to keep records of the total number 
of hours they spent in the field collecting adas data. That 
year, area coordinators contacted all participants around 1 
June. At that time, if some observers already had been able 
to cover their block(s) adequately, they were encouraged to 
help with other areas still in need of coverage. If partici- 
pants felt they would not have time to finish the necessary 
field work in their block(s), another observer was assigned 
to help complete the coverage that season. Adequacy of 
coverage was judged qualitatively by die overall coordinator 
as described below. In addition to regular communication 
between area coordinators and participants, close contact 
was maintained between area coordinators and the overall 
coordinator. 



Gathering Additional Information 

Because different methods work best for gathering data on 
certain groups of species, or because a diversity of methods 
can enhance data collection for all species, a number of 
additional mediods were employed to gather data that 
supplemented the standard adas procedures. These strate- 
gies included the following: 

1. Owling routes. With the realization diat owls and 
poorwills would be inadequately covered compared with 
other species, special owling routes were drawn up in 1977 
and 1982 and assigned to various nocturnal enthusiasts. 
These routes were of varying length and covered virtually 
all the major roads in the sparsely inhabited parts of the 
county. Several walking routes were also covered along 
trails in the Inverness Ridge area of the Point Reyes 
National Seashore and within the Mount Tamalpais water- 
shed. 

2. Spring Bird Counts. Following the tradition of the 
Christmas Bird Counts (CBC) published in the National 
Audubon Society journal American Birds, three Spring 
Bird Counts (SBC)— Southern Marin, Point Reyes Penin- 



sula, and Even Cheaper Thrills— were conducted in Marin 
County in May or early June in various years beginning in 
1978 (Appendix A). 

Aside from seasonal timing, these counts were con- 
ducted in an identical manner to the CBCs, except that, in 
addition to counting die number of individuals of each 
species, each area leader assigned the highest category of 
breeding observed that day (Table 5) to each potential 
breeding species in an area. To channel some of this 
energy into the adas project, the Marin adas grid was 
overlain on the South Marin SBC circle in 1977 and over 
all diree SBC circles in 1 982. Each of the area leaders, many 
of whom were already adas participants, was provided with 
all the information given to regular adas participants along 
with adas cards for all the blocks or partial blocks that fell 
within his or her normal SBC area. In addition to collecting 
the standard numerical and breeding information 
described above, area leaders were also asked to fill in all 
breeding evidence for each individual adas block in their 
area. This resulted in large amounts of atlas data being 
collected on single days, and sometimes parts of blocks 
being covered by the SBC team that otherwise might not 
have been covered by the adaser already assigned that 
block. 

3. Casual observations. Besides using the systematic adas 
data collected in specified blocks from 1976 through 1978 
and in 1982, other available data were incorporated in the 
data base. In advertisements and instructions to partici- 
pants, everyone was encouraged to submit additional data 
on breeding evidence for any species (especially rare or 
unusual ones) that were observed in an adas year but not 
in an assigned adas block. Available data were also solic- 
ited for the intervening years of 1979 through 1981, when 
no organized effort was made to cover adas blocks. Individ- 
uals particularly active in those years were contacted 
direcdy and asked to submit from their field notes and 
memories any and all specific breeding bird sightings. All 
such observations were given the same scrutiny as adas 
card data; a determination of the exact location and type of 
breeding evidence observed was necessary to correcdy 
assign records to specific adas blocks. 

4- Breeding seabirds surveys. Fortuitously, in 1979 and 
1980 personnel from die U.S. Fish and Wildlife Service 
conducted censuses of breeding seabirds along the entire 
California coast and subsequendy published the informa- 
tion as the Catalog of California Seabird Colonies (Sowls et 
al. 1980). Breeding site data from Marin County that had 
not already been confirmed during atlas work were 
extracted from that publication (or field notes and maps on 
file at the California Academy of Sciences in San Fran- 
cisco) and assigned to die appropriate adas block. U.S. 
Fish and Wildlife Service personnel resurveyed breeding 
seabirds on the central and northern California coast in 
1989 (Carter et al. 1990, 1992). These 1989 data were not 



45 



Additional Information 



MARIN COUNTY BREEDING BIRD ATIAS 



Adequacy of Coverage 



used to construct the atlas maps but were used to supple- 
ment the knowledge of distribution and abundance dis- 
cussed in the species accounts. 



Determining Adequacy of Coverage 

Determining when data of sufficient quality have been 
collected in individual blocks or in all blocks in a region is 
a perplexing problem facing all adas projects. Although it 
would be ideal to confirm breeding for all species in each 
block, that goal is unrealistic. On the other hand, adas 
coordinators must attempt to maximize the return of field 
effort given limitations in the number and ability of observ- 
ers, time available, and access problems or other logistical 
constraints. 

On a practical level, an adas coordinator must decide 
when an observer should be shifted to cover another block 
because additional time spent in the original block will 
yield few new species or instances of confirmed breeding 
evidence. More importandy, an atlas project can establish 
scientific credibility only if there is confidence that field 
work has documented a high proportion of the species 
actually breeding in each block. Otherwise there will always 
be the nagging doubts, rife in preadasing days, as to 
whether the distribution patterns or population trend 
indicated is real or just an artifact of insufficient coverage. 

Other Atlases 

A number of methods have been used to assess adequacy 
of coverage in other adases, and all have their shortcom- 
ings. Two popular measures, used singly or in combina- 
tion, assume adequate coverage in a block when a certain 
number of species (and sometimes confirmed breeders) 
have been recorded or when a certain number of field 
hours have been logged. In Vermont, the initial experience 
of adas committee members indicated that most of the 
state's blocks contained 100 breeding species (Laughlin ek 
Kibbe 1985, Kibbe 1986). With this knowledge they 
selected 75 species recorded in a block (and 35 confirmed 
breeders) as the level of acceptable coverage, assuming that 
number would represent 75% of the species actually in 
most blocks. In practice 75 species was only 60%-65% of 
the 1 20 or more species they later found in some blocks. 
The New York adas initially defined adequate coverage as 
76 breeding species per block with half (38) confirmed as 
breeders; they later dropped the 50% confirmation require- 
ment (Andrle ck Carroll 1988). Ontario originally set an 
adequate coverage standard of 1 6 hours of field work with 
the expectation that the effort would identify 75% of the 
breeding species in a block (Cadman et al. 1987). Because 
expectations were not met, coverage goals were modified. 
Based on experience and estimates of habitat diversity in 
each block, coordinators estimated the total number of 

46 



species breeding in each. Retaining the minimum require- 
ment of 1 6 hours of field work, adequate coverage was then 
set at establishing breeding evidence for 75% of the esti- 
mated number of breeding species. 

Setting either an arbitrary number of species detected or 
field hours spent as the measure of adequate coverage is 
problematic. Because habitat diversity, and hence the num- 
ber of breeding species, may vary gready among blocks, a 
preselected regionwide goal of species detection will set 
unrealistically high or low expectations for many blocks. A 
minimum number of hours of field work is an inadequate 
standard because observers varying in field skills will 
consequendy differ in the number of species they can 
detect in a given time period. Even when observer skills are 
comparable, the number of hours needed to detect the 
same number of birds may also vary gready among blocks 
as habitat diversity, ruggedness of terrain, or ease of access 
varies. 

In combination with species and field time goals, Smith 
(1982) suggested measuring adequacy of coverage by plot- 
ting the number of new species detected in each of the 
three breeding categories against time spent in each new 
visit to a block. Termination of coverage was recom- 
mended when the plotted curves leveled off as returns 
diminished with time spent. This method relies on the 
probably unrealistic assumption that most observers will 
keep accurate records, graph them, and correcdy interpret 
die results; regardless, this method, like others, will falter 
because of observer variability. No matter how much time 
is spent to compensate, observers with poor skills in 
identifying bird songs will plateau at lower species totals 
than will more skilled observers. 

Kibbe (1986) suggested using an ACID (Adequate Cov- 
erage Identification) test to evaluate when observers' efforts 
became ineffective and it was time to move on to another 
block. Coverage in this scheme is scored by adding the 
products of three times the number of species with con- 
firmed breeding evidence, two times the number with 
probable evidence, and one times the number with pos- 
sible evidence (ACID score = [3*CO] + [2*PR] + PO). The 
assumption is that as complete coverage is approached, 
scores change less and less between successive surveys. 
With skilled observers this method may actually measure 
"adequate" coverage, but scores of some observers may 
peak at too low a level because they are not recording 
difficult-to-detect species. Because the score is most sensi- 
tive to increases in the number of confirmed species, an 
observer may detect most of the breeding species long 
before the ACID score begins to level off. Thus practicality 
would suggest moving skilled observers to new blocks long 
before their scores peaked, if finding most breeding species 
is a higher priority than establishing confirmed breeding 
evidence for all of them. 



Adequacy of Coverage 



METHODS EMPLOYED IN THE MARIN ATLAS 



Adequacy of Coverage 



Raynor (1983) suggested assessing coverage by first 
plotting the number of species found in each block against 
an informed estimate of the number of breeding species 
probably present in each block. The informed estimate is 
made after the first adas year and is based on identification 
of habitats in each block (from maps or other data), 
knowledge of habitat preferences of expected breeding 
birds, lists of species in nearby blocks with similar habitats, 
and personal knowledge of the block or similar habitats in 
the same area. The informed species estimate can be 
revised or updated annually if coverage continues; also 
results can be evaluated against the estimate and can be 
used to revise it. Once the expected number of species is 
estimated for each block, calculations can be made of ratios 
of the total number of species (or confirmed species) 
recorded to expected species (l"/EX or CO/EX). These 
ratios can be plotted as percentages and classified to define 
quality of coverage. For example, the range of values 
calculated can be partitioned into three equal categories 
corresponding to good, fair, and poor coverage. Raynor 
(1983) proposed a very high standard of listing 95% of the 
expected species and confirming 57% as a goal for ade- 
quate coverage of a block. Although his method is intu- 
itively appealing, it is not without pitfalls. The main 
drawback is that refining estimates over time— based on the 
assumption that with good coverage, counts and estimates 
will tend to converge— may lead to a self-fulfilling prophecy. 
If the species list exceeds the estimate, the estimate will be 
revised upward, but if the list is low and remains low after 
further work is conducted, the estimate will likely be 
revised downward. In other words, once coverage has been 
qualitatively deemed adequate on some level the estimate 
will be revised to fit the actual number of species recorded. 
This is not a test of adequate coverage. Rather, it fits data 
to what one assumes from past knowledge or current field 
work is the "real" number of species breeding in a block. 
Although scientifically fortified with plots and correlations, 
the linchpin of this method is the accuracy of the estima- 
tion of breeding species. Sophisticated data analysis will 
not suffice if the estimates of expected species have low 
accuracy. The people estimating must have a very extensive 
knowledge of local habitat distribution, habitat preferences 
of expected breeding birds, and a fair amount of prior 
distributional knowledge of die birds being studied. In 
atlases covering large geographical areas various sub- 
coordinators are likely to make the species estimates for the 
blocks in their subregion. Hence coverage standards may 
vary with subregions as coordinators vary in knowledge or 
a tendency to be conservative or liberal in their estimations 
of expected breeding species. 

The only way to accurately test for adequate coverage is 
to send a highly qualified observer— well versed in local 
bird songs and willing to hike, if need be, to all available 
habitats— to a block with prior coverage and see if he or she 



can add many new species. If after a day in the field the 
observer adds little to the breeding list, it seems fair to 
assume that the block has been adequately covered. 
Because of limits to observer numbers, ability, and time, 
such a test is usually not possible for most blocks. On the 
other hand, such a test is advised for blocks with lower 
species totals than other adjoining blocks with similar 
habitats and even for a certain percentage of randomly 
selected blocks in various subregions of the adas area. 

The Marin Atlas 

We did not use any a priori standard of coverage nor were 
data from Marin adas blocks formally compared to a 
standard, score, or test. Rather, we assessed adequacy of 
coverage in the Marin County Breeding Bird Adas Project 
on an empirical block-by-block basis in a manner similar 
to that later used in Ohio (Rice 6k Peterjohn 1986). The 
overall coordinator carefully weighed the data for each 
block just prior to and during the final year of adas field 
work in 1982. From the basis of knowledge of habitat 
preferences of expected species and the habitats known or 
expected to be found in each block, species lists from data 
cards for each block widi some coverage were scrutinized. 
Species that were likely still to be found were highlighted 
on cards sent back to observers, who were asked to specifi- 
cally look for those species in the appropriate habitats. To 
further ensure adequate coverage of all blocks, and to 
prioritize the assignment of blocks to participants in the 
final adas season, each area coordinator was given a list of 
all blocks in their area qualitatively divided into the follow- 
ing categories: (1) blocks not covered at all, (2) blocks 
needing much work, (3) blocks needing moderate work, 
and (4) blocks needing spot checking. Blocks in the last 
category appeared already to have been covered "ade- 
quately." Nevertheless, considering that one or more spe- 
cies are almost inevitably missed in every block, no matter 
how well covered, participants were asked to look for 
additional species in these blocks when they had reason to 
suspect that a species had been missed. 

Although throughout the project we emphasized to 
observers the importance of obtaining as much evidence of 
confirmed breeding as possible, we placed a higher priority 
in the final year of at least establishing presence in each 
block of as many of die expected species as possible. This 
approach was taken to ensure that our data would best 
represent the distribution of each species. We did not want 
to miss the presence of species in undercovered blocks at 
the expense of spending too much time confirming breed- 
ing of more species in blocks where most expected species 
had already been found. We assumed that if we spent 
enough time to document the presence of most species that 
die natural by-product would be the observation of consid- 
erable evidence of confirmed breeding. 

47 



Data Summary 



MARIN COUNTY BREEDING BIRD ATI AS 



Quantitative Data 



Mop-up efforts were targeted for blocks in the "needing 
moderate work" category. Skilled observers sent to such 
blocks for one-time intensive visits generally added only a 
few species. The use of mop-up observers to randomly 
sample a small number of blocks is highly recommended 
as a true hands-on test of adequate coverage. The success 
of mop-up efforts also bolstered confidence in the lists of 
expected species generated for blocks where such observers 
were not used. 

In the last year, many blocks were covered for the first 
time, basically by the technique of block-busting, now 
widely promoted by most adas projects. Area coordinators 
or other skilled observers with excellent birdsong identifi- 
cation skills and the willingness to hike off the beaten path 
covered these blocks on at least two field days separated by 
about two weeks to a month. For comparison, block-bust- 
ing teams in some state adas projects with larger block sizes 
than those in Marin averaged about 10 to 20 hours per 
block (Laughlin ck Kibbe 1985, Andrle & Carroll 1988). 
It was felt diat our blocks were of such small size diat an 
observer could easily hike to visit all the major habitats in 
one day. The second visit ensured probable breeding status 
for many species based on hearing or seeing individuals at 
the same site over time. Confirmed breeding evidence was 
usually found for many species on the second visit, which 
we tried to time for the period when many recendy fledged 
young were just out of the nest and still being fed by 
parents. 

Other methods noted above geared toward specific 
groups, such as owls and seabirds, enhanced our ability to 
achieve adequate coverage. Mop-up efforts or Spring Bird 
Count help also had the advantage of obtaining indepen- 
dent coverage of many blocks. This enhanced coverage 
because observers with different skills worked a block, and 
some individuals were likely to visit areas of the block not 
covered by another observer. Given the pitfalls of the many 
methods of assessing coverage described above, there is no 
reason to suspect that our adas necessarily suffered by the 
lack of an a priori standard of coverage. 

Data Summary 

After die completion of atlas field work in 1982, the overall 
adas coordinator made a final check of all adas cards for 
accuracy and asked original observers for details about 
unusual or questionable sightings. The most frequent 
question asked was not whether the observer felt the 
species in question was identified correcdy, but whether 
the species was observed in "appropriate habitat" at die 
right time of year. After being satisfied that all observations 
on the data cards were correct to the best of the 
coordinator's knowledge, the data were transferred from 
the adas cards onto species summary sheets. Each sum- 
mary sheet had a listing of all adas blocks and three 

48 



columns to check off possible, probable, or confirmed 
breeding. The number of blocks that had evidence for the 
three categories was totaled and checked. Bill Lenarz then 
entered all die adas data from the species sheets into 
computer files and checked the breeding category totals 
against those done by hand. Data were summarized for all 
species and all blocks, including the 2.5-, 5-, and 10-km 
block equivalents. Data presented below are from the 
2.5-km block equivalents. Lenarz also wrote additional 
computer programs for more detailed analyses currendy 
slated for future publication. 

Before the 1982 field season, preliminary species maps 
were made by hand, using all the 1976 to 1981 adas data 
and the symbols of breeding evidence subsequendy por- 
trayed on the final maps (see Content of Species Accounts 
p. 73). Copies of these maps were given to the regional 
coordinators to illustrate the detail of the final maps, to 
point out gaps in our knowledge, and to provide encour- 
agement to coordinators and adas participants. After the 
completion of data collection in 1982, final maps were 
constructed by adding to the preliminary maps all the 
information from the checked species sheets. For each 
species, dots on the map were counted and checked against 
the species sheets and, if necessary, adjustments were 
made. It is likely that a very few errors in mapping were 
made, but this should not affect the overall pattern of 
distribution of common species. Maps of rarer species 
were checked and double-checked against the species 
sheets. For several rare and sensitive species, the locations 
of breeding records were protected by moving the dots on 
the map by one to two blocks; such maps were labeled 
accordingly (see Content of Species Accounts p. 73). 

Quantitative Data on Abundance 

Although no organized effort was made to estimate the 
abundance of each species in each block as part of the adas 
project, data on the abundance of birds in Marin County 
collected for other purposes were summarized or identi- 
fied. Sources of this data included the following: 

1 . Spring Bird Counts. Data from the three SBCs 
described above are presented in Appendix A. 

2. Breeding Bird Survey routes. Data from the two 24-5- 
mile USFWS Breeding Bird Survey routes established in 
Marin County are summarized in Appendix B. Eleven 
years of data from the period 1972 to 1986 were used from 
the Fairfax 083 route; coverage dates ranged from 8 May 
to 14 June (median 2 June). Seven years of data from the 
period 1975 to 1986 were used from the Point Reyes 071 
route; coverage dates ranged from 3 to 22 June (median 1 1 
June). 

3. Breeding Bird Census plots. Data on abundance of 
Marin County birds collected on Breeding Bird Census 
plots along the coast (1951-1990) and published in Amer- 



Quantitative Data 



METHODS EMPLOYED IN THE MARIN ATLAS 



Quantitative Data 



ican Birds or the journal of Field Ornithology were not 
summarized, but the names, locations, and citations of the 
published accounts of these plots are listed in Appendix 
C. 

4. Breeding seabird colony surveys. Numbers of seabirds 
estimated at various colonies in Marin County by USFWS 
personnel (Sowls et al. 1980; Carter et al. 1990, 1992) 
were summarized and accompany the seabird accounts. 

5. Heron and egret rookery surveys. Data on the numbers 
of herons and egrets breeding at various Marin County 
rookeries (Pratt 1983, p. 103 this volume) are presented in 
tabular form with the appropriate species accounts. 

6. testing Osprey surveys. Data on the number of 
Ospreys nesting at Kent Lake in the Marin Municipal 
Water District (Evens 1991) accompany the Osprey 
account. 



7- Common Yellowthroat surveys. Data on numbers of 
Saltmarsh Common Yellowthroats from surveys of the San 
Francisco Bay area (Hobson et al. 1986) supplement the 
Yellowthroat account 

In addition, after the adas work was completed, esti- 
mates were made of the relative abundance of each species 
in an "average" block. These qualitative estimates were 
based on the author's detailed field notes on abundance 
gathered while exploring habitats in virtually all parts of 
Marin over an eight-year span from 1975 to 1982. These 
abundance estimates per average block were used in con- 
junction with the adas distribution data to make estimates 
of the relative abundance of each species countywide as 
presented in each species account (see Content of Species 
Accounts p. 73). 



/ 



jm 







49 



MARIN COUNTY BREEDING BIRD ATIAS 
















£&& «4fc£.4dU 



Coastal scrub, grassland, and riparian forest lend a soft-looking texture to the landforms of the Rodeo Lagoon valley and 
Wolf Ridge west of "Hawk Hill." Drawing fry Ane Rovetta, 1 989. 



50 



RESULTS AND DISCUSSION 



all nature is so full, that that district produces the greatest variety which is most examined. 



- Gilbert White, 1768 



Atlas Coverage 

FIELD WORKERS covered all 221 blocks in the Marin 
County Breeding Bird Atlas grid. The amount of time 
spent on field work was tallied only in 1982, when about 
92 observers logged over 2800 hours afield. In only a few 
blocks was coverage considered unsatisfactory. In one case, 
coverage was compromised because we were denied access 
to private land comprising all of one block along the Estero 
del Americano; but observers were able to at least sample 
most habitats in that block via kayak. 

For all atlas blocks combined, possible breeding 
accounted for 33.4% of all records, probable for 34-0%, 
and confirmed for 32.6%. Although these data fall short 
of the ideal of confirming all species as breeders in all 
blocks, they are consistent with our efforts to obtain 
accurate distribution maps by documenting the presence 
of most breeding species in each block (see Adequacy of 
Coverage p. 46). Excluding nightbirds, we probably found 
some evidence of breeding for 90%-95% of all species 
actually breeding in most blocks. Owls, poorwills, and 
other secretive species such as rails were not surveyed as 
well as other species, but their basic distribution patterns 
were established. For example, the Great Horned Owl was 
recorded in about two-thirds of the potential blocks though 
it likely occurred in almost all of them. Nonetheless, the 
owl's adas map clearly shows that it breeds throughout the 
county; notably, a high proportion of the blocks without 
documentation of Great Horned Owl were away from 
roads, where coverage was scant. Extraordinary efforts 
would have to have been made to bring the completeness 
of data collection for nocturnal birds up to that of diurnal 
species. Since the basic distribution patterns of nocturnal 
species were established, the expenditure of such effort 
seems warranted only in the case of Endangered or Threat- 
ened species such as the Spotted Owl (see account). 

Patterns of Species Richness of the 
Breeding Avifauna 

During the field work for the Marin County Breeding Bird 
Atlas, we found breeding evidence for 1 57 species of birds. 
For 143 of these we established confirmed breeding evi- 



dence. Of the remaining 14 species, 9 species— Blue- 
winged Teal, Rhinoceros Auklet, Tufted Puffin, Northern 
Pygmy-Owl, Burrowing Owl, Vaux's Swift, California 
Thrasher, Yellow-breasted Chat, and Red Crossbill— still 
lack confirmed breeding evidence. Currendy, all of these 
species except Burrowing Owl, Yellow-breasted Chat, and 
Red Crossbill probably breed in the county annually in 
small numbers. 

Of the 14 species that remained unconfirmed during 
die years of adas field work, 5 species were confirmed 
breeding in the county at other times: Northern Shoveler 
was confirmed subsequent to atlas work, bodi Common 
Poorwill and Rock Wren prior to adas work, and both 
MacGillivray's Warbler and Black-chinned Sparrow both 
before and after the atlas period. An additional 6 species 
not recorded during adas field work have been confirmed 
as breeders in the county: Peregrine Falcon both prior to 
and after adas work; American Avocet after adas work; 
Spotted Sandpiper both prior to and after adas work; and 
Greater Roadrunner, Cassin's Kingbird, and American 
Dipper all prior to atlas work. Of these, Greater Roadrun- 
ner has been entirely extirpated from the county, and 
American Dipper appears to have been extirpated here as 
a breeder. In recent years, Dippers have been recorded in 
the county only as irregular migrants or winter visitants; it 
is possible they may still breed here irregularly in high 
runoff years, though recent summer records are lacking. 

In all, in historical times Marin County has supported 
at least 1 63 species of breeding birds— 1 54 based on 
confirmed evidence and 9 based on suspected evidence. 
Including naturally irregular/irruptive breeders such as 
Long-eared Owl, Black-chinned Sparrow, Red Crossbill, 
and Lawrence's Goldfinch, but excluding extirpated breed- 
ing species (Greater Roadrunner and American Dipper), 
extralimital breeders (Say's Phoebe, Cassin's Kingbird, 
and Northern Parula) or otherwise very infrequent breed- 
ers (Double-crested Cormorant, American Avocet, Spotted 
Sandpiper, Burrowing Owl, Short-eared Owl, and Yellow- 
breasted Chat), the county's breeding avifauna currendy 
numbers about 152 more or less regularly breeding spe- 
cies. 

A number of additional species not mentioned above 
may also have bred here historically and either went 

51 



Patterns of Species Richness 



MARIN COUNTY BREEDING BIRD ATLAS 



Patterns of Species Richness 



undetected or were inadequately documented; some of 
these may once have been part of the regular breeding 
avifauna (see Species of Unclear Breeding Status or Poten- 
tial Breeders pp. 429-434)- Certain other species breeding 
elsewhere in die San Francisco Bay Area are the most likely 
potential future colonizers to Marin County. One of the 
most likely species to soon become established is Wild 
Turkey which was introduced to die county in 1988. 

Based on the number of blocks in which they were 
recorded, 20 of the 157 breeding species during the adas 
period were classified as nearly ubiquitous in their distri- 
bution here, 15 as very widespread, 17 as widespread, 10 
as fairly widespread, 9 as somewhat local, 23 as local, and 
63 as very local (Table 6). A ranking of the 157 species by 
their Overall Population Index yielded 5 species with 
extremely large populations, 15 with very large popula- 
tions, 12 with large populations, 18 with fairly large popu- 
lations, 6 with moderate-sized populations, 19 with small 
populations, and 82 with very small populations (Table 7). 
The two methods of ranking species— by distribution and 
a combination of distribution and abundance— each 
showed a disproportionate number of species with rela- 
tively restricted distributions and relatively small popula- 
tions. These patterns are typical of many avifaunal 
assemblages that have been studied (e.g., Wiens 1989). 

Countywide, the number of breeding species recorded 
per block ranged from 22 to 84 and averaged 56.7 (SE = ± 
0.79) (Figure 10). Sixty-two percent of the blocks had 
between 50 to 70 species each. The areas of the county that 
tended to have the highest breeding species richness per 
block were the south-central interior ridges (Figure 11) with 
a mix of hardwood, conifer, scrub, and grassland habitats. 
On the whole, blocks in the grassland-dominated regions 
of outer Point Reyes and around Tomales supported the 
lowest species richness. Habitat diversity was not meas- 
ured, but it undoubtedly would have shown a positive 
relationship with species richness per block. 

Recognizing that comparisons of species richness 
between areas of different size, or where data were collected 
differendy, poses some problems (Wiens 1989), it is still 
instructive to compare the size of Marin's breeding avi- 
fauna with that of other regions. Preliminary comparisons 
show that the species richness of the entire Marin County 
breeding avifauna is roughly similar to that of most other 
counties in coastal northern California (Shuford in prep.). 
Comparisons also show that the breeding avifauna of 
Marin County (latitude about 38°, 588 square miles) is 
greater than that of interior areas at roughly the same 
latitude of similar or even much greater size. At about 
38°45' latitude, Yolo County encompasses 1034 square 
miles, ranges from about 100 to 3000 feet in elevation, and 
extends from the east slope of the Interior Coast Range 
across the west side of the Sacramento Valley floor. Yolo 
County has a breeding avifauna of about 133 species 

52 



80-1 



60* 



o 
o 



E 
in 

CVJ 



40* 



20- 



" I I I I I I I 

20 30 40 50 60 70 80 90 

No. species/block 

Figure 10. Frequency distribution of the number of species 
found per atlas block. 

(Gaines &. Beedy 1987). At about 37°45' latitude, the west 
slope of the Yosemite region is roughly the size of Yolo 
County, ranges from about 1 200 to over 1 3,000 feet on the 
west slope of the Sierra Nevada, and currendy supports 
about 141 species of breeding birds (Gaines 1988). The 
east slope of the Yosemite region, roughly equal in size to 
the west slope, ranges from the Sierran crest down the east 
slope to about 6400 feet in the Great Basin and currendy 
sustains about 149 species of breeding birds. The com- 
bined Yosemite region spanning the west and east slopes 
harbors about 187 species of breeding birds. It is not 
surprising that the greater Yosemite region supports a 
more numerous breeding avifauna than Marin County, 
considering that the former is much larger, ranges over 
almost 12,000 feet in elevation, and straddles two of 
California s major biogeographical regions, the Sierra 
Nevada and the Great Basin. Further adas work in Califor- 
nia will provide needed insight into patterns of breeding 
bird species richness in this diverse state. 

The number of breeding species in Marin County's 
avifauna compares favorably even with a few states and 
provinces in North America: Kentucky (164 species), 
Arkansas (157), Delaware (157), Mississippi (153), Prince 
Edward Island (146), and Hawaii (131) (DeSante & Pyle 
1986). Clearly, Marin County supports a large avifauna for 
its size. This can be attributed to the county's diverse array 



RESULTS AND DISCUSSION 

Table 6. Relative distribution ranking of all species recorded on the Marin County Breeding Bird Adas. Species are listed 
in descending order, by the number and percentage of total blocks (No. - %) in which they were detected, with respect to 
the seven categories of the Relative Distribution Index (RDI); see Content of Species of Accounts (p. 75). 



NEARLY UBIQUITOUS 

Scrub Jay (214-96.8) 
Turkey Vulture (213-96.4) 
Red-tailed Hawk (213-96.4) 
Mourning Dove (212-95.9) 
California Quail (208-94.1) 
Bushtit (207-93.7) 
House Finch (207-93.7) 
Bewick's Wren (205-92.8) 
Barn Swallow (203-91.8) 
Cliff Swallow (202-91.4) 
California Towhee (201-91.0) 
Allen's Hummingbird (198-89.6) 
American Robin (197-89.1) 
Chestnut-backed Chickadee (195-88.2) 
Rufous-sided Towhee (195-88.2) 
European Starling (194-87.8) 
Brewer's Blackbird (193-87.3) 
Song Sparrow (192-86.9) 
Orange-crowned Warbler (191 -86.4) 
Dark-eyed Junco (188-85.1) 

VERY WIDESPREAD 

Hutton's Vireo (184-83.2) 
Wrentit (182-82.4) 
Violet-green Swallow (177-80.1) 
Purple Finch (177-80.1) 
Brown-headed Cowbird (173-78.3) 
American Goldfinch (173-78.3) 
Red-winged Blackbird (172-77.8) 
Common Raven (170-76.9) 
Western Bluebird (170-76.9) 
American Crow (169-76.5) 
Black Phoebe (167-75.6) 
Warbling Vireo (164-74-2) 
Anna's Hummingbird (163-73.8) 
Pacific-slope Flycatcher (163-73.8) 
Wilson's Warbler (161-72.8) 

WIDESPREAD 

Steller'sjay (154-69.7) 
Pine Siskin (151-68.3) 
Ash-throated Flycatcher (150-67.9) 
Great Horned Owl (149-67.4) 
Killdeer (146-66.1) 
Downy Wood pecker (145-65.6) 
Lesser Goldfinch (145-65.6) 
House Sparrow (144-65.2) 
Northern Flicker (143-64.7) 
Swainson's Thrush (137-62.0) 
Western Wood-Pewee (136-61.5) 
Western Meadowlark (135-61.1) 
Tree Swallow (132-59.7) 
Plain Titmouse (129-58.4) 
Black-headed Grosbeak (129-58.4) 
Northern Oriole (129-58.4) 
Brown Creeper (128-57.9) 



FAIRLY WIDESPREAD 

Lark Sparrow (124-56.1) 
Horned Lark (123-55.6) 
American Kestrel (122-55.2) 
Band-tailed Pigeon (117-52.9) 
Lazuli Bunting (115-52.0) 
Savannah Sparrow (115-52.0) 
Mallard (110-49.8) 
N. Rough-winged Swallow (104-47.0) 
Olive-sided Flycatcher (96-43.4) 
Grasshopper Sparrow (96-43.4) 

SOMEWHAT LOCAL 

Chipping Sparrow (93-42.1) 
Rock Dove (90-40.7) 
Acorn Woodpecker (86-38.9) 
White-crowned Sparrow (85-38.5) 
Hairy Woodpecker (82-37.1) 
Western Kingbird (74-33.5) 
Belted Kingfisher (73-33.0) 
Nuttall's Woodpecker (72-32.6) 
Northern Mockingbird (67-30.3) 

LOCAL 

White-breasted Nuthatch (59-26.7) 
American Coot (58-26.2) 
Red-shouldered Hawk (56-25.3) 
Winter Wren (56-25.3) 
Cinnamon Teal (52-23.5) 
Rufous-crowned Sparrow (52-23.5) 
Osprey (49-22.2) 
Northern Harrier (48-21.7) 
Hermit Thrush (48-21.7) 
Western Screech-Owl (42-19.0) 
White-throated Swift (42-19.0) 
Golden-crowned Kinglet (42-19.0) 
Pygmy Nuthatch (40-18.1) 
Loggerhead Shrike (40-18.1) 
Red-breasted Nuthatch (39-17.6) 
Golden Eagle (38-17.2) 
Common Yellowthroat (38-17.2) 
Cooper's Hawk (36-16.3) 
Pied-billed Grebe (34-15.4) 
Black-shouldered Kite (34-15.4) 
Barn Owl (34-15.4) 
Marsh Wren (34-15.4) 
Northern Saw-whet Owl (32-14.5) 

VERY LOCAL 

House Wren (29-13.1) 
Pileated Woodpecker (28-12.7) 
Green-backed Heron (24-10.8) 
Purple Martin (24-10.8) 
Black-throated Gray Warbler (23-10.4) 
Yellow Warbler (22-10.0) 
Ruddy Duck (20-9.0) 
Yellow-rumped Warbler (19-8.6) 



Virginia Rail (17-7.7) 
Spotted Owl (16-7.2) 
Blue-gray Gnatcatcher (16-7.2) 
Rock Wren (15-6.8) 
California Thrasher (15-6.8) 
Tricolored Blackbird (15-6.8) 
Hooded Oriole (15-6.8) 
Northern Pintail (14-6.3) 
Western Gull (14-6.3) 
Great Blue Heron (12-5.4) 
Vaux's Swift (12-5.4) 
Western Tanager (12-5.4) 
Red Crossbill (12-5.4) 
Pelagic Cormorant (11-5.0) 
Snowy Plover (11-5.0) 
Hermit Warbler (11-5.0) 
Gadwall (10-4-5) 
Sora (10-4.5) 

Black Oystercatcher (10-4-5) 
Pigeon Guillemot (10-4-5) 
Blue-winged Teal (9-4.1) 
Ring-necked Pheasant (9-4.1) 
Black-necked Stilt (9-4.1) 
Solitary Vireo (8-3.6) 
MacGillivray's Warbler (8-3.6) 
American Bittern (7-3.2) 
Sharp-shinned Hawk (7-3.2) 
Black Rail (6-2.7) 
Great Egret (5-2.3) 
Wood Duck (5-2.3) 
Common Merganser (5-2.3) 
Clapper Rail (5-2.3) 
Red-breasted Sapsucker (5-2.3) 
Black-chinned Sparrow (5-2.3) 
Brandt's Cormorant (4-1-8) 
Northern Shoveler (4-1-8) 
Sage Sparrow (4-1.8) 
Lawrence's Goldfinch (4-1.8) 
Common Murre (3-1.4) 
Common Moorhen (2-0.9) 
Northern Pygmy-Owl (2-0.9) 
Burrowing Owl (2-0.9) 
Long-eared Owl (2-0.9) 
Common Poorwill (2-0.9) 
Yellow-breasted Chat (2-0.9) 
Ashy Storm-Petrel (1-0.4) 
Double-crested Cormorant (1-0.4) 
Snowy Egret (1 -0.4) 
Black-crowned Night-Heron (1-0.4) 
Canada Goose (1-0.4) 
Rhinoceros Auklet (1 -0.4) 
Tufted Puffin (1 -0.4) 
Short-eared Owl (1-0.4) 
Say's Phoebe (1 -0.4) 
Northern Parula (1-0.4) 



53 



MARIN COUNTY BREEDING BIRD ATLAS 

Table 7. Abundance ranking of all species recorded on the Marin County Breeding Bird Adas. Species are listed in 
descending order in seven categories with respect to their Overall Population Index (OPI); see Content of Species Accounts 
(p. 75). 



EXTREMELY LARGE POPULATION 

Cliff Swallow (1010) 
Warbling Vireo (984) 
Song Sparrow (960) 
Orange-crowned Warbler (955) 
Dark-eyed Junco (940) 

VERY LARGE POPULATION 

Red-winged Blackbird (860) 
Scrub Jay (856) 
Mourning Dove (848) 
House Finch (828) 
Bewick's Wren (820) 
Barn Swallow (812) 
Wilson's Warbler (805) 
California Towhee (804) 
Allen's Hummingbird (792) 
American Robin (788) 
Chestnut-backed Chickadee (780) 
Rufous-sided Towhee (780) 
European Starling (776) 
Brewer's Blackbird (772) 
Wrentit (728) 

LARGE POPULATION 

Violet-green Swallow (708) 
Purple Finch (708) 
American Goldfinch (692) 
Swainson's Thrush (685) 
Pacific-slope Flycatcher (652) 
Turkey Vulture (639) 
Red-tailed Hawk (639) 
California Quail (624) 
Bushtit (621) 
Pine Siskin (604) 
Ash-throated Flycatcher (600) 
Lesser Goldfinch (580) 

FAIRLY LARGE POPULATION 

House Sparrow (576) 
Savannah Sparrow (575) 
Hutton's Vireo (552) 
Western Meadowlark (540) 
Brown-headed Cowbird (519) 
Plain Titmouse (516) 
Black-headed Grosbeak (516) 
Northern Oriole (516) 
Brown Creeper (512) 
Western Bluebird (510) 
American Crow (507) 
Black Phoebe (501) 
Lark Sparrow (496) 
Horned Lark (492) 
Anna's Hummingbird (489) 
Steller's Jay (462) 
Great Horned Owl (447) 
Killdeer (438) 



MODERATE-SIZED POPULATION 

White-crowned Sparrow (425) 
Western Wood-Pewee (408) 
Band-tailed Pigeon (351) 
Lazuli Bunting (345) 
Common Raven (340) 
Mallard (330) 

SMALL POPULATION 

Downy Woodpecker (290) 
Olive-sided Flycatcher (288) 
Grasshopper Sparrow (288) 
Northern Flicker (286) 
Chipping Sparrow (279) 
Rock Dove (270) 
Tree Swallow (264) 
Acom Woodpecker (258) 
American Kestrel (244) 
Winter Wren (224) 
Western Kingbird (222) 
Nuttall's Woodpecker (216) 
N. Rough-winged Swallow (208) 
Northern Mockingbird (201) 
White-breasted Nuthatch (177) 
American Coot (174) 
Hairy Woodpecker (164) 
Cinnamon Teal (156) 
Common Yellowthroat (152) 

VERY SMALL POPULATION 

Belted Kingfisher (146) 
Hermit Thrush (144) 
Marsh Wren (136) 
Western Screech-Owl (126) 
Pygmy Nuthatch (120) 
Red-shouldered Hawk (112) 
Rufous-crowned Sparrow (104) 
Osprey (98) 
Northern Harrier (96) 
Black-throated Gray Warbler (92) 
White-throated Swift (84) 
Golden-crowned Kinglet (84) 
Loggerhead Shrike (80) 
Red-breasted Nuthatch (78) 
Pied-billed Grebe (68) 
Black-shouldered Kite (68) 
Barn Owl (68) 
Yellow Warbler (66) 
Northern Saw-whet Owl (64) 
House Wren (58) 
Yellow-rumped Warbler (57) 
Western Gull (56) 
Pileated Woodpecker (56) 
Pelagic Cormorant (55) 
Green-backed Heron (48) 
Purple Martin (48) 
Blue-gray Gnatcatcher (48) 



Tricolored Blackbird (45) 
Hooded Oriole (45) 
Pigeon Guillemot (40) 
Golden Eagle (38) 
Great Blue Heron (36) 
Cooper's Hawk (36) 
Virginia Rail (34) 
Snowy Plover (33) 
Spotted Owl (32) 
Rock Wren (30) 
California Thrasher (30) 
Brandt's Cormorant (28) 
Northern Pintail (28) 
Black-necked Stilt (27) 
Great Egret (25) 
Western Tanager (24) 
Hermit Warbler (22) 
Common Murre (21) 
Ruddy Duck (20) 
Black Oystercatcher (20) 
Ring-necked Pheasant (18) 
Black Rail (18) 
MacGillivray's Warbler (16) 
Solitary Vireo (16) 
Clapper Rail (15) 
Red Crossbill (12) 
Vaux's Swift (1 2) 
Gadwall (10) 
Sora (10) 

Blue-winged Teal (9) 
Sage Sparrow (8) 
American Bittern (7) 
Snowy Egret (7) 
Sharp-shinned Hawk (7) 
Black-crowned Night-Heron (6) 
Common Poorwill (6) 
Wood Duck (5) 
Common Merganser (5) 
Red-breasted Sapsucker (5) 
Black-chinned Sparrow (5) 
Northern Shoveler (4) 
Lawrence's Goldfinch (4) 
Ashy Storm-Petrel (3) 
Common Moorhen (2) 
Rhinoceros Auklet (2) 
Tufted Puffin (2) 
Northern Pygmy-Owl (2) 
Long-eared Owl (2) 
Yellow-breasted Chat (2) 
Double-crested Cormorant (1) 
Canada Goose (1) 
Burrowing Owl (1) 
Short-eared Owl (1) 
Say's Phoebe (1) 
Northern Parula (1) 



54 



Patterns of Species Richness 



RESULTS AND DISCUSSION 



Distributional Highlights 



SPECIES RICHNESS 



Number of 
Species per Block 



73-84 



48-59 




Figure 1 1. Map of countywide patterns of species richness of breeding birds. 



of habitats, resulting from varied topography and the sharp 
moisture and temperature gradients over the short distance 
from the cool, moist coast to the hotter, drier interior. 

Distributional Highlights of Atlas Work 

The adas field work documented distribution patterns for 
a number of Marin County's breeding species diat prob- 
ably would not have been predicted beforehand. Foremost 
among these was the distribution pattern here of the 
Grasshopper Sparrow. McCaskie et al. (1979) considered 
the Grasshopper Sparrow an uncommon to rare breeding 
species in northern California as a whole, and in preadas 
days that classification seemed to fit Marin County as well. 
Thorough coverage of our previously litde-birded grass- 
lands revealed Grasshopper Sparrows breeding in over 
40% of ail atlas blocks and the vast majority of blocks near 
the coast with extensive grassland (adas map and Figure 5). 
The concentration of breeding Grasshopper Sparrows in 
the moister, less disturbed grasslands toward the immedi- 
ate coast matched the pattern noted along the San Mateo 
County coast in the 1960s and 1970s (D.F. DeSante pers. 
comm.); in contrast, Sibley (1952) had reported most 
nesting records in the soudi San Francisco Bay region were 
from the Inner Coast Range, 20 to 25 miles from the coast. 
Before the atlas period, the Cooper's Hawk was a species 



very rarely reported in Marin County in the breeding 
season, but we now know it is a secretive but regular 
breeder here in broadleaved mixed evergreen forests. 

Of course a number of rare or newly established breed- 
ers were documented nesting in the county for the first 
time during the adas years. Among others, these included 
American Bittern, Common Merganser, Sharp-shinned 
Hawk, Black Rail, Black-necked Stilt, Short-eared Owl, 
Say's Phoebe, and Northern Parula. The first and subse- 
quent breeding records of Red-breasted Sapsucker in the 
county documented the existence of a disjunct breeding 
population here (Shuford 1986). The adas also provided 
documentation of the distribution of several species that, 
direcdy or indirecdy as a result of activities of an expanding 
human population, have become well established here in 
appropriate habitat in the last few decades. These include 
Northern Mockingbird, European Starling, Brown-headed 
Cowbird, and Hooded Oriole. The reader is encouraged 
to comb the species accounts for other noteworthy breed- 
ing records or distribution patterns uncovered by the adas 
work. 

The previously little surveyed region of the county 
around Novato proved to support radier widespread breed- 
ing populations of species characteristic of die Inner Coast 
Range, such as Nuttall's Woodpecker, Western Kingbird, 
and Northern Oriole, and a restricted population of a 
formerly unknown breeder in the county— Blue-gray Gnat- 



55 



Distributional F/igKfigKts 



MARIN COUNTY BREEDING BIRD ATLAS Composition of Breeding Avifauna 



catcher. That the Novato area also supported populations 
of such species as Olive-sided Flycatcher, Pacific-slope Ely- 
catcher, Steller's Jay, Chestnut-backed Chickadee, Brown 
Creeper, Wilson's Warbler, and Purple Finch indicated 
that all of die county is tempered to some degree by die 
cool and moist coastal climate. Although hinted at by 
previous work (Orr 1937, Miller 1951), the slopes of 
Mount Tamalpais and some surrounding ridges proved to 
host the county's only or main breeding populations of 
Solitary Vireo, Yellow-rumped Warbler, Black-throated 
Gray Warbler, Hermit Warbler, and Western Tanager. 
The chaparral on Carson Ridge proved to be an important 
habitat in the county for Common Poorwill, California 
Thrasher, Rufous-crowned Sparrow, Sage Sparrow, and, 
irregularly, Black-chinned Sparrow. A number of 
waterbirds (ducks, rails, and shorebirds) were added to die 
county's breeding list, but the restricted breeding distribu- 
tion of most of them documents the limited extent of 
freshwater, brackish, and saline wedands in the county. 

A number of other studies conducted concurrendy with 
or subsequendy to the adas project have provided valuable 
data for the county on the distribution and abundance of 
breeding seabirds (Sowls et al. 1980, Carter et al. 1990), 
herons and egrets (Pratt 1983, p. 103 this volume), Black 
Rails (Evens et al. 1989), Snowy Plovers (Page 6k Stenzel 
1981), Ospreys (Evens 1989), and Common Yellowdiroats 
(Hobson et al. 1986). Quantitative data on an array of 
Marin's breeding birds has been contributed by Spring 
Bird Counts, Breeding Bird Plots, and Breeding Bird 
Surveys (Appendixes A-C). 

Composition of the Breeding Avifauna 

The 163 species of Marin County's breeding avifauna 
represent 43 families of birds. Of these, 41 are aquatic 
species (including Osprey, Belted Kingfisher, and Ameri- 



can Dipper), and 122 are landbirds. All of the waterbirds 
can be found in Marin County year round, although 22 
species have substantially greater populations in winter 
and 7 species have substantially greater populations in 
summer (Table 8). Of the landbirds, 30% (37) are summer 
residents (breeding migrants) and 70% (85) are year-round 
residents, although at least 23 of these are generally more 
numerous in winter (Table 9). Of 1 54 wintering landbird 
species recorded for Marin County (Shuford 1982), 67% 
(103) are year-round residents (including 15 species of 
lingering summer residents) and 33% (51) are wintering 
migrants (including lingering individuals of 13 species of 
typically passage migrants). These patterns of a relatively 
high percentage of resident species and a moderate percent- 
age of summer and winter resident migrants are typical of 
the central and southern coast and foothill regions of 
California (Tangren 1977). On the whole, these regions 
tend to have relatively moderate temperatures and rainfall. 
Areas in California with a high percentage of summer 
residents and a low percentage of winter residents (e.g., 
high mountains) tend to have relatively low June and 
December temperatures and relatively high precipitation. 
Areas with a high percentage of winter residents and a low 
percentage of summer residents (e.g., Central Valley and 
southern deserts) tend to have relatively high June and 
December temperatures and relatively low precipitation. 
Areas with a high percentage of both summer and winter 
residents (e.g., northern Sacramento Valley) tend to have 
large amounts of June rainfall coupled with mild winters, 
whereas areas with low percentages of both of these com- 
ponents (e.g., some southern California mountains) tend 
to have relatively low June precipitation and relatively more 
severe winters (Tangren 1977). 

Miller (1951) partitioned California's breeding birds 
into four avifaunal groupings— one of Boreal (conifer) affin- 
ity and three of Austral (lowland, nonboreal) affinity. 



Table 8. A list of 41 species of waterbirds breeding in Marin County, California, with annotations on seasonal status; all 
species occur year round to some degree. List includes one extirpated breeder— American Dipper. 



Pied-billed Grebe* 
Ashy Storm-Petrel 
Double-crested Cormorant 
Brandt's Cormorant 
Pelagic Cormorant 
American Bittern* 
Great Blue Heron 
Great Egret* * 
Snowy Egret 
Green-backed Heron * * 
Black-crowned Night-Heron 
Canada Goose* 
Wood Duck* 
Mallard 



Northern Pintail* 

Blue-winged Teal* * 

Cinnamon Teal** 

Northern Shoveler* 

Gadwall* 

Common Merganser* 

Ruddy Duck* 

Osprey* * 

Black Rail* 

Clapper Rail 

Virginia Rail* 

Sora* 

Common Moorhen* 

American Coot* 



Snowy Plover* 
Killdeer* 

Black Oystercatcher 
Black-necked Stilt* * 
American Avocet* 
Spotted Sandpiper* 
Western Gull 
Common Murre* 
Pigeon Guillemot* * 
Rhinoceros Auklet* 
Tufted Puffin* 
Belted Kingfisher 
American Dipper* 



* Relatively more numerous in winter. 
'* Relatively more numerous in summer. 



56 



Composition of Breeding Avifauna 



RESULTS AND DISCUSSION 



Composition of Breeding Avifauna 



Table 9. Classification of seasonal status of 122 species of breeding landbirds in Marin County, California. List includes 
one extirpated breeder— Greater Roadrunner— and three extralimital breeders— Say's Phoebe, Cassin's Kingbird, and 
Northern Parula. 



YEAR-ROUND RESIDENTS 

(85 Species) 

Turkey Vulture 
Black-shouldered Kite 
Northern Harrier* 
Sharp-shinned Hawk* 
Cooper's Hawk* 
Red-shouldered Hawk 
Red-tailed Hawk* 
Golden Eagle 
American Kestrel* 
Peregrine Falcon* 
Ring-necked Pheasant 
California Quail 
Rock Dove 
Band-tailed Pigeon* 
Mourning Dove 
Greater Roadrunner 
Barn Owl 

Western Screech-Owl 
Great Horned Owl 
Northern Pygmy-Owl 
Burrowing Owl 
Spotted Owl 
Long-eared Owl* 
Short-eared Owl* 
Northern Saw-whet Owl 
White-throated Swift 
Anna's Hummingbird 
Acom Woodpecker 
Red-breasted Sapsucker* 
Nuttall's Woodpecker 
Downy Woodpecker 



Hairy Woodpecker 

Northern Flicker* 

Pileated Woodpecker 

Black Phoebe 

Say's Phoebe* 

Homed Lark 

Steller's Jay 

Scrub Jay 

American Crow 

Common Raven 

Chestnut-backed Chickadee 

Plain Titmouse 

Bushtit 

Red-breasted Nuthatch* 

White-breasted Nuthatch 

Pygmy Nuthatch 

Brown Creeper 

Rock Wren 

Bewick's Wren 

Winter Wren* 

Marsh Wren 

Golden-crowned Kinglet* 

Western Bluebird 

Hermit Thrush* 

American Robin* 

Wrentit 

Northern Mockingbird 

California Thrasher 

Loggerhead Shrike 

European Starling 

Hutton's Vireo 

Yellow-rumped Warbler* 



Common Yellowthroat 
Rufous-sided Towhee 
California Towhee 
Rufous-crowned Sparrow 
Lark Sparrow 
Sage Sparrow 
Savannah Sparrow 
Song Sparrow 
White-crowned Sparrow* 
Dark-eyed Junco* 
Red-winged Blackbird 
Tricolored Blackbird* 
Western Meadowlark* 
Brewer's Blackbird 
Brown-headed Cowbird 
Purple Finch 
House Finch 
Red Crossbill* 
Pine Siskin 
Lesser Goldfinch 
American Goldfinch 
House Sparrow 



SUMMER RESIDENTS 
(37 Species) 

Common Poorwill** 
Vaux's Swift 
Allen's Hummingbird 
Olive-sided Flycatcher 
Western Wood-Pewee 



Pacific-slope Hycatcher 
Ash-throated Hycatcher 
Cassin's Kingbird 
Western Kingbird 
Purple Martin 
Tree Swallow 
Violet-green Swallow 
N. Rough-winged Swallow 
Cliff Swallow 
Barn Swallow 
House Wren 
Blue-gray Gnatcatcher 
Swainson's Thrush 
Solitary Vireo 
Warbling Vireo 
Orange-crowned Warbler* * 
Northern Parula 
Yellow Warbler 
Black-throated Gray Warbler 
Hermit Warbler* * 
MacGillivray's Warbler 
Wilson's Warbler 
Yellow-breasted Chat 
Western Tanager 
Black-headed Grosbeak 
Lazuli Bunting 
Chipping Sparrow 
Black-chinned Sparrow 
Grasshopper Sparrow 
Hooded Oriole 
Northern Oriole 
Lawrence's Goldfinch 



Relatively more numerous in winter than summer. 
Small numbers regularly winter. 



Members of these avifaunas have similar centers of distri- 
bution and often similar areas of origin as species. The 
four avifaunas are classified as follows: 

1. Boreal avifauna. Species of northern derivation or 
distribution centered in coniferous forest areas and habi- 
tats. 

2. Great Basin avifauna. Chiefly Great Basin and Great 
Plains species. This avifauna in California is concentrated 
east of the Sierran crest in grassland, sagebrush, pifion- 
juniper woodland, riparian woodland, and aquatic and 
semiaquatic environments. 

3. Sonoran avifauna. Desert-dwelling species and those 
that range into the state from the Southwest and Mexico. 
In California, this avifauna occupies mosdy Colorado and 
Mohave desert habitats of desert scrub, arid woodland, 
riparian woodland, and marshes. 



4- Cali/ornian avifauna. Lowland species that are essen- 
tially confined to the state or are centered there and have 
their most continuous and dense populations in Califor- 
nia. These species are found principally in oak woodlands, 
riparian forest, chaparral, and marshlands west of the 
Cascade-Sierra axis. 

In addition, California's breeding avifauna hosts a large 
list of unclassified forms, chiefly species of marine environ- 
ments, species of general continental or holarctic distribu- 
tion, and species or races of widespread western North 
American distribution. 

Marin County's avifauna is generally dominated by 
Boreal and Californian forms. Nonetheless, the Boreal 
and the combined Austral elements are of similar impor- 
tance here at the species level, while Austral (mosdy Cali- 
fornian) elements predominate at the racial and combined 



57 



Composition of Breeding Avifauna MARIN COUNTY BREEDING BIRD ATI AS Composition of Breeding Avifauna 



(species and race) levels (Table 10). The Boreal avifauna 
provides Marin with 27 species and 13 races; the Califor- 
nian avifauna, 11 species and 33 races; the Great Basin 
avifauna, 1 1 species and 3 races; and the Sonoran avi- 
fauna, 5 species and 1 race. Only four Great Basin species 
(Western Meadowlark, Brewer's Blackbird, Northern Ori- 
ole, and Brown-headed Cowbird) and no Sonoran species 
were classified as having fairly large or larger populations 
in the county (cf. Tables 7 and 10), further indicating the 
minor contribution of these avifaunas to that of Marin 
County. The 27 Boreal species of Marin's avifauna are 
matched by 27 species of Austral origin; the 13 Boreal 
races compare to 37 Austral races. 

Miller (1951) stressed the greater importance in Califor- 
nia of the strong west-east moisture gradient versus the 
weaker north-south moisture and temperature gradient in 
influencing the differentiation of the state's Boreal avi- 
fauna. Similarly, the strong west-east (coast-interior) mois- 
ture and temperature gradients in Marin County are the 
prime factors affecting regional distribution of die county's 
avifauna, Boreal or non-Boreal. As detailed above, die 
county has two main climate zones: (1) a coastal zone of 
relatively high winter rainfall, cool summer temperatures, 
and high summer humidity from summer fog and (2) an 
interior zone with less rainfall, higher summer tempera- 
tures, and lower summer humidity. These climatic zones 
correspond to two main biogeographical regions of the 
county with differing avifaunas (Figure 1 2). The coastal 
zone has a dominant Boreal avifaunal element and the 
interior zone a dominant Austral/Californian element. 
Inverness Ridge, Bolinas Ridge, Mount Tamalpais, and 
several other spur ridges in central Marin have similar 
avifaunas because they share many species that prefer the 
conifer forests or dense mixed evergreen forests that pre- 
dominate in the moist coastal zone. Characteristic 
landbirds of the coastal zone are Band-tailed Pigeon, Spot- 
ted Owl, Northern Saw-whet Owl, Hairy Woodpecker, 
Pileated Woodpecker, Olive-sided Flycatcher, Red-breasted 
Nuthatch, Pygmy Nuthatch, Winter Wren, Golden- 
crowned Kinglet, Hermit Thrush, Wilson's Warbler, and 
Pine Siskin. The upper slopes of Mount Tamalpais and 
nearby ridges support another subset of Boreal species. 
These areas have relatively high winter rainfall but are 
sheltered from intense summer fogs by either an inversion 
layer of warmer air at higher elevation or by die barrier of 
adjacent coastal ridges. In these somewhat drier areas, die 
conifer and mixed conifer forests are more open and hence 
support such boreal species as Solitary Vireo, Yellow- 
rumped Warbler, Western Tanager, and a non-Boreal but 
allied species, Black-throated Gray Warbler. All of these 
are lacking from Inverness Ridge, except Yellow-rumped 
Warbler, which breeds there in smaller numbers than on 
Mount Tamalpais. 



58 




Figure 12. Preliminary map of the two main biogeographical 
regions of Marin County. 

The avifaunas of outer Point Reyes and the Tomales 
area are also similar to each other. Although lacking most 
of the true conifer birds, their combined avifauna is pri- 
marily just a depauperate subset of that of the 
Boreal/coastal avifaunal region of Marin County. Hence, 
the Point Reyes/Tomales area should be included in that 
avifaunal region rather than in the Austral/interior region 
of Marin. The outer Point Reyes/Tomales area shares with 
the conifer-dominated coastal areas species such as 
Swainson's Thrush, Savannah Sparrow, Grasshopper 
Sparrow, White-crowned Sparrow, Pine Siskin, and 
American Goldfinch diat are lacking or breed in smaller 
numbers in the drier interior of Marin. 

The other true biogeographical region of Marin County 
is die oak woodland-dominated area from north of San 
Rafael dirough Novato to the Sonoma County line and 
west to around Hicks Valley. This Austral/interior region 
of Marin County supports a primarily Austral/Californian 
avifauna. Characteristic members of the avifauna of this 
region are Western Screech-Owl, Nuttall's Woodpecker, 
Plain Titmouse, White-breasted Nuthatch, House Wren, 
Blue-gray Gnatcatcher, and Lesser Goldfinch. As noted 
above, even in this region the coastal influence is felt as 
such Boreal species as Steller's Jay, Chestnut-backed 
Chickadee, Brown Creeper, Wilson's Warbler, Dark-eyed 
Junco, and Purple Finch are rather widely distributed, 
though more locally than on the immediate coast. 

Miller (1951) divided California geographically into a 
number of faunal districts, provinces, and areas. He 
included the conifer-dominated areas of Marin County, as 
well as outer Point Reyes, in the Central Coast District of 
the Coastal Province of Boreal avifaunal regions of the 
state. He included the rest of Marin, including the Tomales 
area, in the San Francisco Bay District of the Californian 
Province of Austral avifaunal regions of the state. His 
inclusion of die Tomales area in the San Francisco Bay 
district, while at the same time including outer Point Reyes 
in the coastal Boreal province, seems unwarranted because 
of the number of moist habitat-adapted species found in 



RESULTS AND DISCUSSION 
Table 10. Avifaunal affinities of Marin County's breeding birds, after Miller (1951). 



BOREAL AVIFAUNA (27 species) 

Canada Goose 
Common Merganser 
Northern Pygmy-Owl 
Spotted Owl 
Northern Saw-whet Owl 
Red-breasted Sapsucker 
Olive-sided Flycatcher 
Steller's Jay 

Chestnut-backed Chickadee 
Red-breasted Nuthatch 



Pygmy Nuthatch 
Brown Creeper 
Winter Wren 
Golden-crowned Kinglet 
Swainson's Thrush 
Hermit Thrush 
Solitary Vireo 
Yellow-rumped Warbler 
Hermit Warbler 
MacGitlivray's Warbler 



Wilson's Warbler 
Western Tanager 
White-crowned Sparrow 
Dark-eyed J unco 
Purple Finch 
Red Crossbill 
Pine Siskin 



1 A small population probably of introduced stock. 



White-throated Swift 
Say's Phoebe 
Rock Wren 
Western Meadowlark 



AUSTRAL AVIFAUNA 

Great Basin Avifauna (1 1 species) 
Blue-winged Teal 
Black-necked Stilt 
American Avocet 
Common Poorwill 

Race in Marin of Californian affinity. 
Single extralimital breeding record. 
Race in Marin of Sonoran affinity. 

Sonoran Avifauna (5 species) 
Greater Roadrunner 
Cassin's Kingbird 

Extirpated. 

Single extralimital breeding record. 

Populations expanded into residential plantings in Marin in last few decades. 

Race presumed in Marin of Californian affinity. 

Race in Marin of Californian affinity. 

Calt/ornian Avifauna (11 species) 
California Quail 



Northern Mockingbird 
Black-chinned Sparrow 



Anna's Hummingbird 
Allen's Hummingbird 
Nuttall's Woodpecker 

California Condor perhaps formerly bred in Marin. 
Race in Marin of Boreal affinity. 



Plain Titmouse 
Bushtit 
Wrentit 
California Thrasher 



Brewer's Blackbird 
Brown-headed Cowbird 
Northern (Bullock's) Oriole 



Hooded Oriole 



Hutton's Vireo 
Tricolored Blackbird 
Lawrence's Goldfinch 



UNCLASSIFIED SPECIES (109 species) 

Pied-billed Grebe 
Ashy Storm-Petrel 
Double-crested Cormorant 
Brandt's Cormorant 
Pelagic Cormorant 
American Bittern 
Great Blue Heron 
Great Egret 
Snowy Egret 
Green-backed Heron 
Black-crowned Night-Heron 



Wood Duck 
Mallard 

Northern Pintail 
Cinnamon Teal 
Northern Shoveler 
Gadwall 
Ruddy Duck b 
Turkey Vulture 
Osprey 

Black-shouldered Kite 
Northern Harrier 



Sharp-shinned Hawk 
Cooper's Hawk 
Red-shouldered Hawk 1 " 
Red-tailed Hawk 
Golden Eagle 
American Kestrel 
Peregrine Falcon 
Ring-necked Pheasant 
Black Rail 
Clapper Riil 
Virginia Rail 



(Continued) 



59 



MARIN COUNTY BREEDING BIRD ATI^S 



Table 10. (Continued) 



UNCLASSIFIED SPECIES (Continued) 

Sora 

Common Moorhen 
American Coot 
Snowy Plover 
Killdeer 

Black Oystercatcher 
Spotted Sandpiper 
Western Gull 
Common Murre 
Pigeon Guillemot 
Rhinoceros Auklet 
Tufted Puffin 
Rock Dove 
Band-tailed Pigeon 
Mourning Dove 
Barn Owl 

Western Screech-Owl 
Great Homed Owl 
Burrowing Owl 
Long-eared Owl 
Short-eared Owl a 
Vaux's Swift 
Belted Kingfisher 
Acom Woodpecker 
Downy Woodpecker 
Hairy Woodpecker 



Northern Flicker 
Pileated Woodpecker 
Western Wood-Pewee 
Pacific-slope Flycatcher 
Black Phoebe 
Ash-throated Flycatcher 
Western Kingbird 
Horned Lark 
Purple Martin 
Tree Swallow 
Violet-green Swallow 
N. Rough-winged Swallow 
Cliff Swallow 
Barn Swallow 
Scrub Jay 
American Crow 
Common Raven 
White-breasted Nuthatch 
Bewick's Wren 
House Wren 
Marsh Wren 
American Dipper 
Blue-gray Gnatcatcher 
Western Bluebird 
American Robin 
Loggerhead Shrike 



European Starling 
Warbling Vireo 
Orange-crowned Warbler 
Northern Parula 
Yellow Warbler 
Black-dtroated Gray Warbler 6 
Common Yellowthroat 
Yellow-breasted Chat 
Black-headed Grosbeak 
Lazuli Bunting 
Rufous-sided Towhee 
California Towhee 
Rufous-crowned Sparrow 
Chipping Sparrow 
Lark Sparrow 
Sage Sparrow 
Savannah Sparrow 
Grasshopper Sparrow 
Song Sparrow 
Red-winged Blackbird 
House Finch 
Lesser Goldfinch 
American Goldfinch 
House Sparrow 



Race in Marin of Boreal affinity. 
Race in Marin of Great Basin affinity. 
Race in Marin of Californian affinity. 
Race of general western distribution. 
Species of general western distribution. 



60 



Breeding Bird Communities 



RESULTS AND DISCUSSION 



Factors Limiting Species Richness 



both the outer Point Reyes and Tomales areas. On the 
other hand, a number of species with interior affinities 
occur in the Tomales area to a limited or greater extent but 
are essentially lacking on outer Point Reyes. These species 
include Nuttall's Woodpecker, Western Kingbird, Lark 
Sparrow, and Northern Oriole. Quantitative analyses 
using matching coefficients and cluster analysis (Johnson 
& Cicero 1986, Taylor 6k Smith 1986), or other multi- 
variate techniques, are needed to refine the subjectively 
described regions of avifaunal similarity in Marin County 
and perhaps to elucidate others. 

Marin County Breeding Bird 
Communities 

Knowledge of the requirements of individual species as 
described in the species accounts probably provides the 
most insight into species' distribution patterns. Looking at 
species' relationships may stimulate additional discussion 
of both local and broad-scale distribution patterns. For this 
reason, the county's nesting avifauna has been grouped 
into 22 main breeding bird communities (Table 11). 
Wiens (1989) has indicated that published definitions of 
biotic communities range from discrete, closely integrated 
assemblages of species that possess properties paralleling 
those of individuals, to the fortuitous overlapping of spe- 
cies responding independendy to environmental condi- 
tions, to any set of organisms living near each other about 
which it is interesting to talk (Wiens 1989). The listing of 
species in Marin's bird communities implies no relation- 
ship among the species, or lack diereof, but simply diat 
species in a particular community are likely to co-occur in 
similar habitats with a fair degree of certainty. 

An important lesson learned from the exercise of cate- 
gorizing Marin's birdlife into communities is that it is a 
difficult and not entirely satisfying task. Marin County, 
and California in general, are characterized by a diverse 
array of habitats arranged in a patchy mosaic over the 
landscape. Consequendy, Marin hosts many single habitat 
specialists, many multihabitat generalists, and many edge- 
adapted species. Because many species may use several 
habitats, linger on the edge of two or more, or fall any- 
where along the continuum among these strategies, they 
are difficult to categorize as to community affiliations. 
Hence, such species may not be listed as primary members 
of any community, but rather as secondary or tertiary 
members of several communities. 

Most aquatic species of necessity use more than one 
habitat type— one for foraging and another for nest sites. 
For these species the foraging habitat is the community of 
which they are considered primary members. Among 
landbirds, raptors often use open habitats to forage in and 
nearby forests or cliffs for foraging perches, shelter or roost 
sites, and nest sites. Such species include Turkey Vulture, 



Black-shouldered Kite, Red-shouldered Hawk, Red-tailed 
Hawk, Golden Eagle, American Kestrel, Peregrine Falcon, 
Barn Owl, Great Horned Owl, Northern Pygmy-Owl, and 
Long-eared Owl. Aerial insectivores such as swifts and 
swallows forage almost exclusively on the wing but must 
come to earth to nest. Among Marin's landbirds it is 
possible to list quite a number of strongly edge-adapted 
species or multihabitat generalists. These include Califor- 
nia Quail, Mourning Dove, Common Poorwill, Anna's 
and Allen's hummingbirds, Northern Flicker, Black 
Phoebe, Western Kingbird, Scrub Jay, American Crow, 
Common Raven, Bushtit, Western Bluebird, American 
Robin, Northern Mockingbird, Loggerhead Shrike, Euro- 
pean Starling, Lazuli Bunting, California Towhee, Rufous- 
crowned Sparrow, Lark Sparrow, Red-winged Blackbird, 
Tricolored Blackbird, Brewer's Blackbird, Brown-headed 
Cowbird, House Finch, Pine Siskin, the three goldfinch 
species, and House Sparrow. The large number of such 
species and the abundance and widespread distribution of 
many of them attest to the success of a generalist or 
edge-adapted lifestyle. 

Factors Limiting Species Richness 
of the Avifauna 

A great number of factors influence the number of species 
that breed in a given area, but several of these are of 
paramount importance (MacArthur 6k Wilson 1967, 
Wiens 1 989). The pool of species available to colonize an 
area is a function of the distance from source populations, 
the size of source populations, and the dispersal ability of 
those species. Clearly, if populations of potential colonizers 
are isolated from an area by long distances, are small in 
size, and have poor dispersal abilities, they are unlikely to 
provide colonizers or become established if they arrive in 
the area in question. The availability of suitable habitat, 
nest sites, shelter, or odier resources also influences the 
ability of an area to support colonizers that do reach the 
area. Local extinction (extirpation) can also reduce the 
number of breeding species, whether by unpredictable 
chance events such as volcanic activity, forest fires, or 
droughts or by competition with similar species or from 
predation. 

At present the size of the Marin County breeding 
avifauna seems limited largely by availability of suitable 
breeding habitats. Many species of waterbirds that have 
bred elsewhere in the San Francisco Bay Area and poten- 
tially could colonize Marin County (see Potential Breeders 
pp. 429-434) occur here on a regular basis in winter or 
during migratory periods (Shuford et al. 1989). Presum- 
ably if suitable habitats were available in the county, some 
of them would establish themselves as breeders here. On 
die other hand, some species may be poor colonizers 
because of strong attachment to traditional breeding 

61 



MARIN COUNTY BREEDING BIRD ATIAS 

Table 11. Species membership of twenty-two Marin County breeding bird communities. Primary, secondary, and tertiary 
membership assigned by subjective assessment of species' relative abundance among habitat types and with respect to other 
members of a particular community. Many additional species may be found in the Urban/Suburban Community 
depending on the mix of native and exotic vegetation. Community use codes— N = nesting, F = foraging, E = use of edge 
of community— may apply to single species or a whole community. Nesting and foraging codes designate species that use a 
particular habitat for only one of those needs; in terrestrial habitats, all species that lack N and F codes satisfy both breeding 
and foraging needs in their respective habitats. 

OCEANIC (PELAGIC) WATERS COMMUNITY (F) 

Primary Members Secondary Members 

Ashy Storm-Petrel Western Gull 

Rhinoceros Auklet Common Murre 

Tufted Puffin 



NERITIC (CONTINENTAL SHELF) WATERS COMMUNITY (F) 
Primary Members Common Murre 

Brandt's Cormorant Pigeon Guillemot 

Pelagic Cormorant 
Western Gull 



Ternary Members 
Rhinoceros Auklet 
Tufted Puffin 
Osprey 



ESTUARINE WATERS AND TIDAL FLAT COMMUNITY (F) 

Primary Members Secondary Members 

Great Blue Heron Mallard 

Great Egret Northern Pintail 

Snowy Egret Clapper Rail (E) 

Black-crowned Night-Heron Snowy Plover 

Osprey Killdeer (E) 

Western Gull Belted Kingfisher 

American Crow (E) 
Song Sparrow (E) 



Tertiary Members 

Double-crested Cormorant 
Canada Goose 
Peregrine Falcon 
Brewer's Blackbird (E) 



COASTAL SALT MARSH COMMUNITY 

Primary Members 
Mallard 

Northern Harrier 
Black Rail 
Clapper Rail 
Savannah Sparrow 
Song Sparrow 



Secondary Members 
Northern Pintail 
Cinnamon Teal 



Tertiary Members 

Black-necked Stilt (N,E) 
Bam Owl (F) 
Common Raven (F) 



COASTAL BEACH AND DUNE COMMUNITY 



Primary Members 
Snowy Plover 
Western Gull (F) 
Common Raven (F) 
Horned Lark 
White-crowned Sparrow 
House Finch 
American Goldfinch 



Secondary Members 
Northern Harrier 
Killdeer 
Song Sparrow 
Brewer's Blackbird 
Brown-headed Cowbird 



GRASSLAND COMMUNITY 

Primary Members 
Turkey Vulture (F) 
Red-tailed Hawk (F) 
Mourning Dove (F) 
Great Horned Owl (F) 
Western Kingbird (F) 



Horned Lark 
American Crow (F) 
Common Raven (F) 
Western Bluebird (F) 
European Starling -(F) 
Lirk Sparrow 



Savannah Sparrow 
Grasshopper Sparrow 
Red-winged Blackbird (F) 
Western Meadowlark 
Brewer's Blackbird (F) 
Brown-headed Cowbird (F) 



(Continued) 



62 



Table 1 1 . (Continued) 



RESULTS AND DISCUSSION 



GRASSLAND COMMUNITY (Continued) 
Secondary Members 

Black-shouldered Kite (F) 
Northern Harrier (F) 
Golden Eagle (F) 
American Kestrel (F) 
Rock Dove (F) 
Barn Owl (F) 
Loggerhead Shrike (F) 
Tricolored Blackbird (F) 



Tertiary Members 

California Quail (E/F) 
Burrowing Owl 
Short-eared Owl 
Black Phoebe (E/F) 
Say's Phoebe (F) 
Lazuli Bunting (E/F) 
California Towhee (E/F) 
Rufous-crowned Sparrow (E) 



Chipping Sparrow (E/F) 
Northern Oriole (E/F) 
House Finch (E/F) 
Pine Siskin (E/F) 
Lesser Goldfinch (E/F) 
American Goldfinch (E/F) 
Lawrence's Goldfinch (E/F) 



NORTHERN COASTAL SCRUB COMMUNITY 



Primary Members 
California Quail 
Allen's Hummingbird 
Scrub Jay 
Bushtit 

Bewick's Wren 
Wrentit 

Rufous-sided Towhee 
Song Sparrow 
White-crowned Sparrow 
Brown-headed Cowbird 
American Goldfinch 



Secondary Members 
Great Horned Owl (E) 
Western Bluebird (E) 
Swainson's Thrush 
European Starling (F) 
Orange-crowned Warbler 
Wilson's Warbler 
California Towhee (E) 
Rufous-crowned Sparrow 
House Finch (E) 



Tertiary Members 
Mourning Dove (F) 
Anna's Hummingbird 
Northern Flicker (F) 
American Robin (E/F) 
Lazuli Bunting (E) 
Black-chinned Sparrow 
Brewer's Blackbird (E) 
Purple Finch (E) 
Pine Siskin (F) 



FRESHWATER POND OR LAKE COMMUNITY 

Primary Members 
Pied-billed Grebe 
Great Blue Heron (F) 
Mallard (F) 
Cinnamon Teal (F) 
American Coot 

Secondary Members 
Great Egret (F) 
Snowy Egret (F) 
Green-backed Heron (F) 
Black-crowned Night-Heron (F) 
Wood Duck (F) 



Northern Pintail (F) 
Gadwall (F) 
Ruddy Duck (F) 
Osprey (F) 
Virginia Rail (E/F) 
Sora (E/F) 
Killdeer (E/F) 
Black-necked Stilt (F) 
Belted Kingfisher (F) 
Black Phoebe (E/F) 
Song Sparrow (E/F) 
Red-winged Blackbird (E/F) 
Brewer's Blackbird (E/F) 



Tertiary Members 
Canada Goose (F) 
Blue-winged Teal (F) 
Northern Shoveler (F) 
Common Merganser (F) 
Common Moorhen (F) 
American Avocet (F) 
Spotted Sandpiper (E) 



FRESHWATER MARSH COMMUNITY 

Primary Members 
Northern Harrier 
Virginia Rail 
Sora 

Marsh Wren 
Common Yellowthroat 
Song Sparrow 
Red-winged Blackbird 



Secondary Members 
Great Blue Heron (F) 
Mallard 

Cinnamon Teal 
American Coot 
Black Phoebe (E) 
Tricolored Blackbird 



Tertiary Members 
American Bittern 
Black Rail 
Common Moorhen 
Short-eared Owl 



FRESHWATER STREAM COMMUNITY (F) 
Primary Members 
Great Blue Heron 
Belted Kingfisher 

Secondary Members 
Green-backed Heron 



Wood Duck 
Mallard 

Cinnamon Teal 
Gadwall 
Killdeer (E) 
Song Sparrow (E) 



Red-winged Blackbird (E) 
Brewer's Blackbird (E) 

Tertiary Members 

Common Merganser 



(Continued,) 

63 



MARIN COUNTY BRHLDING BIRD ATIAS 



Table 1 1 . (Continued) 



COASTAL RIPARIAN FOREST COMMUNITY 
Primary Members 

Red-shouldered Hawk 
California Quail (E) 
Mourning Dove 
Great Horned Owl 
Anna's Hummingbird 
Allen's Hummingbird 
Downy Woodpecker 
Hairy Woodpecker 
Western Wood-Pewee 
Pacific-slope Flycatcher 
Black Phoebe (E) 
Tree Swallow (N) 
Steller's Jay 
Scrub Jay 

Chestnut-backed Chickadee 
Bushtit 

Bewick's Wren 
Swainson's Thrush 
American Robin 
Wrentit 



European Starling 
Warbling Vireo 
Orange-crowned Warbler 
Yellow Warbler 
Wilson's Warbler 
Black-headed Grosbeak 
Rufous-sided Towhee 
Song Sparrow 
Brown-headed Cowbird 
Purple Finch 
American Goldfinch (E) 

Secondary Members 
Green-backed Heron 
Wood Duck 
Northern Saw-whet Owl 
Belted Kingfisher 
Nunall's Woodpecker 
Northern Flicker 
Ash-throated Flycatcher 
American Crow (N) 



Plain Titmouse 
Winter Wren 
Western Bluebird (E) 
Hutton's Vireo 
Common Yellowthroat 
I .c-i ill Bunting (E) 
California Towhee (E) 
Red-winged Blackbird (E) 
Northern Oriole 
House Finch (E) 
Pine Siskin 
Lesser Goldfinch (E) 

Tertiary Members 
Cooper's Hawk 
Red-breasted Sapsucker 
House Wren 
MacGillivray's Warbler 
Yellow-breasted Chat 



MIXED EVERGREEN FOREST COMMUNITY 

Primary Members 
Cooper's Hawk 
Band-tailed Pigeon 
Great Horned Owl 
Northern Saw-whet Owl 
Anna's Hummingbird 
Allen's Hummingbird 
Downy Woodpecker 
Hairy Woodpecker 
Western Wood-Pewee 
Pacific-slope Flycatcher 
Ash-throated Flycatcher 
Steller's Jay 
Scrub Jay 

Chestnut-backed Chickadee 
Bushtit 

Brown Creeper 
Bewick's Wren 
Swainson's Thrush 
American Robin (E) 
Hutton's Vireo 
Warbling Vireo 
Orange-crowned Warbler 
Wilson's Warbler 
Black-headed Grosbeak 
Dark-eyed J unco 
Purple Finch 



Secondary Members 

Black-shouldered Kite (N,E) 
Red-shouldered Hawk (N,E) 
Red-tailed Hawk (N,E) 
American Kestrel (N,E) 
California Quail (E) 
Mourning Dove (N,E) 
Western Screech-Owl 
Spotted Owl 
Acorn Woodpecker 
Northern Flicker (N,E) 
Olive-sided Flycatcher 
Violet-green Swallow (N) 
Plain Titmouse 
Red-breasted Nuthatch 
White-breasted Nuthatch 
Pygmy Nuthatch 
Winter Wren 
Golden-crowned Kinglet 
Western Bluebird (N,E) 
Hermit Thrush 
Wrentit 

European Starling (N) 
Solitary Vireo 
Yellow-rumped Warbler 
Black-throated Gray Warbler 
Western Tanager 



Lazuli Bunting (E) 
Rufous-sided Towhee 
California Towhee (E) 
Chipping Sparrow (N,E) 
Lark Sparrow (N,E) 
Song Sparrow 
Brown-headed Cowbird (E) 
House Finch (N,E) 
Pine Siskin 

Tertiary Members 

Great Blue Heron (N) 
Great Egret (N) 
Snowy Egret (N) 
Black-crowned Night-Heron (N) 
Wood Duck 
Turkey Vulture (N) 
Northern Pygmy-Owl 
Long-eared Owl (N) 
Red-breasted Sapsucker 
Tree Swallow (N) 
American Crow (N) 
Common Raven (N) 
Northern Parula 
White-crowned Sparrow (E) 
American Goldfinch (E) 



(Continued^ 



64 



Table 1 1 . (Continued) 



RESULTS AND DISCUSSION 



BISHOP PINE FOREST COMMUNITY 

Primary Members 

Northern Saw-whet Owl 
Allen's Hummingbird 
Hairy Woodpecker 
Pacific-slope Flycatcher 
Steller's Jay 

Chestnut-backed Chickadee 
Pygmy Nuthatch 
Brown Creeper 
Bewick's Wren 
Wrentit 

Wilson's Warbler 
Rufous-sided Towhee 
Dark-eyed Junco 
Purple Finch 
Pine Siskin 



Secondary Members 
Mourning Dove (E) 
Spotted Owl 
Great Horned Owl (E) 
Violet-green Swallow (N) 
Bushtit 
Winter Wren 
Swainson's Thrush 
European Starling (N) 
Hutton's Vireo 
Song Sparrow 
White-crowned Sparrow (E) 

Tertiary Members 
Osprey (N) 
California Quail (E) 
Band-tailed Pigeon 
Anna's Hummingbird 



Northern Flicker (E) 
Pileated Woodpecker 
Olive-sided Flycatcher 
Purple Martin (N) 
Tree Swallow (N) 
Red-breasted Nuthatch 
Golden-crowned Kinglet 
Western Bluebird (E) 
Hermit Thrush 
American Robin (E) 
Orange-crowned Warbler 
Yellow-rumped Warbler 
California Towhee (E) 
Chipping Sparrow 
Brown-headed Cowbird 
House Finch (E) 
Red Crossbill 
American Goldfinch (E) 



COAST REDWOOD FOREST COMMUNITY 

Primary Members 
Spotted Owl 
Northern Saw-whet Owl 
Allen's Hummingbird 
Hairy Woodpecker 
Pileated Woodpecker 
Pacific-slope Flycatcher 
Steller's Jay 

Chestnut-backed Chickadee 
Red-breasted Nuthatch 
Brown Creeper 
Winter Wren 



Golden-crowned Kinglet 
Hermit Thrush 
Wilson's Warbler 
Dark-eyed Junco 
Purple Finch 
Pine Siskin 

Secondary Members 
Band-tailed Pigeon 
Vaux's Swift (N) 
Olive-sided Flycatcher 
American Robin 



Tertiary Members 

Great Blue Heron (N) 
Great Egret (N) 
Snowy Egret (N) 
Turkey Vulture (N) 
Osprey (N) 
Sharp-shinned Hawk 
Pygmy Nuthatch 
Hermit Warbler 
Red Crossbill 



DOUGLAS FIR FOREST COMMUNITY 
Primary Members 
Band-tailed Pigeon 
Spotted Owl 
Northern Saw-whet Owl 
Allen's Hummingbird 
Hairy Woodpecker 
Pileated Woodpecker 
Olive-sided Flycatcher 
Pacific-slope Flycatcher 
Steller's Jay 

Chestnut-backed Chickadee 
Red-breasted Nuthatch 
Pygmy Nuthatch 
Brown Creeper 
Winter Wren 
Golden-crowned Kinglet 
Wilson's Warbler 



Dark-eyed Junco 
Purple Finch 
Pine Siskin 

Secondary Members 
Bushtit (E) 
Hermit Thrush 
American Robin 
Wrentit 

European Starling (N) 
Yellow-rumped Warbler 
Lazuli Bunting (E) 
Rufous-sided Towhee 
Chipping Sparrow (N,E) 
Song Sparrow 
Brown-headed Cowbird (E) 
Red Crossbil 



Tertiary Members 

Great Blue Heron (N) 
Great Egret (N) 
Turkey Vulture (N) 
Osprey (N) 
Sharp-shinned Hawk 
California Quail (E) 
Northern Pygmy-Owl 
Acorn Woodpecker (N,E) 
Red-breasted Sapsucker 
Purple Martin (N) 
Bewick's Wren 
Swainson's Thrush (E) 
Orange-crowned Warbler (E) 
Northern Parula 
Hermit Warbler 
White-crowned Sparrow (E) 



(Continued) 



65 



MARIN COUNTY BRITDING BIRD ATI AS 



Table 1 1 . (Continued) 



OAK WOODLAND AND OAK 

Primary Members 
Red-tailed Hawk 
Mourning Dove 
Anna's Hummingbird 
Western Screech-Owl 
Great Horned Owl 
Acorn Woodpecker 
Nuttall's Woodpecker 
Ash-throated Flycatcher 
Western Kingbird 
Violet-green Swallow 
Scrub Jay 
Plain Titmouse 
Bushtit 

White-breasted Nuthatch 
Bewick's Wren 



SAVANNAH COMMUNITY 

Western Bluebird 
European Starling 
Orange-crowned Warbler 
Chipping Sparrow 
Brown-headed Cowbird 
Northern Oriole 
Lesser Goldfinch 

Secondary Members 
Turkey Vulture 
Black-shouldered Kite (N) 
Red-shouldered Hawk (N) 
Golden Eagle 
American Kestrel 
California Quail 
American Crow (E) 



House Wren 
Blue-gray Gnatcatcher 
I lutton's Vireo 
Lazuli Bunting (E) 
Rufous-sided Towhee (E) 
California Towhee (E) 
Lark Sparrow (N,E) 
Western Meadowlark 
House Finch 

Tertiary Members 
Long-eared Owl (N) 
Lawrence's Goldfinch 



CHAPARRAL COMMUNITY 

Primary Members 

California Quail (E) 

Common Poorwill 

Anna's Hummingbird 

Scrub Jay 

Bushtit 

Bewick's Wren 

Wrentit 

California Thrasher 

Rufous-sided Towhee 

Rufous-crowned Sparrow (E) 

Sage Sparrow 



Secondary Members 
Turkey Vulture (E) 
Mourning Dove (E) 
Great Horned Owl (E) 
Orange-crowned Warbler 
Lazuli Bunting (E) 
California Towhee (E) 
Brown-headed Cowbird 
Lesser Goldfinch (E) 



Tertiary Members 

Ash-throated Flycatcher 
Blue-gray Gnatcatcher (E) 
Black-chinned Sparrow 



EUCALYPTUS GROVE COMMUNITY 

Primary Members 
Great Horned Owl 
Allen's Hummingbird 
Olive-sided Hycatcher 
Chestnut-backed Chickadee 
Bewick's Wren 
American Robin 
Northern Oriole 
House Finch 
Pine Siskin 
American Goldfinch 

Secondary Members 

Red-shouldered Hawk (N) 
Red-tailed Hawk (N) 
Mourning Dove (N) 
Downy Woodpecker 
Northern Hicker 



Western Kingbird (N,E) 
Tree Swallow (N) 
Scrub Jay 

American Crow (N) 
Common Raven (N) 
Bushtit 

Brown Creeper 
Western Bluebird (N,E) 
Swainson's Thrush 
European Starling (N) 
Lazuli Bunting (E) 
California Towhee 
Song Sparrow 
Dark-eyed Junco 
Brown-headed Cowbird (E) 
Purple Finch 
Lesser Goldfinch 



Tertiary Members 

Great Blue Heron (N) 
Great Egret (N) 
Black-shouldered Kite (N) 
California Quail 
Pacific-slope Hycatcher 
House Wren 
Winter Wren 
Warbling Vireo 
Chipping Sparrow 
White-crowned Sparrow (E) 



WEEDY HELD COMMUNITY 

Primary Members 
California Quail 
Mourning Dove 
European Starling 
California Towhee 



66 



Song Sparrow 
Red-winged Blackbird 
Brewer's Blackbird 
Brown-headed Cowbird 
House Finch 



Pine Siskin (F) 
Lesser Goldfinch (F) 
American Goldfinch 



(Continued,) 



Table 1 1 . (Continued) 



RESULTS AND DISCUSSION 



WEEDY HELD COMMUNITY (Continued) 
Secondary Members 

Black-shouldered Kite (F) 
Northern Harrier 
Red-tailed Hawk (F) 
American Kestrel (F) 
Great Horned Owl (F) 
Black Phoebe (E/F) 
Bewick's Wren 
Western Bluebird (F) 
Rufous-sided Towhee 
Western Meadowlark 
House Sparrow 



Tertiary Members 
Mallard (N) 
Northern Pintail (N) 
Cinnamon Teal (N) 
Northern Shoveler (N) 
Gadwall (N) 
Ring-necked Pheasant 
Barn Owl (F) 
Northern Flicker (F) 
Western Kingbird (F) 
Bushrit (F) 
Loggerhead Shrike 



Common Yellowthroat 
Lazuli Bunting 
Tricolored Blackbird 



URBAN/SUBURBAN COMMUNITY 

Primary Members 
Rock Dove 
Mourning Dove 
Anna's Hummingbird 
Northern Mockingbird 
European Starling 



California Towhee 
Brewer's Blackbird 
Brown-headed Cowbird 
House Finch 
House Sparrow 



Tertiary Members 
Hooded Oriole 



AERIAL COMMUNITY (F) 

Primary Members 

Violet-gTeen Swallow 
Cliff Swallow 
Barn Swallow 



Secondary Members 
Tree Swallow 
N. Rough-winged Swallow 



Tertiary Members 
Peregrine Falcon 
Vaux's Swift 
White-throated Swift 
Purple Martin 



CLIFF, SLOPE, BLUFF, OR BANK COMMUNITY (N) 



Primary Members 

Brandt's Cormorant 
Pelagic Cormorant 
Black Oystercatcher 
Western Gull 
Common Murre 
Pigeon Guillemot 
Belted Kingfisher 



Secondary Members 
White-throated Swift 
Pacific-slope Flycatcher 
Black Phoebe 
N. Rough-winged Swallow 
Cliff Swallow 
Barn Swallow 
Common Raven 
Rock Wren 
Bewick's Wren 



Tertiary Members 
Ashy Storm-Petrel 
Turkey Vulture 
Peregrine Falcon 
Rhinoceros Auklet 
Tufted Puffin 
Rock Dove 



67 



Factors Limiting Species Richness 



MARIN COUNTY BRFFDING BIRD ATI AS 



Factors Limiting Species Richness 



grounds. For example, Canada Geese (apparendy from 
captive stock) have only recendy become established as 
breeders in the San Francisco Bay Area (Lidicker & Mc- 
Collum 1979), including Marin County, although they 
have long wintered in the region (Grinnell 6k Wythe 1927, 
Grinnell &. Miller 1944). 

Many species of landbirds that breed elsewhere in 
California pass through Marin County as regular migrants 
or strays (Shuford 1982), but the few that have established 
themselves in Marin in recent decades either were formerly 
habitat limited or were introduced species expanding into 
vacant niches. Northern Mockingbirds and Hooded Ori- 
oles have both expanded their breeding ranges in Califor- 
nia as a whole during this century. Both began to breed in 
Marin County as their statewide population numbers 
increased and the residential plantings they favor in this 
part of their range became available with the expanding 
human population (see accounts). Brown-headed Cow- 
birds also expanded into the San Francisco Bay Area, 
including Marin County, by exploiting habitat changes 
caused by extensive livestock grazing and other human 
habitat modifications. Introduced Rock Doves, European 
Starlings, and House Sparrows also expanded to exploit 
unoccupied niches (or were better competitors) in agricul- 
tural, pastoral, or urban/suburban habitats made suitable 
by human endeavors. 

Extralimital breeders such as Cassin's Kingbird and 
Say's Phoebe have bred here only once each, presumably 
because of a lack of suitable breeding habitat. Aldiough the 
breeding record of Cassin's Kingbird is also die only 
record of the species for the county, the Say's Phoebe is 
fairly common here each year in winter and thus provides 
a large pool of potential colonizers. It is unclear whether 
the Northern Parula is limited here by a lack of suitable 
habitat or by the fact that few potential colonizers stray 
from their eastern breeding grounds. Other species with 
small or irregular breeding populations in Marin such as 
American Avocet, Spotted Sandpiper, Burrowing Owl, 
Short-eared Owl, and Yellow-breasted Chat are probably 
limited by suitable habitat (e.g., lack of many ground 
squirrels to provide burrows for Burrowing Owls); coloniz- 
ers are in short supply only for die Chat. 

One species that very likely is absent as a breeder in 
Marin County because of its limited dispersal ability is 
Mountain Quail. Mountain Quail breed in the outer Coast 
Range both to the north and south of Marin County 
(Grinnell & Miller 1944), and seemingly suitable habitat 



is available on Mount Tamalpais and elsewhere in Marin. 
However, Mountain Quail disperse on foot, and the clos- 
est breeding population in Sonoma County is cut off from 
Marin County by a large stretch of unsuitable grassland 
and marshland habitat along the border of the two coun- 
ties. It is not clear if Blue Grouse, which also breeds in 
Sonoma County, has not colonized Marin County because 
of poor dispersal capabilities or because of unsuitable 
climatic or habitat factors. Red-breasted Sapsuckers for- 
merly were thought to reach their breeding limit on the 
California coast in central Mendocino County but are now 
known to breed in small numbers in Sonoma and Marin 
counties (Shuford 1986). Limited observer coverage of the 
southern part of the coastal breeding range suggests that 
these small sapsucker populations formerly may have gone 
undetected, but then again the species may have colonized 
from the regular wintering population in this region, an 
option not available to Blue Grouse. Now that it is extir- 
pated in the county, the Greater Roadrunner is as unlikely 
as the Mountain Quail to recolonize Marin County. The 
Roadrunner appears to be constrained by its limited dis- 
persal abilities, the small size of the nearest breeding 
populations, and the inhospitability of the intervening 
habitats between Marin and source populations, rather 
dian by lack of suitable breeding habitats in Marin. The 
American Dipper, another extirpated breeder and cur- 
rendy an irregular winter visitant here, seems unlikely to 
recolonize because of habitat degradation caused by dam- 
ming of the one known former breeding stream. 

The lack of a breeding population of a particular species 
may also be due to the time lag between extirpation and 
recolonization. Double-crested Cormorants formerly bred 
on the outer coast of Marin County and only recendy have 
recolonized that area (see account), at a time when the 
coastal California breeding population as a whole was 
increasing. The potential pool of colonizing cormorants 
was large because of the species' presence in Marin year 
round, but nevertheless it took several decades to become 
reestablished here as a breeder. Peregrine Falcons formerly 
bred all along the Marin County coasdine but were extir- 
pated when the species population crashed in the 1970s 
from reproductive failures caused by pesticide pollution. 
Peregrines recolonized Marin in 1990. The time lag 
between extirpation and recolonization may have been a 
function of both a limited pool of colonizers and chance 
factors that led to reestablishment of breeding populations 
in odier counties before Marin. 



68 



CONSERVATION APPLICATIONS 



Come now, let us make a truce with tke children of life, and share with them the good things which we plentifully enjoy. There 
is happiness enough for all; and some of us there are who cannot be happy unless all are. 

— William Leon Dawson, 
The Birds of California 

The birds and animals, trees and grasses, rocks, water and wind are our allies. We need to see them with our hearts as well 
as our mind's, to let them speak to us of where we have come and where we are going, of three-and-a-half billion years of shared 
evolutionary travel, of our place on this planet. 

— David Gaines, 
Birds of Yosemite and the East Slope 



How to Use This Book as a 
Conservation Tool 

IT IS ONE THING to state that a breeding bird adas will be 
useful for conservation or management but quite 
another to articulate exacdy how to put it to its best use. 
Perhaps it is appropriate to start by stating what a breeding 
bird adas will not do. An adas will not substitute for 
environmental impact statements and reports involving 
studies of local habitat needs of birds or potential human 
impacts on birds at specific sites, particularly if local condi- 
tions have changed between the time of adas work and 
these studies. Because an adas tries to document the 
distribution, and sometimes abundance, of all species of 
birds in an area, it cannot be expected to be as accurate as 
similar studies that concentrate on single species (e.g., Page 
&. Stenzel 1981 for Snowy Plovers) or small groups of 
closely related species of birds (e.g., Sowls et al. 1980 and 
Carter et al. 1990 for seabirds). An adas will, however, if 
conducted and written with care, serve as a very important 
reference tool to conservationists, consultants, and manag- 
ers. All adases should first and foremost provide an accu- 
rate picture of distribution of most bird species in the 
chosen area and thus should serve as the primary reference 
that defines which breeding species are of restricted distri- 
bution there. These are the species that should be given 
special consideration in any development, mitigation, habi- 
tat enhancement, or habitat acquisition projects. Because 
species of limited range also often occur in small numbers, 
it should not be assumed that such species were found 
during the adas years in every atlas block in which diey 
actually breed. Hence, when a habitat is direatened widi 
degradation, specific searches should be made for relatively 
rare species found in equivalent habitats in nearby blocks, 



based on the assumption that some such species missed in 
the initial adas work may be found if additional effort is 
made. 

The further usefulness of an adas book will then depend 
on the types and extent of additional information provided 
to supplement the adas maps of each breeding species. 
This will of course vary from adas to adas. Because the 
present book documents each species' current and former 
status, it lends a historical perspective to evaluations of the 
need for protection of species. For example, all other things 
being equal, greater consideration should be given to 
protection of a species that has already declined in num- 
bers, especially if it is in trouble throughout its entire range. 
The extensive information on habitat, nesting, and food 
requirements will not only acquaint the reader with basic 
breeding needs of any species but will also direct them to 
further, more detailed literature. Information on popula- 
tion threats will also alert the reader to problems various 
species have faced in the past or might face in the future. 
In short, an adas book can be most useful as a source of 
detailed information upon which decisions can be based. 
Often those decisions will require further field work and 
literature research, as rarely will this or any other atlas book 
provide all the information needed to understand and deal 
with a specific environmental problem. 

Identification of Breeding Bird Species 
of Special Concern 

A number of birds diat breed in Marin County can be 
found on various lists of species that warrant or need 
consideration for protection at the state or national level 
(Table 12). Populations of these species are declining to 
various degrees and in extreme cases face possible extinc- 
tion. At present only state and federally Threatened or 

69 



Species of Special Concern 



MARIN COUNTY BREEDING BIRD ATIAS 



Species of Special Concern 



Table 12. Species or subspecies of birds that breed in Marin County that are currendy listed as Endangered, Threatened, 
or of management concern in diis region by state or national organizations. 



FEDERAL AND STATE ENDANGERED 1 ' 2 

American Peregrine Falcon 
California Clapper Rail 

FEDERAL THREATENED 1 

Northern Spotted Owl 

Western Snowy Plover (coastal population) 

FEDERAL CANDIDATE 3 , Category 2 

Black Rail 

California Homed Lark 

Loggerhead Shrike 

Saltmarsh Common Yellowthroat 

Bell's Sage Sparrow 

San Pablo Song Sparrow 

Tricolored Blackbird 

FEDERAL MANAGEMENT CONCERN 4 , Region 1 
(California) 

Black Rail 
Snowy Plover 
Olive-sided Flycatcher 
Loggerhead Shrike 

STATE THREATENED 2 

California Black Rail 



SLATE SPECIES OF SPECIAL CONCERN' 

Ashy Storm-Petrel 

Double-crested Cormorant 

Osprey 

Northern 1 larrier 

Sharp-shinned Hawk 

Cooper's Hawk 

Golden Eagle 

Snowy Plover 

Rhinoceros Auklet 

Tufted Puffin 

Burrowing Owl 

Long-eared Owl 

Short-eared Owl 

Purple Martin 

Yellow Warbler 

Saltmarsh Common Yellowthroat 

Yellow-breasted Chat 

San Pablo Song Sparrow 

Tricolored Blackbird 

AUDUBON BLUE LIST 6 

American Bittern 
Northern Harrier 
Sharp-shinned Hawk 
Cooper's Hawk 
Red-shouldered Hawk 
Short-eared Owl 
Loggerhead Shrike 
Grasshopper Spanow 



U.S. Fish and Wildlife Service (1989a), Federal Register. 

2 Calif. Dept. Fish and Game (1991a). 

3 U.S. Fish and Wildlife Service (1991). 



4 U.S. Fish and Wildlife Service (1987b). 

5 Calif. Dept. Fish and Game (1991b). 

6 Tate (1986). 



Endangered species are afforded special legal protection, 
aldiough many Candidate or Forest Service "sensitive" 
species are treated on federal lands as if they were "listed." 
Other management categories of concern are just diat— diey 
express concern over apparent declines in species' popula- 
tions but do little to protect diem beyond raising aware- 
ness, an important first step. Although these protection 
efforts should be lauded, diey may not do enough. Are the 
state and federal levels the only valid ones for considera- 
tion of protection of species? Biodiversity has recendy 
become a fashionable concept to promote, but should we 
try to enhance biodiversity just at the state and federal level 
and not at the county level or even in our backyards? 
Should we settle for small populations of species in distant 
parts of our state and nation, when with protection viable 
populations could exist as well in our own neighborhoods? 
Preservation and enhancement of habitats is now also an 
often championed approach to retaining biodiversity, but 



again why not at die local as well as the state and federal 
levels? These topics will be hody debated in the years to 
come. 

As a starting point for consideration of preservation of 
biodiversity of breeding birds in Marin County, it seems 
logical to first promote protection of all the species that 
breed in die county diat have already been given a state, 
federal, or other national management designation (Table 
1 2). This study also identifies an additional preliminary list 
of Marin County Breeding Bird Species of Special Con- 
cern (Table 13), not on any state or national list, that 
should at a minimum be given consideration for protection 
at the county level. An emphasis is placed on the prelimi- 
nary nature of the Marin list and die need for refining it, 
as others will undoubtedly disagree widi die author over 
which species to include on the list or if such a county list 
is even needed. The species in Table 13 are regular native 
breeding species widi overall population indices in the 



70 



Species of Special Concern 



CONSERVATION APPLICATIONS 



Species of Special Concern 



lower 25% of those calculated for all species recorded in 
the adas project (Table 7). They are vulnerable because 
they nest here in very small numbers, generally in 
restricted, often imperiled, habitats or are colonial nesting 
species that concentrate at very few sites. Some species 
falling in the lower 25% of population indices were 
excluded from the list because they were irregular or 
extralimital breeders (Canada Goose, Blue-winged Teal, 
Say's Phoebe, Northern Parula, Black-chinned Sparrow, 
Red Crossbill, and Lawrence's Goldfinch) or had small 
recendy established populations in human-created habitat 
(Hooded Oriole). 

If we are to maintain viable diverse communities of 
birds in Marin County, it is clearly necessary to protect 
extensive areas of the full range of the county s natural 
habitats. Fortunately much of our land has already been 
preserved in parks or open space (Figure 6). The effect of 
habitat fragmentation on bird populations is now a trendy 
topic in ecological and conservation circles, but our knowl- 
edge of these effects is still in its infancy. In that light, it 
seems prudent to err on the side of caution and preserve 



large rather than small areas of habitat. The bird habitats 
in Marin County that most deserve protection are ones 
that conservation efforts are also currendy focused on 
elsewhere in the state and nation— wedands, marshlands, 
and riparian forests. Of the 63 species from the county on 
various management lists (Tables 12 and 13), 51% are 
marsh-dependent species, other waterbirds, or seabirds; 
21% are miscellaneous landbirds; 19% are raptors (two 
species are also marsh dependent); 5% are chaparral- 
dependent species; and 5% are riparian-dependent species. 
As discussed in the land use section, loss or degradation 
of important habitats in the county is very evident. Many 
of these species, particularly those dependent on wedands, 
face uncertain futures without preservation or enhance- 
ment of their habitats. 

The knowledge presented in this and other scientific 
studies can inform concerned citizens, but only if they 
repeatedly and forcefully express the value and importance 
that wildlife plays in enhancing their lives will habitat 
preservation and enhancement efforts succeed. 




Trail winding through the dimly-lit understory of the Douglas fir forest on Inverness Ridge. Drawing b} Ane Rovetta, I 986. 



71 



MARIN COUNTY BREEDING BIRD ATLAS 

Table 13. A preliminary list of Breeding Bird Species of Special Concern in Marin County. Does not include species 
already given state, federal, or national protection or recognition (see Table 1 2). 



BRANDT'S CORMORANT — county breeding population con- 
centrated at only six colonies; vulnerable to disturbance at colonies 
and oil pollution. 

PELAGIC CORMORANT — breeding population well scattered 
along the coast but particularly vulnerable to nearshore oilspills in 
breeding season. 

GREAT BLUE HERON — currently breeding at only seven colonies 
and numbers breeding in county have declined in recent years; 
vulnerable to loss of wetlands, disturbance at colonies, and pesticide 
contamination. 

GREAT EGRET — currendy breeding at only five colonies; depend- 
ent on dwindling wedands and vulnerable to pesticide contamina- 
tion and disturbance at colonies. 

SNOWY EGRET — virtually entire county breeding population 
concentrated at one colony; dependent on vanishing wedands and 
vulnerable to disturbance and pesticides. 

GREEN-BACKED HERON — a very small population dependent 
on overgrown borders of streams and marsh edges; threatened by 
degradation and loss of riparian and freshwater marsh habitats. 

BLACK-CROWNED NIGHT-HERON - entire county breeding 
population concentrated at one colony; dependent on shrinking 
wedands and vulnerable to disturbance at colonies and pesticides. 

WOOD DUCK — very small population dependent on freshwater 
ponds and streams with overgrown borders; numbers apparendy 
reduced over former times. 

NORTHERN PINTAIL — very small breeding population depend- 
ent on freshwater, brackish, and saline wedands for breeding. 

NORTHERN SHOVELER - currendy known to breed in the 
county at only one managed freshwater wedand. 

GADWALL — very small breeding population dependent on 
scarce freshwater and brackish marshes and ponds. 

COMMON MERGANSER — very small breeding population on 
reservoirs and streams. 

RUDDY DUCK — very small breeding population in freshwater 
ponds and marshes. 

VIRGINIA RAIL — very small breeding population restricted to 
freshwater marshes. 

SORA — very small breeding population restricted to freshwater 
marshes. 

COMMON MOORHEN —very small breeding population depend- 
ent on limited freshwater ponds, sloughs, and marshes. 

BLACK OYSTERCATCHER - very small breeding population 
restricted to rocky shores primarily on the outer coast; intertidal food 
supply vulnerable to oil pollution. 



BLACK-NECKED STILT - very small breeding population 
restricted to a few freshwater and brackish wedands along the San 
Francisco and San Pablo bay shorelines; vulnerable to heavy metal 
contamination. 

AMERICAN AVOCET — has attempted to breed a few times in 
brackish or freshwater wedands along the San Francisco and San 
Pablo bay shorelines; vulnerable to heavy metal contamination. 

COMMON MURRE — currendy breeding in the county at only four 
colonies; populations have been severely reduced in recent years by 
oil pollution, gill netting, and severe El Nino. 

PIGEON GUILLEMOT — breeding population well scattered along 
coast but particularly vulnerable to nearshore oilspills in breeding 
season. 

NORTHERN PYGMY-OWL - inexplicably scarce as a breeding 
bird in the county; dependent on clearings in conifer and mixed 
evergreen forests. 

COMMON POORW1LL - very small breeding population 
restricted to chaparral-covered ridges. 

VAUX'S SWIFT — very small breeding population apparendy 
dependent on fire-hollowed nesting snags in conifer forests. 

RED-BREASTED SAPSUCKER - very small disjunct breeding 
population dependent on moist conifer forests and bordering ripar- 
ian zones. 

PILEATED WOODPECKER - very small breeding population 
dependent on old-growdi or mature second-growth conifer forests. 

ROCK WREN — very small population breeding at few sites; may 
be vulnerable to predation as is the Farallon population. 

BLUE-GRAY GNATCATCHER - very small breeding population 
restricted mosdy to live oak woodlands. 

CALIFORNIA THRASHER - very small breeding population 
restricted to a few chaparral-covered ridges. 

SOLITARY VIREO — very small breeding population restricted to 
relatively dry open mixed evergreen woodlands on Mount Tamalpais 
and vicinity. 

HERMIT WARBLER — very small breeding population inhabits 
Douglas fir or mixed Douglas fir/redwood forests on Mount 
Tamalpais and nearby ridges. 

MACGILLIVRAY'S WARBLER - very small breeding population 
inhabits brushy riparian borders mosdy on the coastal slope. 

WESTERN TANAGER — very small population breeding in 
relatively open Douglas fir or mixed evergreen hardwoods on Mount 
Tamalpais and nearby ridges. 



72 



CONTENT OF SPECIES ACCOUNTS 



Birds . . . had many magical properties . . . they u/ere thought to know the secret of all living things, to have great foresight, 
and to fill with wisdom the hearts of those who took the trouble to learn their language and listen. 

— Laurens van der Post, 
A Story Like the Wind 



THE SPECIES ACCOUNTS section of the book provides 
basic, though detailed, information for all of Marin 
County's breeding birds. The key sections of each account 
include (1) an adas distribution map, (2) adas data accom- 
panying the map, and (3) the species account text. These 
materials are presented in a standardized format as 
described below. 



Atlas Breeding Distribution Maps 

A distribution map is presented for each species that was 
confirmed or believed to breed in Marin County during 
the period of adas field work, 1976 to 1982. Species 
lacking adas maps were found breeding in Marin County 
prior to or after the period of adas field work. Each map 
has the adas grid of 221 blocks overlain on a standard map 
of Marin County. Broken lines within the county bound- 
aries denote major roads— further orientation can be 
obtained by reference to the place name map of Marin 
County (Figure 2). 

Three symbols are used within the blocks of the adas 
maps to denote the three categories of breeding evidence 
(Table 5): 

O — Possible Breeding 

© — Probable Breeding 

• — Confirmed Breeding 

Blocks lacking any of the above symbols indicate that no 
evidence of breeding was observed in that block for that 
species during the period of adas field work. Asterisks in 
certain blocks of the map for Nuttall's Woodpecker denote 
records of that species in late June and July indicative of 
postbreeding dispersal; these data demonstrate the impor- 
tance of completing field work before the postbreeding 
period (which varies among species) to ensure that adas 
maps accurately portray breeding distribution. 

The symbol P next to the map of several rare and 
sensitive species denotes diat locations of breeding records 
have been protected by moving dots on the map by one to 
two blocks in any direction (see Data Summary p. 48). 



Key to Abundance and Distribution 
Data Accompanying Atlas Maps 

This key describes the information that accompanies the 
adas map preceding the species account text, with exam- 
ples for a colonial nesting species— Great Blue Heron— and 
a solitary nesting species— Swainson's Thrush (Figure 13). 
Accounts for former, or recendy documented, breeding 
species of course lack adas maps or data and therefore are 
preceded only by information on former or current sea- 
sonal status. 

Seasonal Status 

Information is presented on whether a breeding species 
occurs in Marin County as a year-round resident or only 
as a summer resident (Figure 1 3). For year-round residents, 
periods of peak occurrence (if any) are indicated and 
whether the species occurs primarily in a seasonal role 
other than as a breeder. For example, the seasonal status 
of the Sharp-shinned Hawk is "Occurs year round, though 
almost exclusively as a winter resident and transient from 
Sep through Apr; numbers swell substantially during fall 
migration from Sep through mid-Nov." Information is also 
provided on the periods when colonial waterbirds gather 
at their breeding colonies (e.g., Great Blue Heron, Figure 
13). 

Breeding Status 

This section gives a verbal description of the relative 
abundance of the species in an average block and its 
distribution countywide, based, respectively, on categories 
of the Fine-Scale Abundance Rating and the Relative Dis- 
tribution Index listed below. In addition, the overall popu- 
lation size of the species in the county is described by the 
verbal categories of the Overall Population Index also listed 
below. For example, Swainson's Thrush (Figure 13) is 
termed "a very common [Fine-Scale Abundance Rating = 5], 
widespread [Relative Distribution Index = 1 37] breeder; 
overall breeding population large [Overall Population 
Index = 685]." 

73 



MARIN COUNTY BREEDING BIRD ATLAS 
Great Blue Heron Ardea herodias 



A year-round resident; occupies breeding 
rookeries mosdy from late Jan or early 
Feb thr ough la te lun or mid -jul. 

ACmirly commor^(very locaD breed er; 
overall^reedingpopuTation^ery smatt) 

Recorded in(O)(80) or 5A%j362%) 
of 221 blocks (see Methods). 

O Possible 68 (85%) 

€ Probable (0%) 

• Confirmed = 12 (15%) 



Fine-Scale 
Abundance Rating 




seasonal 
status 

breeding 
status 



Blocks recorded/Relative Distribution Index (RDl) 



Number (and percentage) of blocks in which 
the three breeding categories were recorded. 



Overall 
Population Index 
(FSARx RDl) 



Confirmation Index 



?o + (P T x 2) + (Co x 3) 
total blocks recorded 



Swainson's Thrush Catharus ustulata 



A summer resident from late Apr 
mrougliearlyOct. 

A(very commor)^widespread2breeder; 
overaUbreeding population(Targe. 

Recorded in(O7)(62.0%) 6T221 
blocks. 



O Possible 
€ Probable 
w Confirmed 



17 (12%) 
95 (69%) 
25 (18%) 



OPI = 685) CI = 2.06 



Fine-Scale 
Abundance Rating 




seasonal 
status 

breeding 
status 



Blocks recorded/Relative Distribution Index (RDl) 



Number (and percentage) of blocks in which 
the three breeding categories were recorded. 



Overall 
Population Index 
(TSAR* RDl) 



Confirmation Index 



P + (P r x 2) + (C x 3) 
total blocks recorded 



Figure 13. Examples of data presented with the atlas map of each breeding species during the Marin County Breeding Bird Atlas 
project. Data presentation is sligktl} different for colonial breeders (e.g., Great Blue Heron above) than solitary breeders (e.g., 
Swainson's Thrush above) because most colonies have been located and their size determined (see textj. The circled abundance and 
distribution terms are trie verbal equivalents (from index scales, p. 75), respectively, of the circled FSAR, RDl, and OP/ values below 
them. 



74 



Key To Atlas Map Data 



CONTENT OF SPECIES ACCOUNTS 



Key to Atlas Map Data 



Blocks Recorded 

For each species, the number of atlas blocks in which it 
was recorded is listed along with what percentage of the 
total number of blocks (221) that represents. Because we 
felt we documented most, if not all, heron and egret 
colonies, for those species, data on blocks recorded is listed 
first as the number of blocks widi confirmed breeding 
based on known active colonies. Following in parentheses 
is the number of blocks in which these wedand-dependent 
species were recorded, as an indication of the importance 
of foraging habitat away from colonies and often outside 
the adas block where actual nesting activities were centered 
(e.g., Great Blue Heron, Figure 13). Similarly, for herons 
and egrets, the percentage of "total" blocks is listed first as 
the percentage of confirmed to total blocks and in parenthe- 
ses as the percentage of recorded to total blocks. Since the 
adas grid did not sample most foraging habitat of breeding 
seabirds, the number of blocks recorded (and percentage 
of total blocks) for those species is based on only die 
number of blocks with active colonies. 

Fine-Scale Abundance Rating (FSAR) 
For all but colonial waterbirds, this rating qualitatively 
defines the abundance of a species (based on notes and 
impressions gathered by the audior over a number of years) 
in an "average" adas block in which it was recorded. The 
scale from 1 to 7 is based on the number of pairs an 
observer would expect to encounter by sight and/or sound 
while on foot during four hours afield in one block during 
prime daily or nighdy hours of activity for the species 
during the height of its breeding season. Because the 
categories were assigned qualitatively, diey may be off by 
plus or minus one (or more?) category. For colonial 
waterbirds, rating categories are assigned based on average 
population sizes of known colonies in Marin County 
during the adas period (Tables 14-17). The categories 
(numbers seen/4 hrs) and their verbal equivalents are 
based on a log scale like that used by DeSante and Ainley 

(1980): 

1 < 1 pair very rare (irregular; does not occur 

every year) 
rare (regular; occurs yearly) 

2 1-3 pairs uncommon 

3 4-9 pairs fairly common 

4 10-27 pairs common 

5 28-81 pairs very common 

6 82-243 pairs abundant 

7 > 243 pairs very abundant 

Relative Distribution Index (RDl) 

For all but colonial waterbirds, this index is a measure of 
the relative breeding distribution of a species in the county 
based simply on the number of blocks in which it was 
recorded during the period of atlas field work. The index 
for colonial waterbirds is the number of blocks with active 



breeding colonies. For example, the Great Blue Heron 
(Figure 13) has a Relative Distribution Index of 12 (blocks 
with colonies) even though it was recorded in a total of 80 
blocks. The total number of potential blocks in the county 
(221) in which a species could occur were divided into 
seven categories with verbal equivalents: 
1-31 blocks very local 

32-62 blocks local 

63-93 blocks somewhat local 

94-124 blocks fairly widespread 
125-155 blocks widespread 
156-186 blocks very widespread 
187-221 blocks nearly ubiquitous 

Overall Population Index (OPl) 

This index is derived by multiplying the Fine-Scale Abun- 
dance Rating for a species times the number of blocks in 
which it was recorded during die adas project (times blocks 
with colonies for colonial waterbirds). The range of actual 
values— from 1 to 1010— was divided into seven categories 
with verbal descriptions: 

1-146 very small population 

147-290 small population 

291-434 moderate-sized population 

435-578 fairly large population 

579-722 large population 

723-866 very large population 

867-1010 extremely large population 

Breeding Criteria Categories 

For each of the three Breeding Criteria Categories of 
Possible (O), Probable (©), and Confirmed (9) there is 
listed the number of blocks in which that category was 
recorded and, in parentheses, the percentage of the total 
number of recorded blocks (of all categories) that figure 
represents. Colonial waterbirds are treated the same as 
other species even though they probably were confirmed in 
most, if not all, the blocks they were breeding in. 

Confirmation Index (Cl) 

This index is a measure of how difficult each species was, 
relative to other species, to confirm as a breeder. The index 
for each species was derived by multiplying the number of 
blocks with Possible evidence of breeding by 1 , the number 
of blocks with Probable evidence by 2, and the number of 
blocks with Confirmed evidence by 3. These three prod- 
ucts were dien summed and divided by the total number 
of blocks in which the species was recorded. Though used 
for a different purpose, our Confirmation Index is similar 
mathematically to die "ACID (Adequate Coverage Identi- 
fication) test used by some to evaluate whedier an adas 
block has received enough observer coverage (Kibbe 1986, 
p. 46 this volume). 

75 



Content of Species Account Text 



MARIN COUNTY BREEDING BIRD ATLAS 



Content of Species Account Text 



Content of Species Account Text 

Although the atlas maps do stand alone in documenting 
the breeding distribution of each species, they do not 
provide information that might help to explain the 
observed distribution or a knowledge of habitat and forag- 
ing requirements necessary for conservation efforts. These 
functions are served by the species account text. Selected 
(but detailed) biological/ecological, distributional, and his- 
torical information is presented in three standard sections: 
(1) Ecological Requirements, (2) Marin Breeding Distribu- 
tion, (3) Historical Trends/Population Threats (sometimes 
combined with section 2), and a fourth infrequendy used 
Remarks section. The accounts primarily are meant to be 
informative and to point the reader in the right direction 
when additional information is needed. They are by no 
means meant to be the last word on the subject. 

Because an attempt has been made to standardize the 
accounts and make them accessible, without sacrificing 
detail, the reader may be under the illusion that most 
aspects of each species' breeding ecology are well known. 
This is far from the case. Much still needs to be known 
about the basic biology of even the most common and 
well-studied species. The species accounts vary gready in 
length for the simple reason that the amount of informa- 
tion available varies widely among species. The reader is 
cautioned to interpret and use this information carefully. 
If information is critical for the conservation efforts of a 
species, primary sources should be consulted directly and, 
if possible, local studies should be undertaken. 

Although an effort was made to use information from 
local studies whenever possible, the data presented in the 
species accounts may have been collected far from Marin 
County or even outside California, and its applicability 
may suffer accordingly. The reader may wonder why the 
text provides detail on aspects of a species ecology derived 
from distant studies when in fact local studies are available. 
Such information is presented because insight often comes 
from comparing the biology of die same species in different 
habitats. A classic example of this is Bob Stewart's compar- 
ative studies of Wilson's Warblers at Palomarin on the 
Point Reyes National Seashore and at Tioga Pass in the 
Sierra Nevada (see account). The comparative approach 
works on the local level as well. I did not realize that a basic 
habitat requirement of breeding Wilson's Warblers in 
Marin County was moist, low dense cover until I observed 
them breeding here in moist stands of coastal scrub that 
lacked the canopy of Douglas fir, bishop pine, alder, or 
willow typically associated with their more widespread 
forested breeding haunts here. 

Although the accounts focus on the important aspects 
of each species ecology, they cannot begin to convey ade- 
quately the grace, spirit, intensity, drama, humor, or exhil- 
arating beauty of our feathered friends and their lives. 
Much of avian essence can be captured in prose and 

76 



poetry, as such authors as Dawson (1923) and others have 
so admirably demonstrated, but much cannot. One has to 
experience birds in the raw, on their own terms. The 
accounts that follow may in some small way transmit an 
appreciation for the ecological factors that are important to 
birds, but only by spending time widi die birds themselves 
can we appreciate their importance, and that of the rest of 
the natural world, to our lives. 

Ecological Requirements 

The more one knows about the basic breeding biology of 
a species, the better able one is to interpret its pattern of 
distribution. Nevertheless, an understanding of certain 
aspects of the ecology of breeding birds appears more 
crucial in this regard. Hence, particular ecological require- 
ments have been emphasized here and others have been 
deliberately ignored, even though the latter factors might 
provide additional insight as well. Each species account 
tries to describe the range of local habitats the species 
occupies for breeding, special features of the habitat(s) it 
needs, where it locates its nest, what type(s) of nest it builds 
and of what materials, what kinds of food it requires, and 
what foraging styles it uses to obtain its food. For all but 
habitat requirements, information on seasonal, sex-, and 
age-related variation in nesting and foraging requirements 
are presented when available. Various biological character- 
istics of each species such as types of breeding displays or 
clutch size are not presented because they do not bear 
direcdy on the issue of niche requirements and because 
this information is already summarized in such standard 
references as Harrison (1978) and Ehrlich et al. (1988). 

Marin Breeding Distribution 

This section gives a verbal description of the species' 
breeding distribution, any geographical trends of distribu- 
tion or abundance in the county, and any factors that 
might help explain the observed distribution. Also 
included are specific documented records from the period 
of adas field work, though particularly noteworthy records 
from outside that period are occasionally listed as well. The 
format of these "representative" breeding records is as 
follows: (breeding code, date of observation, and observer 
initials). For example, a record (NY 5/22-28/82 -TO) 
would read that a nest with young was under observation 
from 22 to 28 May 1982 by Typical Observer. Breeding 
codes (Table 5) joined by a slash indicates that more than 
one type of breeding behavior was observed on the same 
date, whereas codes joined by a hyphen indicates that 
different categories of breeding evidence were observed 
over a period of days or weeks. 

Historical Trends/Population Threats 
Although the historical record of changes in the distribu- 
tion and abundance of birds in Marin County and Cali- 
fornia as a whole is incomplete, any and all apparent 
population trends are discussed, from the local to the 



Content of Species Account Text 



CONTENT OF SPECIES ACCOUNTS 



Content of Species Account Text 



widespread. The main historic sources consulted were 
Mailliard (1900) and Stephens and Pringle (1933) for 
Marin County; Grinnell and Wythe (1927) and Sibley 
(1952) for the San Francisco Bay Area; and Grinnell and 
Miller (1944) and Remsen (1978) for California as a 
whole. Robbins et al. (1986) and the corresponding 
unpublished Breeding Bird Survey data for California 
collected by volunteers of USFWS provided information on 
recent bird population trends. Additional sources are other 
avifaunal works, published papers or reports on particular 
species or species groups, and the published seasonal 
reports and/or unpublished data on file with the editors of 
the Middle and Southern Pacific Coast regions of Ameri- 
can Birds. 

Remarks 

This section is a catch-all used very infrequendy and only 

when important or interesting material about a species did 

not fit conveniendy into any of the three main subdivisions 

of the species accounts (e.g., Brown-headed Cowbird 

account). 

Observers 

The following individuals are cited in the text for their spe- 
cific observations: Peg Abbott (PA), Sarah G. Allen (SGA), 
Carol Annable (CA), Bob Baez (BoB), Janice Barry (JBa), 
Hal Barwood (HBa), Dennis Beall (DnB), Max Beckwith 
(MB), Gordon Beebe (GBe), Edward C. Beedy (ECB), Lau- 
rence C. Binford (LCB), Barbara Binger (BBi), Tupper 
Ansel Blake (TAB), Gerald Brady (GB), Aubrey Burns 
(ABu), Stan Camiccia (SCa), Scott Carey (ScC), Harry R. 
Carter (HRC), Pam Cleland (PCI), Marna Cohen (MC), 
Peter Colasanti (PCo), Chris Cuder (CCu), Dave DeSante 
(DDeS), Jules G. Evens (J GE), Carter L Faust (CLF), Marc 
Fenner (MFe), Shawneen E. Finnegan (SEF), Richard Franz 
(RFz), Steve Gellman (SG), Al 6k Wilma Ghiorso 
(A&.WG), Manuel ck Lillian Gorin (M6kLG), Keith Han- 
sen (KH), Rob Hansen (RH), Roger D. Harris (RDH), 
Roger Harshaw (RHa), Burr Heneman (BHe), Jim Higbee 
(J H), Emmy Hill (EH), David A. Holway (DAH), Ken How- 
ard (KeHo), Stuart Johnston (SJ), John P. Kelly (JPK), Shir- 
ley 6k Mike Kelly (SckMK), John Kipping (J Kip), Gerry J. 
Kleynenberg (GJK), Rick LeBaudour (RLe), Bill Lenarz 
(BiL), Phil Lenna (PL), R A. Lewis (RAL), John Lovio (J Lo), 
Gary F. McCurdy (GFMc), Flora Maclise (FMa), Grace 
McMichael (GMcM), Eugene Y. Makishima (EYM), Bill G. 
Manolis (BGM), Gloria Markowitz (GMk), Peter J. 
Metropulos (PJM), Andrea Meyer (AM), Grace Miller 
(GMi), Joseph Morlan (JM), Marina Gera Nell (MGN), 
Don Neubacher (DNe), Ed O'Connor (EO), Gary W. Page 
(GWP), Linda Parker (LP), Carmen J. Patterson (CJP), 



Holly Peake (HoP), Susan Claire Peaslee (SCP), Alan 
Pistorius (AP), Point Reyes Bird Observatory personnel 
(PRBO), Helen Pratt (HPr), William M. Pursell (WMP), 
Alton "Bob" Raible (ARa), C. J. Ralph (CJR), Jean M. Rich- 
mond (J MR), Ane Rovetta (ARo), David Ruiz (DRu), R. J. 
Ryder (RJRy), Ellen Sabine (ESa), Mary Ann Sadler (MAS), 
Barry Sauppe (BS), Phil 6k Margaret Schaeffer (PckMSh), 
Dave Shuford (DS), David Sibley (DaS), Dianne Sierra 
(DSi), Sue Smith (SSm), Bruce Sorrie (BSo), Barry Spitz 
(BSp), Rich Stallcup (RS), Jean Starkweather (J St), Lynne 
E. Stenzel (LES), Robert M. Stewart (RMS), Roger Stone 
(RSt), Helen Strong (HS), Merl Sturgeon (MeS), Meryl Sun- 
dove (MSd), Ian Tait (ITa), Gil Thomson (GiT), Irene 
Timossi (ITi), Dorothy Tobkin (DT), Beverly Treffinger 
(BTr), Wayne 6k Susan Trivelpiece (W6kST), Bill Tyokodi 
(BTy), Ed Vine (EV), Nils Warnock (NW), Anne 6k John 
West (AekJWe), Ralph S. Widrig (RSW), Pamela L Wil- 
liams (PLW), Jon Winter (J W), Peg Woodin (PWo), Keiko 
Yamane (KY), Mark Zumsteg (MZ), Clerin Zumwalt (CZ). 

Abbreviations 

The following abbreviations are used for frequendy used 
literature citations: AB = American Birds, formerly Audubon 
Field Notes (AFN); ABN = "American Birds Notebooks"— 
data on file with the regional editors of the Middle Pacific 
Coast Region of American Birds; ACR Report = Audubon 
Canyon Ranch Report; JFOs = journal of Field Ornithology 
Supplement; (G6kM 1944) = (Grinnell 6k Miller 1944); 
(G6kW 1927) = (Grinnell 6k Wythe 1927); (S6kP 1933) = 
(Stephens 6k Pringle 1933). The following abbreviations 
are used in the listing of representative breeding records in 
the Marin Breeding Distribution section or elsewhere in 
parentheses in the text of the species accounts: 

Ave. = Avenue 

CDFG = California Department of Fish 

and Game 

E, W, N, S = compass directions 

ft. = foot (feet) 

in. = inch(es) 

km = kilometer (s) 

mi. = mile(s) 

Mt. = Mount 

PRNS = Point Reyes National Seashore 

Rd. = Road 

SP = State Park 

St. = Street 

USFWS = U.S. Fish and Wildlife Service 

USFS = U.S. Forest Service 

yd. = yard(s) 

yr. year(s) 



77 



MARIN COUNTY BRHHDING BIRD ATIAS 







Coast redwoods toivering above the lusk, /em;) understory at Samuel 
P. Taylor State Parle. Drawing fc>} Ane Rovetta, 1 989. 



78 



SPECIES ACCOUNTS 



Greb 



es 



Family Podicipedidae 



PIED-BILLED GREBE Podilymbus podiceps 











Occurs year round, though primarily as 










a winter resident from Sep through Mar. 


/•A liV\\ \^3f 


^VCTS--. \ VT~ 






An uncommon local breeder; overall 


~^<i\ ^Sr^\v> 




r^\°3rN L <^- - 




breeding population very small. 


<r\^\^T\ 








Recorded breeding in 34 (15.4%) of 




'3c\Sr s Q?^^^^ 


^V3?n^M 




221 blocks. 




yl^\J^\\^\}^\^' 


^\°3r\ J\ 






hv 




-Vx^-VaSA^— 




O Possible = 12 (35%) 




^\>^-''\» ^c\ '■l-^^^V/Y 


^\ Jf^V Jr-^\ f 




€ Probable = 5 (15%) 




^K^\^^^^^P^ 


V^V><^T\^A\i»X " 


. -- 


• Confirmed = 17 (50%) 




















FSAR=2 OPI = 68 CI = 2.15 




^^^~^^^kK 


V*n\Mjr^ 


v?0> 






--?"/ voV^V 












__/ ^~^~^^^r\^ 







Ecological Requirements 

These floating submersibles are breeding inhabitants of 
Marin County's marshy-edged freshwater ponds and lakes, 
freshwater marshes with open water, and, sparingly, brack- 
ish water impoundments. In much of their range, Pied- 
billed Grebes also nest on sloughs and marshy areas of 
slow-flowing rivers. Occasionally, they breed on estuarine 
waters with slight tidal fluctuations (Palmer 1962, 
Johnsgard 1987), but there appear to be no reports of 
nesting in this habitat in coastal California. Infrequent 
sightings of birds on estuarine waters in summer (e.g., 3 
birds at Drake's Estero 23 Jun 1981 — DS) may represent 
breeders foraging away from nesting ponds or perhaps 
oversummering nonbreeders. Of major importance in all 
breeding habitats is the availability of fairly dense emergent 
vegetation used for nest construction, anchorage, or con- 
cealment. Breeding ponds range in size from x h to more 



than 100 acres and in depth from a few inches to 10 feet 
(usually less than 3-5 ft.); smaller ponds (1 -5 acres) are 
used most frequently (Bent 1919, Johnsgard 1987). 

Pairs are generally solitary while nesting. They defend a 
small area around die nest site, but they will feed with other 
nesting Pied-billeds in deeper, open-water areas of ponds 
and marshes (Glover 1 953a). These grebes build nests that 
are sodden masses of decaying aquatic vegetation. They 
usually conceal the nests in varying amounts of emergent 
vegetation but still allow for underwater approach; some- 
times diey locate nests in open water (Miller 1942, Glover 
1953a, Stewart 1975). Nests are usually anchored to, or 
built up around or among, dead or growing reeds, rushes, 
or, infrequendy, bushes, logs, or dead trees. The nests are 
not rigidly anchored as some rooted plant stalks always 
project through them, preventing drift, yet allowing 

79 



Grebes 



MARIN COUNTY BREEDING BIRD ATLAS 



Greh 



a 



enough up-and-down play to accommodate changing water 
levels (Miller 1942). In shallow water, Pied-billeds usually 
build their nests up from the bottom. Below water level, 
the nest foundation is a bulky mass of vegetation culminat- 
ing above water in a smaller hollowed platform in which 
the female lays the eggs. Nest materials are a wide variety 
of available dead, and sometimes green, materials includ- 
ing cattail flags, rushes, sedges, grasses, algae, and, if the 
nest is in shallow water, occasionally mud. Although they 
lay eggs in only one nest, Pied-billeds construct two to 
several nest structures, and they may continue to add 
considerable nest material during incubation. 

Pied-billed Grebes capture their aquatic prey in their 
bills by foot-propelled pursuit dives of short duration in 
shallow water; by picking individual items from the water's 
surface; by skimming the surface for masses of floating 
invertebrates; or even by snatching insects from the air 
(Johnsgard 1987). Overall, the North American diet is 
about 46.3% insects (especially damselflies, dragonflies 
and nymphs, grasshoppers, water boatmen, back- 
swimmers, waterbugs, predaceous diving beetles, flies, and 
hymenoptera), 27% crayfish, 24-2% fish (especially catfish, 
eels, perch, and sunfish), and 4.1% other crustaceans 
(brine shrimp, crabs, shrimp, etc.) (Wetmore in Palmer 
1962 and Johnsgard 1987, n= 174). Other food items 
include frogs, salamanders, snails, leeches, spiders, and 
seeds and soft parts of aquatic plants. The stout bills and 
heavy jaw musculature of Pied-billed Grebes are well 
adapted for killing heavy-bodied fish, as well as crayfish 
and frogs, which they eat in greater proportions than do 
other species of North American grebes (Zusi & Storer 
1969). For unknown reasons, grebes ingest their own 
feathers. Perhaps the feathers function to prevent bones in 
the stomach from passing into and puncturing the intes- 



tine, to retain bones in the gizzard until they can be 
digested, or to promote the regurgitation of pellets (Jehl 
1988). Hatching is asynchronous. Initially, young often 
ride on their parents' backs, where they are sometimes fed, 
and remain there when the adults dive at signs of danger 
(Palmer 1962). 

Marin Breeding Distribution 

During the adas period, Pied-billed Grebes were patchily 
distributed in Marin County, reflecting the distribution of 
suitable ponds and marshes. Representative breeding loca- 
tions were Nicasio Reservoir (NB-FL 5/16-7/17/82 — 
DS); Phoenix Lake (FL 6/16/76 -RMS); a brackish pond 
at Spinnaker Point, San Rafael (NE Jun 1982 — HoP); and 
a freshwater pond above Rodeo Lagoon (FL 7/27/82 
-DS). 

Historical Trends/ Population Threats 

The number of Pied-billed Grebes breeding in Marin 
County has undoubtedly increased in historical times 
because of the impoundment of streams for catde and 
human water needs, since natural freshwater ponds and 
marshes are rare here and in coastal California in general. 
For California as a whole, numbers of Pied-billed Grebes 
have likely decreased because of loss of most of our 
wedands. Numbers of Pied-billed Grebes were relatively 
stable on Breeding Bird Surveys in California from 1968 
to 1989 (USFWS unpubl. analyses). Since the early 1980s, 
government agencies and private groups in California have 
made management decisions leading to increasing areas of 
permanent and summer-flooded wetlands for waterfowl, 
and Pied-billed Grebes will undoubtedly benefit from these 
new habitats (M.R. McLandress pers. comm.). 



80 



Storm-Petrels 



SPECIES ACCOUNTS 



Storm-Petrels 



Storm-Petrels 

Family Hydrobatidae 



ASHY STORM-PETREL Oceanodroma homochroa 





A year-round resident on pelagic waters 




(peak Sep-Jan); petrels occupy the Faral- 


A~^>^^ \ jOia 


lon Island colony (and probably Bird 




Rock, Marin Co.) almost year round 


"3rA^r\3r\^ 


(irregular late Nov-early Dec). 


^^^^^K^K^x^C^^^c^C^, 


A fairly common, very local breeder; 


overall breeding population very small. 


\^~ >><\v \ ~>^r \ ^-V^\ jV^\ -V^\ \^^\ \--^^**'^i 


Recorded in 1 (0.4%) of 221 blocks 




(see Methods). 




/ti3r'\OcC^5£'v 


O Possible (0%) 




j3MYlMPr^^ 


€ Probable = (0%) 




^p^^"~^^Or^^ 


• Confirmed = 1 (100%) 






FSAR =3 OPI = 3 CI = 3.00 



Ecological Requirements 

Adapted to a lifetime at sea, during the breeding season 
these diminutive ocean waifs obtain their sustenance from 
the edge to about 1 5 miles seaward of the continental shelf 
within the cool waters of the California Current— waters 
strongly influenced by coastal upwelling (Ainley et al. 
1974; Briggs et al. 1987; Ainley ck Boekelheide 1990, 
Chap. 4). From spring to fall, Ashy Storm-Petrels in this 
region most consistendy frequent the warm side of thermal 
fronts bordering upwelled waters (Briggs et al. 1 987). 

Ashy Storm-Petrels nest in loose colonies on islands and 
offshore rocks. Pairs occupy cavities among loose rocks of 
talus slopes, in stone walls, in caves, or under driftwood 
(Bent 1922, Dawson 1923, Ainley et al. 1974). Petrels 
usually occupy breeding sites for a few days one to two 
weeks prior to egg laying before finally settling down to the 
chores of their lengthy breeding season (W.J. Sydeman 
pers. comm.). Generally females lay the single egg on tihe 
floor of the cavity, though occasionally they place it on a 
rough foundation of weeds or pebbles (Bent 1922). Char- 
acteristics of nesting islands selected by die petrels are 
suitable nest cavities, a lack of terrestrial predators, and 
reasonable proximity to productive ocean feeding grounds. 
Nonetheless, the presence of avian predators, particularly 



the Western Gull, affects colony attendance patterns and 
other aspects of the biology of uhese petrels. At the Farallon 
Islands, Ashy Storm-Petrels approach their nesting 
grounds only under the cloak of darkness and make fewer 
visits on full-moon nights than on new moon-nights 
(Ainley & Boekelheide 1990, Chap. 4). As adaptations to 
avoid predation, adults feed their chicks more frequendy 
and chicks fledge more often during dark phases of the 
moon; most chicks that fledge during light phases of the 
moon do so on overcast nights. When disturbed, storm- 
petrels also discharge foul-smelling oil from the mouth and 
nostrils, presumably another antipredator defense. 

Ashy Storm-Petrels usually occur solitarily at sea, but 
large numbers sometimes gadier around concentrations of 
food, which they may locate by their well-developed sense 
of smell. Foraging birds hover on outstretched wings and 
patter on the sea's surface with outspread webbed feet, then 
catch their prey by dipping, surface seizing, or shallow 
plunges. They forage during the day and also probably 
extensively at night (Ainley ck Boekelheide 1 990, Chap. 3), 
as suggested by their consumption of some cnjstaceans 
diat ascend to surface waters mosdy at night (McChesney 
1988). While feeding chicks, Ashy Storm-Petrels at the 

81 



Storm-Petrels 



MARIN COUNTY BREEDING BIRD ATI.AS 



Storm-Petrels 



Farallon Islands prey mainly on small fish and 
euphausiids, and to a limited degree on other crustaceans 
(decapods and amphipods) and cephalopods (McChesney 
1988, n = 30); they are also known to scavenge and con- 
sume fish oil (Ainley et al. 1974, Ainley 1984a). Admirably 
suited to the vagaries of finding food far at sea or to the 
uncertainties of returning landward during adverse 
weather, adults often spend several days away at sea during 
nesting. Upon returning to nesting sites, they feed small 
young an energy-rich stomach oil or, later in the season, 
partially digested prey. Because adults attend their eggs and 
young infrequendy and irregularly, the incubation and 
nesding phases are long and variable, even by seabird 
standards. 

Marin Breeding Distribution 

Ashy Storm-Petrels breed in Marin County at Bird Rock 
off Tomales Point, the northernmost locale in the species' 
breeding range where nesting has been confirmed. These 
petrels were first confirmed breeding there on 3 July 1972 
when an adult was found incubating an egg in a rock 
crevice; a petrel chick found there on 23 August 1969, 
though unidentified, was undoubtedly this species (Ainley 
& Osborne 1972). Sowls et al. (1980) found them still 
breeding there during the adas period as did Carter et al. 
(1992) in 1989 (see below). 

An adult Ashy Storm-Petrel with a brood patch was 
captured in a mist net on the night of 5-6 August 1989 in 
the vicinity of suitable crevice-nesting habitat on an off- 
shore islet in Van Damme Cove, Mendocino County; 
additional evidence of breeding is needed to substantiate 
this probable extension of the breeding range 85 miles 
north of Bird Rock (Carter et al. 1992). 

Historical Trends/ Population Threats 

Although long-term data are lacking, the Marin County 
breeding population of Ashy Storm-Petrels probably has 
always been small. Ainley and Osborne (1972) initially 



estimated a maximum of 20 to 24 birds nesting on Bird 
Rock in 1972, but Ainley and Whitt (1973) later revised 
the estimate downward to 10 birds. During the adas 
period, Sowls et al. (1980, NE 7/1/79) estimated the 
breeding population on Bird Rock at 14 birds. A higher 
estimate of 74 petrels breeding there in 1 989 was attributed 
to greater effort expended that year than in others to 
determine the size of this colony (Carter et al. 1992). 

Virtually all of the northern and central California 
breeding population of Ashy Storm-Petrels is concentrated 
on the Farallon Islands (Sowls et al. 1980, Carter et al. 
1992). A population estimate at the Farallones of 4000 
birds in 1972 (Ainley 6k Lewis 1974) has also been 
reported by other workers (Sowls et al. 1980, Carter et al. 
1992), but additional censusing efforts are needed to assess 
the accuracy of this estimate and the trend of the regional 
population (H.R. Carter 6k W.J. Sydeman pers. comm.). 
The Ashy Storm-Petrel is currendy a Bird Species of 
Special Concern in California (Remsen 1978, CDFG 
1991b). 

Ashy Storm-Petrels breeding at Bird Rock are vulnerable 
to disturbance by humans crossing from Tomales Point at 
low tide or by boat. Occasional intruders, however, are 
quite unlikely to even notice these furtive, nocturnal cavity- 
dwellers, though they would undoubtedly disturb other 
nesting seabirds. Coulter and Risebrough (1973) detected 
high pesticide levels and eggshell thinning in Ashy Storm- 
Petrels at the Farallon Islands, but breeding biology studies 
have not disclosed any adverse effects of this contamina- 
tion (Ainley 6k Lewis 1974; Ainley 6k Boekelheide 1990, 
Chap. 4). Perhaps the greatest threat to the species would 
be a catastrophic event at sea. An oil spill to the south in 
Monterey Bay, where thousands of Ashies concentrate in 
fall, could inflict severe, perhaps irreparable, damage to the 
population (Ainley 1976, Sowls et al. 1980, Roberson 
1985). 



82 



Cormorants 



SPECIES ACCOUNTS 



Cormorants 



Cormorants 

Family Phalacrocoracidae 



DOUBLE-CRESTED CORMORANT Phalacrocorax auritus 





A year-round resident; numbers swell (at 




least on Pt. Reyes estuaries) from Aug 


A>\^$r^^\ N JC^ 


through Dec. At Farallon Islands (and 


xv-^f^^ 


S.F. Bay bridges), birds occupy nest sites 


^Vm^Y^VmJ^ \^\J\^C \^\l^\ ~ 


mosdy from mid- to late Mar (rarely 


*\s^\X2\\J^^^ 


beginning early Apr) through Aug (rarely 


\\^\>)^^\^KX^ 


Sep). In poor food years, Farallon birds 


Vv^c^Cx >^\\ \^\ \^\\^\\^\\^\ 


desert colonies during midseason. 


\&\*^^^ 


A very rare, very local breeder; overall 


\u<^\^^^ 


breeding population very small. 




^v^a^aA^<v3p^\^^ 


Recorded in 1 (0.4%) of 221 blocks 




<35h^^ 


(see Mediods). 






O Possible = (0%) 
© Probable = (0%) 


) s^** ^^\ / ^-^^^^s^T- \^^*V*^^\ J*? 


• Confirmed = 1 (100%) 




FSAR =1 OPI = 1 CI = 3.00 



Ecological Requirements 

The West Coast version of this piscivorous phalacrocorid 
was originally dubbed the Farallon Cormorant, despite 
being the only one of our locally breeding cormorants that 
makes its living in both estuarine or inshore waters and 
inland lakes, reservoirs, and rivers. 

Farallon Island breeders feed within about 20 to 50 
miles of the colony in nearshore coastal waters and in 
estuaries and lagoons on Point Reyes and in San Francisco 
Bay (Ainley & Boekelheide 1990, Chap. 3). Although 
most foraging waters are no more than about 35 feet deep, 
Double-crests apparendy can, if need be, dive foot-pro- 
pelled from 65 to 260 feet, intermediate depdis for diving 
seabirds. Pelagic and Brandt's cormorants generally are 
deeper divers. Double-crested Cormorants usually feed 
singly or in small flocks of less than 20 but sometimes up 
to hundreds of birds (Bartholomew 1942). Smaller flocks 
are often roughly circular in formation and coalesce by 
swimming after diving. Larger flocks arrange themselves in 
long, compact lines perpendicular to the direction of move- 
ment of schooling fish and "leapfrog" (by swimming or 
flying) to the front of the flock after surfacing from dives. 
In marine waters along die West Coast, Double-crests feed 



mostly on schooling fish found from the surface to near 
(but not on) flat sand or mud bottoms (Ainley et al. 1981; 
Ainley ck Boekelheide 1990, Chap. 3). They also feed 
somewhat over rocky or gravelly substrates (Lewis 1929, 
Palmer 1962). Farallon breeders feed almost exclusively on 
neritic and estuarine fish, predominately two species of 
surfperch (Embiotocidae') and in particular the shiner surf- 
perch fC^nuztogaster aggregata) (Ainley et al. 1981; Ainley 
& Boekelheide 1990, Chap. 3, n = 2815). At the Faral- 
lones, the diet of Double-crests varies little between years 
and overlaps little widi that of Brandt's or Pelagic cormo- 
rants. Overall the Double-crested Cormorant feeds primar- 
ily on a wide variety of marine and freshwater fish, usually 
of no commercial value (Lewis 1929, Palmer 1962, Robert- 
son 1974, Ainley et al. 1981). Other dietary items include 
shrimp, squid, salamanders, frogs, and watersnakes, 
whereas crayfish may be important at some inland sites. 
Crabs, mollusks, seaworms, aquatic insects, and odier 
invertebrates are likely first consumed by the cormorants' 
fish prey. The young are fed by regurgitation and, if 
possible, they creche (gather together in clusters) after 
leaving their nests but before fledging. 

83 



Cormorants 



MARIN COUNTY BREEDING BIRD ATLAS 



Cormorants 



On the California coast, nest sites of Double-crests are 
more varied than those of Pelagic or Brandt's cormorants. 
Double-crests nest on the moderately steep, rocky slopes of 
offshore islands or rocks; on inaccessible mainland cliffs; 
in trees; and on structures such as bridges, wharf pilings, 
abandoned dredges, and electrical power towers. At inland 
sites (and sometimes coastal salt ponds), birds nest near 
fresh water, in trees, usually surrounded by water, or on 
islands, where the nest may be placed on the ground; on 
rock ledges or pinnacles; or in bushes or trees (Bent 1922, 
Lewis 1929, Palmer 1962, D. Shuford pers. obs.). Tree 
nests may be in crotches or well out on horizontal limbs 
and range to over 100 feet above the ground. Double-crests 
are highly colonial except where die availability of suitable 
nest foundations precludes closer spacing. Where Double- 
crests cohabit nesting islands with Brandt's Cormorants, 
Double-crests often nest on the steeper, more broken 
terrain of higher slopes, crests of ridges, and summits of 
rocks or islands (Dawson 1923; Ainley 6k Boekelheide 
1990, Chap. 6). A prime requisite of Double-crested Cor- 
morants' nest sites appears to be at least one side of die 
ground or rock falling abrupdy away (Bent 1922). On the 
other hand, Double-crests tend to avoid the narrow shelves 
of precipitous cliffs inhabited by Pelagics (Dawson 1923; 
Ainley 6k Boekelheide 1990, Chap. 6). 

The bulky, cup-shaped body of a Double-crested Cor- 
morant nest is composed primarily of coarse sticks and 
twigs when available. Otherwise seaweed, kelp, dead tules, 
and weed stalks are typical substitutes. The large stick nests 
found in trees may serve as much as landing platforms as 
they do egg baskets for diese heavy, ungainly landing craft 
(Ainley 1984b). Various soft materials such as grasses, 
straw, seaweed, moss, green leaves or conifer sprays, bark 
strips, and feathers (along with the odd bone, dead crab, 
or human artifact) are incorporated in the wall of the nest 
or form a pseudolining (Bent 1922; Dawson 1923; Palmer 
1962; Ainley 6k Boekelheide 1990, Chap. 6). Items such 
as plastic rope, packing tape, paper, cigarette butts, rags, 
and even a gun holster have been found in nests on the 
girders of the San Rafael-Richmond Bridge, Contra Costa 
County; one nest there was built on a hubcap (R.P. 
Henderson 6k M. Rauzon pers. coram.)! Nests built on the 
remains of previous years' nests may become quite large 
from the addition of material throughout the nesting sea- 
son and over the course of many years. 

Marin Breeding Distribution 

The only Marin County breeding record for Double- 
crested Cormorants during the adas period was of a single 
nest observed in a California bay tree amidst a Great Blue 
Heron colony on an island in Stafford Lake, Novate, in the 
breeding season of 1978 (ScC). The only two known 
colonies of this cormorant in the county were established 

84 



on the outer coast prior to and subsequent to the adas 
project (see below). 

Historical Trends/ Population Threats 

Formerly, a nesting colony (size unknown) was located in 
Marin County on a flat shelf of the cliff at Point Resistance, 
about one mile north of the ocean end of Bear Valley; nests 
with large young were observed there on 30 May 1929 
(Bolander 6k Bryant 1930). Surveys in 1979 and 1980 of 
seabirds breeding on the outer coast of California revealed 
no colonies of Double-crested Cormorants in Marin 
County (Sowls et al. 1980), but repeat surveys in 1989 
found a new colony of 14 birds on 5 June at Dillon Beach 
Rocks (Carter et al. 1992). 

Breeding populations of Double-crested Cormorants in 
California declined during historical times at the Farallon 
Islands because of disturbance from commercial collectors 
harvesting Common Murre eggs and from island occu- 
pants (Ainley 6k Lewis 1974; Ainley 6k Boekelheide 1990, 
Chap. 6), on islands off southern California and the west 
coast of Baja California because of pesticide contamination 
and human disturbance (Gress et al. 1973), and in interior 
California because of disturbance from lake development 
and recreation (G6kM 1944, Sowls et al. 1980). Eggshell 
diinning has also been documented at the Old Areata 
Wharf, Humboldt County {fide Sowls et al. 1980). 

Ainley and Lewis (1974) suggested that marine breeding 
populations of Double-crested Cormorants failed to re- 
cover from their decline because populations of their prey 
base— the Pacific sardine (Sardinops caerulea)— were over- 
exploited by humans in the late 1940s at a time of unfavor- 
able environmental conditions. The sardines were 
replaced by the northern anchovy (Engraulis mordax), a 
possibly less desirable prey of the cormorants. Recent 
dramatic increases of Double-crested Cormorants in Cali- 
fornia, despite no rise in sardine populations, suggest that 
other factors may also have been limiting the cormorants. 
The Farallon population of this cormorant began to re- 
cover slighdy in the 1970s (Ainley 6k Boekelheide 1990, 
Chap. 6), declined substantially after the 1982-83 El Nino 
Southern Oscillation event, but by 1 989 had increased to 
about 1140 breeding birds (Carter et al. 1992). Channel 
Island populations, which may have declined substantially 
since die turn of the century, now also are increasing 
(Carter et al. 1992). Breeding numbers in San Francisco 
Bay— swelled gready by the establishment (primarily since 
1984) of colonies on bridges— totaled 2789 birds in 1989 
to 1991, representing 37% of the northern and central 
coastal California population (Carter et al. 1992, H.R. 
Carter pers. comm.). Numbers on the outer coast of this 
region alone increased from 1466 birds in 1979 to 1980, 
to 4785 in 1989, as a result of expansion at old colonies 
and the establishment of 1 1 new ones (Carter et al. 1992). 
Numbers of Double-crested Cormorants also increased on 



Cormorants 



SPECIES ACCOUNTS 



Cormorants 



Breeding Bird Surveys in California from 1968 to 1989, 
though they were relatively stable from 1980 to 1989 
(USFWS unpubl. analyses). These trends presumably 
reflect changes in the population of the interior, where 
most BBS routes are located. 

Because of continentwide population declines, the Dou- 
ble-crested Cormorant was placed on the Audubon 



Society's Blue List from 1972 to 1981 (Tate 1981) and on 
its Species with Special Concerns list in 1982 (Tate ck Tate 
1982). Although numbers now appear to be increasing 
widely (e.g., Tate 1986, Carter et al. 1992), this cormorant 
is still considered a Bird Species of Special Concern in 
California (Remsen 1978, CDFG 1991b). 



BRANDT'S CORMORANT Phalacrocorax penicillatus 





A year-round resident; numbers 




depressed somewhat on ocean waters 




from Dec through Apr. Farallon Island 


A^\y^A^^ \ ypv. 


(and probably Marin) breeders occupy 


"^^\-^\Ot^v \^\ \-?^r <: ^ , ^rTV-^\\^^\?t-\(_^' 


nesting colonies starting mid- to late Mar 


J\^A £c\ y<^\ J^^. \5t<r\ \^\ \^\ \^r\ 


(extremes early Mar and early May) 


\^^^\jr^(^^^^^^^^%^ 


through Aug (rarely through Sep and 


Oct). In poor food years, few birds 


V5" J^x^Ca ~z>^\\ ^\\ ^<r\ ^*c\ y<i\ \^\ 


occupy nest sites and all desert early in 


\)y<^^-§rx^^\^\j£^ 


the season. 


\\^*\^^ 


A very abundant, very local breeder; 


u^c^A^rj)^^^^ '" 


overall breeding population very small. 


V^X^Oe^\^}\-^\Jr^\)^^ 


Recorded in 4 (1.8%) of 221 blocks 




ix^r/ vVf v-^r \^\ \^ 3 T^v^(p i \^\ x>\ s-^ 


(see Methods). 




■^^^^^^Ps^^i^s^^^^ 


O Possible = (0%) 






C Probable (0%) 




• Confirmed = 4 (100%) 




FSAR =7 OPI = 28 CI = 3.00 



Ecological Requirements 

Flight lines of Brandt's Cormorants merge with multi- 
species feeding flocks, alerting birds and humans alike to 
the bountiful productivity of the ocean waters near 
California's seabird breeding colonies. The Brandt's Cor- 
morant is one of two stricdy marine cormorants breeding 
along the California coast and inhabiting waters over the 
continental shelf. Of the two, its distribution indicates it is 
the most characteristic of the cool upwelling waters of the 
California Current (Ainley ck Boekelheide 1990, Chap. 
5). Brandts feed primarily in nearshore waters, but also 
well offshore and in deep coastal bays. The importance of 
these different foraging areas to Farallon Island breeders 
varies seasonally and yearly (Ainley <St Boekelheide 1 990, 
Chap. 3). Although breeders range up to 50 miles from the 
Farallon colony on feeding trips (Ainley ck Boekelheide 
1990, Chap. 3), foraging birds seldom stray more than 6 
miles from land, except in transit (Briggs et al. 1987). 

Brandt's Cormorants make relatively deep, foot- 
propelled foraging dives. Along die mainland coast, they 
forage over sand and mud bottoms at depths of about 30 



to 200 feet and offshore over rocky bottoms, as well, 
apparendy up to about 400 feet in depth (Ainley ck 
Boekelheide 1990, Chap. 3). Brandts use equal propor- 
tions of schooling and nonschooling prey and show great 
dietary diversity. A study of their diet up and down the 
Pacific Coast indicates that although the majority of their 
prey live on or just above the bottom over both rocky and 
flat substrates, appreciable numbers range from mid- 
depths to the surface and others hide in the substrate 
(Ainley et al. 1981). Brandts vary dieir feeding habits from 
nordi to south along the West Coast. To the north in areas 
of overlap widi Pelagics, Brandts eat the same prey as 
Pelagics, but they feed just above rocky substrate or near 
substrate without relief, whereas Pelagics feed primarily in 
rocky substrate. To the south, in areas where Pelagics are 
absent, Brandts feed almost exclusively in rocky habitat or 
near rocks on flat bottoms (Ainley et al. 1981). 

Brandt's Cormorants are gregarious foragers. In years of 
high oceanic productivity (rockfish abundance), they tend 
to feed in large flocks, often with Western Gulls and 

85 



Cormorants 



MARIN C:OUNTY BRLHDING BIRD ATIAS 



Cormorants 



Common Murres; in unproductive years they feed in 
smaller flocks by themselves (Ainley 6k Boekelheide 1990, 
Chap. 3). Brandts prey on a wide variety of marine fish and 
occasionally take octopus and market squid. At die Faral- 
lon Islands, the diet is dominated by midwater schooling 
rockfish, mainly Sebasles flavidus and S. jordani. Other 
important prey are flatfish (bothids and pleuroncctids), 
Pacific tomcod (Microgadus jnoximus), midshipmen (Porich- 
thys notatus), and spotted cuskeels (Chilara taylori) (Ainley 
et al. 1981; Ainley & Boekelheide 1990, Chap. 3, n = 
11,190). In most years, dietary diversity is low because of 
the reliance on juvenile rockfish, but in unproductive 
(usually warm-water) years it is higher. In cool-water years, 
the pre-egglaying diet is more diverse and overlaps little in 
species composition with the later-season diet. Overlap of 
the Brandt's Cormorant diet with that of die Double- 
crested Cormorant is minimal, though slightly greater in 
warm-water years. At the Farallon Islands, Brandt's and 
Pelagic cormorants eat many of the same prey species 
(except in warm-water years), because they bodi rely heavily 
on juvenile rockfish, though Pelagics tend to take smaller- 
sized prey. Like our other cormorants, Brandts feed their 
young by regurgitation, and the young creche (gadier to- 
gether) before and after leaving the nest and before fledging 
(Carter & Hobson 1988). 

Brandts form the largest and densest colonies of our 
locally breeding cormorants where they nest on offshore 
islands, sea stacks, and inaccessible mainland cliffs. Where 
Brandts' colonies overlap with those of Double-crested 
Cormorants, Brandts often prefer the gentler terrain of 
high, rounded shoulders of rock; gradual, sloping inclines; 
and flattops of rocky islands (Bent 1922, Williams 1942). 
Aldiough Brandts occasionally nest on wide ledges or 
niches on a cliff face, these sites are never as precarious as 
diose chosen by Pelagic Cormorants. Brandt's Cormo- 
rants build large, bulky, cup-shaped nests diat solidify from 
compaction and the accumulation of fecal droppings. 
Nests are generally built on the rotted debris and guano of 
the previous year's effort, but, rarely, diey expand to tall 
cylinders widi yearly additions of material. Typical nest 
materials are land plants such as Farallon weed, grasses, 
and mosses and marine plants, including algae, eelgrass, 
and surfgrass (Bent 1922; Dawson 1923; Palmer 1962; 
Ainley 6k Boekelheide 1990, Chap. 5). 

Marin Breeding Distribution 

During die adas period, Brandt's Cormorants bred at six 
colonies along the outer coast of Marin County (NE, Sowls 
et al. 1 980; Table 1 4, Figure 1 4). In 1 989, the total number 



of occupied colonies remained the same, but the small 
colony at die "Sonoma-Marin County Line" was aban- 
doned and a new colony was established at Bird Rock, off 
Tomales Point (Carter et al. 1992). 

Historical Trends/ Population Threats 

Partial surveys in 1969 to 1972 estimated a total of 1330 
Brandt's Cormorants were breeding at diree sites along the 
Marin County coast (Ainley 6k Whitt 1973). From com- 
plete surveys, numbers of this cormorant breeding in the 
county totaled 3204 birds in 1979 to 1980 (Sowls et al. 
1980) and 1935 birds in 1989 (Carter et al. 1992). The 
recent change in local population size may in part reflect 
differences between the two survey periods in oceano- 
graphic conditions, which can dramatically affect the num- 
ber of Brandt's Cormorants that breed in a given year 
(Ainley 6k Boekelheide 1990, Chap. 5). Also, because part 
or all of.die birds at a colony often shift breeding locations 
among years, Carter et al. (1990) stressed the importance 
of assessing population trends over large rather than small 
areas. 

During the mid-1 880s, the Brandt's Cormorant popu- 
lation at the Farallon Islands declined drastically from 
disturbance caused by commercial egg collectors gathering 
Common Murre eggs, but cormorant numbers there have 
since increased dramatically (Ainley & Lewis 1974). Num- 
bers of Brandt's Cormorants attempting to breed at the 
Farallones in the 1970s and 1980s have fluctuated gready 
from year to year, reflecting varying oceanographic condi- 
tions and hence food supplies (Ainley 6k Boekelheide 
1990, Chap. 5). On the central and northern California 
coast as a whole, estimates of the population size of this 
cormorant varied from 58,290 breeding birds at 61 sites in 
1979 to 1980 (Sowls et al. 1980) to 54,029 birds at 71 sites 
in 1989 (Carteret al. 1992). Sowls et al. used a 1979 PRBO 
estimate for the South Farallon Islands of 28,000 Brandt's 
that Ainley and Boekelheide (1990) revised to about 
19,000. 

Populations of Brandt's Cormorants on the Channel 
Islands have also declined historically, first from human 
disturbance and later presumably from the accumulation 
of pesticides as indicated by eggshell thinning (Hunt et al. 
1979). Chick deformities that often result from pollutants 
are rarely observed at the Farallon Islands (Hobson 6k 
Carter 1988). Significant numbers of Brandt's Cormo- 
rants are caught in gill nets (H.R. Carter pers. comm.), but 
few die during oil spills (Page et al. 1990). 



86 



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MARIN COUNTY BRHHDING BIRD ATLAS 




Colony codes used are: 

01 = "Sonoma-Marin County Line" 

02 = "Dillon Beach Rocks" 

03 = Tomales Point 

04 = Bird Rock 

05 = "Elephant Rock Complex" 

06 = Point Reyes 

07 = Coast Campground South 

08 = Point Resistance 

09 = Millers Point Rocks 

10 = Double Point Rocks 



11= Stinson Beach to Rocky Point 
12= Gull Rock Area 

13 = Muir Beach Headlands to Tennessee Cove 

14 = Bird Island 

15 = Point Bonita 

16 = Bonita Cove 

17 = Point Diablo Bluffs and Needles 

18 = Yellow Bluff 

19 = Sausalito Point Area 

20 = Peninsula Point and Cone Rock 



21 = Angel Island 

22 = Bluff Point to Paradise Cay 

23 = Richmond-San Rafael Bridge 

24 = Point San Quentin 

25 = Marin Islands 

26 = The Sisters and Point San Pablo 

27 = Rat Rock 

28 = Southwest San Pablo Bay Duck Blinds 

29 = Marin County- West San Pablo Bay Ship 

Channel 



Figure 14- Map of marine bird colony sites in Marin County. See Carter et al. (1992) for detailed colony maps and standard 
California and USFWS colony codes (not used here). 



88 



Cormorants 



SPECIES ACCOUNTS 



Cormorants 



PELAGIC CORMORANT Phalacrocorax pelagicus 





A year-round resident. Pelagics occupy 
breeding cliffs at the Farallon Islands 
(and probably Marin) in high numbers 
mosdy from Mar (extremes Dec or Jan 
and Apr or May) through Aug (rarely to 
early Oct). In poor food years, birds fail 
to lay eggs or desert nests as early as Jun 


r\* 


'/' VatV L\r"'\ JV""\ \r^\^£T\ \^efC^\^V\Y"-'\ 


or Jul. 

A very common, very local breeder; 
overall breeding population very small. 

Recorded in 11 (5.0%) of 221 blocks 
(see Methods). 




ir^^^ ^^T» \^\A\Jl)'X\ ^i-V < ^ / rv 


O Possible (0%) 




C Probable = (0%) 




• Confirmed = 11 (100%) 




FSAR = 5 OPI = 55 CI = 3.00 



Ecological Requirements 

These sleek iridescent cormorants choose lofty, precarious 
nest sites free of acrophobics. In the watery realm, they are 
exclusively marine inhabitants, but despite what their 
name implies they occupy nearshore waters within 12 
miles (mosdy within 6 mi.) of mainland or island shores 
(Briggsetal. 1987). 

Like Brandts, Pelagic Cormorants are foot-propelled 
divers that perhaps reach 400 feet in depth. In contrast to 
Brandts, Pelagics typically feed alone on solitary nearshore 
prey that hide in rocky reef substrates (Ainley et al. 1981; 
Ainley 6k Boekelheide 1990, Chap. 3). In years of high 
ocean productivity and superabundant prey, Pelagics at the 
Farallon Islands tend to feed in the company of small 
multispecies flocks of seabirds on midwater schools of 
rockfish. The Brandt's Cormorant account contains fur- 
ther comparison of the feeding niches of these two species. 
The diet of the Pelagic Cormorant is most similar to the 
Pigeon Guillemot, another species feeding mosdy in rocky 
substrates (see account). The predominant prey of Pelagic 
Cormorants at the Farallon Islands are several species of 
sculpin (cottids), juvenile rockfish (mosdy Sebastes flavidus 
and S. jordani), and a mysid shrimp (Spirontocaris sp.); 
other fish, crustaceans, octopuses, and marine worms are 
minor components of the diet there (Ainley et al. 1981; 
Ainley 6k Boekelheide 1990, Chap. 3, n = 6839). Juvenile 
rockfish are most important in cold-water years, and scul- 
pins in warm-water years. Like our other cormorants, 
Pelagics feed their young by regurgitation. 



Of the California cormorant clan, Pelagics are the dare- 
devils, nesting on narrow ledges and niches of precipitous 
cliffs and sea caves on coastal bluffs or offshore rocks and 
islands. Because these sites are limited, Pelagic Cormorant 
colonies are generally smaller and looser aggregations than 
diose of Brandt's and Double-crested cormorants (Sowls et 
al. 1980, Carter et al. 1984). Presumably because of the 
precariousness of cliff nests, Pelagics cement them to the 
ledge by their own excrement. Nests are often semicircular, 
radrer than round, where they abut the cliff face, but like 
Brandts' nests, they rarely become large from yearly recon- 
struction and additions. Nest materials include seaweeds, 
grasses, mosses, and, rarely, sticks, with dry grasses and, 
occasionally, feadrers used as a lining (Bent 1922; Dawson 
1923; Palmer 1962; Ainley 6k Boekelheide 1990, Chap. 
6). 

Marin Breeding Distribution 

During the adas period, Pelagic Cormorants bred in 14 
main colony sites along the outer coast of Marin County 
(NE, Sowls et al. 1980; Table 14, Figure 14). In 1989, the 
number of colony sites was reduced to 12 by the abandon- 
ment of the "Elephant Rock Complex" and Point Resis- 
tance colonies (Carter et al. 1992). 

Historical Trends/ Population Threats 

Partial surveys in 1969 to 1972 estimated 800 Pelagic 
Cormorants were breeding at four sites along die Marin 
County coast (Ainley 6k Whitt 1973). Based on complete 

89 



Cormorants 



MARIN COUNTY BRHKDING BIRD ATIAS 



Cormorants 



surveys, numbers of this cormorant breeding in Marin 
declined from an estimated 1672 birds in 1979 (Sowls et 
al. 1980) to 902 in 1989 (Carter et al. 1992). At the 
Farallon Islands, numbers of breeding Pelagic Cormorants 
vary greatly from year to year depending on oceanographic 
conditions and food supply (Ainley 6k Boekelbeide 1990, 
Chap. 6). In particularly poor food years, virtually the 
whole population there may desert their nests early in die 
breeding season— 1989 was such a year. Hence, the low 
numbers at Marin County colonies in 1989 are probably 
indicative of short-term variation in ocean conditions in 
die Gulf of the Farallones rather than a long-term decline 
of the cormorant population. Again, because Pelagic Cor- 
morants, like Brandts, frequendy shift colony sites, popu- 
lation trends should be assessed for large rather than small 



areas (Carter et al. 1990). Sowls et al. (1980) estimated a 
breeding population of 1 5,458 Pelagic Cormorants at 166 
sites along the central and northern California coast in 
1979 to 1980, whereas Carter et al. (1992) estimated 
1 1 ,658 birds at 169 sites (including 1 in S.F. Bay) in 1989 
to 1990. Population trends between die two surveys varied 
among several large segments of the coast. 

Numbers of Pelagic Cormorants breeding on the Faral- 
lon Islands declined in the late nineteenth century because 
of disturbance but subsequently increased (Ainley 6k Lewis 
1 974). Pelagic Cormorants generally are less vulnerable to 
disturbance than are Brandt's or Double-crested cormo- 
rants because they are more widely dispersed in less acces- 
sible nesting sites. 




^Ci+h Anysu.n 



Nesting cormorants must be ever watchful for the predatory shenanigans of 
Western Gulls. Drawing by Keith Hansen, 1989. 



90 



Bitterns and Herons 



SPECIES ACCOUNTS 



Bitterns and Herons 



Bitterns and Herons 



Family Ardeidae 



AMERICAN BITTERN Botaurus lentiginosus 









A year-round resident; numbers swell 




\ Vr~V, 




slightly from Sep through Apr. 


-)^\\':.j^\^\ c^^\ J^"""---^ 






A very rare (perhaps rare), very local 


JV\\JV\S-V\SA^ 




(- - 


breeder; overall breeding population very 
small. 




^^^ScVw; 




Recorded in 7 (3.2%) of 221 blocks. 








O Possible 4 (57%) 


\E)^v^^^ 


<r *""^A • ' \^\ \*^\ \^\ V--" 




€ Probable 2 (29%) 






\^^><^%\j<(\ 


^\>^V •=-' 


• Confirmed = 1 (14%) 




j?|y^ 






FSAR=1 OPI = 7 CI = 1.57 








>^^P°" 





Ecological Requirements 

With their frozen sky-pointing postures, cryptic reedlike 
coloration, and booming, ventriloqual calls, American 
Bitterns can be difficult to spot in their breeding haunts of 
freshwater marshes and coastal swales. They prefer cattail 
and tule marshes over much of the range, but their main 
requirement seems to be dense marsh vegetation within 
the first two to three feet of the ground. Although Ameri- 
can Bitterns breed in brackish and saltwater marshes 
elsewhere in North America (Bent 1926, Palmer 1962), 
there appear to be no definitive breeding records for these 
habitats in coastal California; Bitterns do frequent them at 
other seasons, however. 

American Bitterns are solitary feeders that usually hunt 
from a standing or slow walking position (Hancock 6k 
Kushlan 1984). They hold their bills low and strike with a 
quick jab while wading through water or low marsh, or 
while peering down from a bank or a perch in marsh 
vegetation. American Bitterns also walk quickly or run in 
tall grass, gleaning insects from grass stems or flycatching 
them from the air. Overall, the diet includes about 20.3% 
fish, 19.0% crayfish, 23.1% aquatic and land insects, 
20.6% frogs and salamanders, 9.6% mice and shrews, 



5.5% snakes, and 2.2% crabs, spiders, and miscellaneous 
invertebrates (Palmer 1 962). Geographic and seasonal vari- 
ation in the diet has been noted. 

Unlike most of the heron and egret clan, American 
Bitterns breed solitarily. They usually locate their nests in 
wet places in a marsh; they build them up as much as eight 
inches above shallow water or mud, or they lodge them in 
marsh vegetation (Bent 1926, Palmer 1962). Nests are also 
found occasionally on dry ground in grassy meadows or 
hayfields (though in proximity to marshlands) or on float- 
ing islands in lakes. Concealment is provided by surround- 
ing vegetation, but more often than not the nest is open 
above radier than screened by arched-over stalks; new 
growth may further seclude the nest as the season pro- 
gresses. The nest is a small, flat platform made of materials 
such as dead cattail flags, bulrushes, other sedges, rushes, 
and reeds, as well as grasses, weeds, or small sticks; eggs 
may sometimes be laid practically on bare ground. Young 
hatch asynchronously and are fed by regurgitation. Cir- 
cumstantial evidence suggests that young may move to a 
second nearby platform after 20 days of age and that 
previous years' nests may sometimes be reused (Palmer 
1962). 

91 



Bitterns and Herons 



MARIN COUNTY BREEDING BIRD ATLAS 



Bitterns and Herons 



Marin Breeding Distribution 

The only confirmed breeding record for American Bittern 
in Marin County was a sighting of two recendy fledged 
young at Abbott's Lagoon on 28 July 1981 (DS) in an area 
where adults had been seen diroughout the breeding 
season. The rarity here of Bitterns in the breeding season 
seems largely attributable to the scarcity of extensive fresh- 
water marshes. 



Historical Trends/Population Threats 

American Bitterns probably always have been scarce breed- 
ers in Marin County, but diey may have declined with the 
historic loss of freshwater marshes, particularly around 
San Pablo and San Francisco bays. The American Bittern 
was included on die Audubon Society's Blue List in seven 
years from 1976 to 1986 (Tate 1981, 1986; Tate 6k Tate 
1982). Numbers of American Bitterns were relatively sta- 
ble on Breeding Bird Surveys in California from 1968 to 
1989 (USFWS unpubl. analyses). 



GREAT BLUE HERON Ardea herodias 











A year-round resident; occupies breeding 










rookeries mostly from late Jan or early 
Feb through late Jun or mid-Jul. 




jf-^- 






A fairly common, very local breeder; 




< ?-A 






overall breeding population very small. 


\*Z>^ \ A. A Jr. \ w Jr \ J^^ \ O \^Z-\-~ Jt^i* \ \^^\^ .1 

\vN5rf\ °^c\ 9-V\ ^-V\ ■ lc lV^ r A ( ^V\ °J>^ 


^oj 






Recorded in 12 (80) or 5.4% (36.2%) 


\^-\XA-Jr\ Jr\ A--\ 3r^\ 3r\ 








of 221 blocks (see Methods). 


\^v >-^h >A^^\ v-^\ o V--i\ \^-^\ i^\ v 








O Possible = 68 (85%) 
€ Probable = (0%) 


V^^J^rx^V^V^^A^c^^JV^xS-T 






• Confirmed = 12 (15%) 


r-T7 ^C\^\°3<>^ 


^54^ 




^& 


FSAR = 3 OPI = 36 CI = 1 .30 



Ecological Requirements 

Great Blue Herons are stately denizens of shallow tidal and 
freshwater feeding grounds and adjacent uplands. Great 
Blues have the widest range of foraging habitat of Marin's 
breeding herons and egrets. To seize unsuspecting prey, 
diey most commonly stand still or wade slowly in die 
shallow waters or along the shores of estuaries, lagoons, 
bays, freshwater ponds, streams, and, less frequently, tide 
pools; they also perch in nearshore kelp beds in die 
Monterey Bay area, riding the swells (Roberson 1985). 
Less frequendy, in aquatic habitats, they hover over water 
and stab at prey below, dive into water from die air 
headfirst, drop into water from perches feet first, float or 
swim on die water's surface stabbing at or picking up prey, 
wing-flick to disturb prey, and dash after prey with dieir 
wings used for balance, lift, or braking (Hancock &. 
Kushlan 1984). Additional aquatic, shoreline, or terrestrial 

92 



feeding techniques include pecking, probing, and even 
flycatching. Great Blues spend a fair amount of time 
stalking, poised motionless, in pastures and fields in 
search of rodents (especially pocket gophers), lizards, and 
insects. Although dtey forage mosdy during the day, Great 
Blues also feed at night, especially in tidal habitats (Han- 
cock ck Kushlan 1984). In some areas, especially on 
islands, Great Blues visit human habitations for scraps of 
food put out for them. Great Blues forage singly or in 
aggregations and in some circumstances defend feeding 
territories (Pratt 1 980). It has been suggested that colonial 
nesting Great Blues will follow one anodier to exploit food 
diat is unevenly distributed and concentrated in areas of 
temporary abundance. However, adult birds at the 
Audubon Canyon Ranch rookery on Bolinas Lagoon do 
not appear to follow odier herons on foraging flights; 



Bitterns and Herons 



SPECIES ACCOUNTS 



Bitterns and Herons 



instead they land where other herons are already feeding 
or go to familiar feeding grounds (Pratt 1980). Long- 
distance foraging flights are common in Ciconiformes, and 
some authors believe Great Blues may fly up to about 50 
miles to feeding areas. Although most adults departing the 
Audubon Canyon Ranch rookery go to Bolinas Lagoon to 
feed, many fly out of sight in several directions to unknown 
foraging areas (Pratt 1980). See Marin Breeding Distribu- 
tion section for the extent of the foraging range in the 
county. 

Overall in the U.S., this heron's diet is about 71.6% 
fish, 8.9% crustaceans, 8.2% insects, 4.7% mice and 
shrews, 4.2% amphibians and reptiles, and 2.5% miscel- 
laneous animal and vegetable matter (Palmer 1962). Most 
vegetable fare is probably taken incidentally, but Great 
Blues apparendy eat the seeds of water lilies. They also 
capture marshbirds as large as Black-necked Stilts and 
Clapper Rails (Palmer 1962, Hancock 6k Kushlan 1984); 
rails are particularly susceptible to herons and egrets when 
forced out of marshes at high tides (Evens 6k Page 1986). 

Great Blues breed in small to large colonies or, rarely, 
solitarily. In Marin County, they place their nests high in 
large trees such as redwoods, Douglas fir, California bay, 
coast live oak, and eucalyptus, often in mixed colonies with 
Great Egrets (Pratt 1983). In mixed heronries, Great Blues 
typically, but not invariably, nest in the highest parts of 
trees (up to 130 ft., Bent 1926), with other species below 
them (Palmer 1962). In other areas, Great Blues also nest 
in shrubs, on the ground, on tule platforms, on rock ledges 
or sea cliffs, and on duck blinds or other artificial struc- 
tures. Nests are flat platforms of sticks with inner shallow, 
saucer-shaped depressions. They may vary considerably in 
bulk since they are reused repeatedly; nest material is 
added throughout incubation and early in the nesding 
phase. The nests may be lined widi fine twigs, mosses, pine 



needles, reeds, weed stalks, marsh grasses, or leaves (Bent 
1926, Palmer 1962, H.M. Pratt pers. comm.). The young 
hatch asynchronously and are fed by regurgitation. 

Marin Breeding Distribution 

During the adas period, Great Blues were confirmed nest- 
ing in 12 colonies scattered throughout Marin County 
(Pratt 1 983; Table 1 5, Figure 1 5, and adas map). Only two 
of these colonies (at Nicasio Reservoir and Drake's Head) 
were newly discovered by adasers. Great Blues have also 
nested at several other sites either prior to or after the adas 
period (Pratt p. 103 this volume; Table 15, Figure 15). 
Although most local heron colonies are adjacent to estuar- 
ies, Great Blues forage throughout the lowlands of Marin 
in the breeding season. In contrast, the other colonial 
breeding herons and egrets forage primarily in the estuaries 
and marshes along Marin County's bay and ocean shores 
(see atlas maps). 

Historical Trends/ Population Threats 

Grinnell and Miller (1944) did not list any breeding sites 
for Great Blue Herons in Marin County, but this was 
undoubtedly because of limited coverage in this area. Pratt 
(1983) reported the known history of Marin County colo- 
nies, noting that the Audubon Canyon Ranch colony was 
"well established and active in 1941." Although data are 
lacking, it seems likely diat heron populations have been 
reduced by die historic loss of extensive marshlands in the 
San Francisco Bay system. Pratt (p. 103 this volume) 
describes a decline in Great Blue numbers in Marin since 
1968. For California as a whole, numbers were relatively 
stable on Breeding Bird Surveys from 1968 to 1989 
(USFWS unpubl. analyses). Disturbance at nesting colonies 
can cause abandonment (Werschkul et al. 1976), and 
pesticide contamination poses threats to reproductive suc- 
cess (see Hancock 6k Kushlan 1984). 



93 



MARIN COUNTY BREEDING BIRD ATLAS 



Table 15. Numbers of breeding pairs of Great Blue Herons at 16 Marin County colonies from 1967 to 1991 (see Figure 15). 
U = Nest counts unavailable— herons may or may not have been nesting. In addition, at least one heron nest was at an inaccessible 
site off Bel Marin Keys Boulevard, Novato, in 1985 (based on the sound of large young being fed). 





1967 


1968 


1969 


1970 


1971 


1972 


1973 


1974 


1975 


1976 


1977 


1978 


1979 


Audubon Canyon Ranch 


50 


62 


55 


50 


44 


46 


58 


48 


45 


40 


41 


43 


35 


De Silva Island 






































5 


Drake's Head 


U 


u 


U 


U 


U 


U 


U 


U 


u 


U 


U 


U 


U 


Home Bay 


u 


2 


3 


U 


U 


u 


u 


U 


u 


U 


u 


U 


u 


Inverness Park 


u 


U 


U 


u 


U 


16 





U 


4 


3 


4 


3 


u 


Nicasio Reservoir 


u 


u 


u 


u 


u 


U 


u 


U 


U 


U 


U 


U 


u 


Nick's Cove 


u 


u 


u 


23 


u 


u 


u 


23 


28 


23 


24 


16 


u 


North San Pedro Road 














1 


I 


1 








1 


2 


1 


4 


Phoenix Lake 









































Sand Point 


u 


u 


u 


u 


u 


u 


u 


25 


7 


u 


U 


U 


u 


Schooner Bay A &. B 


u 


u 


u 


u 


u 


u 


15 


15 


12 


9 





7 


7 


Stafford Lake 


8 


u 


u 


5 


u 


u 


5 


14 


21 


U 


21 


19 


16 


Olema 


u 


u 


u 


26 


u 


19 




















U 


Bolinas-Fairfax Road 


u 


u 


u 


U 


u 


U 


U 


u 


U 


u 


U 


U 


U 


Smiley's "Preserve" 









































West Marin Island 


u 


u 


u 


U 


u 


u 


u 


u 


u 


u 


U 


u 











1980 


1981 


1982 


1983 


1984 


1985 


1986 


1987 


1988 


1989 


1990 


1991 




Audubon Canyon Ranch 


33 


27 


26 


13 


16 


18 


21 


18 


13 


16 


9 


7 




De Silva Island 


5 


3 








1 


4 


3 


3 


7 


6 


9 


9 




Drake's Head 


U 


U 


11 


3 


4 


1 + 


U 


U 


3 


U 










Home Bay 





1 











U 


U 


U 





U 










Inverness Park 


12 


8 


6 


3 


6 


u 


u 


u 


10 


U 


7 


9 




Nicasio Reservoir 


U 


U 


6 


6 


5 


u 


u 


u 


13 








U 




Nick's Cove 


18 


15 


15 


19 


16 


u 


12 



















North San Pedro Road 


4 


3 


2 


2 


4 


u 


u 


u 


8 


16 


21 


17 




Phoenix Lake 











1 


U 


u 


u 


u 





u 





U 




Sand Point 


U 


16 


13 


10 


12 


u 


7 


u 


8 


13 


18 


16 




Schooner Bay A & B 


7 


3 











u 


u 


u 





U 










Stafford Lake 


27 


23 


27 


29 


28 


30 


31 


u 


30 


27 


32 


16 




Olema 


U 


U 


U 


U 


U 


U 


U 


u 


U 


U 


U 


U 




Bolinas-Fairfax Road 


u 


U 


U 


u 


U 


U 


u 


u 


U 


1 


1 


1 




Smiley's "Preserve" 
































6 


10 




West Marin Island 








1 























1 


2 





94 



SPECIES ACCOUNTS 




Colony codes used are: 

1 = Audubon Canyon Ranch 

2 = De Silva Island 

3 = Drake's Head 

4 = Home Bay 

5 = Inverness Park 

6 = Nicasio Reservoir 



7 = Nick's Cove 

8 = North San Pedro Road 

9 = Phoenix Lake 
10 = Sand Point 

11= Schooner Bay A 6k B 
12 = Stafford Lake 



13 = Olema 

14 = Bolinas-Fairfax Road 

15 = Smiley's "Preserve" 

16 = West Marin Island 



Figure 15. Map of heron and egret colony sites in Marin County. Rookeries denoted b> bold numbers were known to be active in 
1991; all other sites were active prior to 1991 (see Tables 15 and 16). 



95 



Bitterns and Herons 



MARIN COUNTY BREEDING BIRD ATIAS 



Bitterns and Herons 



GREAT EGRET Casmerodius albus 



'vgtft^ 


^r^-^ ^ 






Occurs year round, though numbers 
swell substantially when birds occupy 


X\^V 


T V^V Jk^\9^ 


\ ^V\ ^V\ ^-V\ °>A ' 




breeding rookeries, mosdy from mid-Mar 
(rarely mid-Apr) until late Jul to mid-Aug. 


^Y^ 




\^vC \r\ 3r\ 3r^A^ 




A very common, very local breeder; 
overall breeding population very small. 

Recorded in 5 (37) or 2.3% (16.7%) of 
221 blocks (see Methods). 




5\ ^-Y"\ i-'TX L' 


■>-'\>i \^\ ' v-"\ )£--"v 


\ '""jsxrv .— ' 


O Possible = 32 (86%) 






■*\ . 3r-^\-J J^T\ JV""\ -A"" 


£rvT' 


© Probable = (0%) 




-^SS^t-V^ 


^"\ ^-Ya c lV\. JVv"- 


IS? 


• Confirmed = 5 (14%) 




^s=> 




t^V^'A' vx 


FSAR=5 OPI = 25 CI = 1.27 



Ecological Requirements 

Great Egrets present elegant lines and exude a ghosdy aura 
as they forage in a variety of shallow-water habitats, includ- 
ing estuaries; lagoons; bays; saltwater, brackish, and fresh- 
water marshes; as well as ponds and streams, irrigation 
ditches, and wet meadows. In Marin County, breeding 
Great Egrets prefer estuarine and bay habitats. They use 
inland freshwater habitats and pasturelands only to a 
limited degree compared with Great Blue Herons (see adas 
maps). 

Great Egrets feed singly or in groups and form large 
aggregations at concentrations of prey. Solitary birds vigor- 
ously defend foraging sites. Aggressive encounters occur 
when gregariously feeding birds attempt to steal prey cap- 
tured by other individuals (Hancock & Kushlan 1984). 
Great Egrets forage primarily by slowly walking in shallow 
water, along shorelines, or in dry habitats; they also poise 
motionless to dart out and seize prey from a crouched 
posture with their heads drawn in. Infrequent foraging 
tactics include startling or activating prey by vibrating their 
feet in water (moving their feet up and down on the 
substrate) or by wing flicking. Great Egrets also forage 
actively by hovering over the water and stabbing at prey 
below, by flying along and periodically reaching into the 
water to pick up prey, or by diving into the water from the 
air headfirst. More complex foraging tactics include foot 
paddling while flapping the wings violendy up and down; 
and periodically hopping from the water, stabbing prey 
brought to the surface. Great Egrets also glean insects from 

96 



plants. They feed primarily on fish, frogs, salamanders, 
snakes, snails, crustaceans, insects, small mammals, and, 
occasionally, small birds. Fish usually comprise the bulk of 
the diet in the wet season, but there is considerable local 
variation (Palmer 1962, Hancock & Kushlan 1984). Great 
Egrets specialize in capturing small to medium-sized rails 
forced from cover at high tides (Evens & Page 1986). Birds 
apparendy travel considerable distances to forage. In the 
breeding season, Great Egrets frequendy fly along the 
shoreline northwest of Bolinas, presumably commuting 
from the nesting colony at Audubon Canyon Ranch on 
Bolinas Lagoon to alternate feeding grounds at Limantour 
and/or Drake's esteros (Shuford et al. 1989). 

Great Egrets breed solitarily or, more often, in small to 
large colonies, often in association with other species of 
wading birds. In Marin County, they nest in tall trees 
(redwoods, Douglas firs, eucalyptus, California buckeyes), 
often alongside Great Blue Herons (Pratt 1983; see Great 
Blue Heron account). Elsewhere, they sometimes nest low 
to the ground in small willows or on bent-down bulrushes 
(Bent 1926, Palmer 1962). Nests are flat platforms of sticks 
or rule stalks and are usually flimsier and flatter than those 
of Great Blue Herons; nests from previous years may be 
reused. Many nests lack a lining or cavity, but sometimes 
they are considerably hollowed and are well lined with fine 
twigs, vines, or weed stems. The young hatch asynchro- 
nously and are fed by regurgitation. 



Bitterns and Herons 



SPECIES ACCOUNTS 



Bitterns and Herons 



Marin Breeding Distribution 

During the atlas period, Great Egrets nested at five colonies 
along the Marin County shoreline (Table 16, Figure 15, 
and adas map). 

Historical Trends/ Population Threats 

From the 1880s to the 1890s, Great Egret numbers in 
California were gready reduced by hunters for the feather 
trade. They began to recover by 1911, and by 1943 the 
species was "common in the remaining suitable portions 
of its former range" (G&M 1944). Numbers probably 
failed to reach historic levels because of the extensive loss 
of the state's wedand habitat. Great Egrets reappeared in 
the San Francisco Bay Area in 1924 (Stoner 1934), and in 
Marin County with seven birds at Bolinas on 7 May 1929 
(Stoner 1934) and one bird at Drake's Estero on 7 June 
1931 (Stephens 1931). The birds at Bolinas were likely 
breeding then at what is now known as Audubon Canyon 



Ranch (Pratt 1983). Human insensitivity was still evident 
in July 1955, when 53 egrets (mosdy Greats, a few Snow- 
ies) were "wantonly slaughtered ... by rifle-bearing target 
shooters" at West Marin Island; the culprits were arrested 
(AFN 10:51). The West Marin Island rookery had been 
active for "many years" prior to this incident (Ralph ck 
Ralph 1958). Pratt (p. 103 this volume) describes recent 
trends in numbers of Great Egrets at Marin County 
colonies. On the whole, Great Egret numbers increased on 
Breeding Bird Surveys in California from 1968 to 1989 
(USFWS unpubl. analyses). 

Great Egrets reproduced poorly in the late 1960s and early 
1970s because of DDT-induced eggshell thinning (Faber et 
al. 1972, Ives 1972, Pratt 1972), but since then a decrease 
in the rate of egg loss during incubation suggests the 
species is recovering (Pratt 1974). Disturbance at colonies 
can, of course, cause abandonment. 



Table 16. Numbers of breeding pairs of Great Egrets at five Marin County colonies from 1967 to 1991 (see Figure 15). U = 
Nest counts unavailable— egrets may or may not have been nesting. See Pratt (1983) for numbers of Great Egrets seen on or 
feeding near West Marin Island, 1973 through 1981. 





1967 


1968 


1969 


1970 


1971 


1972 


1973 


1974 


1975 


1976 


1977 


1978 


1979 


Audubon Canyon Ranch 


70 


74 


86 


85 


85 


96 


99 


96 


85 


65 


84 


88 


98 


Inverness Park 


























3 


4 


12 


2 


U 


Nick's Cove 


U 


u 


u 





u 


U 


u 


5 


15 


13 


6 


12 


U 


Sand Point 


U 


u 


u 


U 


u 


u 


u 








U 


U 


U 


u 


West Marin Island 


U 


u 


u 


u 


u 


u 


u 


U 


u 


U 


U 


u 


58 








1980 


1981 


1982 


1983 


1984 


1985 


1986 


1987 


1988 


1989 


1990 


1991 




Audubon Canyon Ranch 


103 


148 


150 


97 


110 


113 


98 


113 


113 


102 


91 


100 




Inverness Park 


1 














U 


U 


U 


1 + 


U 










Nick's Cove 


6 


5 


































Sand Point 


U 


6 


13 


19 


19 


u 


20 


u 


20 


25 


25 


49 




West Marin Island 


u 


75 


187 


190 


139 


84 


160 


89 


66 


79 


119 


90 





On-site counts found 155 pairs in 1990 and 131 pairs in 1991 (R.L. Hothem/USFWS pers. comm.); counts from this site reported in table from 
these and previous years were taken from a boat 



97 



Bitterns and Herons 



MARIN COUNTY BREEDING BIRD ATLAS 



Bitterns and Herons 



SNOWY EGRET Egretta thula 



xV'YdV' 




A year-round resident; birds occupy 
breeding colonies mosdy from mid-Mar 
dirough mid-Aug. 
A very abundant, very local breeder; 




<"\ ^\\ ^-V\ ^&\ J^c\ S^\°^c \ 


overall breeding population very small. 

Recorded in 1 (22) or 0.4% (10.0%) of 
221 blocks (see Methods). 


\ V- 1 




O Possible = 21 (95%) 




\i\^K\^^P^ — ■*" 


© Probable = (0%) 




. )b<^\ V^\" X^S. V^Y-J V-'A \^\ \^-\~P^S 

>"Ak tfV^C \^\ J^\ \^df- v-'V i^olK" 


• Confirmed = 1 (5%) 






FSAR=7 OPI = 7 CI = 1.09 



Ecological Requirements 

These dashingly handsome little egrets with "golden slip- 
pers" forage in a variety of shallow saltwater, brackish, and 
freshwater habitats comparable to those frequented by 
their larger cousin the Great Egret. Snowies also occasion- 
ally forage in pastures and fields and, like Cattle Egrets, will 
follow cattle and other livestock to pick up insects dis- 
turbed by their grazing (Palmer 1962, Hancock ck Kushlan 
1984)- In the breeding season in Marin County, Snowy 
Egrets forage primarily in tidelands and marshlands along 
the San Francisco and San Pablo bayshores near their only 
regular nesting colony (see adas map). 

Snowies are extremely active feeders "moving about with 
great show of nervous energy, yet much poise and grace" 
(Palmer 1 962). They also have the most diverse repertoire 
of foraging behaviors of any heron or egret so far studied 
(Hancock ck Kushlan 1984)- Snowies typically feed in 
conspecific or multispecies flocks, principally while walk- 
ing slowly or quickly or while standing. They are especially 
adept at startling or attracting prey by vibrating their legs 
and yellow feet, by scratching their toes across or inserting 
them into the substrate, or by moving their feet up and 
down on the substrate. In addition, Snowies attract fish by 
placing their bills in the water and rapidly opening and 
closing them. Snowies are also accomplished aerial forag- 
ers. From flight they periodically reach into the water for 
prey, trail their toes in die water, and while hovering stab 
at prey below or pat, stir, or rake the water with their feet. 
Snowies also dash about rapidly, extending their wings for 
a few seconds at a time, with short flights interspersed. 
Their gende looks belie their kleptoparasitic tendencies. I 

98 



have watched Snowies chase White-faced Ibis at Los Banos 
wildlife refuge and force them to drop crayfish, which the 
Snowies prompdy ate. Their diet includes small fish, 
crustaceans (especially crayfish), frogs, lizards, snakes, 
worms, snails, insects, and, occasionally, small rodents 
(Palmer 1962). 

Breeding Snowies are highly colonial, and they typically 
nest widi other species of egrets or herons. Only occasion- 
ally do pairs breed alone. At the only well-established 
Marin County colony at West Marin Island, Snowies nest 
at varying heights in live oak and buckeye trees, in coastal 
scrub, and on the ground (Pratt 1983, H.M. Pratt ck R 
Hothem pers. comm.). Birds breeding irregularly at 
Audubon Canyon Ranch nest 60 to 70 feet up in coast 
redwood trees (H.M. Pratt pers. comm.). Throughout their 
range, nests usually are situated from the ground to 30 feet 
up (most 5-10 ft.) in a variety of trees; various bushes, 
cacti, and broken-down reeds and bulrushes also serve as 
nest supports (Bent 1926, Palmer 1962). The typically 
elliptical, somewhat loosely woven nest has a foundation 
of sticks and a rather flat body of twigs with a shallow 
cavity. Nests are sometimes lined with finer twigs, stalks of 
marsh plants, or roodets. On occasion, birds construct no 
nest and instead lay eggs in a depression in the broken and 
matted-down tules of the previous year. Dead canes, reeds, 
rushes, tules, sage, holly, birch, and other plants may be 
used in the nest depending on availability. Snowies prob- 
ably do not, or only infrequendy, reuse former nests, 
although they may use the same site and sticks from other 



Bitterns and Herons 



SPECIES ACCOUNTS 



Bitterns and Herons 



old nests in construction of the new one (Palmer 1962). 
Young hatch asynchronously and are fed by regurgitation. 

Marin Breeding Distribution 

Snowy Egrets breed consistendy in Marin County in large 
numbers only at the West Marin Island rookery (Table 1 7, 
Figure 15, and adas map). About five pairs nested at 
Audubon Canyon Ranch in 1969, 1988, and 1989; four 
pairs nested there in 1990 and 1991 (Pratt 1983, H.M. 
Pratt pers. comm.). 

Historical Trends/ Population Threats 

Snowy Egrets were locally common in California prior to 
1880, but because of the ravages of plume hunters diey 
declined to the brink of extinction by the early 1900s 
(G6kM 1944). By 1908 they were recorded again, and by 
1943 they were fairly common in favored places (G&.M 
1944), though as late as 1932 a bird seen at Richardson 
Bay, Marin County, was still worthy of note in the Condor 
(Swanton 1933). Snowy Egrets probably have not recov- 



ered to historical population levels because of the extensive 
loss of California's wedands. At the time of Ralph and 
Ralph's (1958) visit to the active Snowy Egret colony at 
West Marin Island in 1957, local residents claimed that 
Snowies had been established there "for at least five years. 
See Great Egret account regarding a slaughter of 53 egrets, 
including a few Snowies, on West Marin Island in July 
1955. Pratt (p. 103 this volume) describes recent trends in 
the West Marin Island Colony. On the whole, Snowy 
Egret numbers increased on Breeding Bird Surveys in 
California from 1968 to 1989 (USFWS unpubl. analyses). 
In San Francisco Bay, Snowy Egret eggs show concentra- 
tions of organochlorine pesticide residues and mercury, 
but below "critical" levels that cause adverse effects on 
reproduction (Ohlendorf et al. 1 988). Monitoring of Bay 
Area colonies should be continued as reproductive failure 
in Idaho colonies has been linked to DDE contamination 
(Findholt 1984). Like our other colonial nesting waders, 
Snowy Egrets are also highly susceptible to nest loss from 
disturbance. 



Table 17. Estimates of the number of breeding pairs of Snowy Egrets and Black-crowned Night-Herons on West Marin Island 
from boat censuses from 1979 to 1991. 





1979 


1980 


1981 


1982 


1983 


1984 


1985 


1986 


1987 


1988 


1989 


1990 


1991 


Snowy Egrets 


262 


U 


325 


500 


400 


400 


161 


126 


239 


212 


245 


300 


277 


Black-crowned Night-Herons 


98 


U 


109 


80 


89 


54 


79 


40 


41 


35 


61 


37 


45 



On-site counts found 463 pairs in 1990 and 487 pairs in 1991 (R.L. Hothem/USFWS pers. comm.). 
On-site counts found 306 pairs in 1990 and 294 pairs in 1991 (R.L. Hothem/USFWS pers. comm.). 



99 



Bitterns and Herons 



MARIN COUNTY BREEDING BIRD ATLAS 



Bitterns and Herons 



GREEN-BACKED HERON Butorides striatus 




A year-round resident; numbers swell 
slightly from Apr through mid-Oct 

An uncommon, very local breeder; 
overall breeding population very small. 

Recorded in 24 (10.8%) of 221 blocks. 

O Possible = 20 (83%) 
C Probable = 1 (4%) 
• Confirmed = 3 (13%) 

FSAR = 2 OPI = 48 CI = 1.29 



Ecological Requirements 

This compact, dapper heron blends in well with the 
forested margins of the quiet waters of streams, ponds, and 
freshwater marshes. Green-backed Herons use brackish 
marshes and estuarine borders to a limited extent in 
California, as they do commonly elsewhere, but most 
breeding birds here inhabit freshwater habitats. They for- 
age mosdy by day but also sometimes at night; at coastal 
sites, the timing of foraging bouts may vary with tidal 
heights (Hancock &. Kushlan 1984). Green-backs are 
patient feeders. They usually stand crouched and motion- 
less on a branch or rock, or they walk slowly along the 
shoreline waiting to stab their prey. Birds also flycatch 
from a standing position and startle prey by vibrating their 
feet in water or by raking their toes across the substrate. 
More active foraging techniques include launching from a 
perch feet first, or from the air headfirst, into the water, and 
floating or swimming on the surface of the water while 
securing prey. They even hang from branches while stab- 
bing in the water. Green-backed Herons also ingeniously 
place items such as bread and feathers on the water's 
surface to lure in prey (Lovell 1958, Sisson 1974). In many 
situations they vigorously defend feeding territories 
(Palmer 1962, Hancock ck Kushlan 1984). Their diet 
includes small fish, crustaceans, aquatic and land insects, 
amphibians, reptiles, other invertebrates, and small mam- 
mals (Palmer 1962, Hancock ck Kushlan 1984). 

In Marin County, Green-backed Herons nest solitarily. 
Elsewhere, they also nest in small groups or, rarely, large 
colonies; they rarely mix widi other species for nesting. 
Nests are placed in a variety of situations. These include 
from on the ground to up to 30 feet in trees in dry woods 



or orchards; in low trees (most 10-15 ft.) or bushes over 
or near water; or amid marsh vegetation (Bent 1926, 
Palmer 1 962). Nests vary from round to oval and from very 
flimsy platforms (usually new nests) to very tighdy woven, 
bulky structures (mostly old, reworked nests). Green- 
backed Heron nests built on the foundations of other 
species' nests, such as those of Black-crowned Night-Her- 
ons or crows, are usually rather flimsy. The body of the 
nest is built of twigs or, rarely, of coarse weeds, reeds, or 
cattails. It is commonly unlined or lined with finer twigs, 
vines, and bits of reeds or other plant material. The young 
hatch asynchronously and are fed by regurgitation. 

Marin Breeding Distribution 

The spotty breeding distribution of Green-backed Herons 
in Marin County during the atlas period reflected the 
limited distribution of their preferred breeding habitat. 
Representative nesting locations were Nicasio Reservoir 
(NB 5/9/81 — JE) and just north of Inverness Park (FL 
7/18/82 — DS). An earlier breeding record was of a nest 
with diree young observed at San Anselmo Creek in Ross 
on 5 July 1935 (GuIIl 7, No. 7). 

Historical Trends/ Population Threats 

Loss and degradation of marsh and riparian habitats must 
have gready reduced the state's historic populations of this 
heron. On the whole, numbers of Green-backed Herons 
increased on Breeding Bird Surveys in California from 
1968 to 1989, though numbers were relatively stable from 
1980 to 1989 (Robbins et al. 1986, USFWS unpubl. 
analyses). 



100 



Bitterns and Herons 



SPECIES ACCOUNTS 



Bitterns and Herons 



BLACK-CROWNED NIGHT-HERON Nycticorax nycticorax 









A year-round resident; birds occupy 




^v^T-^ N PO^~- 




breeding colonies mosdy from mid-Mar 


j\^\j><^ 




through mid-Aug. 




\^\ Jk^\ J>r\ \^\ Jk^\ J^\°i^\ 




An abundant, very local breeder; over- 


V\Jr 


'^<^^\^\\^^^^\\^\^i^^f°^ 




all breeding population very small. 




PcjAa^o^^V!^^^ 




Recorded in 1 (1 7) or 0.4% (7.7%) of 


^X^~ 


^x^t^^^ 




221 blocks (see Methods). 


\ \^ 






O Possible = 16 (94%) 






— -r" 


© Probable = (0%) 




Ai^TV^pV-^CJ^ 




• Confirmed = 1 (6%) 






^ 


FSAR = 6 OPI = 6 CI = 1.12 




!vL, ^ 7 ^^^"^S^c^X 







Ecological Requirements 

The handsome portly profiles of Black-crowned Night- 
Herons appear posed for die painter at their communal 
daytime roosts and nesting colonies. The seasoned natural- 
ist need not raise binoculars in the fading light of dusk to 
identify their eerie silhouettes as these herons give their 
characteristic wok-wok calls while flying out to feed in a 
variety of shallow marine and freshwater habitats, includ- 
ing bays, estuaries, tidal flats, lagoons, freshwater ponds, 
and marshes. 

Unlike our other herons and egrets, Black-crowned 
Night-Herons are primarily nocturnal or crepuscular forag- 
ers. Although they occasionally come out to feed in broad 
daylight, they usually do so only on overcast or foggy days. 
They forage either solitarily, maintaining exclusive feeding 
territories, or in aggregations, and they usually hunt their 
prey from a poised stance or from a slow stalking gait 
(Hancock & Kushlan 1984). While standing or walking, 
they sometimes put dieir heads under an opened wing, 
which may reduce glare, making prey more visible, or 
attract prey to the shade of the wing. Additionally, Black- 
crowns occasionally hover over water, stabbing at prey 
below; dive headfirst into water; and float or swim on the 
water's surface, stabbing or picking up prey. On occasion 
they also rapidly open and close their bills in algae-covered 
water to attract prey (Palmer 1 962). 

The diet is diverse and varies with locality. Overall in 
the U.S. it consists roughly of 51 .5% fish, 22% crustaceans 
(shrimp and crayfish), 16% aquatic insects, 6% frogs, 3% 
rodents, and the remainder mosdy spiders and worms; 
other food items include tadpoles, snakes, salamanders, 



mollusks, marine annelids, vegetable matter, and small 
birds (Palmer 1962, Wolford 6k Boag 1971). While walk- 
ing around in colonies of nesting waterbirds, Black-crowns 
will take the young of terns, other herons, and ibises 
(Hancock 6k Kushlan 1984)- At one locale, Black-crown 
young were initially fed shrimp, followed by fish after the 
young reached three weeks of age (Palmer 1962). 

Black-crowned Night-Herons nest in small to very large 
colonies, usually with other herons or egrets. Nest sites are 
diverse and range in height from the ground to 160 feet up 
in trees. Colonies are variously located in trees or brush in 
mainland or island woodlands, forests, or swampland; in 
old orchards or in city parks; in stands of cattails and tules 
and on floating dead vegetation anchored to emergent 
cattail stalks; and on the ground among tufts of tall grass 
on islands (Bent 1926, Palmer 1962). Black-crowns at 
Marin County's West Marin Island colony build their 
nests mostly in coastal scrub but also in California buckeye 
trees (H.M. Pratt 6k R. Hothem pers. comm.). Nests vary 
from radier frail platforms to solid, bulky structures (some- 
times deeply cupped) that are used for several years. Coarse 
twigs, sticks, reeds, or weed stalks make up the body of the 
nest, while finer materials such as small twigs or rootlets 
form the lining or are woven into the top; sticks from old 
nests are reused in die construction of new ones. 

Marin Breeding Distribution 

Black-crowned Night-Herons currently nest in Marin 
County only at West Marin Island (Pratt 1983; Table 17, 
Figure 1 5, and adas map). 

101 



Bitterns and Herons 



MARIN COUNTY BREEDING BIRD ATLAS 



Bitterns and Herons 



Historical Trends/ Population Threats 

Little information is available on the historical status of this 
species throughout California, though Grinnell and Miller 
(1944) termed it "formerly abundant, now greatly depleted 
locally." Moffitt (1939a) reported that about 25 pairs of 
Black-crowned Night-Herons nested in Marin County in 
live oaks and California bays on die north end of Belvedere 
Island from at least 1918 to 1938. He expressed concern 
at that time over the fate of the colony because of recent 
nearby house building and brush clearing. That colony is 
no longer extant, and the birds probably abandoned it 
because of further human encroachment soon after 
Moffitt's report. Disturbance to colonies can, of course, 
cause abandonment (Tremblay 6k Ellison 1979). Ralph 
and Ralph (1958) observed Black-crowns breeding at the 
West Marin Island rookery, which had been active for 
"many years." It is unknown if all, part, or any of that 



colony was established by emigrants from the Belvedere 
Island colony. Pratt (p. 103 this volume) describes recent 
trends in the West Marin Island colony. On the whole, 
Black-crowned Night-Heron numbers were relatively stable 
on Breeding Bird Surveys in California from 1968 to 1989 
(USFWS unpubl. analyses). 

The Black-crowned Night-Heron was included in the 
Audubon Society's Blue List from 1972 to 1981 (Tate 
1981) and on its list of Species with Special Concerns in 
1982 (Tate & Tate 1982). It was down-listed to a Species 
of Local Concern in 1986 (Tate 1986). In addition to 
habitat loss and disturbance, organochlorine (and mer- 
cury) contamination poses a widespread threat to Black- 
crowned Night-Heron populations, including Bay Area 
colonies, but so far it has had only limited local effects on 
their reproductive success (Ohlendorf et al. 1978, 1988; 
Custer et al. 1983; Findholt 1984). 




Whether foraging or attending young, Black-crowned Night-Herons exude unwavering concentration. Photograph by Ian Tait. 



102 



Bitterns and Herons 



SPECIES ACCOUNTS 



Bitterns and Herons 



RECENT POPULATION TRENDS OF MARIN COUNTY 
HERON AND EGRET COLONIES 

Helen M. Pratt 



The history of Marin County's heron and egret colonies 
was previously chronicled by Pratt (1983). Ongoing 
monitoring of these colonies provides additional informa- 
tion on the population trends of Great Blue Herons, Great 
Egrets, Snowy Egrets, and Black-crowned Night-Herons. 
See Pratt (1983) for census methods. The recent initiation 
of a monitoring program of heron and egret colonies 
throughout much of die San Francisco Bay Area (J. P. Kelly 
pers. comm.) should provide a broader perspective with 
which to evaluate future population trends of the Marin 
colonies. 

Great Blue Heron 

Since the high of 62 pairs in 1968, the Great Blue Heron 
population at Audubon Canyon Ranch has declined by 
about 89% (Table 15). During die past 25 years, various 
other colonies were newly formed, first discovered, aban- 
doned, or have increased or decreased in size. Preceding 
the discovery of the Drake's Head and Nicasio Reservoir 
colonies, a countywide census in 1974 revealed 125 heron 
nests. In 1982, the first year after all the notable colonies 
were discovered, 107 nests were counted. In 1989, the nest 
count was 79, though additional birds were nesting then 
at Inverness Park, where poor visibility through the trees 
precluded a census. Thus the overall heron population in 
Marin County has declined since 1968, largely attributable 
to the drop in numbers at Audubon Canyon Ranch and 
the abandonment of the Nick's Cove and Schooner Bay 
colonies. Recent events at the Stafford Lake colony— die 
county's largest in 1990— may presage further declines of 
the Marin heron population. The Stafford population 
declined by 50% from 1990 to 1991. Also in 1991, all 
nests there failed because a temporary lowering of the water 
level in the lake, to enable repair of die irrigation system of 
the neighboring golf course, apparendy allowed raccoons 
to invade the colony (H.M. Pratt pers. obs.). 



Great Egret 

The Great Egret population at Audubon Canyon Ranch 
has increased since 1967 (Table 16). In two instances, 
sharp declines from one year to the next— from 85 pairs in 
1975 to 65 in 1976 and from 150 pairs in 1982 to 97 in 
1983— occurred the year following raccoon predation on 
die colony. After installation of raccoon barriers at the base 
of nesting trees prior to the 1984 nesting season, the 
Audubon Canyon Ranch population has since remained 
at a plateau of about 100 to 110 pairs. On West Marin 
Island, the Great Egret population has fluctuated widely 
since the first census there in 1979 (Table 16). 

Snowy Egret 

Like those of Great Egrets, Snowy Egret numbers on West 
Marin Island have fluctuated widely since 1979— from a 
low of 126 pairs in 1986 to a high of 500 pairs in 1982 
(Table 1 7). Such fluctuations are characteristic of Snowy 
Egrets at other colonies as well (e.g., Thompson et al. 
1979). Determinations of general population trends would 
require coordinated censuses over a wide area. 

Black-crowned Night-Heron 

Based on counts from a boat, the Black-crowned Night- 
Heron population on West Marin Island has decreased 
since the first census in 1979 (Table 17). A fire on the 
island in July 1981 may have been responsible for a decline 
from 109 nests that year to 80 in 1982. Night-heron nests 
are hidden deep widiin die coastal scrub, and probably 
most of them are impossible to see from a boat. Figures 
from these censuses may be too inaccurate to be useful. 
Recent on-site counts provide better population estimates 
(see Table 1 7). 



103 



Waterfowl 



MARIN COUNTY BREEDING BIRD ATLAS 



Waterfowl 



Waterfowl 

Family Anatidae 



CANADA GOOSE Branta canadensis 



J^\\\ 3>2*\ 


^C^\ \ jf\ 


Occurs year round, though primarily as a 
winter resident from Sep through early 
Apr. 

A rare, very local breeder, overall breed- 




r>A^oiV^J^ 


ing population very small. 
Recorded in 1 (0.4%) of 221 blocks. 






O Possible = (0%) 




prO ' \ . \^\^~^-^^\"^ r^\ C-A^^ \ ^K^^ \ ^*\ \ ^^^ \ ~^/ o 


C Probable = (0%) 




"\: \^\ Ar^V ^~^^S^\\^^\jP^C\e^^r — r ' 


• Confirmed = 1 (100%) 




0/ ^-^*^\ 2vy^-i^V^\ y^\ ^V^\ 


FSAR =1 OPI = 1 CI = 3.00 




i^^ V-^_i ^~~^^JO<-'''\ ^r^-. 





Ecological Requirements 

The echoing sounds of geese calling in flight overhead stir 
nostalgic feelings for times when California was wilder and 
untrammeled. The only native subspecies of Canada 
Goose known to breed in California is the Western, or 
Great Basin, Canada Goose, Branta canadensis moffitti (see 
Bellrose 1980). As native breeders, "Honkers" are pres- 
endy restricted in California to the Modoc Plateau, Great 
Basin Desert, and valleys of die adjoining Klamath, Cas- 
cade, and Sierra Nevada mountains (GckM 1944). There 
they breed on the marshy borders of freshwater and alka- 
line lakes, reservoirs, streams, and in extensive marshes 
and wet meadows. In recent years, birds (probably mosdy 
or entirely from plantings) have begun breeding locally in 
a wild state in the San Francisco Bay region on estuarine 
borders or islands, and at reservoirs, where they may also 
mix with domestic birds (see below). Captive birds estab- 
lished in California are largely B. c. moffitti, especially stock 
from near Reno, Nevada (M.R. McLandress pers. comm.). 
Canada Geese nest in a greater variety of sites than all 
other species of waterfowl (Palmer 1976a, Bellrose 1980). 
They sometimes give the appearance of being semicolonial 
nesters, but this seems to reflect the concentration of birds 
at limited suitable nesting sites rather than inherent social 

104 



tendencies of the species; they do sometimes nest in or 
near California or Ring-billed gull colonies (Palmer 
1976a). The nest site must be firm and dry and include 
freedom from disturbance, cover for the nest, and unob- 
structed visibility in all directions for the incubating bird 
(sometimes island nests may be in woods or under scrub). 
Nearby must be a guard site for the gander (up to 0.25 mi. 
away on open terrain), a grazing area (usually close at hand, 
but up to 1 -5 mi. away), and proximity to permanent water 
(to which the young are led). Nests are usually within a few 
feet of water, and about 90% are within 50 yards; excep- 
tionally, a nest may be 300 yards from water. Canada 
Geese nest most frequendy on islands or islets. They also 
commonly select hummocks on peninsulas, lakeshores, 
streamsides, or in marshes or fields; mats of bullrushes in 
marshes, or the tops of muskrat or beaver houses; hay- 
stacks; dikes and ditch banks; gravel bars, talus slopes, 
river bluffs, ledges, or cliffs; clusters of low scrubby growth, 
stumps, or trees (particularly in abandoned nests of her- 
ons, Ospreys, and other hawks); and a variety of elevated 
artificial structures, including washtubs, tires, wicker bas- 
kets, wooden boxes or platforms, and anchored floating 
rafts. The female forms the nest scrape in the earth or other 



Waterfowl 



SPECIES ACCOUNTS 



Waterfowl 



soft substrate by "wallowing" and collects twigs, reeds, 
weed stems, and grasses for the base and rim from, at most, 
a few feet from the nest (Dawson 1923, Palmer 1976a, 
Bellrose 1980). 

Canada Geese feed primarily by grazing in marshes, 
meadows, and fields; they forage in cultivated fields more 
often on autumn staging areas and wintering grounds than 
on spring or summer habitats. Aquatic feeding is inciden- 
tal except in coastal birds. They feed by tipping up in water, 
mainly when grazing forage is scarce, and, rarely, by diving 
from the surface (Palmer 1976a). Breeding birds feed 
singly or in flocks away from nesting territories. They feed 
mosdy during the day, but also at night during periods of 
fattening (McLandress & Raveling 1981a) or when they 
are unable to secure adequate food during daylight hours 
because of shortages or excessive disturbance while feeding 
(M.R. McLandress pers. comm.). The diet of breeding 
birds includes mainly the shoots, foliage, stems, seeds, 
roots, and rhizomes of grasses, sedges, and aquatic plants, 
berries, and cultivated grains (particularly succulent, high 
protein sprouts and mature seed heads). Insects, crusta- 
ceans, mollusks, and fish form a minor part of the diet, 
and perhaps all but the latter are consumed when attached 
to food plants. Canada Geese shift from a winter diet of 
mosdy corn or other carbohydrates to a diversity of food 
items in spring before migrating (McLandress 6k Raveling 
1981a,b). Although they still continue to eat some corn, 
protein-rich new-growth grass is important for laying on fat 
stores necessary for the migration and breeding effort. 
Females may have to obtain minerals (perhaps some from 
snail shells), and possibly protein, for egg formation from 
food sources on the breeding grounds. Goslings also need 
high-protein grass for growth (M.R. McLandress pers. 
comm.). 

Marin Breeding Distribution 

The only known breeding location for Canada Geese in 
Marin County was at West Marin Island near San Rafael. 
During the adas period, an adult was seen there from a 
boat in the spring/summer of 1982 (HPr). Goslings seen 
nearby at McNear's Beach by Point San Pedro in May or 
June of 1982 (DT) may have come from a nesting attempt 
on West Marin Island, the Sisters (small islets direcdy off 
Pt. San Pedro), or from any of a number of duck blinds 
along the shoreline to the north. Subsequendy, nesting 



was confirmed on West Marin Island by the observation 
there of a nest with eggs on 27 March 1983 (PCI) and two 
adults with two goslings on the water close to the island on 
24 May 1 983 (HPr et al.). A semidomestic flock also lived 
nearby at Peacock Gap golf course. In recent years, non- 
breeding oversummering individuals or escapees have 
occasionally been seen during the breeding season at 
various Marin County locations. A flock of up to 35 birds 
at Bolinas Lagoon in June and July each year since 1984 
may represent birds dispersing after breeding at sites else- 
where in Marin County or the San Francisco Bay Area 
(Shuford et al. 1989). 

Historical Trends/Population Threats 

The Canada Goose was first recorded nesting on the 
California coast in 1932 when two pair bred at Crystal 
Springs Reservoir, San Mateo County (Bird Lore 35:112, 
Moffitt 1939b). It is now well established as a breeder in 
small numbers at several sites around San Francisco Bay 
(Lidicker 6k McCollum 1979) and appears to be increasing 
(ABN). It seems likely that most, if not all, coastal nesting 
records pertain to birds of introduced stock for the follow- 
ing reasons: (1) the major gap in the breeding range 
between the main California population in the northeast- 
ern corner of the state and that of the recendy established 
population in the Bay Area, Suisun Marsh (1970s), the 
Delta (1980s), and Yolo County (1970s) (M.R. 
McLandress pers. comm.); (2) the strong attachment to 
traditional breeding grounds (Palmer 1976a); and (3) the 
ease with which this species adapts to captivity. It is also 
possible that a few winter residents pioneered a new 
breeding outpost on their own, perhaps as cripples from 
hunting casualties. The current coastal population may 
have originated from birds bred in captivity in the Bay Area 
in the early 1900s from eggs collected at Lake Tahoe 
(Grinnell et al. 1918); from a semicaptive flock of B. c. 
moffitti that has bred at Lake Merritt, Oakland, since at 
least 1954 (AFN 10:276, AB 27:91 3); or from captive stock 
derived from near Reno, Nevada, and released in Suisun 
Marsh, the Delta, or the Sacramento Valley. Released birds 
from captive stock from the Tahoe-Reno area seem to adapt 
to the mild conditions of Bay marshes and will probably 
continue to increase until hunters perceive numbers to be 
great enough to hunt (M.R. McLandress pers. comm.). 



105 



Waterfowl 



MARIN COUNTY BREEDING BIRD ATLAS 



Waterfowl 



WOOD DUCK Aixsponsa 













A year-round resident; numbers swell 






\ 






somewhat from Sep th rough Apr. 


rAvA 






\^A\3A\ 


[- - 


A rare, very local breeder; overall breed- 
ing population very small. 
Recorded in 5 (2.3%) of 221 blocks. 


\<5 


^A^A^ 








O Possible 4 (80%) 




p^^v 




*OVAP*fA^ 




€ Probable (0%) 




V-^V-^>^ 


^V^CvA 




V--^C " 


• Confirmed = 1 (20%) 






VA°^cA^ 


\^s(~\^\\^\ 


Or^^V" — '" r " 






Cm 


\SV\*V 


C3A\Afv3r 


3^5^^ 


FSAR =1 OPI = 5 CI = 1 .40 




J h^Z> 











Ecological Requirements 

Gaudily bedecked male Wood Ducks and their cryptic 
mates are wary recluses of the quiet waters of ponds, 
slow-flowing streams, wooded swamps, marshes, and res- 
ervoirs that provide overhanging secluding, woody vegeta- 
tion along their margins. Wood Ducks rarely venture into 
deep open or fast-flowing waters. In most cases, they seem 
to prefer smaller water bodies to larger lakes and rivers for 
nesting (Naylor 1 960). Wood Ducks are the most wood- 
land- and forest-inhabiting of our local breeding ducks. 
Expert at flying between trees or through their crowns, 
Woodies use tree trunks and branches for perches, nest 
sites, and part of their foraging beat. In Marin County, 
riparian, broadleaved evergreen, mixed coniferous, or 
coniferous forests surround the wedands used for nesting. 
Prime feeding areas are shallow (<1 ft. deep) or contain 
much floating or emergent vegetation as substrate for 
invertebrates (Drobney & Fredrickson 1979). Decompos- 
ing deciduous leaves provide excellent substrate for the 
midge larvae and other invertebrates important in the diet 
of Wood Ducks, particularly egg-laying females and grow- 
ing ducklings (M.R. McLandress pers. comm.). A diversity 
of habitats is important to Wood Ducks to provide them 
with a broad array of plant and invertebrate foods (Landers 
et al. 1977). As well as providing foraging needs, ideal 
habitat for brood rearing and summer molting should 
include a spreading brushy overstory for concealment from 
above; small open-water passages; and scattered fallen dead 
limbs, trees, stumps, exposed roots, or muskrat houses for 
perching (Palmer 1976b). 



Pairs or small groups of Wood Ducks generally feed 
from the surface in shallow water. They prefer to forage in 
wooded wedands, though they occasionally feed along 
nonwooded shorelines next to open water (Drobney 6k 
Fredrickson 1979). Foraging birds move constandy and 
rapidly, using sizable areas during the course of feeding. 
There is no evidence that birds are attached to specific 
feeding areas or that they establish feeding territories. 
Woodies forage primarily by pecking at foods on the 
surface or by surface dabbling and, infrequendy, by sub- 
surface dabbling or bottom feeding (Drobney ck Fredrick- 
son 1979). Rarely, they dive to catch fish (Palmer 1976b). 
While afloat or ashore, Wood Ducks are adept at catching 
nearby airborne insects (Palmer 1976b) and probably 
glean others from emergent vegetation, stumps, logs, and 
water margins (Landers et al. 1977). In fall and winter, they 
prefer to procure acorns and other mast from shallow 
flooded swamps and bottomlands, but they also search for 
diese foods under trees, and even among shrubbery, in 
upland forests (Palmer 1976b, Bellrose 1980). At times 
they fly into trees twined with grapevines and snatch the 
grapes from arboreal perches. Wood Ducks sometimes 
also feed in fields of corn, wheat, or other cereal grains, 
and, at least in Ohio, in farmers' hog lots. 

The diet from fall di rough early spring is about 90.2% 
vegetable matter and 9.8% animal matter (Mabbott in 
Palmer 1976b, n = 399). In South Carolina, vegetable 
foods account for over 90% of the diet in all months except 
March, when they comprise 77% (Landers et al. 1977, n = 
200). In Missouri, animal foods comprise about one-third 



106 



Waterfowl 



SPECIES ACCOUNTS 



Waterfowl 



of the diet of males in spring (n = 55) and of males and 
females in fall (n = 40) (Drobney 6k Fredrickson 1979). In 
preparation for breeding, females enter a period of 
hyperphagia, when they concentrate on protein-rich inver- 
tebrates and spend twice as much time feeding as do males 
(Drobney 6k Fredrickson 1979; Drobney 1980, 1982). 
During the breeding season in Missouri, consumption of 
animal foods by females averages about 58% (n = 60) and 
reaches a peak of 79% (n = 20) during laying (Drobney 6k 
Fredrickson 1979). Females of breeding pairs there eat 
more invertebrates, a greater diversity of invertebrates, and 
more aquatic (vs. aquatic-associated and nonaquatic) inver- 
tebrates than do males. The latter difference is perhaps 
attributable to the fact that males are more alert to their 
surroundings while foraging and therefore feed from a 
more erect posture. Hence they might be expected to feed 
more on fallen branches and tree trunks than do females, 
which feed more on or below die water's surface. The shift 
of Woodies from eating mostly aquatic invertebrates in 
spring to mosdy nonaquatic invertebrates in fall may be a 
result of changing availability (Drobney 6k Fredrickson 
1979) or perhaps more time spent foraging in upland areas 
when mast and fruit crops have ripened. The young 
initially eat almost exclusively animal matter (mostly 
insects), but by six weeks of age they have gradually 
switched to a diet comparable to that of adults (Palmer 
1976b, Bellrose 1980). 

The main vegetable fare includes the seeds and other 
parts of aquatic plants and the seeds, nuts, and fruits of 
trees; fleshy fruits may be important in summer (Palmer 
1976b, Landers et al. 1977, Drobney 6k Fredrickson 
1979). Acorns may be a particularly important food in fall 
and winter, depending on the crop. Animal foods consist 
primarily of aquatic and land insects and other inverte- 
brates. Important items are adult and larval dragonflies, 
damselflies, mayflies, midges, caddisflies, crane flies, horse 
flies, beedes, and true bugs; less important are odier 
insects, spiders, snails and slugs, isopods, crustaceans, 
and, very rarely, amphibians, fish, and mice. 

Wood Ducks nest in natural cavities of trees, in Nordi- 
ern Flicker or Pileated Woodpecker cavities more or less 
enlarged by the decay of wood, in wooden and metal nest 
boxes, in barns (in hay) and abandoned camps, and in 
hollow trees (especially fallen ones) (Palmer 1976b). In 
Merced County, Wood Ducks use natural cavities in trees, 
since woodpecker (even flicker) cavities there are too small, 
though most use wooden nest boxes erected in the last 16 
years (S. Simmons pers. comm.). Exceptionally, Wood 
Ducks prospect for nests in chimneys, or nest in crevices 
or fissures in rocks; one very unusual twig and leaf nest 
they used was supported by small branches high in a tree 
(Palmer 1976b). No nest material is added to the nest 
cavity except down (Bellrose 1980). The eggs are laid (and 
initially covered by debris) in a depression hollowed or 



scratched out by the female in the soft, dry rotted wood or 
in other bits of bark, twigs, and leaves that have fallen into 
the cavity or have been brought in by squirrels (Bent 1923, 
Dixon 1924, Palmer 1976b, Bellrose 1980). Wood Ducks 
prefer nest boxes with sawdust spread on the bottom of the 
cavity (S. Simmons pers. comm.). They also prefer nest 
trees over water or in open stands along small streams or 
ponds, though they sometimes nest in dense woodlands 
and up to 200 yards from water (Dixon 1924, Naylor 
1960, Palmer 1976b, Bellrose 1980). In Merced County, 
Wood Ducks most readily accept nest boxes close to and 
facing the water since they typically fly along waterways and 
are more likely to see nest holes so situated (S. Simmons 
pers. comm.). A preference is shown for nest boxes on 
vertical or forward-leaning trees. The height of natural 
cavities in trees ranges from 2 to 65 feet above the ground 
(Bellrose 1980). In California, 12 natural nest sites (in 
willows, cottonwoods, or valley oaks) ranged from 6 to 30 
feet above the ground, and 10 of these were below 15 feet 
(Dixon 1924). In Illinois, the height of 158 nest cavities 
ranged from 6 to 55 feet above the ground (Bellrose et al. 
1964). The average height there was 25 feet, but nests over 
30 feet were actually preferred, based on occupancy rates 
relative to availability. Wood Ducks will use nest cavities 
year after year and are most likely to use previously occu- 
pied nests. Females prefer entrance holes as small as they 
can easily pass through, and there are instances of females 
cracking an egg still in the oviduct while squeezing through 
a narrow crevice (Dixon 1924)! Frequendy, more than one 
female will "dump" eggs in the same cavity (Palmer 1976b, 
Bellrose 1980). In Merced County, up to 62 eggs have 
been dumped in a single nest box, and as many as six 
different females have laid in the same box on the same 
day (S. Simmons pers. comm.)! Egg dumping occurs there 
mostly from the middle to the end of the nesting season 
and may involve mosdy juvenile females. Only rarely will 
two female Wood Ducks incubate in the same cavity or will 
one lay jointly with another species of cavity-nesting duck 
(Palmer 1976b, Bellrose 1980). 

The day after the young hatch, the female coaxes them 
to spring out of the cavity and flutter to the ground or water 
by calling to them from the entrance cavity, a nearby limb, 
or from below. After first leading the young to water, the 
female is likely to keep moving them (Bellrose 1980). 
Females and broods may move to a series of ponds, 
traveling as much as 1 .5 miles direcdy from an initial open 
nest pond to a vegetated one. Before developing flight 
capabilities, they may journey as far as 4 miles to another 
watershed. In areas of rivers and oxbow lakes, broods may 
move an average of 1 .5 to 3 miles in die first two days after 
leaving the nest and a maximum of 6.5 miles in four days 
(Smith 6k Flake 1985). Wood Ducks sometimes produce 
two broods in a season, a rare phenomenon in North 

107 



Waterfowl 



MARIN COUNTY BREEDING BIRD ATIAS 



Waterfowl 



American waterfowl (Bellrose 1980). The production of 
two broods in a season by banded females has been noted 
repeatedly in Merced County (S. Simmons pers. comm.). 

Marin Breeding Distribution 

Although seen at scattered locations in Marin County 
during the atlas period, Wood Ducks were confirmed 
breeding only once. A female with two downy young was 
observed on 5 May 1980 at Mill Pond on the Stewart 
Ranch about two miles south of Olema (DS). Had we 
contacted the California Department of Fish and Game 
during our adas work, we undoubtedly would have con- 
firmed nesting of more Wood Ducks. Fish and Game 
initiated a pilot nest box program by erecting 1 52 boxes 
throughout the state from 1 952 to 1 956 (Naylor 1 960). Of 
the 5 boxes in Marin County, at least 2 were occupied 
"immediately" by pairs of nesting Wood Ducks; on the 
basis of a photograph in Naylor (1960:247), these were 
apparendy at Mill Pond. Of 1 2 nest boxes in Marin in the 
1960s, Wood Ducks "used" 1 at Mill Pond in 1967, 3 of 
7 along Lagunitas Creek (19??), and 2 of 4 along Olema 
Creek in 1966 (MeS fide GiT). California Department of 
Fish and Game personnel also put up 3 nest boxes at 
Nicasio Reservoir in both 1983 and 1984, but as of 1991 
it is not known if Wood Ducks have nested in diem yet. 
In the 1970s, females widi small young were seen on Pine 
Gulch Creek, on Mill Pond, and on Papermill Creek north 
of Tocaloma (EO). Adults with small young were seen at 
Five Brooks Pond in the Olema Valley from late May 
through July each year from 1987 dirough 1989 (ABN). 



Historical Trends/ Population Threats 

In Marin County, Wood Ducks formerly "nested" on 
Gallinas Creek in 1872 or 1873 and occurred along 
Papermill and Lagunitas creeks up to about 1886 (GckW 
1927). Mailliard (191 1) observed that Wood Ducks were 
"plentiful" in Marin County in the 1870s and 1880s, but 
that they were "extremely scarce" there in 1910. Grinnell 
and Wydie (1927) considered them extirpated in the San 
Francisco Bay region. Numbers also declined throughout 
California, with a low ebb in 1915; thereafter, numbers 
increased at least through 1943 (G&.M 1944, Naylor 
1960). Naylor (1960) attributed the early declines to over- 
shooting from sport hunting, to market hunting (especially 
for the valuable, highly colored feathers of the male), and 
to habitat destruction. The latter included draining and 
reclaiming marshes, dredging and mining activities, along 
with clearing of riparian vegetation resulting in the loss of 
nesting cavities. Though recent nest box programs will 
likely aid their recovery, it is doubtful that Wood Ducks 
will ever regain their former "abundance," given the his- 
tory of habitat degradation and continuing increases in 
human development and recreational uses of waterways. 
Pesticides from agricultural runoff may also be affecting the 
supply of aquatic insects that are crucial during the nesting 
season (S. Simmons pers. comm.). Numbers of Wood 
Ducks were relatively stable on Breeding Bird Surveys in 
California from 1968 to 1989 (USFWS unpubl. analyses). 




108 



Waterfowl 



SPECIES ACCOUNTS 



Waterfowl 



MALLARD Anas platyrhynchos 




A year-round resident. 

A fairly common, fairly widespread 
breeder; overall breeding population of 
moderate size. 

Recorded in 110 (49.8%) of 221 
blocks. 



O Possible 
© Probable 
• Confirmed 



43 (39%) 
10 (9%) 
57 (52%) 



FSAR = 3 



OPI = 330 CI = 2.13 



Ecological Requirements 

Mallards, our most familiar and adaptable ducks, frequent 
a wide variety of shallow freshwater ponds, marshes, sew- 
age ponds, reservoirs, slow-moving streams, and brackish 
marshes and estuaries. Mallards need wedands for resting 
and loafing, feeding, waiting, and brood rearing; they also 
require upland nesting sites with good cover (Dzubin 
1969). Wedand feeding sites are generally near to, but up 
to five miles from, nesting areas (M.R. McLandress pers. 
comm.). An important requisite for breeding is space and 
freedom from interference from conspecifics. Although the 
home range throughout the whole breeding cycle may 
include up to six to ten ponds, the pair soon localizes its 
activity to one or two ponds (or a part of a large pond). This 
waiting area is a temporarily exclusive territory from which 
the male chases intruding pairs (and pursues lone females), 
especially during the brief period (1 3-22 days) just prior to 
laying until early incubation (Titman 1983). The male 
defends his mate from other males intent on forced copu- 
lation, and he alerts her to the presence of potential 
predators, allowing her to feed with a minimum of inter- 
ruption during a period when she has special nutrient 
requirements. An important function of the waiting area is 
the reestablishment of the pair bond whenever the female 
is away from the nest (Dzubin 1969). Territories some- 
times overlap, but more than one pair are usually not seen 
at the same place at the same time; some males move 
beyond the territory once the female begins incubating. 

Breeding Mallards feed primarily in shallow water by 
tipping up for aquatic plants in marshes and for mast in 
flooded swamps. Although capable of diving for food, they 



rarely do so (Palmer 1976a). They also feed in agricultural 
crops such as corn, rice, or waste grain in stubble fields, 
especially from late summer dirough winter. In addition, 
they may obtain grain during breeding from ephemeral 
ponds in tilled land (Swanson et al. 1979). On warm 
summer nights, hens and broods may feed after dark on 
concentrations of emerging midges and mayflies (Swanson 
ck Sargeant 1972). The diet overall consists of about 90% 
vegetable matter, including the stems and, particularly, the 
seeds of aquatic plants, cultivated grains, and mast 
(McAtee in Palmer 1976a, n = 1 578). Animal matter com- 
prises 10% of the diet in the form of aquatic insects 
(caddisfly larvae, dragonflies, damselflies, predaceous div- 
ing beedes, water boatmen, mosquito larvae, and midge 
larvae), earthworms, snails, crustaceans, tadpoles, fish 
eggs, and, rarely, frogs and small fish (Palmer 1976a, 
Swanson et al. 1979). Mallards obtain a significant part of 
their energy and lipid (fat) requirements for reproduction 
at sites occupied prior to arrival on the breeding grounds 
(Krapu 1981). The protein for egg formation, however, is 
obtained principally from the diet during the nesting 
period. Females spend more than twice as much time 
foraging during the laying period as do males (Dwyer et al. 
1979). During laying, females increase their consumption 
of animal matter to about 70%; snails are important dien 
for both protein and calcium (Krapu 1979; Swanson et al. 
1979, n = 15). See Northern Shoveler account regarding 
the importance of fat reserves for egg formation. Young 

109 



Waterfowl 



MARIN COUNTY BREEDING BIRD ATLAS 



Waierfowl 



Mallards switch from an initial exclusive dependence on 
invertebrates to vegetable fare when diey are about half 
grown (M.R. McLandress pers. comm.). 

Mallards are catholic in dieir choice of nest sites but 
generally prefer upland sites to marshes (Bellrose 1980). 
Availability of fairly dense cover about two feet high 
appears to be the main requirement. Nests range from a 
few feet to as much as a mile and a half away (Palmer 
1976a, Bellrose 1980). Reports of most nests being within 
100 to 350 yards of water (Dzubin 6k Gollop 1972, Palmer 
1976a, Bellrose 1980) are likely an artifact of extensive 
research in the pond-studded prairie pothole regions of the 
U.S. and Canada. At Grizzly Island in the Suisun Marsh, 
nest densities are highest about one-half mile (or as far as 
possible) from water (M.R. McLandress pers. comm.). The 
distance of nests from the water is related not only to the 
dispersion of water, but also to the availability of nesting 
cover close by and the intensity of harassment of females 
by males as a function of population density (Dzubin & 
Gollop 1972). Because of these considerations, the closest 
water to the nesting site is not necessarily the pond used as 
a waiting site or the pond to which the brood is moved at 
hatching. 

Typical nest sites include weed fields, hayfields and 
pastures, grain stubble fields, grassy and weedy edges of 
roadsides, weedy and brushy levee and ditch banks, dense 
marsh vegetation (sometimes over water), small islands, 
under piles of brush, under fallen logs in dense brush, 
under clusters of trees, and, less frequendy, in heavy timber 
at the base of a large tree (Bent 1923, Palmer 1976a, 
Bellrose 1980). Atypical sites include tree crotches up to 25 
feet above the ground, on tree limbs, in hollows of trees, 
on stumps, on muskrat houses or fallen logs, in old magpie 
nests, on the understructure of a bridge, and in a box on 
a barn roof. Mallards also accept artificial nest baskets, 
especially those located three to four feet over water in areas 
free of heavy vegetation (Bellrose 1980). The female forms 
the nest bowl in plant litter (already in place or gathered at 
the site) or in moist earth and adds pieces of marsh plants, 



weeds, and grasses reachable from the nest. Most nests 
have a distinct canopy or cover, either natural or bent over 
die nest by the female (M.R. Mcl^andress pers. comm.). 
After hatching, the female may lead the precocial young 
overland to more than one body of water; distances trav- 
eled by hens and broods may be up to three and a half 
miles in two days, and five miles in nine days (Dzubin 6k 
Gollop 1972, Palmer 1976a, Bellrose 1980). In large 
ponds and marshes, several stretches of shoreline are used 
during this period. The factors responsible for brood 
movement are unclear (Dzubin ck Gollop 1972). They 
probably involve searching for favorable food and cover 
and moving to more permanent ponds and may be influ- 
enced by drying of wetlands, food shortage, and distur- 
bance. 

Marin Breeding Distribution 

During the adas period, the Mallard was the most numer- 
ous and widespread duck nesting in Marin County. It bred 
at freshwater sites scattered throughout the interior, at 
brackish marshes along the coast, and on the San Fran- 
cisco and San Pablo bayshores. In urban-suburban set- 
tings, the breeding status of free-flying birds can be difficult 
to establish because of the presence of domestic stock and 
their hybrids and of "self-tamed," unconfined wild birds 
on the same ponds with truly wild stock. Fortunately, these 
considerations did not pose problems in establishing 
breeding in particular blocks because wild Mallards were 
such widespread breeders. Representative nesting loca- 
tions were a pond by the beach at Limantour Estero (FL 
4/8/80 — DS); Bahia Drive ponds near the Petaluma River 
mouth, Novato (NE 6/7/80 — DS); and Nicasio Reservoir 
(FL 7/18/82 -DS). 

Historical Trends/ Population Threats 

In the period 1968 to 1989, populations of breeding 
Mallards were increasing in California (Robbins et al. 
1986, USFWS unpubl. analyses). 



110 



Waterfowl 



SPECIES ACCOUNTS 



Waterfowl 



NORTHERN PINTAIL Anas acuta 







Occurs year round, though primarily as a 






winter resident from late Aug through 


j\^^%^ 


early Apr. 


N- \s*\'\ \f^ 


\\\2^XX^c^\^^^i- - 


An uncommon, very local breeder; 


r^VVrC 


\^\ \^\ \<v\^\ \^\\^Kio\ 


overall breeding population very small. 




-"\ Jr\ jx^X it^-K- \-<Z\ V^V o V"^ \ 


Recorded in 14 (6.3%) of 221 blocks. 




^^^X^^X-A^Cl^r^A^A^x^^^CsA^-^ 


O Possible = 8 (57%) 




i^y^F^^^^ 


© Probable = 2 (14%) 






• Confirmed = 4 (29%) 




^J^V^^^ 


FSAR = 2 OPI = 28 CI = 1.71 









Ecological Requirements 

These sleek, elegant ducks characteristically nest in open 
country that contains many scattered small bodies of water 
(Palmer 1976a)— typically shallow, temporarily flooded 
basins harboring an abundance of aquatic invertebrates for 
foraging and brood-rearing habitat (Krapu 1 974a). Ephem- 
eral spring ponds, vernal pools, and flooded uplands are 
particularly attractive (M.R. McLandress pers. coram.). 
Apparent adaptations by Pintails to the use of ephemeral 
wedands are smaller clutches, shorter incubation periods, 
and shorter fledging periods than Mallards and Gadwalls 
(Bellrose 1980, M.R. McLandress pers. comm.). Surpris- 
ingly, open flooded areas are used, seldom near trees. In 
Marin County, breeding Pintails mosdy frequent wedands 
in or near extensive brackish marshes along the bayshore, 
though they sometimes also use inland bodies of water. 
How the niches of the Mallard and Gadwall differ from 
that of the Pintail in Marin County is not clear, though 
Pintails are fussier in their choice of wedands and appear 
to choose larger, but shallower bodies of water. Compared 
with other dabbling ducks, Pintail drakes are highly mobile 
and least attached to their waiting sites or "activity centers" 
(Palmer 1976a). Drakes will chase other females and loaf 
or feed with other drakes while their mates are at their 
nests. With this low level of aggression, many individuals 
can use preferred ponds. 

Pintails prefer to feed in very shallow water at the surface 
or by tipping up (Palmer 1976a). Rarely, large numbers of 
Pintails will dive for preferred foods (Miller 1983). Their 
long necks and tipping feeding style enable them to sift 
seeds and benthic invertebrates from the detritus and 



sediments on or near the bottoms of ponds (Krapu 1974a). 
They also pick seeds or invertebrates from concentrations 
on the water's surface (Krapu 1974a, Euliss & Harris 
1987). While foraging, they tend to spend more time 
tipping up than moving, then move quickly over short 
distances between tip-ups (Eadie et al. 1979). Pintails also 
feed extensively in rice, corn, and stubble fields on waste 
grain, though primarily in fall and winter (Bellrose 1980). 
Pintails often feed in both agricultural fields and wedands 
at night (Euliss & Harris 1987, Miller 1987). 

The diet in North America is about 87.2% vegetable 
and 12.8% animal (Mabbott in Palmer 1976a, n = 790), 
but percentages vary and animal matter sometimes pre- 
dominates geographically or seasonally. For example, in 
die Central Valley the fall diet of Pintails is 97% or more 
vegetable matter, but by mid- to late winter it may be about 
30%-85% animal matter (Connelly ck Chesemore 1980, 
Euliss &. Harris 1987, Miller 1987). Vegetable fare con- 
sists largely of the seeds of pondweeds, sedges, alkali 
bulrush, grasses, smartweeds, and several species of "moist 
soil plants" such as brass buttons, fat hen, swamp timothy, 
and purslane (Palmer 1976a, Miller 1987, M.R. 
McLandress pers. comm.). Animal fare includes various 
aquatic insects (midge larvae, water boatmen, caddisfly 
larvae, dragonfly and damselfly larvae and nymphs, preda- 
ceous diving beetles, mosquito larvae), earthworms, snails 
and other mollusks, crustaceans (brine shrimp, crabs, 
crayfish, shrimp), and, very rarely, miscellaneous items 
such as frogs and fish (Krapu 1974a,b; Palmer 1976a; 
Swanson et al. 1979; Miller 1987). Pintails arrive on the 



111 



Waterfowl 



MARIN COUNTY BREEDING BIRD ATEAS 



Waterfowl 



breeding grounds with large fat reserves necessary for the 
reproductive effort (Krapu 1974a). In North Dakota, 
breeding female Pintails in insect-rich habitats consume 
79% animal matter compared with 30% by males (Krapu 
1974b). Overall in various habitats, females consume 60% 
animal foods, reaching a peak of 77% during laying, when 
there is the greatest need for calcium (from snails) and 
protein. See Northern Shoveler account regarding the 
importance of fat reserves for egg formation. 

Pintails choose relatively dry, open nest sites with low or 
sparse vegetation and generally nest farther from water 
than other ground-nesting ducks (Duncan 1987). In 
Alberta, most nests are from 0.6 to 1.2 miles from water, 
with some as far as 1 .9 miles from water; many nests are 
also on pond edges or on islands. The low average dis- 
tances of nests from water in other studies (about 200 yds. 
or less) may be because nest searches were conducted close 
to water (Duncan 1987) or because studies were in areas 
with extensive pond systems, where it is difficult for birds 
to nest far from water (M.R. McLandress pers. comm.). 
Female Pintails apparendy nest far from water to reduce the 
probability of predation, rather than to avoid harassment 
by males near ponds as some authors have suggested 
(Duncan 1987). Spacing in available upland areas seems 
to be more important than vegetation for nest site selection 
(M.R McLandress pers. comm.). Although Pintails may 
select nest sites in bare earth, they more often choose sites 
near, or in, some vegetation, such as weeds, grasses, brush 
clumps, low willows, or beds of marsh plants (Palmer 
1976a, Bellrose 1980). They generally avoid timbered or 
extensive brushy areas. In the brackish Suisun Marsh, 
Pintails often nest in the cover of pickleweed two to diree 
feet high (M.R. McLandress pers. comm.). More than 



other species of waterfowl, Pintails use farmland habitats 
such as stubble fields, hayfields and pastures, roadsides, 
fallow fields, grain fields, and field edges for nesting 
(Palmer 1976a, Bellrose 1980). Females lay their eggs in a 
natural or hollowed-out depression lined with grass, bits of 
straw, weed stems, leaves, sticks, or mosses mixed with 
down (Bent 1923). Pintail hens usually lead their broods 
farther overland to water than other puddle ducks, and 
frequently from one pond to another (Bellrose 1980). 

Marin Breeding Distribution 

During the adas period, most nesting Pintails in Marin 
County were found in wedands along the San Francisco 
and San Pablo bay shorelines. Representative nesting local- 
ities included Spinnaker wedands, San Rafael (FL 5/6/80 
— DS); Bahia Drive ponds near the Petaluma River mouth, 
Novato (FL 6/1 6/79 & 6/14/80 -Gil); and fish-breeding 
ponds near the Cheese Factory, Hicks Valley (FL 6/21 &. 
7/1 4/82 — DS, ScC). The presence of a pair of birds at a 
freshwater pond at the head of Home Bay, Drake's Estero, 
on 1 7 June 1981 (DS) suggested that Pintails may occasion- 
ally breed along the outer coast of Marin County. 

Historical Trends/ Population Threats 

Formerly, Pintails were not known to breed in Marin 
County (GekW 1927, S&P 1933, G&lM 1944), but they 
were probably overlooked because of limited observer 
coverage since small numbers were known to breed then 
around San Francisco Bay (G&.M 1944). Numbers of 
Northern Pintails were relatively stable on Breeding Bird 
Surveys in California from 1968 to 1989 (USFWS unpubl. 
analyses). 



112 



Waterfowl 



SPECIES ACCOUNTS 



Waterfowl 



BLUE- WINGED TEAL Anas discors 







Occurs year round, though primarily as a 






spring transient from late Jan through 






Jun (mostly Apr-Jun). 


^K\^\^^^\^3^^C^ 




A very rare (perhaps rare), very local 
breeder; overall breeding population very 


Y^O\ \zVA^T \^-^\ \ *a** , "r \ ^-^\'' \"~><* 


^Vw^cA 


small. 


\^S^^^k\^\v\\. 


^\3r\ 3r^C^V^_ 


Recorded in 9 (4.1%) of 221 blocks. 


\ uyf^A ■ i^-^^-^^^i c-\^\ 


^^p^3^^ 


O Possible 8 (89%) 


^pV^V^T^V^^^Or 




C Probable = 1 (11%) 


IS^^Tr^vV 


• Confirmed = (0%) 




-^^^^^^? (> 


FSAR=1 OPl = 9 CI = 1.11 









Ecological Requirements 

This dapper eastern counterpart of the Cinnamon Teal 
occurs irregularly in the breeding season in Marin County 
in shallow, marshy-edged ponds, in freshwater marshes, in 
slow-moving streams and sloughs, and, sparingly, in brack- 
ish water impoundments. The drake defends both his 
waiting site and his mate (Palmer 1976a). Both the hen and 
the drake stay within a circumscribed site, which may 
contain more than one water area; the drake prefers to wait 
at the one nearest the nest. Other nearby areas are used for 
communal feeding. Blue-winged and Cinnamon teal over- 
lap broadly in habitat use and are often found in each 
other's company. Both species prefer to feed in very shal- 
low water, usually with much emergent or floating vegeta- 
tion (Bent 1923, Palmer 1976a). Although bodi species 
feed frequendy in both open water and among emergent 
vegetation, Blue-wings feed more often in open water than 
Cinnamons do; both species feed to a limited degree on 
mudflats on pond edges (Connelly & Ball 1984). At times, 
Blue-winged Teal feed over deep water on emerging aquatic 
insects or on invertebrates using the substrate of vascular 
plants that extend to the surface (Swanson et al. 1974). 
Blue-wings sometimes gather to feed at night on emerging 
insects that concentrate in large numbers on die water's 
surface at the approach of darkness. Bodi of these teal 
species feed mosdy near the surface by prolonged immer- 
sion of part or all of the bill below the water's surface (eye 
above water) or by picking items off the surface. To a 
limited degree, they feed by immersing the head past the 
eye or by tipping up, but they seldom dive (Swanson et al. 



1974, Palmer 1976a, Connelly & Ball 1984). They also 
visit grain fields in the fall (Bent 1923). 

The Blue-winged Teal diet year round is about 70% 
vegetable matter and 30% animal matter (Mabbott in 
Palmer 1976a, n = 319). Vegetable food consists mainly of 
seeds of grasses and sedges, and seeds, stems, and leaves 
of pondweeds. Animal foods include aquatic insects 
(midge larvae, caddisfly larvae, nymphs of damselflies and 
dragonflies, predaceous diving beedes, water boatmen, 
mosquito larvae), mollusks (mosdy snails), various small 
crustaceans, and a few spiders, water mites, and, very 
rarely, fish (Swanson et al. 1974, 1979; Palmer 1976a). As 
with other dabbling ducks, dietary changes occur season- 
ally. Animal matter is particularly important in die breed- 
ing season when it can amount to 89% (increasing from 
45% in spring to 95% in summer) of die diet of males and 
females combined (Swanson et al. 1974, n = 107). At that 
time, females feed more intensely and consume more 
animal matter (more snails, less crustaceans) than do 
males. In North Dakota, consumption of animal matter 
(especially midge larvae and snails) by females (n = 20) 
peaks at 99% of the diet during laying, when calcium and 
protein needs are high for egg formation (Swanson et al. 
1974, 1979; Krapu 1979). See Northern Shoveler account 
regarding die importance of fat reserves for egg formation. 
The Blue-winged Teal diet also varies considerably 
between local habitats (Swanson et al. 1974). Compared 
with Green-winged Teal, Blue-wings feed more on animal 
matter and more heavily on vegetative parts of plants than 
on seeds (Bellrose 1980). Relative to Shovelers, Blue-wings 



113 



Waterfowl 



MARIN COUNTY BREEDING BIRD ATLAS 



Waterfowl 



eat larger crustaceans (Swanson et al. 1974). Unlike other 
dabbling clucks, Blue-winged Teal feed on amphipods tbat 
concentrate on the terminal buds and other parts of 
vascular plants. 

Blue-winged Teal typically nest in dry sites widi fairly 
tall, dense grass, in sedge meadows, in brackish marshes 
of cord grass and salt grass, or in hay or alfalfa fields (Bent 
1923, Palmer 1976a, Bellrose 1980). They also occasion- 
ally nest on soggy ground, on islands, in dense cattail 
growth, and in cavities in and upon muskrat houses. Nest 
sites range from the water's edge up to a mile away, but 
average roughly 125 feet from water (Bellrose 1980). The 
nest may be set well into a dense clump of rank grass on 
the surface or may be sunk in a cavity flush with the ground 
(Bent 1923). The nest bowl is lined with fine dead grass 
(less frequendy with cattail blades or other wedand vegeta- 
tion in damp places) and the obligatory down; growing 
grass often arches over the nest cavity. Females with broods 
may travel overland about 100 to 1600 yards (maximum 
2.25 mi.) from the nest site to water but once established 
are more likely to remain at a site than are many other 
species of ducks (Bellrose 1 980). 

Marin Breeding Distribution 

Blue-winged Teal occur in Marin County in small num- 
bers most years in May and June (DS), but die species has 
not yet been adequately documented to breed here. Docu- 
mentation is difficult to obtain because of the species' 
relative scarcity in Marin, the great similarity of female 
Blue-winged Teal and the much commoner Cinnamon 
Teal, and the tendency of these two species to hybridize in 
the wild (Harris ck Wheeler 1965; relatively frequent 
sightings of males showing apparent hybrid characters 
— ABN). During the adas years, we obtained suggestive 
evidence of nesting by observations of a female Blue- 
winged/Cinnamon-type female and downy young accom- 
panied by a male Blue-winged Teal on Americano Creek 
on the Marin/Sonoma County border near Valley Ford, 
Sonoma County, on 10 June 1977 (CJP), and at the Bahia 
Drive ponds near the Petaluma River mouth, Novato, on 
14 June 1980 (GiT) (but see comments below). During the 
adas period, there were also scattered sightings of male 
Blue-winged Teal on Point Reyes and in wedands along the 
San Pablo and San Francisco bay shorelines. 

Adasers in California should be very cautious about 
interpreting the significance of observations of male Blue- 
winged Teal accompanying female Blue-winged/Cinna- 
mon Teal and young. First of all, Blue-wings likely breed 
one to two months later than Cinnamon Teal (M.R. 
McLandress pers. comm.). Secondly, male Cinnamon 
Teal attend females with young only infrequendy (5%- 



10% of broods), and Blue-winged Teal probably do so 
even less frequently since they are one of the northern 
ducks, which tend to have weak pair bonds. Blue-winged 
Teal males in these cases may be ready to mate, but the 
hens were likely mated earlier in the season to Cinnamon 
Teal males, hence the offspring would be Cinnamon Teals 
(M.R. McLandress pers. comm.). Even the observation of 
a carefully identified Blue-winged Teal female with young, 
even if attended by a male Blue-winged Teal, is not positive 
proof of nesting of this species because of the uncertainty 
of parentage. Nesting of Blue-winged Teal in areas of 
marginal occurrence of that species within the heart of the 
range of the Cinnamon Teal can probably best be consid- 
ered valid only after a number of carefully identified Blue- 
winged Teal females have been seen with broods, thus 
lessening the likelihood that all sightings represented off- 
spring of mixed-species pairs. 

Historical Trends/ Population Threats 

Grinnell and Miller (1944) did not consider the Blue- 
winged Teal a breeding species in coastal northern Califor- 
nia, although they did list a 21 June record for Areata, 
Humboldt County. Recendy, McCaskie et al. (1979) con- 
sidered the species a rare to uncommon breeder on the 
northern California coast. This reported change in status 
may simply be the result of more thorough recent coverage 
of this region, but odier evidence suggests a possible range 
expansion. Wheeler (1965) and Connelly (1978) reported 
that from the 1930s to the 1960s, Blue-winged Teal pio- 
neered new breeding areas and increased in numbers on 
the Pacific Coast, especially north of California. On the 
other hand, the species' notorious tendency to abandon 
drought-stricken areas to pioneer newly available habitat 
far from the center of its breeding range (Bellrose 1980) 
perhaps explains periodic influxes that occur in Marin 
County and elsewhere along the northern California coast 
that might be interpreted as a true range expansion. For 
example, in May and June of 1980, one observer (DS) saw 
23 Blue-winged Teal in Marin County compared with 
about 1 to 4 birds per year in several other years, with 
roughly equivalent time spent in the field. Similarly, John- 
son and Yocum's (1966) report of a ratio of 42 Blue- 
winged Teal males to 36 Cinnamon Teal males at Lake 
Earl, Del Norte County, from 1 June to 20 July is certainly 
not typical of most years since Yocum and Harris (1975) 
considered Blue-wings to be rare breeders in that region. 
Numbers of Blue-winged Teal on Breeding Bird Surveys 
in California did increase from 1968 to 1989 but were 
relatively stable from 1980 to 1989 (USFWS unpubl. anal- 
yses). 



114 



Waterfowl 



SPECIES ACCOUNTS 



Waterfowl 



CINNAMON TEAL Anas cyanoptera 







Occurs year round, though primarily as a 


A^l^>^ N «~i 




spring transient from Jan through May 


^V^TvJrt^Ow^^-dV- 


^^voj^ 


(especially Mar and Apr) and secondarily 


x^^^Pc^^^X^coA 




as a summer resident from May through 


r^Swr^C^\\%^\^r 


A>VO\ 


Sep. 


\VSQcM^^ 


YSkcCi 


A fairly common, local breeder; overall 


\^\><v» V-^\ h^\ \/f \yf I 
KNjiA^iM _A"\ Jr\ 3f\ 




breeding population small. 




r^\ A^A^W— 


Recorded in 52 (23.5%) of 221 blocks. 




'lA^x^A^^v^x 


O Possible = 23 (44%) 




^\ujV\°JV\^^\ 3t^^v 


-V"s J^c^\ ^Vfe\ — -v 


© Probable = 6 (12%) 




^w3^5y\3r\3r€ 




• Confirmed = 23 (44%) 












Jj>^ \^7 ^^LA^>K 


FSAR = 3 OPI = 156 CI = 2.00 



Ecological Requirements 

The stunning Cinnamon Teal drakes and their cryptic 
mates are studies in contrast in the freshwater and brackish 
ponds and marshes they inhabit. Their lifestyles, habitat 
preferences, and foraging methods are so similar to those 
of their primarily eastern congeners, the Blue-winged Teal, 
that the reader is referred to the descriptions in the preced- 
ing account. The Cinnamon Teal diet is roughly 80% 
vegetable matter, mosdy seeds and other parts of sedges, 
pondweeds, and grasses; and 20% animal matter, divided 
about equally between aquatic insects (beedes, true bugs, 
damselflies, dragonflies, larval midges, etc.), and mollusks 
(snails and small bivalves) (Martin et al. 1951, n = 59; 
Mabbott in Palmer 1976a, n = 41). Like other dabbling 
ducks (see accounts), Cinnamon Teal appear to change 
their diet seasonally and consume more animal matter in 
summer (especially females). 

Cinnamon Teal usually select nest sites in dense vegeta- 
tion, such as grasses, weeds, bulrushes, and sedges in 
uncultivated land, meadowlands, marshes, swales, and 
grain fields, and on dikes and islands (Bent 1923, Dawson 
1923, Palmer 1976a, Bellrose 1980). Females usually 
scratch out a shallow depression on dry land that they line 
with dead grasses and plant stems. They also frequendy 
establish marsh nests and, more commonly than other 
species of dabbling ducks, nests over water. Marsh nests 
are bulkier baskets or platforms made of dried cattails, 
sedges, or marsh grasses. These nests may be under dense 
matted vegetation of the previous year's growth, may be 



reached by a tunnel burrowed out by the female, or may be 
suspended over water in emergent vegetation. If threatened 
by rising water levels, females sometimes raise ground 
nests by adding materials. Although nests range from 
direcdy over water to 220 yards away from it, most are 
widiin 75 yards of water. The best brood habitats are small 
bodies of water, such as ponds, ditches, and canals, with 
plentiful submerged aquatic vegetation for feeding and 
emergent vegetation for protective cover (Bellrose 1980). 
The hen may move broods as far as a mile in a few days, 
but, if cover and feeding conditions are good, their activi- 
ties may involve only a few acres (Palmer 1976a). 

Marin Breeding Distribution 

During the adas period, the Cinnamon Teal, next to the 
Mallard, was the second most widespread breeding duck 
in Marin County. Although some breeding Cinnamon 
Teal were scattered throughout the county, most were 
concentrated in coastal and bayshore wedands. Represen- 
tative breeding locations included the pond at the Drake's 
Beach visitor's center, PRNS (FL 5/27/80 -DS); McGinnis 
Park, San Rafael (FL 6/3/80 -DS); and Laguna Lake, 
Chileno Valley (FL 6/28/81 -DS). Numbers of Cinna- 
mon Teal in early spring belie actual breeding abundance 
since there is a peak migratory period from February 
through April with numbers falling to summer levels by 
May (Shuford et al. 1989). 



115 



Waterfowl 



MARIN COUNTY BRHHDING BIRD ATLAS 



Waterfowl 



Historical Trends/Population Threats 

Cinnamon Teal were not reported historically from Marin 
County as breeding birds (G&W 1927, SckP 1933, 
G&.M 1944), probably because of limited observer cover- 
age or an author's impression that such records lacked 



regional significance. Numbers of Cinnamon Teal were 
relatively stable on Breeding Bird Surveys in California 
from 1968 to 1989 (USFWS unpubl. analyses). 



NORTHERN SHOVELER Anas clypeata 









Occurs year round, though almost exclu- 




^>-x^^ \ jr^i. 




sively as a winter resident from late Aug 


_/ ^^Ai^'A -t>**k 






through Apr. 


r^Vv^n 


\r^\ Jk^\°y<r\ V^\OrC 


-••Vb V\/ s 


A very rare (perhaps now rare), very 
local breeder; overall breeding popula- 




rX^Ofv^rS^v 




tion very small. 


\d~^ 


Of <: v\ >-V^\ ^-V^\ jV^\ J^<^\ 


r\ V^A°A-^ 


Recorded in 4 (1.8%) of 221 blocks. 




^^^_V^V^^Va> 


WAj^AprC^^r^^ 






m^\ . '^^"^-^V^A" C^\^\ 3<*^\ 


^^V^ \ ^^^\-J^y oo 


O Possible 4 (100%) 




^C^rC^S^r^wV^ 


^^^^V^v^v <- 


€ Probable = (0%) 




^Ps^?>^ 


p^^O^^Jo 


• Confirmed = (0%) 








FSAR =1 OPI = 4 CI = 1 .00 




i >^=> s , 







Ecological Requirements 

With such an outsized bill, one might expect the "Spoon- 
bill" to ply its trade in waters of a different ilk, but, like 
many of the dabbling duck clan, its breeding haunts are 
shallow ponds and open marshy areas with shallow water- 
ways and abundant aquatic vegetation (along widi sur- 
rounding dry meadows). Soft, slimy mud is another 
requisite (Palmer 1976a). Shovelers do not appear to care 
if the water is clear, clean, muddy, flowing, or stagnant; and 
they nest near fresh, alkaline, brackish, or estuarine waters. 
Breeding Shovelers are the most territorial of the prairie- 
nesting dabbling ducks, presumably because of their need 
to secure high-quality feeding sites for reproduction 
(Ankney ck Afton 1988). Shovelers select a home range 
with a "core area or "waiting area,' a nest site, and several 
(3-13) "peripheral" ponds (Poston 1974)- Pairs spend 
60%-90% of dieir time on the core area, which appears to 
supply the basic resources for nesting and pair isolation. 
The core area is a restricted portion of the home range, 
such as a pond, a section of a pond, or several adjacent 
small ponds. It contains a loafing area (the waiting area) 
and feeding areas. The male waits at the core area for the 
return of the female during incubation and defends it 



against other intruding Shovelers at all times from the 
onset of laying until the waning of the pair bond or until 
the female hatches the brood, whichever comes first. Home 
ranges overlap, with neighboring pairs sharing "neutral" 
areas of nesting cover, peripheral ponds, and sometimes 
core areas in the absence of the residents. Nest sites are 
usually close to core areas but may be at some distance. 

The Shoveler's large, spatulate bill with well-developed 
lamellae is designed for filter feeding. Shovelers often feed 
in very shallow water, continuously moving slowly about 
with their heads and necks partly or fully submerged. Thus 
engaged, they sweep their bills from side to side, skimming 
above the bottom or filtering the water to obtain small 
animal life and seeds (Palmer 1976a, Bellrose 1980). A 
number of Shovelers will feed socially in this manner, 
paddling rapidly together in a circular or elliptical orbit on 
the water, apparently stirring up the plankton-laden waters 
and straining it through the lamellae of the bill. In breed- 
ing areas in Alberta, Shovelers prefer to feed in the shal- 
lows of ponds containing submergent and surface 
vegetation (Poston 1 974). They also feed in deeper water 
than most dabbling ducks do, by swimming along, with 



116 



Waterfowl 



SPECIES ACCOUNTS 



Waterfowl 



the bill slightly submerged, skimming and filtering the 
surface waters (Palmer 1976a, Bellrose 1980). Shovelers 
gather in large numbers to feed in this manner on sewage 
ponds. They also filter below the surface by tipping up, but 
infrequendy compared with other dabblers; they seldom go 
to fields to forage. Like most others dabblers, they rarely 
dive. On warm summer nights, hens and broods feed after 
dark on emerging midges and mayflies and on water fleas 
making vertical migrations to the water's surface (Swanson 
6kSargeantl972). 

The diet of North American birds (fall to spring) is 
roughly 66%-72% vegetable matter, including seeds and 
soft parts of grasses, sedges, pondweeds, waterlilies, algae, 
and smartweeds. The remaining 28%-34% of the diet is 
animal matter, especially mollusks (mostly freshwater uni- 
valves), insects (water boatmen, backswimmers, water 
tigers, dragonfly nymphs, flies, and caddisfly and mayfly 
larvae), small crustaceans (ostracods, copepods), and cray- 
fish and fish (probably rare); in certain areas, animal matter 
may predominate in the diet (Martin et al. 1951, n = 101; 
McAtee in Palmer 1 976a, n = 70). The bill is particularly 
well adapted for feeding on microscopic phytoplankton 
and zooplankton, but these food items are rapidly digested, 
hence their importance is undoubtedly underestimated in 
many diet studies (Bellrose 1980). In one study of post- 
breeding Shovelers, their diet was 78% animal matter, of 
which 90% was zooplankton (Dubowy 1985). Like other 
dabbling ducks, Shovelers change their diet seasonally and 
rely more on animal matter in summer. In North Dakota, 
females (n = 1 5) consume 99% animal matter (mosdy 
microcrustaceans and small snails) during laying when the 
need is high for protein and calcium for egg formation 
(Swanson et al. 1979, Krapu 1979). Ankney and Afton 
(1988) reported that prelaying (n = 14) and laying (n = 23) 
female Shovelers in Manitoba ate over 90% animal matter, 
again mosdy snails and crustaceans. They concluded that 
while protein intake is important to nesting birds that the 
size of fat reserves is the factor limiting clutch size of 
Shovelers and probably also other dabbling ducks, such as 
Wood Ducks and Mallards. During the prelaying period, 
male Shovelers ate a similar proportion of animal matter 
to that consumed by females but only 67% (n = 10) during 
the laying period. 

Northern Shovelers usually nest in dry upland sites, 
sometimes in moist meadowland, and, rarely, in wet 
marshes. Nesting cover is typically grasses (especially short 
varieties and salt grass), sometimes hay, and, rarely, weeds, 
bulrushes, sedges, or woody vegetation, such as willows, 



poplars, or rosebushes; concealment can be minimal, 
especially early in the season (Bent 1923, Dawson 1923, 
Poston 1974, Palmer 1976a, Bellrose 1980). The nest is a 
hollow, lined with dead grasses, weeds, or broken reeds, 
and, of course, down. Apparendy females sometimes re- 
locate nests to higher ground when threatened by rising 
waters (Poston 1974). Although they may range up to a 
mile from water, most nests are from about 75 to 300 feet 
from water; nests immediately adjacent to water are usually 
on islands or levees (Poston 1974, Palmer 1976a, Bellrose 
1980). In the prairies of Canada, broods seldom remain 
on one pond longer than seven to ten days, and they may 
move with the hen up to a mile through a series of ponds 
in about two weeks (Poston 1974). Younger broods are 
kept in shallow shoreline areas near emergent vegetation; 
older broods prefer larger bodies of permanent water 
(Palmer 1976a). 

Marin Breeding Distribution 

During the atlas period, we obtained circumstantial evi- 
dence of breeding via sightings of a male at McGinnis 
Park, San Rafael, on 3 June 1980; a male at the Bahia Drive 
ponds near the Petaluma River mouth, Novato, on 7 June 
1980; two to three birds on outer Point Reyes from 16 to 
18 June 1981; and a female on Laguna Lake, Chileno 
Valley, on 19 July 1982 (all DS). Subsequendy, breeding 
was confirmed at the Las Gallinas sewage ponds, San 
Rafael, with the observation of a female with downy young 
from 28 May to 7 July 1985 (DT, CLF, DAH); Shovelers 
also bred there at least in 1986, 1987, and 1988 (ABN). 

Historical Trends/ Population Threats 

Formerly, Northern Shovelers were not known to breed in 
Marin County (GckW 1927, S&P 1933, G&M 1944), 
but they may have gone undetected because of limited 
observer coverage since small numbers did breed dien 
nearby on San Francisco Bay (G&W 1927, GckM 1944). 
Recent increases in the number of breeding Shovelers, and 
dieir broods, at Grizzly Island Wildlife Area, Solano 
County, have coincided with increases in the number and 
total acreage of summer ponds following extensive flood- 
ing in 1983 and subsequent management for summer 
water (M.R. McLandress pers. comm.). Shoveler numbers 
increased on Breeding Bird Surveys in California from 
1968 to 1989 but were relatively stable from 1980 to 1989 
(USFWS unpubl. analyses). 



117 



Waterfowl 



MARIN COUNTY BREEDING BIRD ATI AS 



Waterfowl 



GAD WALL Anas strepera 







Occurs year round, though primarily as a 




^V^r-^ K ^C2>u. 


winter resident from late Aug through 


-V^\\> ;; \^ 




Apr. 






A rare, very local breeder; overall breed- 




y<c\ JV\ \^\^<r\ yr\\<r\^^\ 


ing population very small. 


\\^<r\ 


\\\\^Qx^D<^ 


Recorded in 10 (4.5%) of 221 blocks. 




^\^^\ 3rV \±\. \^\ y<^\ \^\°\~-~^ 


O Possible 6 (60%) 




n'^\^^^3^^r^v^^\^^V^\^-^^^\^^A\>-^ ; ^ •° 


€ Probable (0%) 




^\ 3r^\ A^V^ '■'"•J^\Vi Jf^/V "\-^\ Jt-^v jr-^^>— "*"" 


• Confirmed = 4 (40%) 






FSAR=1 OPI = 10 CI = 1.80 









Ecological Requirements 

The Gadwall's subde yet striking beauty graces Marin 
County's shallow freshwater ponds and marshes, sluggish 
streams, and brackish marshes, which abound in sub- 
merged aquatic plants. During the breeding season, it 
avoids woods or thick brush (and waters widi such bor- 
ders) and, along with the Shoveler, uses alkaline or brack- 
ish waters more than other dabbling ducks do (Palmer 
1976a). Breeding pairs often use areas that include a broad 
stretch of open water as well as small ponds. Like odier 
dabbling ducks (see accounts), Gadwall usually occupy a 
home range with a nest site, waiting area, and feeding areas 
that may overlap with those of neighboring pairs; aggres- 
sion of the male centers around a smaller defended area or 
just the mate (Gates 1962, Dwyer 1974, Palmer 1976a). 
When nesting densities are very high on islands, intense 
aerial-pursuit activity causes territorial defense of a female 
or section of habitat to break down (Duebbert 1966). 
Under these conditions, hens leaving nests for relief peri- 
ods may be forced to fly more than a mile to feed and rest 
free of pursuing drakes. 

Gadwall usually feed on shallowly submerged and float- 
ing vegetation, and more often in open water than odier 
dabblers do (Palmer 1976a). They feed at all hours of the 
day and after dark on abundant supplies of emerging 
midges and water fleas making vertical migrations to open 
surface waters. Gadwall feed mosdy by dabbling, tipping 
up, and picking, and by fdtering from the surface or while 
swimming with their heads immersed; they apparendy do 
not feed direcdy on the bottom (Serie ck Swanson 1976). 

118 



Small planktonic crustaceans are most frequently ingested 
by fdtering (though not as effectively as Shovelers do), and 
most insects and fdamentous algae are obtained by surface 
picking or tipping up. Gadwall are adept at separating 
small midges and beede larvae from a substrate of algae 
and detritus. Sometimes they feed from shore on 
windrowed detritus or concentrated plankton. While feed- 
ing, Gadwall spend more time moving than tipping up and 
move slowly over long distances between bouts of tipping 
up (Eadie et al. 1979). They also dive well for food when 
necessary (though infrequendy), occasionally forage in 
grain stubble, and, rarely, forage in woods for acorns 
(Palmer 1976a). 

Like wigeon, but unlike most dabbling ducks, Gadwall 
generally prefer the succulent leaves and stems rather than 
the seeds of aquatic plants. Filamentous algae are very 
common in Gadwall diets, making the Gadwall somewhat 
unique among ducks (Serie ck Swanson 1976, Swanson et 
al. 1979, M.R. McLandress pers. comm.). The North 
American fall and winter diet is about 98% vegetable 
matter and 2% animal matter (Martin et al. 1951, n = 371; 
Mabbott in Palmer 1976a, n = 362). Like odier dabblers, 
Gadwall depend more on animal matter in summer. Dur- 
ing breeding on saline lakes in North Dakota, the diet of 
adults is 46% animal and 54% vegetable matter; females 
consume more animal matter than males do (Serie ck 
Swanson 1976, n= 107). Consumption there of animal 
matter (mosdy aquatic insects and crustaceans) by females 
in both freshwater (n = 35) and saline lakes (n = 20) 



Waterfowl 



SPECIES ACCOUNTS 



Waterfowl 



reaches a peak at 72% of the diet during laying when 
demand is high for calcium and protein for egg formation 
(Serie 6k Swanson 1976, Krapu 1979, Swanson et al. 
1979). Throughout North America, animal matter con- 
sumed includes aquatic insects (larvae and adults of 
midges, caddisflies, beetles, other flies, true bugs, dragon- 
flies, damselflies, and grasshoppers), crustaceans, mollusks 
(mosdy snails), and a few fish. Vegetable fare consists 
primarily of leaves, stems, rootstocks, and, sparingly, seeds 
(important locally or seasonally) of grasses, sedges, pond- 
weeds, and other aquatic plants, including algae. Preflight 
young initially eat chiefly surface invertebrates but gradu- 
ally switch to aquatic invertebrates and plants until, by 
three weeks of age, they are essentially herbivores; import- 
ant invertebrates to young are midges, aquatic beedes, 
water fleas, and water boatmen (Palmer 1 976a). 

Although widely distributed in nesting habitat, Gadwall 
show a tendency toward colonial breeding (on islands, 
especially where surrounded by open water) (Palmer 
1976a). Exceptionally, on islands, many nests may be 
within a few feet of each other, some less than one foot 
apart (Duebbert 1 966). Gadwall nest a month to a month 
and a half later than Mallards (M.R. McLandress pers. 
comm.). This may be related to their dependence on 
nesting in dry and dense upland herbaceous vegetation, 
which becomes increasingly available as spring advances 
(Gates 1962). Late nesting may also be due to a depen- 
dence on insects that develop later in the season in semi- 
permanent wedands (Serie 6k Swanson 1976). Gadwall 
usually select nest sites on well-drained or, occasionally, 
damp ground, but rarely in emergent or matted floating 
vegetation. Reports of most nests being within 100 yards 
of water (sometimes nearly a mile from water) may be 
because of the tendency of Gadwall to nest on small 
islands, dikes, and channel banks with requisite cover 
(Gates 1962, Palmer 1976a, Bellrose 1980). At Grizzly 
Island in the Suisun Marsh, highest nest densities were 
about one-half mile (or as far as possible) from water (M.R. 
McLandress pers. comm.). For nesting, Gadwall favor 
transitions in habitat, such as the interface of water and 
land, breaks or openings in cover, patches or clumps 
within uniform stands of vegetation, or sites near mounds, 
stones, or other landmarks (usually within 10 ft.). Gadwall 
generally seek taller and more leafy vegetation for nesting 
than do other dabbling ducks and do not nest in stubble 
fields unless other herbaceous vegetation is growing there 
(Palmer 1976a, Bellrose 1980). The cover they select is 



usually dense, coarse herbaceous vegetation (especially net- 
des and thisdes), taller grasses, alfalfa or hay, sedges, 
bushes, or willows. The nest is a hollow lined with dry 
grasses, weed stems, or strips and pieces of reeds, and, of 
course, down;, it is usually arched over by vegetation or is 
beneath leafy, herbaceous plants (Bent 1923, Dawson 
1923, M.R. McLandress pers. comm.). Females sometimes 
build up the nest to escape rising water (Palmer 1 976a). An 
exceptional nest site was one in a crow's nest in a tree. On 
islands, females sometimes lay their eggs in nest bowls of 
the preceding year(s?) (Duebbert 1966). Broods do not 
remain long on small water bodies; instead, they prefer 
open-water areas of moderate to large size having sub- 
merged aquatic plants for food and deep channels for 
escape by diving (Palmer 1976a). Females may lead broods 
as much as 500 yards across open water or over a mile from 
upland nest sites to favorable deep-water marshes or 
impoundments (Gates 1962, Duebbert 1966). In Utah, 
the distance that hens moved their broods during the 
rearing period averaged 0.56 miles (range 0.26-1.15 mi.) 
(Gates 1962, n = 13). 

Marin Breeding Distribution 

During the adas period, Gadwall bred locally in Marin 
County along the San Pablo Bay shoreline and, occasion- 
ally, on Point Reyes and in the interior of the county. 
Representative breeding locations included San Antonio 
Creek, W of Point Reyes-Petaluma Rd. (FL 5/6/79 -SG); 
Bahia Drive ponds near the Petaluma River mouth, 
Novato (FL 6/20/80 -DS); and Horseshoe Pond, E of the 
Drake's Beach visitor's center, PRNS (FL 7/8 6k 8/15/81 
-DS). 

Historical Trends/ Population Threats 

Historically, Gadwall were not known to breed in Marin 
County or the San Francisco Bay Area (G6kW 1927, S6kP 
1933, G6kM 1944). Subsequent confirmation of breeding 
in these areas (Gill 1977, ABN, records above) may partly 
reflect increased observer coverage or the dramatic con- 
tinentwide post-1 950s rise in breeding and wintering pop- 
ulations (Johnsgard 1978, Bellrose 1980). Wintering 
numbers have also increased on Point Reyes since the early 
1970s (Shuford et al. 1989). Gadwall numbers increased 
on Breeding Bird Surveys in California from 1968 to 1989 
(USFWS unpubl. analyses). 



119 



Waterfowl 



MARIN COUNTY BREEDING BIRD ATLAS 



Waterfou/l 



COMMON MERGANSER Mergus merganser 







A year-round resident; numbers swell 




^>-\ v ^o^^ 


slightly from Nov through Mar. 


J\^\%^ 


A rare, very local breeder; overall breed- 




A ^K^\. j^y\ JK \ 3r^^\ ^<i\ xVV •£ - - 


ing population very small. 


^Vvtv 


o\^\\^\ \r\i\r\ JV^\ JkV>A 


Recorded in 5 (2.3%) of 221 blocks. 




OcvVjPrtEV^^^ 


O Possible 3 (60%) 


\\ i 


5JvWVa3^ 


€ Probable = (0%) 




^V\3r\SV^\^V\3c r:> A^^ -' 


• Confirmed ■ 2 (40%) 




■ i^^vvrv-A^^ 


FSAR=1 OPI = 5 CI = 1.80 




^ JC <MW»^V() 











Ecological Requirements 

Along the northern California coast, these "fish-hounds" 
ply the cool, clear waters of rivers, large streams, lakes, and, 
particularly toward the south, reservoirs. Because visibility 
plays such a large part in the pursuit of their fish prey, 
Common Mergansers are generally absent from turbid or 
weed-choked waters. Foraging birds work relatively shallow 
waters, varying from riverine rapids to quiet backwaters, 
more or less following shorelines. They feed mosdy near 
the bottom in about 6 feet of water, but their efforts may 
take them to 40 feet in depth (Palmer 1976b). Birds may 
initiate searches for prey by swimming along widi their 
heads submerged. Mergansers propel themselves under- 
water with their feet only (stroked in unison), except 
perhaps in unusual circumstances, and they randomly 
search among, and under, submerged rocks or other 
objects where fish might hide. The diet of adults is over- 
whelmingly fish, but occasionally they eat fish eggs, aquatic 
salamanders, shrimp, and mussels or other mollusks (Pal- 
mer 1976b). Common Mergansers consume at least 50 
species of North American fish, eating what is locally 
available roughly in proportion to their abundance and 
vulnerability. Studies of captive birds indicate Mergansers 
first eat smaller fish if available. The upper limit in size of 
fish they can eat probably ranges from 5 to 6V2 inches in 
girth (Latta & Sharkey 1966). Ducklings at first feed en- 
tirely on insects caught mosdy beneath the surface but 
soon catch small fish; they will also eat some green vegeta- 
tion (Palmer 1976b). 

Common Mergansers are cavity nesters and prefer hol- 
low trees when available; as many as four nests have been 

120 



found in different cavities in the same tree (Palmer 1976b). 
The height of tree cavities does not appear to be important 
(Palmer 1976b), but most are at moderate heights and 
range to 100 feet or more above the ground (Dawson 
1923). Nest sites are used repeatedly, probably by the same 
female (Palmer 1976b). When suitable tree cavities are in 
short supply, Common Mergansers will nest in a variety of 
well-protected holes and dark recesses. Among the alter- 
nate sites reported are cavities on cliff faces, ledges under 
low fir bushes, among the undercut roots of standing trees, 
in remote crevices among loose boulders, under dense 
tangles of gooseberry bushes and nettles on the tops of 
islands, in old nests of hawks or crows, in large nest boxes, 
and in buildings (Bent 1923, Dawson 1923, Palmer 
1976b, Bellrose 1980). Common Mergansers sometimes 
descend chimneys or otherwise enter unoccupied build- 
ings. Nests in human structures include ones in between 
loosely piled bales of hay in an abandoned ice house, in 
depressions scraped in the dirt floor of an abandoned 
lighthouse, and underneath the supports of a covered 
bridge. In Sweden, people provide entrance holes to attics, 
and they have noted up to four nests in an attic. Some 
authors have reported that, in addition to a thick layer of 
down, the nest is also lined with weeds, grasses, roodets, 
moss, or leaves (Bent 1923, Dawson 1923). However, 
Palmer (1976b) claimed that no vegetation is added to the 
nest. Perhaps the vegetation reported in some nests was 
material already present in the cavity before it was occupied 
by a Merganser. The young climb to the entrance of the 
cavity and tumble to the ground while the female calls from 



Waterfowl 



SPECIES ACCOUNTS 



Waterfowl 



nearby. Most nest sites are situated near or over water, but 
females will bring broods down small tributaries to main 
waterways or will lead them overland as much as 200 
yards; one female and her brood were found almost a mile 
from water (Palmer 1976b). Once on the water, part of the 
brood will sometimes ride on the mother s back. As the 
season progresses, adults and young tend to move down- 
stream and, if available, to larger bodies of water, such as 
lakes and estuaries. 

Marin Breeding Distribution 

During the atlas period, Common Mergansers were con- 
firmed breeding only at Kent Lake. A female with a brood 
was seen there on 27 June 1981, as were two females with 
broods in the summer of 1982 (GFMc). An estimated two 
pairs have bred at Kent Lake in most subsequent years 
through 1988 (JOE). A high count of seven adult females 
with 10+ chicks on 7 June 1985 (JGE) may have included 
the previous year's young as Mergansers are known to have 
"gang" broods, and probably extended families and 
delayed maturation (M.R. McLandress pers. comm.). 
Sightings of four females or immatures at Alpine Lake on 



1 August 1981 (DS, ITi), one to three females on Walker 
Creek about one mile east of Hwy. 1 on 1 May and 1 2 June 
1 982 (DS), and one to five birds on Papermill Creek just 
northwest of Tocaloma on 26 March and early April 1982 
(GMk) suggest the possibility of nesting at these sites. A 
more recent nesting confirmation came from the discovery 
of a two-to-three-week-old Common Merganser (one eye 
damaged) near Bon Tempe Lake on 23 July 1991 QBa et 
al. fide RMS). 

Historical Trends/ Population Threats 

Formerly, Common Mergansers were not known to breed 
in Marin County or elsewhere in the San Francisco Bay 
Area (GckW 1927, SckP 1933, G&M 1944). Because of 
their scarcity as breeders here, they may have been over- 
looked. On the other hand, the creation of large, forest- 
bordered reservoirs, such as Kent and Alpine lakes, may 
have allowed wintering birds to pioneer new breeding 
habitat. Numbers of Common Mergansers increased on 
Breeding Bird Surveys in California from 1980 to 1989 
(USFWS unpubl. analyses). 



mrA 




121 



Waterfowl 



MARIN COUNTY BRIiMDING BIRD ATLAS 



Waterfowl 



RUDDY DUCK Oxyura jamaicensis 



^A\^<\* 3r^vAr^\ 3rv \^\ 




Occurs year round, though primarily as a 
winter resident from mid-Sep through 
mid-May. 

A rare, very local breeder; overall breed- 
ing population very small. 

Recorded in 20 (9.0%) of 221 blocks. 


\V^J)^ \a V«*A \- 


\^\\^\2A^^ 


O Possible = 13 (65%) 


\aX° A^^-^CV ^\^K 




© Probable = 2 (10%) 


\ ;j&r^\ V--*A^~ \^\ \^ K v~ ' v- 


A^r^r^vB^ 


• Confirmed = 5 (25%) 


' 


t^M^y^w 


-'\^-v^>-''\''' A^^\V^ Jr^*^ — \ 


FSAR=1 OPI = 20 CI = 1.60 


J '>^ > 


v^_/ ^^~'^?V<©' " 





Ecological Requirements 

The rusty hues of male Ruddy Ducks in breeding plumage, 
widi their brilliant, sky-blue bills, smardy accent Marin 
County's freshwater marshes and marsh-edged ponds. For 
breeding, Ruddies select permanent wedands in semiarid 
environments that support rich concentrations of benthic 
invertebrates for foraging and emergent vegetation for nest 
construction, support, and concealment (Siegfried 1976a, 
Gray 1 980). Nesting habitat and food do not appear to be 
limiting factors, and breeding males do not defend a 
well-defined territory or restrict themselves to discrete areas 
with fixed boundaries. Instead, males are highly mobile in 
search of food and females and defend an area that extends 
about ten feet around their mate, on or off the nest. Males 
tend to stay close to their mates before and during laying 
but will continue to court other females and copulate with 
them if the opportunity arises. During periods of nest 
relief, paired females are escorted by males to favorable 
open-water feeding areas, while unpaired females behave 
secredy and feed near emergent vegetation to avoid harass- 
ment by courting males (Gray 1980). 

Ruddy Ducks procure their food mosdy by diving and 
straining items from the soft muddy ooze on the bottoms 
of ponds as they move along (Siegfried 1973). They also 
skim items from the water's surface widi the head and neck 
stretched out and moving from side to side, the bill halfway 
immersed, and the tongue making rapid pumping move- 
ments. Rarely, they pick at items on the surface. Ruddy 
Ducks forage during the day, but nonincubating birds, at 
least, also feed at night (Siegfried et al. 1976), and perhaps 
females and ducklings do as well (see Swanson 6k Sargeant 

122 



1972). The year-round diet of Ruddy Ducks in North 
America is about 72% vegetable and 28% animal matter 
(Cottam in Palmer 1976b, n = 163). Martin et al. (1951) 
reported animal matter varying from a low of 21% of the 
diet in winter (n = 60) to 41% in summer (n = 25); but 
others have found almost complete dependence on animal 
matter during breeding (see below). The vegetable fare is 
mosdy die seeds, tubers, and leafy parts of pondweeds and 
sedges, with some wild celery and algae. Animal matter 
consists largely of insects (mainly midge larvae; also caddis- 
fly larvae, water boatmen, predaceous diving beedes, and 
nymphs of dragonflies and damselflies), along with small 
numbers of mollusks, crustaceans (mosdy amphipods), 
leeches, and miscellaneous items, such as marine worms, 
water mites, bryozoans, fish, sponges, and hydroids. 

In the breeding season in Manitoba, animal matter 
accounts for 90% of the diet of adult males (82% midge 
larvae and pupae, n = 23), 95% of adult females (63% 
midges, 22% mollusks; n = 19), and 88% of ducklings 
(73% midges, n = 18) (Siegfried 1973). At Tule Lake in 
California, five major invertebrates represent 94% (by 
volume) of the diet of breeding adults (Gray 1980). Midges 
comprise 79% of die diet of males (n = 22), 83% of females 
(n = 22), and 80% of ducklings; snails are consumed only 
by egg-laying females (5%, n = 7). Breeding females con- 
sume more midge larvae than males at every stage of the 
breeding cycle except incubation. At Tule Lake, the first 
peak, or "bloom," of midges in open water coincides with 
the period of prereproductive fattening of females. Most 
clutches are initiated about two weeks after numbers of 



Waterfowl 



SPECIES ACCOUNTS 



Waterfowl 



midge larvae reach a peak in the nesting area. Midge 
densities decline in the breeding areas through the incuba- 
tion and hatching periods and increase in open water. 
Highest prey densities are in open water immediately 
following peak hatching of young and coincide with the 
movement of females with broods out of the nesting area. 
Young initially seek shelter in shallow water and emergent 
vegetation but usually within a week move to open-water 
areas of high food availability. Although Ruddy Duck 
broods generally do not move overland to different ponds 
(Siegfried 1977) and brood movement over water is mini- 
mal when nests are located within favorable brood-rearing 
localities (Joyner 1977a), in large wetlands broods may 
move over three miles from nesting areas to open water 
(Gray 1980). Occasionally, Ruddy Ducks raise two broods. 
Females remain with young only until they are half grown; 
males accompanying females with broods are attracted to 
the female rather than to the young. 

Ruddy Ducks preferentially nest around permanent 
marsh areas in the fringe of dense to moderately dense 
emergent vegetation of bulrushes, cattails, sedges, and 
rushes (Bent 1923, Low 1941, Palmer 1976b, Siegfried 
1976b, Bellrose 1980). The height of nests above water or 
distance of nests from open water can, of course, vary 
because of seasonal and yearly changes in water levels. In 
Iowa, nest sites vary from 1 to 1 33 yards (av. 32 yds.) from 
open water free of emergent vegetation (Low 1941). Nests 
there also vary from about l /5 to 3Vi feet above the mud. 
Average nest heights above water in various habitats range 
from about 1 to 2 feet (Low 1941, Siegfried 1976b). 
Ruddies build nests up from the marsh bottom to above 
the water, use broken and matted emergent vegetation, or 
attach the nests, floating, to surrounding vegetation. The 
choice of nest sites is governed to a large extent by water 
depth at the time of nesting; therefore, the predominant 
species of plant cover used varies from year to year and 
during the course of the nesting season (Low 1941, Sieg- 
fried 1976b). In some studies, Ruddies were found to use 
predominantly green vegetation for nests (Siegfried 
1976b), whereas in others they used mosdy dried residual 
vegetation from the previous year (Low 1941, Joyner 
1977b). Siegfried (1976b) suggested that Ruddies delayed 
breeding until there was enough new plant growth to 
satisfy nesting requirements, but Gray (1980) felt that food 
availability played an important role in determining the 
onset of breeding. 

The female first constructs a flat platform of reeds, 
bulrushes, cattails, or marsh grasses on which she lays her 
outsized eggs (largest, relative to body size, of any water- 
fowl) (Bent 1923, Low 1941, Palmer 1976b, Siegfried 



1976b, Bellrose 1980). When the clutch is complete, she 
adds a rim, forming a bowl-shaped nest, and begins incu- 
bating. She may or may not add a sparse lining of finer bits 
of marsh plants or down. Fresh materials are constandy 
being added to the nests to compensate for a gradual 
setding caused by the decomposition of the underparts of 
the nests (Low 1941). Varying numbers of nests are 
equipped with overhead canopies of bent vegetation for 
concealment and ramps or runways of matted vegetation 
for entering or leaving the nest (Bent 1923, Low 1941). 
Alternate nest sites include abandoned nests of Coots and 
Redheads, muskrat houses or feeding platforms, and the 
hollow side of a floating log (Bent 1923). 

Ruddy Ducks are also noted for their habit of sometimes 
parasitically laying eggs in nests of other Ruddies and other 
marsh-nesting species (such as other ducks, grebes, Amer- 
ican Bitterns, American Coots, and Common Moorhens) 
or dropping them on the ground (Bent 1923; Low 1941; 
Joyner 1973a,b; Palmer 1976b; Siegfried 1976b). Ruddy 
Ducks may respond to loss of nesting habitat and to 
drastically fluctuating water levels (high or low) by aban- 
doning nest sites; building up the bases of their nests to 
compensate for rising water levels; increasing rates of inter- 
and intraspecific nest parasitism; and producing platform 
nests (flattened emergent vegetation) into which they 
deposit "unwanted" eggs (Joyner 1977b). 

Marin Breeding Distribution 

During the adas years, Ruddy Ducks bred at scattered sites 
throughout Marin County. Numbers of breeding birds 
here are not as large as they might at first seem because fair 
numbers of birds oversummer on both salt and fresh water 
(Shuford et al. 1989). Representative breeding locations 
included die pond at Drakes' s Beach visitor's center, PRNS 
(FL 7/10/82 — JGE); fish-breeding ponds near the Cheese 
Factory, Hicks Valley (FL 6/1 5-7/14/82 -ScC, DS); and 
Bahia Drive ponds near the Petaluma River mouth, 
Novate (FL 6/28/80 -DS). 

Historical Trends/ Population Threats 

Grinnell and Miller (1944) reported a decline in the 
California population caused by loss of breeding habitat. 
Numbers were relatively stable on Breeding Bird Surveys 
in California from 1968 to 1989 (USFWS unpubl. analy- 
ses). Tule Lake was an important area of Ruddy Duck 
production in the 1970s, but now few broods survive 
(M.R McLandress pers. comm.). This loss of production 
may be die result of large numbers of predators (raccoons) 
or perhaps the effect of pesticides from agricultural runoff. 



123 



MARIN COUNTY BREEDING BIRD ATEAS 



**•/• 







Does innate courage or naivete propel downy Wood Ducks, fluttering, jrom their lofty nest holes! 

Drawing b} Keith Hansen, I 989. 



124 



blew World Vultures 



SPECIES ACCOUNTS 



New World Vultures 



New World Vultures 



Family Cathartidae 



TURKEY VULTURE Cathartes aura 







A year-round resident. 


HtoiHoV^ k ~^ 
^\°X^\o\>r\ °3rV\o \>A ° V>\o >^o\> J CoO ( ~ 




A fairly common, nearly ubiquitous 




breeder; overall breeding population 
large. 


JV^\ OjV<\ °3r<\ °A^\ ° ; A^\ O V-^A • jAiC O \J>A 




Recorded in 213 (96.4%) of 221 


\^y\ \Jk\^\ °J<^\ ° >-^\o V^\ o \>z\Q- YP<~« \^\ 

\ NCk^^oX °Jt<r\ ° Jv^C\0 \-^A jo p<A(m^r o \>rC O J 




blocks. 


\ >r5A V<C\ ° 3A\ ° Jr-'X \^\ oT^C O V^AO'-V^V 






^^°><^ >^9'iV :> \ o V-Ajo V>A« V-"ao Vi-A o \ 






V55^>#7v\'-8>Tn\ °A<\ °Jv^°A<CC °A^\ ° ><a 






\AJ^H^A^\ 3c^\ >Ki\o \^\ o V-"\ o \AvTo \ 
Vvo 3?<1T\P V'x o \^i?^v\-^\ o v>-a O V> J CQA^--^r7 > r^>^ 




O Possible = 204 (96%) 


\ iV^TX ®si t\\ ° V — \ ° i&>5j& i/A-'J-i^T^j^ o v>^^ 






\ VJk^X & ^?A^Ki : A" 'J^\ o V-"\ • >-<\ o V^C o V-7 




C Probable = 1 (0.5%) 






\ ^\<A °3r<\ cOe^X 90P^ o \^'o\^^fijC^^J\ 


- -r* 


• Confirmed = 8 (3.5%) 


To.oP'ao >-^vO X>Ko y^VSA^V o V>A« \>^v P J 7 






l-^skQfV^\ o,'V>a o £<-A o \>d^n V>\o UAoXK 








i : \J^YMJ^\ A^\ A<\o , 3^r\ o i^rtv o\^\ <sjrr\ 










r* ? 


FSAR=3 OPI = 639 CI = 1.08 




^7 ^^c^^^c^c^ 


^& 



















Ecological Requirements 

The "tippy-glider," wings held in a slight V, soars gracefully 
over virtually all of Marin's landscape, circling and teeter- 
ing gendy in response to the vagaries of the air currents. 
Once aloft, it flaps leisurely but infrequendy as it exploits 
the earth's envelope in its far-ranging searches for animals 
that have met their fate. TVs are usually seen in the air 
singly or in small groups except when congregating at or 
leaving roosts or during migration. In Marin County, they 
feed in virtually any habitat where they are able to reach the 
ground easily. Although TVs, unlike other North Ameri- 
can vultures, forage efficiendy beneath the forest canopy 
(Houston 1986, Jackson 1988), apparendy they descend to 
the forest floor in Marin County only at nest sites. They 
feed here most often in open agricultural country that 
affords an abundance of grazing animals and easy sighting 
of dead "prey" from the air. They also feed frequendy along 
open road corridors on animals cut down by fast-moving 
vehicles and along estuarine shores and beaches on car- 
casses of wave-cast birds and marine mammals. 

Turkey Vultures are primarily solitary breeders, 
although nesting birds sometimes cluster near concen- 
trated food sources (Jackson 1988). They choose secluded 
nest sites consisting of little more than a cleared, trampled 



area within a relatively dark recess. Although nest sites are 
usually at ground level, some are high in tree cavities, in 
caves, under rocks on ledges of cliffs, in old tree nests of 
other raptors or herons, or in upper rooms of buildings 
(Tyler 1937; Jackson 1983, 1988). Prominent forest breed- 
ing habitats are bottomland hardwoods and thickets. 
When available, cliffs are also favored, primarily because 
of cave nesting sites and perhaps because updrafts make 
departures from the nest site easy. Ground nests may be in 
podroles, in crevices among rocks, under or beside logs, 
inside hollow logs, at the base of hollowedout trees, and 
in thickets, tangles of vines, brush heaps, and clumps of 
chaparral with narrow entrance ways. Sometimes nest sites 
are below ground in caves or in hollows of rotten stumps. 
There appear to be no records of Turkey Vultures nesting 
in old tree nests of odrer species or in buildings in 
California (P.H. Bloom pers. comm.). In the West, about 
77% of nests are in caves and 10% on cliff ledges or among 
rocks (Jackson 1983, 1988; n = 324). In die East, nest sites 
are more varied, with about 34% in hollow trees, stumps, 
or logs; 28% in drickets; 1 3% in caves; 8% on cliff ledges 
or among rocks; and 5% in buildings (n = 418). The 
choice probably reflects site availability rather than regional 

125 



hleui World Vultures 



MARIN COUNTY BREEDING BIRD ATlJ\S 



New World Vultures 



preference. In many heavily forested regions of the West, 
nest sites probably are more like those in the East, and 
Marin County is no exception (see records below). Turkey 
Vultures lay their eggs on the ground, on bare stone, in 
rotten wood chips, or sometimes in other debris. Although 
they usually make little or no effort to prepare the nest site, 
one bird using a tree cavity pulled off dry rotten wood from 
the walls of the cavity with its beak, tore it into bits, and 
spread it on the floor (Tyler 1937). TVs often use nest sites 
again in successive years or at intervals (Jackson 1988). 
Since 1920 there have been fewer nests found in the East 
in tree cavities and more in thickets, apparendy because of 
changing forest management practices and the prolifera- 
tion of exotic vines. For obvious reasons, nest sites in 
buildings must have also increased historically. 

Roosting is a ritual aspect of Turkey Vulture existence. 
In spring and summer, some birds roost singly; others 
roost in aggregations year round (Jackson 1988). Roosting 
congregations are generally near stable food resources and 
often near or over water. Turkey Vultures often remain at 
roosts well past sunrise, perhaps for thermoregulatory 
reasons as well as to await favorable winds or updrafts for 
flight. Departure time from roosts is correlated with ambi- 
ent wind speeds, but not with temperature or cloud cover. 
TVs can lower their body temperatures markedly at night, 
thus reducing energy expenditures (however, see Hatch 
1970). At or near the roost, perching birds are statuesque 
with wings outspread, feathers raised, and backs to the 
sun. This behavior may facilitate a return to normal diur- 
nal body temperature. In inclement weather, they may 
remain at roosts for as long as two days, foregoing meals 
until suitable flying conditions return. 

Communal roosting appears to be an adaptation for 
increasing the efficiency of individuals foraging on a dis- 
persed, unpredictable food supply (Rabenold 1983). The 
scavenging lifestyle is one of extended search and patience, 
since these weak-clawed, weak-billed raptors cannot select 
their victims. Birds forage widely and do not patrol a fixed 
breeding range (Houston 1986). Most feeding is done 
from midmorning to midday. Rabenold (1983) speculated 
that birds circle to higher altitudes later in the day to search 
for die next meal, to which they return the following day. 
The degree to which sight and smell guide Turkey Vultures 
in finding their meals has long been the subject of much 
debate. Unlike most birds, Turkey Vultures have a well- 
developed olfactory sense. In tropical forests, Turkey Vul- 
tures mosdy use smell to locate carrion. Vision plays a 
minor role, for they find completely hidden food as quickly 
as visible bait (Houston 1986). Birds cannot detect fresh 
carcasses, probably because they do not yet give off a strong 
odor, but do prefer comparatively fresh meat if given a 
choice. Nevertheless, vision plays an important role (Jack- 
son 1988) and seems to be of great aid in very open 
country. Turkey Vultures also watch each other and odier 

126 



scavengers, such as ravens and crows, that have located 
food. They may sight scavengers on the ground or other 
vulnires circling over carcasses in a distinct "verification 
pattern" (Rabenold 1983). Having located a seemingly 
dead animal, the Turkey Vulture approaches it cautiously 
with a gawky gait to see if it has breadied its last. Smaller, 
thin-skinned carcasses are torn open immediately. Vul- 
tures usually 'attack" carcasses through available orifices- 
eyes, mouth, and anus (Rabenold 1983). For larger, 
thicker-skinned carcasses, the Vultures gather in numbers 
and wait patiendy until other scavengers make the first 
incisions or until time and decay make the carcasses soft 
and ripe. If large animals are set upon immediately, they 
usually cannot be finished while fresh, but purification 
works fast and the naked cathartid head is well adapted for 
avoiding disease while plunging into a body in an 
advanced state of decomposition. 

Turkey Vultures are primarily carrion feeders, and the 
diet reflects what is available. The main items are mam- 
mals, birds, turtles, snakes, and fish. Occasionally Turkey 
Vultures eat insects, such as grasshoppers and mormon 
crickets; dead tadpoles in drying ponds; cow dung (per- 
haps deriving benefit from the beedes it contains); and 
seal, sea lion, or human excrement (Tyler 1937, Jackson 
1988). Turkey Vultures will also occasionally take live prey, 
usually vulnerable young or incapacitated adults. They 
have killed newborn pigs, young and weakened chickens, 
tethered or otherwise entrapped birds, and the young of 
colonial waders. In addition, they sometimes batter nest- 
ling Great Blue Herons, forcing them to disgorge; wade 
into water to stab live fish; and peck out the eyes of cows 
or horses mired in bogs. When hard pressed, Turkey 
Vultures will eat vegetable matter, such as pumpkins and 
palm nuts. Paterson (1984) found plant material in about 
25% of all pellets collected in Virginia in autumn. Vegeta- 
tion comprised as much as 70% of one pellet, suggesting 
it was consumed direcdy, not inadvertendy. TVs will also 
eat salt from blocks left in pastures for cattle. 

Marin Breeding Distribution 

During the adas period, the Turkey Vulture was perhaps 
the most wide ranging of Marin's breeding birds, seen 
soaring over virtually every square inch of the county. But 
because of the difficulty of finding nests, the adas map of 
this species is one of the least satisfying in terms of 
documenting the details of breeding distribution. Because 
of the limited numbers of stable cliffs or caves in Marin 
County, most Turkey Vultures seem to nest here in for- 
ested areas. Representative nesting locations were inside a 
burned out hollow at the base of a redwood (shielded by 
small branches and debris) on the ridge north of San 
Geronimo (NY 5/29-7/11/82 -DS); under a rock over- 
hang among boulders in chaparral off the Yolanda Trail, 
Mt. Tamalpais (NE-NY 4/. ? -6/5/82 — ITi); inside the hoi- 



New World Vultures 



SPECIES ACCOUNTS 



New World Vultures 



lowed-out trunk of a living oak near Stafford Lake, Novato 
(NE 5/6/79 — KH); in a burned-out redwood stump in a 
canyon of Big Rock Ridge above Stafford Lake (NY 5/?/82 
— ScC); and in a crack in serpentine rock under oaks on 
ML Burdell, Novato (NE 5/?/79 -ScC). Prior to the adas 
period, a nest was found under a horizontal log in a logged 
bishop pine forest on Inverness Ridge (NE 5/7-11/73 
-RH). 

Historical Trends/ Population Threats 

Although not well quantified, some population trends are 
evident. Overall, Turkey Vultures seem to have been wide- 
spread and numerous in North America in the 1800s 
(Wilbur 1983, Jackson 1988). Subsequendy, they declined 
in numbers with the depletion of populations of bison and 
other large herbivores. They later increased with the avail- 
ability of road-killed animals along our expanding highway 
system but decreased again as forest breeding sites were 
cleared, organochloride pesticide contamination increased, 
and environmental laws required burial of animal wastes. 
Grinnell and Miller (1944) reported that Turkey Vul- 
tures were "thought to be less numerous now than for- 
merly" in California. Based on Christmas Bird Count 
(CBC) data from 1950 to 1973, Brown (1976) detected a 
decline in wintering populations of Turkey Vultures in the 
United States. California was the only state to show an 
apparent increase, but this was because of a large popula- 
tion on CBCs (Drake's Bay and Pt. Reyes) reported only in 
later years. Garrett and Dunn (1981) suggested that Turkey 
Vultures have declined as breeders in coastal southern 
California. Turkey Vultures were on the Audubon Society 



Blue List for 1972 and 1980, with concern in southern 
California (Tate 1981). They were also on their list of 
Special Concern in 1982 (Tate &. Tate 1982), and on their 
list of Local Concern in 1986 (Tate 1986). Numbers of 
Turkey Vultures on Breeding Bird Surveys in California 
were relatively stable from 1968 to 1989 (USFWS unpubl. 
analyses). 

Turkey Vulture eggshells were 11% thinner in Califor- 
nia in the pesticide era (post-1947) than previously, but this 
degree of thinning is not of the magnitude generally asso- 
ciated with major declines of productivity in other species 
(Wilbur 1978). Other potential contaminants that might 
affect Vultures are systemic organophosphate pesticides 
applied topically to livestock and ingested from carcasses, 
and perhaps heavy metals (Pattee ck Wilbur 1 989). Turkey 
Vultures face additional threats today. Urbanization may 
limit food supplies and nest sites. Although road kills 
provide supplemental food, the Vultures themselves also 
fall victim to speeding vehicles. Recent changes in grazing 
practices and husbandry techniques have meant that fewer 
animal carcasses are left on the range to be eaten by 
Vultures (Wilbur 1983). Tree cavity sites, in which birds 
have high nesting success, may now be limited by forest 
management practices (Jackson 1983). Trees large enough 
to harbor suitable cavities are generally about 150 to 200 
years old and are increasingly rare today. Also these trees 
must be injured before fungus invades to rot out cavities, 
and fire suppression makes this less likely. Although Tur- 
key Vultures do not lead a charming lifestyle, these gende 
creatures deserve our admiration and protection. 




127 



MARIN COUNTY BREEDING BIRD ATLAS 




ft mm 

mm 



e V Ik W^ 

'3 \\. f % 




*K \ 







Although the Turkey Vulture is one of Marin County's most widespread and conspicuous birds, who among us has been honored 
with even a glimpse of the intimacies of its home life? Drawing b} Keith Hansen, 1989. 



128 



Hawks and Eagles 



SPECIES ACCOUNTS 



Hawks and Eagles 



Hawks and Eagles 

Family Accipitridae 



OSPREY Pandion haliaetus 



Uto^ 










Occurs year round, though primarily as a 
summer resident from Feb through Sep. 


J^\\\J^< 








An uncommon, local breeder; overall 


"mA^cT jL- 










breeding population very small (numbers 
increasing). 
Recorded in 49 (22.2%) of 221 blocks. 


\\ { 


O^v^pv^x 


V"\ JV^\*3r^\ -V' 


3^a2-V^_ 




O Possible = 41 (84%) 




i^V^^JtA 








© Probable = (0%) 






■^J^vVv^ 


"Y>V^>^C 


. . -r- 


• Confirmed = 8 (16%) 








^^^V^^v- 








<^SX^_j 




^vJV^X^Ar^ 


r^?o 


FSAR = 2 OPI = 98 CI = 1.33 




xi^* 


^\j?7 









Ecological Requirements 

This cosmopolitan raptor haunts the fish-producing waters 
of bays, estuaries, reservoirs, and large streams or rivers. 
Marin County's breeding birds are concentrated within a 
few miles of the coast, where they forage primarily in 
estuaries, in the ocean near the surf, and in reservoirs. 
Overall, the coastal California breeding population is con- 
centrated along rivers, streams, and bays. Marin County's 
Kent Lake is the only coastal reservoir currendy used for 
nesting. In the interior of the state, 72% of the birds nest 
by lakes and reservoirs and 28% on rivers (Henny et al. 
1978). 

Ospreys nest solitarily or semicolonially. Most nests in 
Marin County are over, or within a half mile of, water and 
are situated about 40 to 1 20 feet up in Douglas fir, coast 
redwood, or bishop pine. Tree characteristics, elevation, 
slope, distance from water, and isolation from predators 
are likely the main factors influencing the choice of nest 
sites. From 1981 to 1990, 53 nest sites at Kent Lake were 
almost evenly split between Douglas fir (53%) and coast 
redwood (47%) and between dead (51%) and live (49%) 
trees; 10 of the 26 live trees had dead crowns (Evens 1991 , 
in press). Of 25 sites in use from 1981 to 1984, 76% were 
in Douglas firs and 24% were in redwoods; of 28 sites 



established since 1984, 32% were in Douglas firs and 68% 
were in redwoods. The reason for this shift in tree species 
use is unclear (Evens 1991 , in press), but it may reflect the 
changing availability of suitable nest sites as prime sites 
become occupied. In this regard, it should be noted that 
the period of increasing use of redwoods as nest sites 
coincided with the most rapid growth of the Kent Lake 
Osprey population (Evens 1 991 , in press; Table 1 8). Of 1 8 
nest sites in 1990 in the Tomales Bay area (from Five 
Brooks north to Mt. Vision), 10 were in bishop pines (7 
dead, 3 live), 5 in Douglas firs (all dead), and 3 on artificial 
structures. The greater use of bishop pine reflects the 
dominance of this pine along almost all of Inverness Ridge 
adjacent to Tomales Bay, except at the south end of the 
bay, where it is replaced by Douglas fir. 

Ospreys may prefer dead trees with a minimum of 
lateral branches because such trees deter climbing preda- 
tors (Evens 1985). Nest sites at Kent Lake range from over 
water to about 0.56 miles (av. 209 yds.) from shore (Evens 
1987, n = 42). One Marin nest in the early 1960s was on 
a rocky coastal bluff. 0( the three nests constructed on 
artificial structures at Tomales Bay in recent years, two 
were on duck blinds and one was on an active power pole. 

129 



MARIN COUNTY BREEDING BIRD ATLAS 

Table 18. Numbers of nests and productivity of Ospreys in Marin County, California, at Kent Lake from 1981 to 1990 and 
at Tomales Bay in 1 989 and 1 990 (Evens 1 991 , in press). Numbers for Kent Lake in parentheses represent adjustments (using 
the ratio of productive to occupied nests of 1:1.4 in subsequent years) for 1982 data to account for underestimation of 
occupied nests because of late season nest counts in that year. 



KENT LAKE 



YEAR 


Occupied 
Nests 


Active 
Nests 


Productive 
Nests 


Total 
Young 


Number of Fledglings per/ 


Occupied 


Active 


Productive 


1981 


15 


11 


11 


23 


1.53 


2.09 


2.09 


1982 


16(20) 


15 


14 


20 


1.25(1.0) 


1.33 


1.43 


1983 


20 


14 


13 


24 


1.20 


1.71 


1.85 


1984 


18 


14 


11 


19 


1.06 


1.36 


1.73 


1985 


22 


20 


19 


37 


1.68 


1.85 


1.95 


1986 


24 


22 


19 


37 


1.54 


1.68 


1.95 


1987 


31 


27 


23 


41 


1.32 


1.52 


1.78 


1988 


30 


26 


23 


36 


1.20 


1.38 


1.56 


1989 


32 


28 


26 


48 


1.50 


1.71 


1.85 


1990 


35 


25 


22 


31 


0.89 


1.24 


1.41 



Me 



1.32(1.29) 



1.59 



1.76 



TOMALES BAY 



YEAR 


Occupied 
Nests 


Active 
Nests 


Productive 
Nests 


Total 
Young 


Number of Fledglings per/ 


Occupied 


Active 


Productive 


1989 


14 


12 


9 


13 


0.93 


1.08 


1.44 


1990 


15 


11 


8 


11 


0.73 


1.00 


1.38 



Me 



0.83 



1.04 



1.41 



Occupied nest = a large apparendy complete nest attended by one or two Ospreys during the breeding season. 

Active nest = an occupied nest with an incubating adult. 

Productive nest = an occupied nest from which at least one young fledged. 



130 



Hawks and Eagles 



SPECIES ACCOUNTS 



Hawks and Eagles 



In 1989, the latter nest was carefully moved during the 
incubation period to a platform specifically constructed 
about 1 50 feet away, but the occupants abandoned it Q.G. 
Evens pers. comm.). In coastal California as a whole, 
Ospreys nest almost exclusively in trees, whereas in the 
interior of the state about 79% use trees and 21% use 
artificial structures (Henny et al. 1978). Tree nests are 
located at the top, exposed to the sky (Henny 1988). 
Elsewhere, Ospreys nest in a wide variety of sites in varying 
proportions, occasionally as much as two to three miles 
from water. Nest height is not as important as inaccessibil- 
ity from mammalian predators and seclusion from distur- 
bance. Nests over water are generally at lower heights than 
those over land, and ground nests, which tend to be on or 
near a prominent object, are prevalent on small islands. 
All nests have relatively unobstructed views of the sur- 
roundings and at least one perch nearby. Other nest sites 
include power towers, unused chimneys or windmills, 
sheds or buildings, channel markers, pilings, road signs, 
boats (sunk or aground), piles of fence rails, fences or 
walls, old stumps, cacti, tops of rock pinnacles, rocks or 
boulders (on land or in water), piles of seaweed, driftwood, 
or other debris on beaches, and even the crossed poles of 
a tepee and a 1000-pound bomb (Bent 1937, Henny 
1988)! Ospreys readily adapt to nest sites provided for 
them and often have greater nesting success on nest plat- 
forms than at natural sites (Henny 1988). 

Ospreys construct large, bulky nests primarily of sticks, 
but they also may use sod, seaweed, cornstalks, bird wings 
or corpses, bones, cow dung, and a wide variety of odier 
natural and human debris. They line the nest widi materi- 
als such as moss, lichen, eelgrass, grass, bark, and even 
mud. Nests can be very large because sticks are added 
throughout the breeding season, and old nests are refur- 
bished and added to year after year. In some cases, nests 
have been continuously occupied for 45 years or more 
(Bent 1937). Pairs will sometimes have several alternative 
nests built within the territory over a period of years, one 
of which is active at a time (Henny 1988). Some birds 
(presumably subadults) build nests but do not lay eggs, and 
adults often build "frustration nests," generally not laying 
in them, after failing to rear young in their original nests. 

The Osprey's mode of foraging combines plunging and 
grasping with its feet, the undersides of which are covered 
with pointed, prickly scales, or spicules, for retaining a 
grasp on slippery fish (Henny 1988). Ospreys generally 
cruise at a height of 50 to 100 feet and look into the water 
at an angle of 45° or less. When Ospreys see fish, they 
maneuver into position above, often hovering, waiting 
until the fish are close to the surface. From a long glide or 
brief hover, they plunge feet first, strike the water at about 
20 to 45 mph, and penetrate to a maximum depth of about 
three feet. Just before the Osprey enters the water, the feet 
are swung forward beyond the head and the wings are 



extended upward and back so that their tips extend past 
the tail. After capture, powerful forward and downward 
strokes of the wings raise the bird to the surface and clear 
of the water. Ospreys sometimes fly down at an oblique 
angle and catch fish while skimming along the water's 
surface (J.G. Evens pers. comm.). In flight, Ospreys adjust 
the fish so that its head points forward, and they often 
must fend off piratical forays from gulls. Ospreys take fish 
up to about two pounds, and, exceptionally, they will 
capture two fish, one in each foot, on the same dive! 

The Osprey diet is almost exclusively live fish, but 
occasionally dead ones are taken. A wide variety of fresh- 
water and saltwater species are secured, but benthic-feeding 
fish of shallow waters are easier to capture, suggesting that 
they are selected over piscivorous fish when equally avail- 
able (Swenson 1979). Larger fish are brought back to the 
nest later in the nesting cycle, not because the larger 
females first begin to forage for young then, but because 
fish have grown in size as the season progresses (DeSante 
6k Scriven 1977). The increase in fish size is advantageous 
to adults then feeding the older, more demanding Osprey 
young. In Marin County, the few prey remains found 
under Osprey nests at Kent Lake were freshwater carp 
(W.C. Follet fide GFMc, Evens 1985), but in fact, Kent 
Lake breeders appear to feed much more extensively on 
saltwater species at Bolinas Lagoon and on the outer coast 
(Evens 1985, 1991, in press). Inverness Ridge breeders 
also appear to feed primarily in salt water. Ospreys also 
occasionally prey on small rodents or rabbits, small to 
medium-sized birds, snakes, frogs, turtles, and inverte- 
brates (Bent 1937, Tait et al. 1972, Wiley 6k Lohrer 1973, 
Henny 1988). Presumably such prey are taken when fish 
are scarce; when foraging is hampered by inclement 
weather or murky water; when young Osprey lack fishing 
skill; or when crippled, captive, or concentrated alternative 
prey are too attractive to pass up. 

Historical Trends/ Population Threats/Marin 
Breeding Distribution 

Osprey populations have declined seriously in historic 
times. In California, Grinnell and Miller (1944) reported 
that the species was "originally common and widespread," 
but by 1944 was "much reduced in number." Continent- 
wide, but most severely in the East, a drastic population 
reduction began to be noticed in the 1960s (Ames 6k 
Mersereau 1964). This decline was attributed largely to 
eggshell thinning from the accumulation of pesticide resi- 
dues, exacerbated by encroachment of humans on nesting 
sites and by shooting (Henny 1977, Ogden 1977). In 
California, the Channel Island population disappeared 
from 1917 to 1968, while concurrently the population on 
die central and soudiern mainland declined virtually to 
extinction (Diamond 1969). The southern California 
population disappeared long before the pesticide era, and 



131 



Hawks and Eagles 



MARIN COUNTY BREEDING BIRD ATLAS 



Hawks and Eagles 



this decline may be attributable to removal of nesting trees, 
degradation of lake and river quality, boating on nesting 
lakes, and shooting (Remsen 1978). With the banning of 
DDT, dieldrin, and other pesticides, Ospreys have begun a 
comeback, though the southern California population still 
remains close to extirpation (I lenny et al. 1978, Garrett ck 
Dunn 1981, Henny &. Anthony 1989). Pesticide contami- 
nation and eggshell diinning were still occurring in Cali- 
fornia from 1973 to 1984, but apparendy were not major 
mortality factors at that time (Littrell 1 986). Ospreys appear 
to have increased in the West after the creation of reser- 
voirs (Swenson 1981, Henny 1983), but it seems unlikely 
that this has completely counterbalanced the loss of spawn- 
ing beds of anadromous fish from reservoir construction 
or the effects of pollution, disturbance, or shooting. In the 
San Francisco Bay Area, the loss of anadromous fish has 
been offset to some degree by the introduction, beginning 
in the 1870s, of fish from the East and Midwest that now 
make up the bulk of our warm-water fish populations 
(Skinner 1962). 

Although historical sightings existed for Lake Lagunitas 
and Tomales Bay (Mailliard 1900, S&P 1933), there were 
no reports of nesting Ospreys in Marin County as of 1943 
(G&.M 1944). Given the limited historical coverage of the 
area, the former "abundance of the species in California, 
and the current breeding distribution, it seems likely that 
Ospreys formerly bred here (though perhaps in smaller 
numbers) but were overlooked. From 1962 to 1963, 
Ospreys nested on the cliffs at Double Point on the Point 
Reyes peninsula (Chan 1979). In 1953, Kent Lake was 
constructed in the Lagunitas Creek watershed north of 
Mount Tamalpais. Although one of several reservoirs in 
that area, it was the largest and most remote. A fire that 
swept the slopes adjoining Kent Lake in 1945 left many 
large snags— prime nesting habitat— in an open Douglas 
fir-coast redwood forest. Many dead redwoods that now 
border the lakeshore apparendy were drowned by the 
rising of the dammed waters (Evens 1985). The first 
Osprey nest was found at Kent Lake in 1967 (S. 
Cammiccia pers. comm.; Evens 1991, in press). Ospreys 
may not have nested there until then because of the time 
necessary for colonization and because copper sulfate was 
added to the lake in 1964 to kill carp, thus reducing or 
eliminating the food supply. The Kent Lake population 
increased rapidly to about 8 "active" nests in 1973 (C. 
Zumwalt pers. comm.), and an estimated 7 occupied nests 
in 1975 (Henny et al. 1978). Osprey studies at Kent Lake 
from 1981 to the present have documented a continued 
dramatic increase of the population to 35 occupied nests 
in 1 990 (Evens 1 991 , in press; Table 1 8). The fact that the 
Kent Lake population expansion began before die ban on 
DDT in 1972 may be because this isolated watershed was 



free of DDT residues. Currendy, breeding Ospreys also 
concentrate in Marin County along central and northern 
Inverness Ridge and the fringes of the south end of 
Tomales Bay, where there were 1 5 occupied nests in 1 990 
(Evens 1991, in press). This population expanded gready 
in the last decade. A sighting of an Osprey in the Tomales 
Bay area in the mid-1970s was considered a red-letter day, 
whereas now it is rare not to see an Osprey in this area on 
a summer day. The first known nest on Inverness Ridge 
was established above the town of Inverness in 1978, but 
pairs had been noted in the area for two or three years 
before that (Evens 1991, in press). Since most Ospreys 
nest within about 20 miles of where they were raised 
(Henny 1977), the Inverness Ridge population may have 
expanded by recruitment from the Kent Lake population. 

During the adas period, new information was added on 
die distribution of Ospreys breeding away from Kent Lake. 
Without a thorough systematic search just for Osprey 
nests, a minimum of 5 occupied nests were located on 
Inverness Ridge in 1982 (DS et al.). In 1981 and 1982, 
there was also a single nest on top of a duck blind over 
water at the southeast end of Tomales Bay (NY each year 
1981-89 — JGE), another on Big Rock Ridge above Staf- 
ford Lake (NY 5/20/82 — ScC), and 16 occupied nests at 
Kent Lake (McCurdy 1983). Because nest surveys at Kent 
Lake in 1982 were not initiated until mid-June, perhaps 
after some nesting failures earlier in the season, there may 
have been up to 20 occupied nests at Kent Lake that year 
(Evens 1985). Using the conservative survey figures for 
Kent Lake and random observations from elsewhere in the 
county, there were a minimum of 23 occupied nests in 
Marin County during the last summer of adas field work 
in 1982. The continued increase of the Kent Lake popula- 
tion (Table 18) and more thorough searches on Inverness 
Ridge, whose population is perhaps also still increasing, 
have produced a high count of 50 occupied nests in Marin 
in 1990 (Evens 1991, in press). The Marin County breed- 
ing population is currently the most southerly stronghold 
for the species in California, although a sizable population 
occurs in Baja California and the Gulf of California 
(Henny &. Anderson 1979, Henny &. Anthony 1989). 

Numbers of Ospreys recorded on Breeding Bird Sur- 
veys in California were relatively stable from 1968 to 1989 
(USFWS unpubl. analyses), but these multispecies surveys 
are not well suited for detecting population trends of 
semicolonial raptors. Surveys solely of Ospreys indicate 
that on the whole their numbers are increasing and the 
breeding range is currently expanding in northern Califor- 
nia (Gould & Jurek 1988, Henny &. Anthony 1989). 
Nonetheless, the Osprey is still a Bird Species of Special 
Concern in California (Remsen 1978, CDFG 1991b). 



132 



Hawks and Eagles 



SPECIES ACCOUNTS 



Hawks and Eagles 



BLACK-SHOULDERED KITE Elanus caeruleus 











A year-round resident; numbers generally 




^c^x^ K 






swell from Sep through Mar but vary 


jOv'^wV^ 


■^\ - : i>--^ * 






irregularly between seasons and years, 


^C\ Jk 


A ^^\ \ -'j^C^' \ ^\ \ 






depending on fluctuating vole popula- 
tions. 




\^^\ \ Jt >>^Y~ \^\^~ 


p^A^VO 




An uncommon, local breeder; overall 


^v^ 








breeding population very small. 

Recorded in 34 (15.4%) of 221 blocks. 






o<<\ ®V^\ ®j?\ ,Jri 


.. 


O Possible = 20 (59%) 




-^x$k^\°J^r\?y^ 






© Probable = 7 (21%) 




2-Mv >V\ \^\ V^ 


^c^^A^^S 




• Confirmed = 7 (21%) 






~\J?/ ^^siv' 


S<y - 


FSAR = 2 OPI = 68 CI = 1.62 



Ecological Requirements 

These elegant graceful kites inhabit Marin County's open 
lowland valleys and low, rolling foothills. They forage in 
moist meadows, grasslands, low marsh vegetation, riparian 
edges, irrigated pastures, and cultivated fields or orchards 
that provide the requisite prey base. Although the sur- 
rounding terrain may be semiarid, Kites often reside near 
water sources, where prey are more abundant. The partic- 
ular characteristics of the nesting site do not appear to be 
as important as its proximity to a suitable food source 
(Hawbecker 1942). With open foraging country nearby, 
Kites often build nests in isolated trees or clumps of trees, 
although they sometimes place them in dense stands on 
steep slopes. They nest in a wide variety of trees of 
moderate height and sometimes in tall bushes. Native trees 
used in California are live and deciduous oaks, willows, 
cottonwoods, sycamores, maples, toyons, and Monterey 
cypress (Pickwell 1930; Hawbecker 1940, 1942; Dixon et 
al. 1957). Kites also use introduced trees, such as orchard 
varieties and eucalyptus. Unlike most tree-nesting hawks, 
Kites do not build their nests in a firm crotch next to a 
trunk or at a limb fork, but instead among the slender 
branches of the crown of the tree (Pickwell 1930). The nest 
is typically screened from view from below but is open 
above, affording easy access to the occupants and aerial 
predators. The nest is usually deep enough in the tree, 
however, to provide at least dappled shade when the young 
are left for long periods (Hawbecker 1940). Heights of tree 
nests range from 15 to 75 feet (most 20-50 ft.) above the 
ground (Pickwell 1930, Bent 1937, Hawbecker 1942). At 



Grizzly Island in the Suisun Marsh of Solano County, 
Kites nest commonly in coyote brush about 6 to 8 feet high 
(D. Fortna pers. comm.). In this area, they nest in bushes 
even though there are many eucalyptus groves nearby in 
which Red-tailed Hawks and Great Horned Owls nest. 
Although sometimes frail looking, most nests are well-built 
platforms varying from a shallow to a deeply hollowed 
bowl (Pickwell 1930, Bent 1937, Hawbecker 1942). Nests 
tend to flatten out as the young develop (Hawbecker 1942). 
Kites build their nests of dry sticks and twigs and line them 
with materials such as grass, straw, roodets, stubble, weed 
stems, lichen, moss, strips of inner bark, and perhaps a few 
feathers (Pickwell 1930, Bent 1937). Unlike many hawks, 
Kites generally build a new nest for each clutch. Occasion- 
ally, they will build on top of old nests of other birds, such 
as Cooper's Hawks and American Crows (Dixon et al. 
1957). Rarely, they will rebuild in the exact spot in the 
following year after removal of a nest, refurbish the last 
year's nest, re-lay in the same nest if the eggs are taken, or 
use one of their nests from a previous year for a second 
brood (Barlow 1897, Hawbecker 1942). 

Kites traverse their foraging grounds in buoyant, airy 
flight. They typically hunt while hovering at about 100 feet 
with legs dangling down. Birds generally face into the wind 
and maintain their position by slowly flapping their 
upstretched wings or remain aloft, "kiting," by the force of 
the wind alone. When prey are spotted, they raise their 

133 



/ lawks and Eagles 



MARIN COUNTY BREEDING BIRD ATLAS 



Hawks and Eagles 



wings even higher in a sharp) V and then slowly descend, 
accelerating in the last few yards when actually making a 
strike from a diagonal or vertical fall. 

Although hunting Kites may range up to about 2 miles 
from perches, they forage mostly within 100 yards (primar- 
ily males near nest sites) and secondarily up to 0.6 miles 
from perches (Warner 6k Rudd 1 975). At all seasons, Kites 
forage mosdy early and late in die day. From die initiation 
of incubation until the young approach fledging, the male 
provides food for both his mate and the young. (See 
Warner and Rudd 1975 for further details of seasonal, 
temporal, or sexual differences in hunting behavior, hunt- 
ing success, and strike efficiency.) Rodents and birds are 
commonly decapitated before delivery to the nest (Palmer 
1988a). The male usually transfers food to the female at or 
near a perch or the nest. If she declines, the male eats the 
prey or sometimes stores it in a shallow hollow of a limb 
or in a split limb of a tree (Dixon et al. 1957). The female 
may eat prey at a perch, but when she delivers it to the 
young she initially tears it into pieces to feed them (Palmer 
1988a). Later in the season, she drops the prey at the nest, 
and the young are able to dismember it themselves. 

Based on analysis of prey remains from their relatively 
large, owllike pellets, Kites in California eat small mam- 
mals almost exclusively (Bond 1940, 1942; Hawbecker 
1940, 1942; Moore 6k Barr 1941; Stoner 1947; Cunning- 
ham 1955; Dixon et al. 1957; Waian & Stendell 1970; 
Stendell 6k Meyers 1973; Warner 6k Rudd 1975; Meserve 
1977; Palmer 1988a). Prey must be diurnally active, have 
a minimum body weight (usually about 1 oz.), and occur 
in some minimal abundance (Meserve 1977). In most 
instances, the meadow mouse (Microtus californicus) over- 
whelmingly dominates the diet, accounting for 50%- 
100% (usually 70%-90%) of the prey items (references 
above). At times, feral house mice (Mus musculus) may be 
equally important or even the dominant prey. Instances of 
Mus accounting for 85%-90% of die diet are from prey 
remains of nonbreeding groups of Kites or from commu- 
nal winter roosts (Meserve 1977)- Because of their much 
smaller size (three times less by weight) than Microtus, Mus 
may not provide sufficient energy for Kites to nest success- 
fully. In two California studies, numbers of Microtus prey 
were highest during summer and lowest in winter, and vice 
versa for Mus (Warner 6k Rudd 1975, Stendell in Meserve 
1977). The harvest mouse (Reithrodontomys megalotis) is a 
distant third in dietary importance, occurring in about 
5%-10% of the pellets. Odier small mammal prey of 
minor importance are pocket gophers, pocket mice, kanga- 
roo rats, white-footed mice, shrews, and die young of 
cottontail rabbits, woodrats, and ground squirrels. Rirely, 
small ground-dwelling birds, snakes, frogs, lizards, and 
large insects are taken. Kites occasionally scavenge (Warner 
6k Rudd 1975). 

134 



Since Black-shouldered Kites exploit cyclic populations 
of meadow mice (Krebs 1966) and irregularly irrupting 
populations of house mice (Pearson 1963), they exhibit 
many adaptations to an abundant but temporally and 
spatially fluctuating food supply. Although Kites may nest 
in an area for many successive years, they generally are 
nomadic seasonally, and their breeding populations may 
increase locally and their nest spacing may decrease with 
increasing vole populations (Palmer 1988a). Most birds 
breed successfully in spring during the peak of vole popu- 
lations, but some will double brood during periods of 
plentiful food (Dixon et al. 1957). In addition, clutch size 
increases with prey density, and the number of successful 
nests and the number of young raised are related to the 
percentage of voles in the Kites' diet (B.A. Wright in 
Palmer 1988a). Eggs hatch asynchronously, and the length 
of the nesting period varies with the food supply (Palmer 
1988a). Also, as noted above, food is stored when plenti- 
ful. 

Marin Breeding Distribution 

During the adas years, breeding Black-shouldered Kites 
were concentrated in Marin County in two general areas: 
(1) the valleys and low hills in the south-central coastal 
sector and (2) similar terrain near Novato. This distribu- 
tion pattern was similar to that of die Red-shouldered 
Hawk and may reflect the productivity of lowland areas that 
remain relatively moist during the summer dry season and 
therefore support abundant prey in the respective habitats 
used by these two species. The lack of adas breeding 
records in the northern part of the county near Tomales 
may have been because of overgrazing, which eliminates 
extensive potential Microtus habitat. The limited breeding 
population on Point Reyes is puzzling. Although much of 
diis area is overgrazed, it supports extensive areas of seem- 
ingly suitable marshy grassland and coastal swale habitat. 
A representative Marin breeding locality was Bolinas (NB 
4/14/77 -DS.CA). 

Because Kites tend to be nomadic and to decrease or 
increase rapidly with fluctuating vole populations, future 
adasers should be very cautious in interpreting any changes 
they detect when they repeat the Marin adas at a later date. 

Historical Trends/Population Threats 

Early historical breeding records for Marin County are 
from near Novato in 1901 and 1902 (Ray 1904, G6kW 
1927) and from Kentfield in 1917 (Squires 1917). Ste- 
phens and Pringle (1933) considered Kites "rather rare" in 
Marin County. However, diose audiors and Stephens 
(1 945) together list 1 5 records for the county from 1 920 to 
1945. This seems a rather respectable total, considering 
the very limited observer coverage at that time. 



Hawks and Eagles 



SPECIES ACCOUNTS 



Hawks and Eagles 



Much has been written about historical population 
decreases, a more recent dramatic upswing in the U.S. 
population, and a major range extension into Central 
America (Eisenmann 1971). Grinnell and Miller (1944) 
reported that Kites were common and widespread in 
appropriate habitat in California prior to 1885, but by 
1943 they were rare in, or extirpated from, many sections, 
despite a slight recent trend of recovery. Several audiors 
have since speculated that the species reached a nadir in 
the late 1920s and 1930s and have stated or implied, based 
on limited evidence, that the species was close to extinction 
at the time (Waian &. Stendell 1970, Warner 6k Rudd 
1975, Larson 1980, Pruett-Jones et al. 1980). However, 
Williams (1940) reviewed 109 published accounts and 1 36 
records from correspondence and interviews and con- 
cluded that "no statement as to its actual increase or 
decrease is justified at the present; nor could we say 
whether the bird is holding its own." For the period 1935 
to 1939 alone, Williams assembled records of 32 definite 
and 39 probable California breeding pairs. Considering 
the few observers then and the lack of systematic surveys, 
these data suggest that the species was not in imminent 
danger of extinction as a breeder in California at that time, 
despite its obvious serious decline earlier in the century. 

Despite the problems of tracking population trends of a 
species tied to rapidly fluctuating food supplies, analyses of 
Christmas Bird Count data documented a rapid increase 
of the California population from the mid-1 940s through 
the mid-1960s (Fry 1966, Waian & Stendell 1970, 
Eisenmann 1971, Larson 1980). Subsequent data suggest 
an increase continuing through the mid-1970s (Larson 
1980, Pruett-Jones et al. 1980), but the population since 
the mid-1960s might just have been fluctuating markedly 
from year to year in response to prey populations. Num- 
bers of Kites on Breeding Bird Surveys in California were 
relatively stable from 1968 to 1989, diough data suggested 



a slight decrease from 1980 to 1989 (USFWS unpubl. 
analyses). 

Early declines were apparendy caused by shooting, habi- 
tat loss, and perhaps by overzealous egg collectors. The 
eggs were highly prized because of their scarcity, because of 
the variability between egg sets, and because Black-shoul- 
dered Kite eggs are among the most beautiful of those of 
all North American birds. Illegal egg collecting continued 
until at least 1940 despite laws passed in 1905 to protect 
the birds and their nests (Williams 1940); further protec- 
tion was afforded by legislation in 1957. California's Kite 
population has apparendy increased because of the birds' 
ability to tolerate habitat fragmentation caused by agricul- 
tural practices, to exploit increased Microtus populations 
thriving in fields irrigated year round (Eisenmann 1971, 
Warner 6k Rudd 1975, Pruett-Jones et al. 1980), and to 
reproduce at a high rate. A clutch size of four to five eggs 
and the ability to double brood in a single year are both 
unusual adaptations for a hawk (Hawbecker 1940, 
Eisenmann 1971). Martin (1989) noted that the amount 
of irrigated agricultural land in California increased by 
42% from 1944 to 1978, coinciding with the period of 
dramatically increasing Kite numbers. 

The great year-to-year fluctuations in Kite numbers 
appear, at least in part, to be tied to similar changes in the 
prey base influenced by rainfall. Pruett-Jones et al. (1980) 
found a significant positive correlation between Kite num- 
bers and rainfall. This perhaps is explained by the fact that 
microtine rodents need standing water to reproduce 
(Church 1966) and that their numbers are usually reduced 
in a drought. The decline in Kite numbers in California 
during the 1975-76 to 1976-77 drought and the substan- 
tial increase in numbers in Oregon at that time, including 
their first breeding record (Henny 6k Annear 1978), fur- 
ther suggests a link between rainfall, vole populations, and 
Kite populations. 



135 



Hawks and Eagles 



MARIN COUNTY BREEDING BIRD ATLAS 



Hawks and Eagles 



NORTHERN HARRIER Circus cyaneus 

















A year-round resident; numbers swell 


/"VdYv^V \ 




\ 










from Sep through mid-Mar. 




\j^\ \^ 


Y^y 










An uncommon, local breeder; overall 
breeding population very small. 




\*\^\ 


vLV\" 


\pp*0x 


\ VvV 






Recorded in 48 (21.7%) of 221 blocks. 




^V^ToV- 




-^\) V^\ ^-\ 








O Possible = 32 (67%) 




^\ o- V^A 


~^^K\''Jk 




X^^ 




C Probable = 9 (19%) 
• Confirmed = 7 (14%) 










3rwr^ 


"t3<~ 




FSAR = 2 OPI = 96 CI = 1.48 




'z^r'^ 




voV--V" 


i „ -a<\ J^t^ 


-"■YOrH 


^2o 






b^"^* 








^^<r\ 







Ecological Requirements 

The Northern Harrier is quick to attract attention with its 
conspicuously low foraging flights and the loop-the-loop 
antics of its roller coaster-like breeding display dives. 
Marin County's breeding Harriers inhabit freshwater 
marshes, coastal swales, wet meadows, moist grasslands, 
salt marshes, and hayfields. Throughout their breeding 
range, Northern Harriers occupy a variety of open terrain 
that typically has herbaceous cover, often intermixed with 
woody growth (Palmer 1988a). They generally occur in 
moist or wet areas, which are more likely to provide 
adequate nesting cover and a good prey base. In addition 
to marshland, swampland, or grain field habitats, diey 
sometimes occupy fallow weed fields, cut woodlands, 
young stands of planted conifers, and sagebrush steppe far 
from water. In general, however, they nest in wetter, less 
exposed sites than their crepuscular ecological equivalent, 
the Short-eared Owl. 

Harriers select ground nest sites in grassy areas, in 
cattails, in mixtures of herbaceous and woody growth, in 
weed patches, among low brush or close beside bushes or 
trees, or in grain fields or other low cultivated croplands. 
They also occasionally nest on muskrat houses in water or 
on accumulated floating vegetation; exceptionally, birds 
have located their nests on haystacks or, once, 20 feet up 
in a willow in an old Swainson's Hawk nest (Palmer 
1988a). Ground nests tend to be well concealed, at least 
from the sides, in densely vegetated areas within a marsh 
or field; rarely, they are fully exposed. 

136 



The nest is generally a shallow, slightly hollowed plat- 
form situated on the ground, perhaps in a depression, or 
on top of flattened, low vegetation (Dawson 1923, Bent 
1937, Palmer 1988a). It may be simply a hollow lined with 
grasses, or, particularly in damp places, it may be built up 
widi sticks, straw, reeds, and weed stems and lined with 
finer vegetative parts and perhaps a little moss or feathers. 
The height of the nest wall varies with the height of water 
in tidal areas, and nests in wet areas often act as bulky, 
floating rafts. Harriers sometimes use nests several years in 
a row and add new materials annually (Palmer 1988a). 
Young over five days old usually leave the nest if disturbed 
or to seek shade in covered portions of runways. The 
young usually return to the nest when the female returns 
with food. With time, these hideouts often become crude 
platforms that the female sometimes uses as a distribution 
center for prey. At wet sites, the young are less likely to 
leave the nest before they can fly. 

Northern Harriers may be colonial, even within tracts 
of apparendy similar habitat (Palmer 1988a). Although 
rather opportunistic feeders, their distribution seems to be 
tied closely to the presence of small, diurnal, primarily 
grassland rodents (mostly microtines) and perhaps birds to 
some extent. In addition to their tendency toward colonial 
nesting, Harriers also exhibit a strong bent toward bigamy, 
or even harem polygamy with males sometimes mated with 
up to seven females. Polygamy tends to be practiced by 
older birds, especially in dense Harrier populations when 
voles are abundant. 



Hawks and Eagles 



SPECIES ACCOUNTS 



Hawks and Eagles 



The male typically caches and delivers all the food 
during incubation and early brood life. About five days 
after the eggs hatch, females begin short hunting flights, 
which they increase and extend throughout the remainder 
of the nesting cycle. Monogamous males generally tend to 
supply food for nesdings longer than polygamous males, 
which often cease prey deliveries to less favored nests. This 
can lead to instances of female attempts to intercept and 
snatch prey from the talons of a male going to feed another 
female, and to piracy attempts between females mated to 
the same male. More typically, the female flies up to seize 
food dropped by the male upon delivery. Food transfers 
usually occur within 100 yards of the nest, though some 
may occur over 0.6 miles away. The altitude or method of 
prey transfer may vary depending on the stage of the 
nesting cycle, the weather, the size of prey delivered, or 
pesticides accumulated by the adults. Before delivery, die 
male typically beheads the prey, sometimes eviscerates it, 
and, if it is a bird, more or less plucks it. Frogs are skinned, 
and the fur of voles is stripped off and discarded. On 
recovering the prey, the female will, if need be, prepare it 
and return to the nest to feed the young; she sometimes 
first eats a portion. The talons of Harriers are best adapted 
for seizing and holding prey. Consequendy, much killing 
is done with the beak, often incidentally as the bird begins 
feeding on the neck and throat of prey. Harriers eat prey 
piecemeal and usually consume everything but the gastro- 
intestinal tract. 

Northern Harriers exhibit marked overlap in their 
home ranges or hunting areas, especially away from nests 
(Palmer 1988a). These areas are not hunted uniformly, as 
birds again and again fly routes that enable diem to 
surprise prey, sometimes hunting as little as 20%-30% of 
their territory. This results in particular prey species com- 
posing a significant portion of the diet for a short period, 
and then possibly not recurring. In one study, radio-tagged 
birds had a daily cruising radius of 1 .25 x 1 .75 miles; they 
used certain areas near the nest much more than others, 
and they departed in the same direction from which they 
had returned with prey. 

The Northern Harrier is our only raptor that seeks small 
and medium-sized prey by low-level, lineal scanning 
(Palmer 1988a). Hearing is also an integral aid to hunting. 
Harriers have angular acoustic resolution within the range 
known for owls and at least four times as great as that of 
"typical" diurnal raptors (Palmer 1988a). Notably, Harriers 
are our only diurnal raptor with well-developed facial discs. 
In the field they are able to locate vole squeaks accurately 
and to attack prey successfully without the aid of visual or 
olfactory cues. Harriers typically hunt from buoyant, sus- 
tained, tilting flight, generally less dian ten feet above die 
vegetation. They do not usually hunt while soaring or 
gliding. Harriers quarter to and fro over short distances 
over fields and marshes, making numerous sharp turns 



and, occasionally, doubling back to reinvestigate likely 
spots. While quartering over dense vegetation, they often 
hover persistendy, or stall midair and drop lower. In such 
instances, they frequendy reach down with their legs and 
foot-stab in an effort to force prey from cover. This tech- 
nique is used particularly against songbirds in bushy vege- 
tation and against rails hiding in wracks of floating debris. 
Harriers will fly from one stranded debris wrack to 
another, hovering and sometimes landing in apparent 
attempts to flush prey. Flying prey are usually taken on the 
first attempt and are not pursued if they flee. Another 
common hunting technique is "border following," in 
which Harriers fly purposefully along ridges and vegetation 
discontinuities, such as fencerows, ditches, or roadsides, in 
efforts to surprise mobile prey. 

Capable of remarkably quick maneuvering at close 
range, Harriers pounce directly on prey from flight, after 
hovering, or they "hook-pounce" in a three-quarter turn 
after overshooting prey (Palmer 1988a). They also make 
slower, deliberate pounces on microtine nests. Males tend 
to border-follow and nest-pounce more and hook-pounce 
less than females do. Generally, males tend to fly lower and 
faster and catch more passerines than females, which catch 
fewer, larger prey. The males' coloration may serve as an 
advantage in hunting sharp-eyed prey in open terrain. 

Harriers also hunt while perched on the ground or, 
occasionally, from stumps or fence posts (Palmer 1988a). 
Rarely, they plunge, Ospreylike, to catch fish in ponds or 
make horizontal passes along the top of streams of flying 
bats. It is unclear whether large birds captured on the water 
are sometimes drowned deliberately or inadvertendy. Har- 
riers also opportunistically take advantage of songbird 
nests uncovered in newly mown hayfields. Additionally, 
they hunt in association with foxes, along the edges of fires, 
and even near targets in active bombing ranges! Harriers 
also pirate food from other species of raptors and vice 
versa. Both sexes appear to hunt throughout the day, but 
activity peaks among locations and years may reflect vari- 
ous activity periods of major prey species taken. Daily 
hunting activity varies widi weather, prey activity, competi- 
tion, and other factors. Harriers perch more in rainy 
weather, tend to hunt more birds when windy, and feed 
on carrion and rob prey more in severe winter weather. 

For North America as a whole, the yearly Harrier diet 
by weight is about 58% mammals, 34% birds, and 8% 
other prey (Clark & Ward in Palmer 1988a). In another 
summary based on number of prey items (n = 2362), 
Snyder and Wiley (1976) reported the North American 
diet was 47.7% birds, 34.8% mammals, 15% inverte- 
brates, and 2.5% lower vertebrates. Harriers, however, 
exploit whatever prey are readily available to dieir hunting 
techniques, resulting in great dietary variation among sea- 
sons and localities. For examples, Errington and 
Breckenridge (1936), reporting on the diet of nesting birds 

137 



Hawks and Eagles 



MARIN c:OUNTY BREEDING BIRD ATLAS 



Hawks and Eagles 



in the Midwest, found drat mammals accounted for 96% 
of the dietary items during a vole outbreak, but only 37% 
at another site during a drought year. Selleck and Glading 
(1943) reported that birds made up 80.6%, mammals 
18%, and reptiles 1 .4% of the total prey items (n 438) at 
four nests in San Luis Obispo County, California. The 
main prey diere were blackbirds, House Finches, other 
passerines, California Quail, and brush rabbits; there was 
considerable variation in the prey delivered to various 
nests. 

Mammal prey may range from shrews to skunks and 
jackrabbits (especially voles and small to medium-sized 
rodents), but, except for their young, larger species are 
probably incapacitated or dead. Bird prey range from small 
sparrows to upland game birds, ducks, and American 
Bitterns, consisting mosdy of small to medium-sized birds 
from sparrows to Mourning Doves; again, larger species 
taken are usually young birds or those found injured or 
dead. Short-eared and Screech owls are occasional prey, as 
are small diurnal raptors such as Sharp-shinned Hawks 
and American Kestrels. Other miscellaneous prey items 
include snakes, lizards, toads, frogs, fish, crayfish, large 
insects (especially grasshoppers), and spiders. Young Har- 
riers may supplement their diet with slow, weak insects and 
snakes that they themselves can catch. In some instances, 
adults catch larger prey for older young. In fall and winter, 
voles and carrion are more important in die diet (especially 
to juveniles). Harriers take the greatest variety of prey items 
in spring and summer when young mammals, young and 
adult birds, and cold-blooded prey are more prevalent. 

Marin Breeding Distribution 

During the adas period, nesting Harriers were concen- 
trated in two sections of Marin County. The stronghold 
was the coastal lowlands, particularly on outer Point Reyes, 
where the extensive grassland and dune system are replete 
with coastal swale marshland. To a lesser degree, breeding 
Harriers concentrated in bayshore marshes and reclaimed 
marshland converted to grain agriculture, particularly in 
the vicinity of the Petaluma River near Novate Represen- 
tative breeding localities were the marshy/brushy border of 
salt marsh at Limantour Estero (NE 4/5/78 —AM); swale 
near McClure's Ranch, Point Reyes (NE 5/11/82 -DS); 
and swale near Brazil Ranch, SE of Dillon Beach (NY 
6/3/82 -DS). 

Historical Trends/ Population Threats 

Mailliard (1900) and Stephens and Pringle (1933) consid- 
ered the "Marsh Hawk" a winter resident in Marin Coun- 
ty, but nesting had been documented on Point Reyes as 



early as 1917 (NE 6/20/17 -GckW 1927). The earlier 
ascriptions of winter residency probably reflected the influx 
of I larriers at drat season and the limited exploration of 
the favored breeding haunts of Point Reyes and bayside 
marshes, radier than any subsequent change in status. In 
fact, much evidence points to declining, rather than 
increasing, populations of breeding Harriers in recent 
decades. 

Grinnell and Miller (1944) noted that breeding Harrier 
populations had been gready reduced by habitat loss in 
"late years." Although there is no numerical documenta- 
tion, Harriers must have continued to decline sharply 
during the period of great human population growth and 
intense diking and filling of the greater San Francisco Bay 
marshes just before and after World War II (Atwater et al. 
1979). As much as 95% of that estuary's tidal marshes 
have been leveed or filled since the Gold Rush, and Harrier 
populations must have plummeted accordingly. Using 
Christmas Bird Count data from 1952 to 1971, Brown 
(1973) documented a continentwide decline in wintering 
Harrier numbers from the early 1950s to the early 1960s, 
when populations leveled off and then increased some- 
what, particularly in California. From 1968 to 1989, num- 
bers of Harriers were relatively stable on Breeding Bird 
Surveys in California (USFWS unpubl. analyses). Concern 
over Harrier population declines has resulted in inclusion 
of the species on the Audubon Society's Blue List every 
year from 1972 to 1986 (Tate 1981, 1986; Tate 6k Tate 
1982); as of 1986 Harriers were considered "down or 
greatly down nearly everywhere." Similar concerns resulted 
in listing the Northern Harrier as a Bird Species of Special 
Concern in California (Remsen 1978). Martin (1989) 
noted the mixed results of reports on population trends of 
Harriers. Though cautioning against the difficulty of inter- 
preting population trends of the species, he felt that Har- 
rier numbers appeared to be stable or increasing slighdy in 
die West. In addition to habitat loss, Harriers have also 
been troubled by eggshell thinning from pesticide accumu- 
lations (Anderson 6k Hickey 1972). Although biocides 
were implicated in declines, at least early on, supporting 
evidence could be stronger. Other postulated causes of 
declines are grazing (Remsen 1978) and, at least formerly, 
shooting (Palmer 1988a). Despite encouraging signs in 
California, continued concern for the fate of this species is 
clearly warranted, as indicated by retention of the North- 
ern Harrier on the state's recent list of Bird Species of 
Special Concern (CDFG 1991b). 



138 



Hawks and Eagles 



SPECIES ACCOUNTS 



Hawks and Eagles 



SHARP-SHINNED HAWK Accipiter striatus 







Occurs year round, though almost exclu- 




^P^-^ N j(\ 


sively as a winter resident and transient 


y\^3^ 


from Sep through Apr; numbers swell 


^-V\ JkK 


\v-Vv3r\ \r\^\^c\t^\' 


substantially during fall migration from 
Sep through mid-Nov. 




\^\\^C\\^D^ p 


A rare, very local breeder; overall breed- 




C\^K^\^ i \\^ > \\ ^V"\ J^\\ J^c\ J^ 


ing population very small. 




^-^X^^nA l"' \ \*^\ \£*%*\ \ ^^\- \-^*^-~-~\z^*\ \ J^ > ^~^. 


Recorded in 7 (3.2%) of 221 blocks. 




ff^\ S^\^-i^^\ ~'c\^\ 3r^\ J^T\ \r\ ></ 


O Possible 6 (86%) 




*J^\^\^\^^X~J^^ 


€ Probable = (0%) 






• Confirmed = 1 (14%) 




Sr^^^^^^^K^^^^ 






p^* \^ i ^^si\/V V-^s. 


FSAR=1 OPI = 7 CI = 1.29 



Ecological Requirements 

This dashing miniature bird-hawk is such a rare breeder in 
Marin County that it is difficult to describe its habitat 
preferences here. The only breeding confirmation for die 
county was of a family group of adults and recently fledged 
young residing in a dense stand of second-growth Douglas 
fir and coast redwood. This forest had an understory of 
mosdy small to medium-sized tanbark oaks, a few sapling 
firs and redwoods, and scattered ground cover of sword 
ferns. Regardless of the dominance of conifers, this habitat 
was noticeably denser than the stands of mixed evergreen 
forest where Cooper's Hawks breed here. 

Grinnell and Miller (1944) knew of few actual nesting 
records for California and hence, apparently based on 
limited evidence, described breeding habitat as "eidier 
deciduous or coniferous woodland, not dense forest but at 
edges or where broken." Continentwide, most birds nest 
in stands of dense young conifers in conifer or mixed 
conifer-deciduous forests; where deciduous trees are the 
dominant cover, they usually select insular conifer stands 
for the actual nest site. Although some authors mention a 
preference for nesting sites near openings (Bent 1937, 
GckM 1944, Palmer 1988a), Sharp-shins nest both in 
broken forests, fragmented naturally or by timber harvests, 
and in large blocks of continuous pristine forests (Reyn- 
olds 1989). Based on extensive surveys in Oregon's conifer 
forests, the three species of Accipiter breeding there all 
select dense stands that provide screening from predators 
and a shady, mild environment (Reynolds et al. 1982, 
Reynolds 1983). In contrast to Cooper's Hawks and Gos- 
hawks, most Sharp-shins in Oregon use denser, younger 



(25- to 60-year-old), even-aged stands for nesting (see Coop- 
ers account for comparison). These stands have shallow, 
single-layered, dense canopies (mean crown closure 68%- 
80%), an abundance of dead limbs on trunks beneath the 
live crowns, and ground cover of patches of ferns, mosses, 
grasses, and low shrubs. A few Sharp-shins nest there in 
dense (mean closure 90%), old-growth (200+ years) stands 
with multi-layered canopies and sparse ground cover; occa- 
sionally, a pair nests in a stand of stunted quaking aspen. 
Sharp-shins there chose nests sites on gende to moderate 
slopes (av. 25%, range 8%-47%), and, unlike Coopers 
and Goshawks, which prefer northerly facing slopes, they 
do not prefer any particular slope aspect. Nest sites tend to 
be near springs or quiet streams, but this may have been 
an artifact of choosing dense forests that tend to be in 
moist situations. 

In Oregon, Sharp-shins place their nests in the denser 
portion of the lower canopy on horizontal branches against 
the trunk or in a crotch of a double or split trunk (Reynolds 
et al. 1982). Nest heights there range from 10 to 80 feet 
above the ground, averaging 75 feet in mature sites and 39 
feet in second-growth sites. Elsewhere, nests are placed in 
similar situations, ranging from 6 to 90 feet above the 
ground (Bent 1937). The nest is a shallow platform of 
interlaced dead conifer twigs lined with finer twigs or outer 
tree bark (Bent 1937, Palmer 1988a). Sharp-shins build 
most nests in conifers but, occasionally, select deciduous 
trees for nest sites. Unusual nest sites include an old Blue 
Jay nest 6 feet up in a sapling; in a hole in a cave; in a 
"hollow prong" of a broken sycamore branch; on high 

139 



Hawks and Eagles 



MARIN COUNTY BRHLDING BIRD ATLAS 



Hawks and Eagles 



rocks; on an old, collapsed magpie nest; and on top of a 
pile of tnmbleweed (Bent 1937, Palmer 1988a, Reynolds 
1989). Sharp-shins sometimes reoccupy nest sites the fol- 
lowing year, usually building a new nest or, rarely, building 
on top of the previous year's nest (Reynolds &. Wight 
1978, Reynolds 1983, Palmer 1988a). In Oregon, Sharp- 
shins reoccupy 40% of nest sites the following year, but 
none thereafter; they build new nests within 100 yards of 
old nests (Reynolds 1983). 

Breeding Sharp-shinned Hawks forage in a wide variety 
of coniferous, mixed, or deciduous forests and woodlands 
(Reynolds 1989). In Oregon, they forage primarily in the 
forest canopy, but in Alaska they also forage extensively in 
the ground-shrub and shrub-canopy zones (Reynolds 
1989). Sharp-shinned Hawks are experts at reckless sneak 
attacks on unsuspecting prey. From concealing foliage, 
usually well up in the forest canopy, diey dash out to seize 
small birds and vanish (Bent 1937, Palmer 1988a); or diey 
drop low to the ground from perches and alternately flap 
and glide, concealing themselves behind vegetation or 
landforms, then pounce on quarry by surprise. Sharp- 
shins do not hesitate to dash fearlessly through dense 
tangles of trees and underbrush in pursuit of prey. They 
also pursue prey on the ground, sometimes jumping or 
running in open areas or through weeds and bushes. The 
male captures all the food for his mate and offspring until 
the midnesding phase, when the female resumes hunting 
(Palmer 1988a). Males will hunt up to nine-tenths of a mile 
from the nest site. Upon capturing prey, they begin tearing 
and plucking at the base of the skull. Favored plucking sites 
(stumps, logs, or horizontal limbs) in Oregon average 1 34 
feet from nest trees (range 69-1 71 ft.; Reynolds et al. 1982, 
Reynolds 1983). Prey is often beheaded before delivery to 
the female or nesdings. 

The Sharp-shinned Hawk diet is over 90% small birds, 
the remainder small mammals, reptiles, amphibians, and 
insects (Palmer 1988a). Snyder and Wiley (1976) reported 
that 93.1% of prey items (n = 1343) were birds, 4.2% 
invertebrates, 2% mammals, and 0.6% lower vertebrates. 
Sharp-shins take birds up to quail size, mosdy sparrows, 
finches, warblers, thrushes, vireos, and swallows. They 
take mammals up to tree squirrels in size, mosdy mice and 
voles, small rabbits, shrews, and bats. Cold-blooded prey 
include frogs, snakes, lizards, and insects, especially grass- 
hoppers, dragonflies, crickets, beedes, large butterflies and 
moths, and caterpillars. During the breeding season, when 
males forage for the family, mean prey size for three sites 
in Oregon and Utah ranged from 0.4 to 1 ounce (Reynolds 
1989). On average, Sharp-shins tend to take smaller prey 
than Cooper's Hawks, but there is controversy as to 



whether female Sharp-shins take larger prey than conspe- 
cific males (Balgooyen 1976, references in Palmer 1988a). 

Marin Breeding Distribution 

The few sightings of Sharp-shinned Hawks during the 
breeding seasons of die adas period were primarily from 
ridges in the Kent Lake area north of Mount Tamalpais. 
The only confirmed breeding record was of the sighting of 
two fledglings accompanied by adults on the east side of 
Bolinas Ridge above Kent Lake on 28 and 30 July 1982 
(GFMc, DS). Although Sharp-shins are definitely localized 
and very scarce breeders here, the true status of the species 
may be masked by its retiring habits and its preference for 
remote areas during the breeding season. 

Historical Trends/ Population Threats 

Mailliard (1900) and Stephens and Pringle (1933) consid- 
ered the Sharp-shinned Hawk a winter resident in Marin 
County. Grinnell and Miller (1944) reported only a few 
breeding records for the San Francisco Bay Area, none of 
which were from Marin. Although the species was scarce 
during the period of intensive field work during the adas 
project, confirmation of breeding then suggests it was 
overlooked as a nesting species in Marin County in earlier 
times. 

Grinnell and Miller (1944) did not report any popula- 
tion declines in California. Based on limited circumstan- 
tial evidence, Remsen (1978) felt that breeding populations 
had declined gready since that time, resulting in placement 
of die species on the state's list of Bird Species of Special 
Concern, where it still remains (CDFG 1991b). Popula- 
tions were apparendy reduced early in the century by 
shooting, particularly in the East (Palmer 1988a). Sharp- 
shin populations declined drastically (mosdy in the East) 
starting in the 1940s, apparendy from pesticide accumula- 
tion and eggshell thinning, documented in the West (Sny- 
der et al. 1973). The species was on the Audubon Society's 
Blue List every year from 1972 to 1986 (Tate 1981, 1986; 
Tate &. Tate 1982). Based on Christmas Bird Counts, 
continentwide declines leveled off in the mid-1960s and 
swung upward by die late 1960s, largely from increases in 
California (Brown 1973). North American breeding pop- 
ulations were relatively stable from 1965 to 1979 (Robbins 
et al. 1986). Numbers increased in California from 1968 
to 1979, but the trend was relatively stable when the 
analysis was extended to 1989 (USFWS unpubl. data). 
Currendy, the most important regional threat to Sharp- 
shinned Hawks is the reduction of nesting and foraging 
habitat from logging (Reynolds 1989). 



140 



Hawks and Eagles 



SPECIES ACCOUNTS 



Hawks and Eagles 



COOPER'S HAWK Accipiter cooperii 













Occurs year round, though primarily as a 




^5^-^ N 








winter resident and transient from Sep 


fSM^S^ 




^w?-^^ 




through Apr; numbers swell substantially 












during fall migration from Sep through 


r^CVnK 


\2A^VA\^V 








mid-Nov. 






r~\^x 


^QrWTJ 


P 


A rare, local breeder; overall breeding 


vs 


\A^>t\x' 


xSOV' 






population very small. 

Recorded in 36 (16.3%) of 221 blocks. 






v>S 




-y.. 


O Possible = 27 (75%) 




'*^%V\°3rO\ 


<Vo 


~q V^f\o Y^C Y><£\- 




€ Probable = 4 (11%) 




2^KVjCVf^v^\ 


viV 


'^\^air\%V^ ; A ? J^r' 


i^V? * 


• Confirmed = 5 (14%) 




V^X2> 








FSAR=1 OPI = 36 CI = 1.39 



Ecological Requirements 

The penetrating red eyes and the harsh, cackling alarm 
calls of a Cooper's Hawk defending its nest site are not 
quickly forgotten by the observer lucky enough to stumble 
upon such forest magic. In Marin County, most Cooper's 
Hawks breed in secluded stands of closed-canopied mixed 
evergreen hardwoods, usually dominated by coast live oak, 
California bay laurel, and madrone (see records below); 
rarely, they breed here in alder-dominated riparian forest 
or woodland. Although Grinnell and Miller (1944) 
emphasized the importance of riparian-deciduous habitat, 
Cooper's Hawks in California breed primarily in live oak 
woodlands (mixed evergreen forests), though they also 
inhabit coniferous forest to a limited degree (Asay 1987). 
Most California nests are in closed-canopied stands of six 
or more trees (rarely in isolated trees) with a subcanopy of 
vertical tree trunks and large branches widi few small 
branches or leaves; ground cover is absent or consists of 
short grass and/or poison oak or a few other shrubs (Asay 
1987, n = 52). Dense canopy cover (about 65%-95% 
closure) is a consistent vegetative characteristic of Cooper's 
Hawk nest sites throughout their range; also, understories 
at nest sites are often relatively open (Palmer 1988a). Marin 
County nest sites resemble the structure of those elsewhere 
in California, though the dominant trees and saplings, 
shrubs, and ferns in the sparse understory/ground cover 
differ (see records below). Most California nest trees (79%) 
are in flat areas, usually bottomlands between hills, and the 
rest (21%) are on steep hillsides (Asay 1987). The flat areas 
appear most favorable for growth of tall live oak trees. 



Cooper's Hawks also breed in coniferous (typically 
second-growth) or mixed forests (Reynolds 1989). In 
Oregon's conifer forests, most pairs of three species of 
Accipiter nest in dense stands on gende to moderate slopes 
with northerly exposures that provide screening and pro- 
tection from predators and a shady, mild environment 
(Reynolds et al. 1982, Reynolds 1983). Most nest sites 
there are also near quiet, ephemeral streams or springs. 
Cooper's Hawks probably select for dense forest growth 
rather than for particular factors (such as water) that 
promote it. These coexisting species use habitats with 
different structures at the nest site associated with the age 
of the forest stand used. Sharp-shinned Hawks use 25- to 
60-year-old even-aged stands; Cooper's Hawks use 30- to 
80-yearold even-aged stands with somewhat larger and 
more widely spaced trees and deeper crowns; and Gos- 
hawks use 1 50+-year-old mature stands ranging from 
closed canopies with few shade-tolerant understory trees to 
stands with more open canopies with many understory 
trees. Cooper's Hawk nest sites in Oregon have an average 
canopy closure of 69% (range 15%-100%, n = 9), many 
dead limbs below the live crowns, and sparse to moderate 
ground cover. The slope gradient at nest trees there aver- 
ages 1 7% (range 0%-80%). In Oregon, yearling females 
nest in younger successional stages than older females do, 
or in stands that have undergone selective overstory 
removal (Moore ck Henny in Palmer 1988a). 

In California's evergreen hardwood forests/woodlands, 
Cooper's Hawks select nest trees that are generally some of 
the most mature trees in the stand, in an area widi the 



141 



/ lawks and Eagles 



MARIN COUNTY BRLTDING BIRD ATLAS 



Hawks and Eagles 



highest canopy cover and the sparsest ground cover (Asay 
1987). Of California nests, 75 of 77 were in live oaks 
(Quercus agrifolia and Q. wislizenii). Cooper's Hawks here 
huild nests in or just below the canopy and, depending on 
die growth form of the tree, either in a fork of the main 
trunk or out on a branch away from the tnink. Cooper's 
Hawks place their nests from three-quarters to four-fifths of 
the way up the tree; the average nest height of 48 California 
nests is 33 feet (range 19-46 ft.). Estimated heights of 5 
Marin County nests ranged from about 25 to 50 feet above 
the ground (records below). In Oregon's conifer forests, 
Cooper's Hawks place their nests either immediately below 
the crown or in the lower crown of the nest tree; nest 
height of 33 nests there averaged 48 feet (range 25-100 ft.) 
above the ground (Reynolds et al. 1982). Most Cooper's 
Hawk nests there are placed on horizontal limbs against 
trunks; and a few are placed out on limbs or in crotches of 
double trunks. In eastern Oregon, many of these hawks 
nest in deformed trees infected with dwarf misdetoe and 
having heavy foliage, "witches brooms," or double trunks. 
Yearling females in Oregon use misdetoe as a nest struc- 
ture significandy less often (50%) than older females (70%) 
do (Moore ck Henny in Palmer 1988a). Throughout 
North America, average nest heights range from 26 to 50 
feet above the ground (Palmer 1 988a); nests may be as low 
as 10 feet or perhaps, exceptionally, on the ground (Bent 
1937). Some nests are built on squirrel, crow, or woodrat 
nests, or on rubble in the fork of a tree; others may be 
incorporated in masses of misdetoe, grapevines, or die 
abnormal, densely branched growth of a limb (Bent 1937, 
Palmer 1988a). 

The nest is a broad, shallow platform of clean, dry sticks 
and twigs, lined with flakes of bark added throughout egg 
laying and incubation. Greenery, usually one or two coni- 
fer sprays, is added to the nest intermittendy. In evergreen 
hardwood habitat in California, the rate of nest site 
reoccupancy was 80% (Asay 1987, n = 41). Birds reused 
the previous year's nest in 32% (1 1 of 34) of the nesting 
areas occupied in consecutive years; half of all nesting 
attempts were in rebuilt nests. In conifer habitat in Ore- 
gon, Cooper's Hawks reoccupied 27% of nest sites in the 
second year and 1 1% in the third year (Reynolds 1983). If 
they reused a nest site, they usually built a new nest; a few 
birds irregularly used alternate nest sites. In another Ore- 
gon study, 10 of 17 (59%) nest sites were reused in the 
following year, and only successfully nesting females over 
two years old returned to the same site (Moore ck Henny 
in Palmer 1988a). 

Cooper's Hawks forage in a variety of cover types— from 
openings to dense forests— though one study in Utah 
showed a preference for foraging in dense stands of small 
to medium-sized trees (Reynolds 1983, 1989). A sugges- 
tion that these hawks forage mosdy in edge situations may 
be an artifact of the ease with which they are seen in open 

142 



settings. On the odier hand, prey is usually more abundant 
in edge situations than deep in dense forests. In Oregon, 
at least, die Cooper's Hawk is more of a generalist than the 
Sharp-shinned Hawk and captures prey in the ground- 
shrub, shrub-canopy, and canopy zones (Reynolds 1989). 

Cooper's Hawks are efficient predators, capable of very 
rapid flight over short distances. Their rounded wings and 
long tails give them great maneuverability, which enables 
them to fly dexterously through dense brush (Brown ck 
Amadon 1968, Palmer 1988a). They rely on concealment 
and surprise to capture quick and agile prey. Cooper's 
Hawks often hunt from a perch and fly down with a 
sudden burst of speed to seize unsuspecting prey (Brown 
ck Amadon 1968, Palmer 1988a). Leaving the perch, they 
often fly low, taking advantage of die contours of the land 
or vegetation for cover, and may fly higher after flying 
quarry. In a typical strike, they stop flapping 12 to 15 feet 
from the prey and begin swinging the feet forward at about 
5 feet (Palmer 1988a). Just before impact, they set their 
wings in a braking movement, dirust the pelvis forward, 
and rapidly extend the feet chest-high, seizing the prey with 
both or, occasionally, only one foot. Cooper's Hawks 
pursue and catch many birds that fly by the trees in which 
they are perched. On occasion, they also fly through 
swarms of bats leaving a cave, singling out one and follow- 
ing its every twist and turn. Coopers sometimes hunt from 
higher flight, stooping falconlike at pigeons in the open. 
They also pursue prey into bushes, stalk or pursue it on 
the ground (by walking, hopping, or half running and half 
flying), and even attempt to flush it from cover. Cooper's 
Hawks use their hearing to stalk quail. They are known to 
down prey in water and hold it underwater until it ceases 
to move. 

Males feed their mates occasionally before egg laying 
and are their sole providers during incubation (Palmer 
1988a); females rarely hunt during the first three weeks of 
the nesding period but do hunt increasingly thereafter 
(Kennedy ck Johnson 1986). The amount of time and the 
time of day the male spends hunting may depend on the 
activity patterns of the prey, and especially the demands of 
the young, which increase to the point requiring hunting 
throughout the day. Males forage out to one and one-half 
to two miles or more from the nest (Reynolds 1983, Palmer 
1988a). The male usually eats the head and viscera and 
does much of the plucking at the kill site (Palmer 1988a). 
He brings the prey to stumps, logs, or large horizontal 
limbs used as plucking sites, where he continues to pluck 
and partially dismember the prey. In Oregon, the most 
frequently used plucking site averaged 177 feet from the 
nest tree (range 138-282 ft.; Reynolds et al. 1982). When 
the young are small, the prey is well plucked, headless, and 
eviscerated, but by the fourth or fifth week, the male brings 
prey only three-quarters plucked and whole (Palmer 
1988a). The female flies out to receive the prey at the 



Hawks and Eagles 



SPECIES ACCOUNTS 



Hawks and Eagles 



plucking site and feeds herself there or at the nest. The 
female initially flies out to the plucking site to retrieve food 
for the young, but later, if she is absent, the male delivers 
prey to the nest. If the male brings more food than is 
needed it is stored for future use on an old nest nearby 
(Brown 6k Amadon 1968). Dead chicks may be eaten, but 
probably larger young do not attack smaller siblings unless 
they are very hungry from a shortage of prey (Palmer 
1988a). 

The Cooper's Hawk diet is about 70.4% birds (mosdy 
medium-sized, such as jays, thrushes, and flickers), 17.9% 
small mammals, 8.9% reptiles (mosdy) and amphibians, 
and 2.1% insects (J ones m Palmer 1988a). In the East, 
birds account for over 80% of the diet, but in the West, 
birds comprise only about 47%-74% of the diet. Cooper's 
Hawks capture larger birds than Sharp-shinned Hawks do, 
although mean weight of birds in die Cooper's Hawk diet 
is highly variable among studies— from 1.5 ounces in the 
eastern U.S. to 4-3 ounces in eastern Oregon (Reynolds & 
Meslow 1984, Reynolds 1989). Cooper's Hawks take birds 
as large as pheasants, grouse, small owls, American Kes- 
trels, Merlins, and crows, and as small as nesding gold- 
finches. Important mammal prey are chipmunks, young 
hares, cottontails, tree and ground squirrels, woodrats (in 
Calif.; P.H. Bloom pers. comm.), voles, deer mice, and 
shrews. Mean weight of mammal prey at three sites in 
Oregon and Utah ranged from 5.2 to 10.4 ounces (Reyn- 
olds 1989). Some Coopers will specialize in certain types 
of prey. For example, in the southern Sierra foothills, a 
breeding pair brought to the nest 63.4% lizards, 29.3% 
birds, and 7.4% mammals (Fitch et al. 1946, n = 41). 
There is controversy as to whether females do (e.g., Storer 
1966) or do not (e.g., Kennedy 6k Johnson 1986) take 
larger prey than males do. 

Marin Breeding Distribution 

During the adas period, Cooper's Hawks were scattered 
widely throughout the forested regions of Marin County in 
the breeding season. The status as indicated by the atlas 
map is probably deceptive, though, considering that 
Cooper's Hawks are very retiring while breeding and prefer 
areas away from human presence. Observers equipped 
with a knowledge of the species' habits and a willingness 
to get off the trail in hilly terrain would likely be able to 
discover many more breeding sites. For example, one atlas 
observer stumbled upon four nests in one breeding sea- 
son—at three of them the hawks were seen only at the 
immediate nest site. Contrary to popular belief, the 
Cooper's Hawk may be a more numerous breeder in 
Marin and other coastal counties than the Red-shouldered 
Hawk. The latter species is much more easily detectable 
than the Cooper's Hawk because it resides primarily in 



lowland areas, where observers are concentrated, and 
because it is very vocal and visible when displaying or 
hunting. 

Representative breeding records are listed with fairly 
detailed nest site descriptions because of the paucity of 
such information for California beyond those found in 
Asay (1987): (1) Bolinas Ridge near Bolinas Lagoon, 
6/2/81 (ARo et al.), NE about 25 ft. up in coast live oak 
in mixed forest of coast live oak, bay laurel, and coast 
redwood with a brushy understory on the edge of a 
redwood-dominated canyon slope; (2) N end of Inverness 
Ridge, 4/29/82 (DS), NE about 50 ft. up in bishop pine 
in a mixed coast live oak, bishop pine, and bay laurel forest 
with a moderate understory of huckleberry, poison oak, 
and hazelnut; (3) on north-facing slope off Marshall- 
Petaluma Rd., 6/21/82 (DS), NY about 45 ft. up in a bay 
laurel in a forest almost exclusively of that species with 
sparse ground cover mosdy of sword ferns; (4) Chileno 
Valley, 7/2/82 (DS), NY about 45 ft. up in a California 
buckeye, in a buckeye, bay laurel, and coast live oak forest 
with sparse understory/ground cover; and (5) canyon off 
Big Rock Ridge, 7/4/82 (DS), NY about 45 ft. up in a bay 
laurel in a mixed forest of about equal proportions of bay 
laurel, coast live oak, and madrone with a sparse under- 
story of sword ferns and hazelnut. 

Historical Trends/ Population Threats 

For Marin County, Mailliard (1900) reported the Cooper's 
Hawk was a "common winter resident"; Grinnell and 
Wythe (1927) listed Inverness as a station of summer 
residence; and Stephens and Pringle (1933) considered the 
Cooper's Hawk a permanent resident, "fairly common, 
more numerous in winter." Prior to the adas work, sum- 
mer reports were few for the county, and the only breeding 
record was of a nest in an alder grove at Muddy Hollow, 
near Limantour Estero in die early 1970s (JH fide GWP). 
Prior data are not sufficient to compare with the status in 
Marin today. Since Grinnell and Miller's (1944) mono- 
graph on California's avifauna, the population of Cooper's 
Hawks breeding in the state has declined to an unknown 
degree, resulting in its listing as a Bird Species of Special 
Concern (Remsen 1978, CDFG 1991b). 

Based on migration counts, Christmas Bird Counts, 
and incidental reports, Cooper's Hawk populations 
declined continentwide, but mosdy in the East, from the 
1920s to 1960s (Palmer 1988a). The Cooper's Hawk was 
on the Audubon Society's Blue List from 1972 to 1981 
and in 1986, and on their list of Species of Special 
Concern in 1982 (Tate 1981, 1986; Tate 6k Tate 1982). 
From 1965 to 1979, Nordi American breeding popula- 
tions were low but relatively stable (Robbins et al. 1986); 
die California Foothills had one of the highest densities. 

143 



Hawks and Eagles 



MARIN COUNTY BREEDING BIRD ATLAS 



Hawks and Eagles 



The California population was relatively stable from 1968 
to 1989, though data suggested a slight decrease from 1980 
to 1989 (USFWS unpubl. analyses). 

Early declines may have been from extensive shooting, 
but declines since the late 1940s appear to be caused by 
DDT accumulation (Henny ck Wight 1972, Snyder et al. 
1973). Populations in the East declined much more dian 
in the West. Eastern Cooper's Hawks carried higher con- 
centrations of DDE than those in the West did, apparendy 
because of the greater reliance of the eastern birds on avian 



prey (see above). California birds have been somewhat 
contaminated with pesticides; the decline of the California 
breeding population probably was caused mosdy by habi- 
tat destruction (Remsen 1978). Today, nesting and forag- 
ing habitat loss from logging remains die main threat to 
breeding populations in the U.S., though indirect human 
disturbance at nest sites and the taking of nesdings by 
falconers pose additional threats (Remsen 1978). Pesticide 
accumulation, loss of wintering habitat, and shooting still 
pose threats in Mexico (Reynolds 1 989). 



RED-SHOULDERED HAWK Buteo lineatus 









A year-round resident; numbers swell 








slighdy during fall migration from Sep 


AdX*rvA 


^^O^^^^qP^^ 




through mid-Nov. 




\\^?^CX^\^\j^O?>^- 




An uncommon, local breeder; overall 


Vfl 






breeding population very small. 




\ Jf\ J*r\ Jt-"\ £> V^AC» \^\ • i><T o J 




Recorded in 56 (25.3%) of 221 blocks. 




3r\ JVA Jr\ j^^o^>\k\Z^0\>^\ 








^^A^\^\\^-V\ J^c\ J<\ < l\\. J\ 






v~Cv 


^S*>£^\^\}>^^ 




O Possible = 29 (52%) 




"x \^\ Jr\'*l^ \^\ \^\ \^\ \^^~-y- 




© Probable 9 (16%) 
• Confirmed = 18 (32%) 




-; \3vyV iA^\^-V\^V\ J-*<La J&$\. \\. J>A 








-i^/V^^A 11 V^\o\><V^V^\^lL--V^V-A^V. 




FSAR = 2 OPI = 112 CI = 1.80 




rj^^Y^nf^ -<T \^\ \-^-\ \>*\ \^\ V^A 


^& 






-V/ ^C.V-^A*T^S# \y\ \i^\ 








i ^^^--x v^s^^^zo \^\ V- 








J* 3 ^ \Jtf ^^~^rX^\ 







Ecological Requirements 

This resplendent woodland hawk inhabits Marin 
County's well-timbered lowland drainages and, secondar- 
ily, adjacent upland slopes. Prime areas include stretches 
of dense riparian forest or woodland, oak-dominated 
mixed evergreen forest, oak woodland, or eucalyptus 
groves, adjacent to or interspersed with openings clothed 
with soft and luxuriant but relatively low vegetation of 
moist grasslands, meadows, swales, or marshland. Forests 
or woodlands provide nest sites, shelter, and some foraging 
opportunities. Although nesting habitat is usually near 
open water, this is not essential as long as moist upland 
openings are available for foraging. The haunts of Red- 
shouldered Hawks here contrast with die drier, more open 
upland habitats frequented by Red-tailed Hawks (see 
account). 



Red-shouldered Hawks place their nests in large trees in 
stands of mature timber, often near openings; there may 
or may not be a well-developed understory at the nest site 
(D. Shuford pers. obs.). Red-shoulders generally situate 
nests more than halfway up the tree, below the canopy 
crown; Red-tails generally build much higher (Palmer 
1988a). The average height of 274 widespread nest records 
is 47 feet, with a range of 8 to 110 feet (Apfelbaum 6k 
Seebach in Palmer 1988a); exceptional nests have been 
found on the ground (Palmer 1988a). Nests are generally 
placed in a main fork where the trunk divides into three or 
more branches (Bent 1937). They are seldom built on a 
horizontal branch against the trunk and very rarely in the 
fork of a branch. The nest is a substantial, well-built 
structure fdling die crotch to a considerable depth. Red- 
shoulder nests are smaller than Red-tail nests and contain 



144 



Hawks and Eagles 



SPECIES ACCOUNTS 



Hawks and Eagles 



more soft material than those of accipiters. Red-shoulders 
sometimes use nests built by Cooper's Hawks or nests 
previously occupied by owls; they also will build over old 
squirrel nests. Red-shoulders build the nest from sticks or 
twigs mixed with strips of bark, dry leaves, lichens, mosses, 
and twigs of evergreens with needles attached. They line it 
with fine shreds of bark, soft mosses or lichens, fresh 
conifer sprays, and, as incubation progresses, downy feath- 
ers. Decorative greenery or other "symbolic" materials 
added to the nest (months before egg laying through the 
nesding stage) may function to indicate active attachment 
to a site (Palmer 1988a). Later, eggs and young would seem 
adequate notice of occupation. An apparent increase in the 
addition of greenery through the nesding phase (Portnoy 
6k Dodge 1979) may support Bent's (1937) contention 
that this material may be for sanitary rather than ornamen- 
tal purposes. Greenery or its substitutes may include coni- 
fer sprays, green leaves, whole plants, blades of cornstalks, 
dried tent caterpillar webs, ears of corn or corncobs, tissue 
paper, and nests of various passerines (Bent 1937, Palmer 
1988a). Red-shoulders tend to build new nests each year, 
but occasionally they use them for two or three successive 
years or, more often, return to the old, alternative nest after 
a lapse of two or more years (Bent 1937). 

The Red-shouldered Hawk sometimes hunts by gliding 
just over the tops of the forest, through the woods, or flying 
low over marshes or meadows (Bent 1937). It can slip 
upon prey at close range in trees or pounce on smaller, 
slower prey on the ground (Bent 1937), relying on surprise 
more than speed (Palmer 1988a). Red-shoulders are most 
frequendy observed waiting patiendy on relatively low 
perches on trees, utility poles, wires, or fence posts, from 
which they drop or swoop down on prey. In California, in 
more than 6000 hours of observation including over 250 
prey capture attempts, Red-shoulders always initiated 
attacks from a perched position (P.H. Bloom pers. comm.). 
This hawk's hearing is extremely keen, and it may rely on 
hearing as much as sight for hunting (Dixon in Bent 1937 
and Palmer 1988a). 

As the moist habitats frequented and the hunting tech- 
niques used suggest, the Red-shouldered Hawk concen- 
trates on cold-blooded vertebrates and, in some places, 
seasonally, on small mammals (Palmer 1988a). Snyder and 
Wiley (1 976) reported that prey items (n = 141 3) consisted 
of 55.6% invertebrates, 21.2% lower vertebrates, 20.2% 
mammals, and 2.8% birds. In southern California, small 
rodents are the principal prey (P.H. Bloom pers. comm.). 
Dietary items include snakes, toads, frogs, or other 
amphibians up to bullfrog size; mammals mostly from 
shrew (more than voles) to chipmunk size; small lizards 
and young turtles; a few small to medium-sized birds; a few 
small fish; a few crayfish; considerable numbers of insects, 
usually cricket and large-grasshopper size; and the odd 
centipede, earthworm, or snail. Red-shoulders are able to 



transport, sometimes drag, or eat in place, surprisingly 
heavy prey, such as small herons, full-grown squirrels, 
ducks, opossums, and muskrats. They also occasionally eat 
carrion. 

Marin Breeding Distribution 

During the adas period, Marin County's breeding Red- 
shouldered Hawks were concentrated in two general areas: 
in the San Andreas fault zone, primarily of the Olema 
Valley; and, more extensively, in the lowlands around 
Novato. The paucity of Red-shoulders in the lowlands of 
the northwestern sector of the county may have reflected 
the lack of appropriate moist grassland and meadow edges 
to riparian areas there caused by heavy grazing. Represen- 
tative nesting sites were in a Douglas fir in Inverness (NY 
May-early Jun 1982 -AckJWe); in Olema Marsh (NB 
2/25/81 — DS); in an alder grove at Stinson Gulch (NE 
4/26/82 — DS); in a eucalyptus along Novato Creek in 
O'Hare Park, Novato (NE 4/26/82 -ScC, DS); and in a 
valley oak along San Jose Creek, E of Hwy. 101, Novato 
(NB-NY 2/19-6/7/78 -MGN). 

Historical Trends/Population Threats 

Mailliard (1900) considered the "Red-bellied Hawk" an 
"occasional winter visitant" in Marin County, and Ste- 
phens & Pringle (1933) considered it a "rather rare" 
resident here. Compared with the current status, this 
would suggest an increase in numbers historically, whereas 
instead numbers have probably decreased. In earlier times, 
the status of this species was apparendy underestimated 
because of the limited ornithological attention focused on 
the Point Reyes area or on the northern sectors of the 
county, including Novato, areas where the Red-shouldered 
Hawk is most numerous today. Although information is 
lacking, Red-shoulders have likely been displaced in recent 
times from former breeding areas in the moist lowlands of 
the now heavily developed Highway 101 corridor in east- 
ern Marin. 

Grinnell and Miller (1944) reported that the species was 
gready reduced throughout California and even extirpated 
locally "due to progressive human occupancy of the land." 
In reviewing the recent status in California, Wilbur 
(1973a) concluded that despite local displacement, extirpa- 
tion had not occurred in any major segment of the original 
range. A major population decline in the Central Valley 
(Wilbur 1973a, Gaines 1974) has reversed itself recendy 
(S.A. Laymon pers. comm.). Suggestions of increases in 
some coastal counties (Wilbur 1973a) may reflect 
rebounds from former declines or, alternatively, an artifact 
of increased observer effort. In southern California, these 
hawks are reoccupying parts of the Los Angeles basin 
where mature trees now provide nesting habitat in certain 
residential areas, parks, and cemeteries; they have also 
expanded into date palm plantations in the Mohave Desert 

145 



Hawks and Eagles 



MARIN COUNTY BRFFDING BIRD ATLAS 



Hawks and Eagles 



(Harlow 6k Bloom 1989). On the whole, Red-shouldered 
Hawk numbers increased in California from 1968 to 1989 
but were relatively stable from 1980 to 1989 (USFWS 
unpubl. analyses). Caution is still warranted, though, as 
the ongoing expansion of the human population in low- 
land corridors puts additional pressure on this species 
(despite its ability to adapt to some residential situations. 

From the late 1950s to the early 1970s, eggshell thick- 
ness of Red-shouldered Hawk populations in southern 
California was reduced by 3%-14% from the accumula- 



tion of pesticide residues (Anderson 6k Hickey 1972, 
Wiley 1975). Reproductive success in southern California 
between 1972 and 1987 appeared to be normal, and the 
eggshell thinning is currcndy being investigated (Harlow 
6k Bloom 1989). The loss of riparian and oak woodland 
nesting habitat is die most serious factor currendy affecting 
the species in California. 



RED-TAILED HAWK Buteo jamaicensis 









A year-round resident; numbers swell 


JO 


\^*\ j£^\ *-J>P\ 0~>\ _i^_X« p-<\^r-^ 




Sep through Feb. 
A fairly common, nearly ubiquitous 




' V^A * JrlA oV-^A °Je^Y *\ r. \ q A^CA ° A^A •JpV^ - 




breeder; overall breeding population 




\^\ •A^\ °V\o> ; ^\ovAoVA ©A^a • V-^T \ 
^\ • V<5\ • V^A o v-'A ° Wto A>A © Y>-A~ o\>-V ° ^A 

V^k5oo\ # JV<\ °\<\ °A<A .•3r^*3r\ ° Jr^A *^ 
\ ^^A nj^A o \^\ • jr-^A * XJaA o \^\ o V-^\ \-^ ' 




large. 
Recorded in 213 (96.4%) of 221 




Vso Ji<?s\ QJirA • \^\ o ,V^\ o \^\ o \>\ © Y^-A 
VV^-^H^V^A® \^oWVe\W\o \A-A © Y>A; • \ 




blocks. 




Va © J<<vvP V"\ ® A^AeA'V^A © V^ ° Y^A ©>^AfT^>^ 








X VV^XonA "tA oV^-A ©i&S^A^vvQAp^TDAf-^© \>^ > 








Vt\ ° Y-^^Ask^x © VTovlfl V-^A © A^x »-V^\ © r^ 
X Vj^X '•;A > aj;v^'Oy>Ao V--A ©> > A» V^x© V<7 
\-+£v-» XPa oA^A© \x\o Wfo A>r\ ■© V-'A • \>r( " 




O Possible = 111 (52%) 






^^ 




API^svA'kaPVA k\o Y>A'© X^A-o LA 
\K\ -,© V--A • A^\ '♦ \c--AA© >^A © V--^\ © jaA o V-A^A> 


. r - 


© Probable = 56 (26%) 




U^v%>-^\* V^A o XS>Ao V^dpjjVM-o wr\ ov^v 

-4"i VU4-A>» X ^--T n \^-"A ».\ ^*T riSV ^--\ n V rVTS X ^V A X — ~v 




• Confirmed = 46 (22%) 






JP<\(OStk^\o Vw\° \^.o\^^=b\^%~6\>\o V>A«^ 










Io^jksjb^ — X° VA»vA° X^A© a^A-® V-"\ • V< 
Q^x ^^ ^<o V>A © wo \£-*\ © -vc\ • V*C2\ 
■<x<y\^ ^-<r o V-^A o v-'A ® A-^A o^A^x^L 


^?& 


FSAR =3 OPI = 639 CI = 1 .70 




IJ^> \*/ ^^*^\^ 







Ecological Requirements 

The Red-tailed Hawk is a bird of myth, song, and legend, 
but even more a part of everyday reality as it soars gracefully 
overhead on outstretched wings. Even our jays, for 
unknown reasons, pay homage to the Red-tail by their 
imperfect screeching vocal imitations. In Marin County 
and elsewhere, Red-tails are primarily birds of forest or 
woodland edges. Here they hunt mosdy in open, relatively 
dry upland grasslands that host suitable foraging perches, 
which may be exposed limbs of isolated trees, trees in 
clumps, woodlots, edges of woodland or forest, utility 
poles, or large rock outcrops. Foraging perches are usually 
scattered throughout the territory, though certain ones are 
favored (Palmer 1988b). Nest site requirements are iso- 
lation from disturbance, a commanding view, and un- 
obstructed access. Red-tails typically nest high in the open 
crown of a tree taller than those surrounding it and 
generally within view of several perch sites. Chosen trees 

146 



are often well up a slope or on a ridge or hilltop, in a clump 
or grove of trees, in a woodlot, or, occasionally, in an 
extensive forest. Tree nests are usually situated at the 
junction of large limbs with the trunk or in crotches 
formed by two or more large limbs (Bent 1937). Red-tails 
also nest on the crossbeams of utility poles. Although 
preferring lofty views, Red-tails select varying nest sites, and 
in the West, nest heights range from a few feet (in desert 
habitats) to 120 teet (Bent 1937). The average height of 22 
nests in sycamores in San Diego County was 55 feet, 
ranging from 43 to 75 feet (Dixon in Bent 1937); this is 
probably fairly typical of most of die wooded sections of 
California. Red-tails use a wide variety of trees for nesting. 
In Marin County, they appear to prefer eucalyptus, particu- 
larly in open ranchlands, and coast live oaks. Red-tails also 
use cliff ledges for nest sites, particularly in arid areas where 
trees are scarce. Red-tails sometimes nest in unused aeries 



Hawks and Eagles 



SPECIES ACCOUNTS 



Hawks and Eagles 



of Golden Eagles; in nests of ravens, crows, and other 
species of buteos; and in nests previously used by owls; or 
they will build on the platforms of old squirrel nests (Bent 
1937, Palmer 1988b). They may build a nest each year for 
at least several years, reuse a nest during successive years, 
or leave a nest vacant for a year or more and then reuse it 
(Palmer 1988b). The nest is a large, bulky affair made of 
sticks and twigs, lined with items such as strips of bark, 
small twigs, and lichens. For a period of weeks, before 
laying to late incubation, Red-tails add greenery or "deco- 
ration" to the nest in the form of conifer sprays, deciduous 
twigs and leaves, corncobs, cornhusks, cornstalks, willow 
and aspen catkins, a variety of other plant material, various 
rubbish, and even oriole nests! It is not always clear what 
is "decoration" and what is nest lining. The addition of 
decorative material appears to be a behavioral derivation of 
prey capture and delivery that serves to indicate active 
attachment to a site (Palmer 1 988b). 

As might be expected for a numerous and widespread 
species, Red-tails are versatile, opportunistic hunters 
(Palmer 1988b). Most frequendy they hunt from an erect 
or forward-leaning stance, high on a perch, waiting for prey 
to reveal itself. For close prey, Red-tails glide downward at 
an angle with few wingbeats; for more distant prey they 
approach with a few rapid wingbeats alternating with 
glides. Watching for movements of the intended prey, on 
the final gliding approach at about a ten-foot distance, they 
extend the legs forward and spread the toes. The strike is 
usually made with one foot farther forward. A number of 
aerial foraging tactics are also used. Generally at an altitude 
under 200 feet, Red-tails will flap and glide, quartering over 
terrain to catch prey in the open; they may dodge among 
trees, brush, or rock outcrops, remaining concealed until 
coming upon their prey at close range. Occasionally, they 
also maneuver through thick stands of trees, accipiterlike, 
usually striking prey on or near the ground. Red-tails also 
swoop down from hovering flight or from an immobile 
position while facing into the wind on set wings (Dunne 
et al. 1988). When streams of bats are departing from 
caves, Red-tails sometimes stoop downward on diem with 
half-closed wings or fly parallel and veer sharply toward 
them (Palmer 1988b). Red-tails may actually run on the 
ground when attacking (especially when the prey is large), 
and bound from one to another in pursuit of grasshop- 
pers, crickets, or other small, relatively slow prey. Two 
Red-tails, presumably paired, sometimes hunt coopera- 
tively with one on each side of a tree attempting to catch 
tree squirrels. Red-tails will also pirate from other hawks, 
such as Northern Harriers, and will eat fresh carrion. 
Upon capture, they carry small prey to a feeding perch, 
which is lower than a hunting perch. Voles are swallowed 
whole; larger mammals may be beheaded and the fur 
partially discarded; small birds are beheaded and plucked. 
Heavy prey, which may struggle and crawl into cover, may 



be dragged a short distance to a suitable spot, where it is 
plucked and fed on; the remains may be carried to an 
elevated perch. 

The bulk of the Red-tail diet, up to 80% in some studies, 
is mammals (Palmer 1988b). Birds make up much of the 
remainder, but the menu also includes snakes, lizards, 
frogs and toads, salamanders, fish (mosdy dead), turtles, 
crayfish, various insects, centipedes, spiders, and some 
carrion. Snyder and Wiley (1976) reported that prey items 
(n = 2224) in the North American diet consisted of 50.5% 
mammals, 36.8% invertebrates, 8.5% birds, and 4-2% 
lower vertebrates. The diet of Red-tails in the Sierra Nevada 
foothills (excluding arthropods probably taken acciden- 
tally, n = 507) is 73.7% mammals (mostly small to 
medium-sized ones, such as ground squirrels, pocket 
gophers, and rabbits), 21 .6% snakes and lizards, and 4-7% 
birds (Fitch et al. 1946, n = 4l54). Some Red-tails there 
specialize on larger prey dian odier individuals do. 

Marin Breeding Distribution 

During the atlas period, the Red-tailed Hawk was the most 
widespread of all of Marin County's breeding diurnal 
raptors and one of our most widespread breeding birds 
overall. Red-tails appeared to be most numerous as breed- 
ers in the central and northern sections of the county, 
where lowland valleys and rolling hills are dominated by 
grassland interspersed with broken woodland and forest or 
extensive tree plantings. Representative breeding localities 
were the eucalyptus grove along Hwy. 1 about 1 mi. S of 
Sonoma County border (NB 2/1 1/78 — SJ, DS); the euca- 
lyptus grove at Brazil Ranch, SE of Dillon Beach (NE 
4/28/82 — DS); and the eucalyptus grove near SE corner 
of Abbott's Lagoon (NE 5/11/82 -DS). 

Historical Trends/ Population Threats 

The planting of cypresses and especially eucalyptus (used 
as hunting perches and nest sites by hawks) in the once 
nearly treeless northwestern part of Marin County appar- 
endy has enabled breeding Red-tails to expand locally in 
historical times. This expansion has probably been offset 
by displacement in parts of the heavily developed areas of 
Marin, particularly along the Highway 101 corridor, as has 
been noted elsewhere in California (G&.M 1944). Red- 
tailed Hawk numbers appeared to increase slighdy on 
Breeding Bird Surveys in California from 1968 to 1989 
but were relatively stable from 1980 to 1989 (USFWS 
unpubl. analyses). Personnel of wildlife agencies respond- 
ing to questionnaires felt that Red-tailed Hawks were 
declining in California. The authors summarizing that 
survey data indicated that most of the state's populations 
were stable, though local populations, such as on die 
southern coast, were declining rapidly, primarily from 
housing developments and fire (Harlow 6k Bloom 1989). 



147 



Hawks and Eagles 



MARIN COUNTY BREEDING BIRD ATLAS 



Hawks and Eagles 



GOLDEN EAGLE Aquila chrysaetos 










A year-round resident. 






A rare, local breeder; overall breeding 
population very small. 
Recorded in 38 (17.2%) of 221 blocks. 


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\ >c^ >^\ Jr\ JrVoV^A V^\°Jv^\ V^\ 


P 


O Possible - 33 (87%) 


^^^^^^^c^x^ 




€ Probable = (0%) 






• Confirmed = 5 (13%) 


vAi3c\S\^^^ 


^KSp. -r- 


FSAR=1 OPI = 38 CI = 1.26 






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b^* *^^J ^^^^X- 







Ecological Requirements 

When watching soaring Golden Eagles at close range it is 
easy to fathom why they inspire folklore, creation myths, 
and, most of all, respect. These aerial masters range from 
the arctic tundra to desert regions. Breeding requisites 
include solitude, extensive open-ground foraging areas, 
and suitable nest sites nearby (Palmer 1988b). In Marin 
County, breeding Golden Eagles inhabit expansive interior 
grasslands and oak savannah interspersed with a mosaic of 
forests or woodlands in adjoining canyons or on nearby 
slopes. 

Depending on availability, Golden Eagles build their 
nests in large trees, on niches or shelves of cliffs or steep 
earthen banks, on boulders, on human structures, or, 
rarely, on the ground (Bent 1937, Palmer 1988b). Tree 
nests range from about 20 (rarely 10) to 100 feet above the 
ground and the species most often used in California are 
pines, oaks, sycamores, eucalyptus, and redwoods (Bent 
1937). Active nests in Marin County during the adas 
period were in planted pines, Douglas firs, and redwoods. 
Ideally the nest site should be where an eagle burdened 
with prey can arrive without hindrance on a favorable wind 
or updraft, and where currents allow landing at slow 
speeds (Dixon 1937, Palmer 1988b). Depending on the 
region, nests are often oriented to provide shelter from 
excessive heat or cold (Palmer 1988b). Pairs frequendy 
have additional nests (up to 1 2 total), and they may nest in 
one nest for a number of consecutive years or alternate nest 
sites; there is no obvious pattern of occupancy among 

148 



various pairs (Palmer 1988b). Certain nest sites are mag- 
nets attracting a succession of owners over perhaps hun- 
dreds of years or more. 

These eagles' nests are very large, bulky affairs that vary 
considerably in size, depending on the amount of material 
added when reused. The body of the nest consists of large, 
dry sticks; the lining is typically of finer material, such as 
coarse roots, leaves, moss, lichens, rabbit fur, dry grass, 
and sprigs of bush or tree greenery. Some nest lining 
oddities are cow bones, deer antlers, barbed wire, burlap 
bags, newspapers, stockings, and other rubbish, including 
a cowboy hat (but no sign of cowboy, horse, or boots)! As 
is typical of raptors, Golden Eagles bring small branches 
or twigs of greenery (pine, deciduous, or other) to the nest 
throughout the reproductive cycle (sometimes to all nests 
in a territory) and at other times (Palmer 1988b). Greenery 
may function to advertise ownership or reinforce attach- 
ment to the territory. 

Golden Eagles usually hunt early and late in the day, 
mosdy when in flight (Palmer 1 988b). Soaring birds sweep 
and circle high above the ground, descend upon spotting 
prey, then fold their wings and plunge headfirst. About 
three yards above ground, they check their flight, follow the 
quarry, and grasp it with one or both feet. These eagles also 
fly low to the contours of the ground, keeping out of sight, 
seizing prey they surprise away from cover. Less frequendy, 
eagles hunt from perches, swooping down on prey that 
they spot moving into the open. They also stoop on or 



Hawks and Eagles 



SPECIES ACCOUNTS 



Hawks and Eagles 



chase birds in the air. Rarely, if an eagle misses a squirrel, 
it may wait on the ground until the squirrel surfaces from 
a burrow, then take wing and catch it. Birds also feint at 
snakes to make them uncoil and tire, then seize them 
behind the head with one foot and grasp them farther 
behind with the other. Golden Eagles may hunt coopera- 
tively in pairs (usually later in the nesting cycle) or in 
groups of up to four (Palmer 1988b). One member of a 
pair usually makes the initial attempt at capture. If unsuc- 
cessful, it may flush the intended victim (during the initial 
stoop or after landing and walking about on the ground), 
which is then caught by the other bird, which has been 
soaring above (Carnie 1954, Palmer 1988b). Groups of 
eagles sometimes cooperate to catch large prey, such as 
foxes, Wild Turkeys, or deer and antelope (disabled by 
snow or in poor condition) (Palmer 1988b). 

Overall, the Golden Eagle's diet is about 83.9% mam- 
mals, 14.7% birds, 1.0% reptiles, and 0.4% fish 
(Olendorff in Palmer 1988b). In terms of biomass, the 
main prey of Golden Eagles are rodents (ground squirrels, 
prairie dogs, marmots), hares, and rabbits (Palmer 1988b). 
Goldens sometimes prey on full-grown deer or prong- 
horns, but probably only when the quarry is already 
injured or handicapped. They also take deer fawns or the 
young of other large mammals, adult and young foxes and 
coyotes, and a wide variety of small mammals (in addition 
to those already mentioned), including opossums, skunks, 
muskrats, tree squirrels, woodrats, and odiers in size down 
to deer mice and voles. Locally, Golden Eagles do occasion- 
ally prey on very young sheep (rarely other livestock or 
domestic animals), but consumption is mosdy of stillborn 
young or other carrion. Birds also eat carrion of a variety 
of other species, particularly road-killed hares and rabbits. 
Bird prey range in size from cranes, swans, Wild Turkeys, 
and Great Blue Herons down to, rarely, larks and spar- 
rows. Important bird prey are open-country game birds 
(grouse and pheasants) and magpies. Golden Eagles also 
eat small numbers of fish, snakes, tortoises and turtles, 
large insects such as grasshoppers and Mormon crickets, 
and the odd frog. In the interior central Coast Range of 
California, the Golden Eagle's diet is 77.3% mammals 
(mosdy jackrabbits, ground squirrels, and black-tailed deer 
fawns), 1 3.5% medium- to large-sized birds (mosdy Yellow- 
billed Magpies, Western Meadowlarks, and Great Horned 
Owls, but also such large species as Great Blue Heron, 
Mallard, Turkey Vulture, Red-tailed Hawk, Greater Road- 
runner, and American Crow), 5.6% snakes, and 3.6% fish 
(Carnie 1954, n = 503). The considerable variation in the 
diet there of pairs hunting close together was apparendy 
due to preference for, or specialization on, certain types of 
prey. 

Males feed females during incubation and, early in the 
nesding phase, supply food for both their mates and the 
nesdings (Palmer 1988b). Incubating females leave their 



nests to obtain food brought by their mates to nearby 
plucking sites. When nestlings hatch, males prepare food 
at plucking sites by removing feathers from birds and 
decapitating or dismembering mammals and more or less 
removing their fur. Males dien deliver food items to their 
nests, or females come to get them. When the young are 
small, generally a great excess of food accumulates at the 
nest. Hence the aerie serves as a food cache against possible 
shortage when hunting becomes difficult during prolonged 
periods of inclement weather. When the young get inade- 
quate food, stronger eaglets sometimes attack weaker ones, 
eventually causing death, and they may sometimes eat 
them. 

Marin Breeding Distribution 

During the adas period, Golden Eagles nested only in the 
northern interior of Marin County, but the exact locations 
are masked on the adas map (see Content of Species 
Accounts p. 73). The relatively even spacing of nests, the 
scattered sightings between nest sites, and the fact that 
home ranges in San Diego County range from 19 to 59 
square miles (av. 36; Dixon 1937) suggest that the total 
breeding population in Marin County was about five pairs. 
In 1982, we found four active nests and a fifth unoccupied 
nest used in previous years, which may have been an 
alternative nest of one of die other four pairs (ScC, DS, 
HBa —data on file). The lack of breeding Golden Eagles in 
die extensive grassland areas of the Point Reyes peninsula 
and the Marin Headlands may have been because of a lack 
of ground squirrels and low numbers of jackrabbits there. 

Historical Trends/Population Threats 

Pressure on Golden Eagles, especially through degradation 
or total loss of habitat, became serious in the West only in 
the present century (Palmer 1988b). Grinnell and Miller 
(1944) noted that the species had been reduced in num- 
bers or extirpated in areas of California closely setded by 
humans, but elsewhere in the state numbers were close to 
normal. The Golden Eagle is currendy listed in California 
as a Bird Species of Special Concern (Remsen 1978, CDFG 
1991b) and as "Fully Protected" (Harlow ck Bloom 1989). 
Thelander (1 974) estimated there were 500 breeding pairs 
in California, and Olendorff et al. (in Palmer 1988b) 
estimated the state's wintering population at about 5046 
birds. Numbers of Golden Eagles appeared to increase 
slighdy on Breeding Bird Surveys in California from 1968 
to 1989 but were relatively stable from 1980 to 1989 
(USFWS unpubl. analyses); odier observations suggest 
declines in the state, particularly on the southern coast 
(Harlow ck Bloom 1989). 

In the West, ranchers have put intense pressure on 
Golden Eagle populations because of reputed extensive 
predation on sheep, though the facts indicate such preda- 
tion is limited and local. Golden Eagles were shot from 

149 



Hawks and Eagles 



MARIN COUNTY BREEDING BIRD ATLAS 



Hawks and Eagles 



airplanes as early as fall to spring of 1935-36, when over 
200 birds were killed by "sportsmen" in Tehama County 
(Dale 1936). At least 20,000 (perhaps 40,000) eagles were 
killed, mosdy from aircraft, from 1940 to 1962 in die 
sheep-raising country of west Texas and New Mexico alone 
(Spofford 1964, Palmer 1988b). Golden Eagles were given 
legal protection in 1963. Permits were still issued for 
livestock protection, though eagles could not be taken from 
aircraft or by poison. Hundreds were still being killed 
illegally in the early 1970s, but the enlightened efforts of 



Audubon societies and state and federal agencies have 
greatly lessened the problem. Accidental electrocution on 
power lines has also been a problem but has been reduced 
in some areas by the redesign of transmission structures. 
Golden Eagles have also experienced eggshell thinning, 
deadi, and contamination from pesticides (Reichel et al. 
1969, Palmer 1988b) and have picked up lead concentra- 
tions from the environment (Harlow 6k Bloom 1989). 
Golden Eagles are very sensitive to disturbance; watching 
of nests (even with telescopes at long range) is best avoided. 




A Golden Eagle looks over its territory from its formidable nest. Photograph by Ian Tait. 



150 






Falc 



SPECIES ACCOUNTS 



Falcons 

Family Falconidae 



Falcons 



AMERICAN KESTREL Falco sparverius 




Occurs year round, though primarily as a 
winter resident from mid-Aug through 
Feb. 

An uncommon, fairly widespread 
breeder; overall breeding population 
small. 

Recorded in 122 (55.2%) of 221 
blocks. 

O Possible = 71 (58%) 
C Probable = 26 (21%) 
• Confirmed = 25 (20%) 



FSAR =2 OPI = 244 



CI = 1.62 



Ecological Requirements 

It is common to see these dainty, boldly marked falcons 
emphatically pump their tails or bob their heads after 
alighting in hunched posture on perches in open country. 
Elevation, moisture, or particular plant communities exert 
little influence on Kestrel distribution, as they range from 
sea level to timberline and from deserts to moist forest 
fringes. Kestrels are edge adapted and inhabit the margins 
of a wide variety of forests and woodlands bordering on 
low, open vegetation of grasslands, meadows, and scattered 
brush; open or burned forests or woodlands; and even 
urban-suburban settings. In Marin County, American Kes- 
trels breed along the edges of, or within extensive openings 
in, all the major forest or woodland communities border- 
ing on grasslands, open weed fields, or meadows; around 
isolated woodlots, windbreaks, and ranchyards in expan- 
sive pasturelands; or in similar urban-suburban environ- 
ments. In winter, the sexes segregate by habitat— females 
mosdy use expansive open habitats with few trees, and 
males mostly use clearings in habitats with more trees and 
brush (Koplin 1973, Mills 1976, Stinson et al. 1981). 
These differences may reflect the dominant females' forc- 
ing the males into less suitable habitats, thereby reducing 
competition for food, or sexual differences in preferred 



habitats harboring preferred prey. Smallwood (1988) attri- 
buted sexual habitat segregation on the wintering grounds 
to earlier migration of females, which occupy habitats of 
superior foraging quality first, leaving poor-quality habitats 
to later-arriving males. A suggestion that habitat separation 
may also be widespread in summer (to a lesser extent than 
in winter; Mills 1 976) is countered by the knowledge that 
females initially center most activities around the nest site 
and are fed by their mates, which range widely throughout 
dieir territories, and that, in some areas at least, there is no 
sexual separation in die foraging niches of breeding birds 
(Balgooyen 1976). The main requirements for breeding 
Kestrels are low, open vegetation for ground foraging, 
suitable foraging perches (high preferred to low), an ade- 
quate prey base, and available nest cavities (Balgooyen 
1976, Palmer 1988b). 

Nest sites in particular can limit Kestrel density. A 
suggestion diat Kestrels breed almost colonially or socially 
where there is an abundance of cavities (Palmer 1 988b) 
may be an exaggeration, as territoriality will limit Kestrel 
numbers before nest sites are exhausted in such a situation 
(Balgooyen 1976). Unlike most raptors, Kestrels prefer 
natural tree cavities or those excavated by large woodpeck- 

151 



Falcons 



MARIN COUNTY BREEDING BIRD ATLAS 



Falc 



ers (particularly Northern Flickers) for nesting (Bent 1938, 
Balgooyen 1976, Palmer 1988b). They also use holes in 
cliffs or eroded stream banks (natural or excavated by 
kingfishers), cavities in buildings, chimneys, drainpipes, 
hollowedout fence posts, old pigeon boxes, magpie nests, 
and bird boxes. Kestrels add little, if any, material to the 
cavities and lay their eggs on the bare door or on whatever 
die previous occupant has left behind (Bent 1938). Nest 
heights range from less than three feet above the ground to 
as high as suitable cavities exist— in one case over 350 feet 
up in a 22-story building (Palmer 1988b). Roest (1957) 
reported nest heights at various locations in North Amer- 
ica ranging from 4 to 50+ feet above the ground (most 
10-35 ft.) and Balgooyen (1976) reported 43 nests in the 
Sierra Nevada ranging from 7 to 80 feet (av. 26 ft.). 

Balgooyen (1976) found most nests in the eastern Sierra 
in the lower reaches of basins, presumably because these 
sites were sheltered from weather and allowed easy flights 
back to die nest with food. In Marin County, most Kestrels 
nest in lowland areas because of the greater availability of 
open areas and, apparendy, nests sites there, and probably 
because of a greater prey base in fertile lowland valleys. In 
the Sierra, Balgooyen (1976) found that both the slope 
exposure of the nest tree and the nest entrance tended to 
face east, providing protection from storms and die advan- 
tage of early morning sun. Raphael (1985) confirmed the 
easterly orientation of Kestrels' nest trees and nest cavities 
in Balgooyen's study area; Kestrels there chose these sites 
despite the availability of cavities oriented in other direc- 
tions. Interestingly, woodpeckers in the same area, which 
might similarly benefit from the thermoregulatory advan- 
tages of easterly oriented nest cavities, tended to choose 
northerly facing cavities even though suitable decay was 
randomly oriented. Kestrels nesting in the tropics may 
avoid heat by selecting nest sites facing into prevailing 
winds and away from direct sunlight (Balgooyen 1990). 

Kestrels hunt more or less regularly and continuously 
throughout the day and may or may not show peaks of 
foraging activity (Palmer 1988b). They seek visible and 
vulnerable prey in short, sparse vegetation, where they 
capture most prey on or near the ground and take most 
flying prey on the wing (Balgooyen 1976, Palmer 1988b). 
These falcons mosdy hunt from perches (when available) 
and secondarily hover and hawk. Birds use a wide variety 
of foraging perches, including dead branches and trees, 
stumps, rock outcrops or cliffs, fence posts, telephone and 
power poles, transmission lines, and an array of artificial 
structures. Average perch height in the Sierra Nevada is 22 
feet (n = 328); birds forage from lower perches in high 
winds (Balgooyen 1 976). From perches, birds face into the 
wind and scan terrain for prey by rotating or tilting die 
head, dien snap the head into a fixed position upon 
locating prey and fly down to make the capture. In the 
Sierra, effective capture distance from the perch extends to 

152 



900 feet, with 86% of captures from to 164 feet (av. 112 
ft.). In areas lacking perches, Kestrels face the wind with 
their bodies angled head upwards and hover with shallow 
wingbeats and tail fanned. Sometimes they hang motion- 
less, then plummet direcdy down or drop lower and hover 
again before die final attack. Kestrels hover mosdy from 
heights of 40 to 100 feet and dive headfirst after mammals 
and feet first after insects (Balgooyen 1976, Palmer 1988b). 
During dives, the birds may partly close their wings or 
flutter downward, checking and controlling their descent 
(Palmer 1988b). Whether hunting from perches or hover- 
ing, Kestrels modify their attacks, depending on the type of 
prey (Balgooyen 1976). They direcdy attack frogs, lizards, 
and small mammals from flight. Kestrels track the flight of 
grasshoppers, and after they land, the birds fly to the spot, 
brake, and usually flush the insect and make the kill on the 
wing. If grasshoppers remain still, Kestrels land and walk 
about, crisscrossing the area to attempt to raise the insect. 
From flight, Kestrels also, rarely, pick large insects or 
lizards deftly from tree trunks or rocks (Balgooyen 1976, 
Palmer 1988b). Sometimes they fly rapidly, low to the 
ground, in pursuit of flying prey (Palmer 1988b). Birds 
also hawk insects by flying direcdy out (up to 35 ft.) from 
perches (Balgooyen 1976) or by soaring and flapping 
upward and away from perches, then stalling and diving 
abrupdy at their targets (Palmer 1988b). They sometimes 
dart out from a perch, tilt their bodies, attempt to strike 
down dragonflies with their wings, then drop to the 
ground to seize their prey (Palmer 1988b). Birds may eat 
prey at the capture site, while transported aerially, or on 
arrival at a nearby perch. 

Kestrels sometimes catch birds in flight and secure bats 
from the bark of trees or by diving down on them or from 
behind in straight flight (Balgooyen 1976, Palmer 1988b). 
Rarely, these falcons obtain small nonflying insects, spi- 
ders, or worms with their beaks while running (occasion- 
ally flapping to gain speed) or jumping on the ground. 
Kestrels sometimes persistendy rob nests and return to get 
the rest of a brood (Palmer 1988b). They have plundered 
burrows of Bank Swallows and nests of Cliff Swallows, 
Barn Swallows, phoebes, bluebirds, and House Sparrows 
by hanging upside down and reaching in with one foot or 
by ripping off the top of the nest. Infrequendy, Kestrels 
take birds captured in mist nests or ground traps and hunt 
along the smoky windward edges of fires. Rarely, Kestrels 
eat fresh or decayed carrion. One Kestrel foraged on the 
ground for bread (initially with feral pigeons). 

Kestrels usually capture prey in the talons and kill them 
by biting them with die beak in the head or neck (Bal- 
gooyen 1976, Palmer 1988b). They usually crush the head 
and discard the wings, legs, and other extraneous parts of 
insects; pluck feathers from birds; discard some rodent 
hair and ingest much; and discard large hard parts and 
alimentary canals of birds and mammals. 



Falcons 



SPECIES ACCOUNTS 



Falcons 



Kestrels are generalized predators of invertebrates and 
small vertebrate animals. Prey items consist of 95.7% 
invertebrates, 2.4% mammals, 1.2% birds, and 0.7% 
lower vertebrates (Snyder 6k Wiley 1976, n = 9242). Prey 
taken can vary considerably with locality, season, or the 
tendency of individuals to concentrate on one particular 
type of prey (Balgooyen 1976, Palmer 1988b). Principal 
prey are large insects (especially grasshoppers, Jerusalem 
crickets, dragonflies, beedes, and caterpillars); small mam- 
mals (from shrew to ground squirrel or rabbit size— espe- 
cially voles); birds (from hummingbird to Mourning Dove 
or quail size, but mosdy moderate, size); and reptiles and 
amphibians (small lizards, snakes, frogs, tadpoles, and 
toads). Miscellaneous prey items include crayfish, centi- 
pedes, scorpions, spiders, earthworms, and snails. The 
Kestrels' diet by weight in the Sierra Nevada is 31.7% 
insects (mostly grasshoppers), 26.0% reptiles, 25.7% 
mammals, and 16.6% birds; by occurrence, it is 88.4% 
insects, 8.1% reptiles, 2.1% mammals, and 1.4% birds 
(Balgooyen 1976). In the early season or during inclement 
weather, Kestrels in the Sierra Nevada concentrate on 
birds and mammals. Later on, they rely more on lizards 
and insects and even more so on insects from midseason 
until fall departure. In Humboldt County, Kestrels feed on 
voles and shrews in the absence of insect prey (Collopy in 
Palmer 1988b). Although die sexes generally take the same 
kinds of prey in about the same proportions, there are 
some exceptions. In the Sierra Nevada, breeding males 
and females choose similar-sized prey (Balgooyen 1976), 
but in southern California, breeding males usually choose 
smaller prey than breeding females (Bryan in Palmer 
1988b). 

Males forage in areas of the territory away from the 
immediate nest site and bring females food from four to 
five weeks before egg laying until one to two weeks after 
hatching of the eggs (Balgooyen 1976). Males transport 
prey to perches near the nest site, where they transfer the 
food to females with their beaks (Balgooyen 1976, Palmer 
1988b). Both sexes may cache surplus vertebrate prey by 
wedging or pushing it with the beak into a suitable site. 
Cache sites may range from the ground up to 65 feet and 
include grass clumps, hollow railroad ties, tree roots, 
bushes, fence posts, building gutters, tree limbs and holes, 
and tops of power poles or burned stumps. Although food 
caching may serve to hide prey, the main function is to 
store it as a reserve for times of inclement weather or low 



prey availability or to meet the demands of growing young. 
Cached food is usually eaten within a few hours or within 
2 or 3 days; some food is held as long as 6 or 7 days. The 
young are initially fed by the female via the male for 7 to 
10 days after hatching, then by both sexes separately 
(Balgooyen 1976). Through the first 7 to 14 days, the 
female initially removes all extraneous material from prey 
and feeds the young only flesh. Thereafter, prey are deliv- 
ered to the young whole and unprepared. 

Marin Breeding Distribution 

During the adas period, Kestrels bred widely in Marin 
County, though breeding numbers were substantially 
smaller than winter numbers. Breeding Kestrels were 
sparse or lacking on much of Point Reyes because of the 
dense forest or scrub cover on most of Inverness Ridge, 
and perhaps because of limited nest sites and small popu- 
lations of large insects in the fog-shrouded, windswept 
grasslands of the outer Point Reyes peninsula. The reasons 
for their spotty distribution elsewhere in Marin, particu- 
larly in the grassland-dominated hills east of Tomales Bay 
where large numbers of Kestrels winter, is less clear. Nest 
sites may be a limiting factor: 40% of the blocks east of the 
Point Reyes peninsula that lack breeding Kestrels also lack 
breeding Northern Flickers, which typically provide many 
Kestrel nest cavities. Representative breeding localities of 
American Kestrels were Bear Valley, PRNS (FY 4/22/76 
-RMS); Ignacio area (FY 5/27/77 -RMS); and Carson 
Ridge (DD 3/9/85 -ITi). 

Historical Trends/ Population Threats 

Grinnell and Miller (1944) did not comment on any 
trends in California populations, but Palmer (1988b) 
reported that "widespread deforestation and land develop- 
ment have facilitated a continentwide population increase 
of this remarkably adaptive species." Numbers of breeding 
Kestrels decreased on Breeding Bird Surveys in California 
from 1980 to 1989 (USFWS unpubl. analyses). Eggshell 
thinning in Kestrels has been correlated with increasing 
levels of DDE (Risebrough ck Monk 1989), but even 
though chemical contaminants have caused deaths, there 
have been no drastic declines in Kestrel numbers as a 
result (Palmer 1 988b). There currendy are no major threats 
to western populations (Piatt 6k Enderson 1989). 



153 



Falc 



MARIN COUNTY BRFFDING BIRD ATI AS 



Falcons 



PEREGRINE FALCON Falco peregrinus 



Formerly a year-round resident and breeder until the 1970s, when extirpated as a nesting bird. Now occurs year round, 
though primarily as a winter resident and transient from mid-Sep through mid-Apr. Following a statewide increase after a 
ban on DDT pesticides and the implementation of a captive breeding program, summer sightings have increased in Marin 
and breeding attempted in 1990 and 1991 (successfully). 



Ecological Requirements 

These swift, spirited falcons inhabit open country, where 
their speed and feather-raising pursuit dives strike fear in 
the hearts of the avian populace. Peregrines hunt in the 
air— over oceans, forests and woodlands, marshes and 
wedands, chaparral, and cities— and very rarely take ground 
prey. In coastal areas, Peregrines hunt a great deal around 
estuaries and seabird colonies. Ideally, they select nesting 
aeries that command a wide view, are near water, have 
plentiful bird prey in the vicinity, and are seldom disturbed 
(Palmer 1988b). Peregrine Falcons formerly nested in 
Marin County on cliffs overlooking the ocean (see below). 
In contrast, Prairie Falcons, which have never nested here, 
tend to inhabit more arid environments, select lower, more 
sheltered nest sites, and feed lower to the ground and 
considerably more on mammals than do Peregrines (Pal- 
mer 1988b). Peregrine Falcons will nest in deserts if near 
marshes, lakes, or rivers, and both Peregrine and Prairie 
falcons may nest on the same cliffs or switch ownership of 
nest sites in different years. 

Peregrines typically select cavities or ledges on cliffs, 
often with a sheltering overhang, for nest sites (Palmer 
1988b). They generally lay their eggs on a fairly level spot 
at least two feet in diameter— a larger space is advantageous 
so the young can move about as they grow. Peregrines 
make a nest scrape in debris, such as soil or rocks, and 
sitting birds may pull some of tins material toward them to 
form a rim sufficient to keep the eggs from rolling away. In 
North America, cliff nest sites are usually unmodified or 
may contain old nests of Common Ravens, Rough-legged 
Hawks, Golden Eagles, Red-tailed Hawks, or cormorants. 
Historically, raven nests were often used in Marin County 
(B.J. Walton pers. comm.). Substitutes for cliff nests in- 
clude church towers, castle ruins, bridges, quarries, raised 
platforms, and assorted buildings (mosdy urban). In very 
open country, such as tundra, Peregrines will nest in 
recesses or level spots at or near the top of eroded river- 
banks or on boulders, hills, slopes, dunes, bog hummocks 
or islets, or even on ground with no topographical relief. 
One pair nested in the straw packing inside a barrel cast 
up in salt marsh in San Francisco Bay near Redwood City, 
San Mateo County (Dawson 1923); other birds reportedly 
bred in bay marshes in Santa Clara County (G&W 1927). 

154 



Peregrines will also appropriate solid (rarely flimsy) stick 
nests (usually disused) of a variety of large birds placed in 
dead or living trees (Palmer 1988b). The use of stick nests 
in trees is only widespread in Australia, and there are only 
one or two such records in North America (occupation of 
Bald Eagle nests in British Columbia). On the whole, the 
use of tree cavities, including hollow tops where limbs have 
broken off, is even rarer, though slightly more frequent in 
North America than the use of stick nests in trees. Pere- 
grines often use alternative nest sites within the same 
territory— one or two is common, and birds have used up 
to seven in 16 years. If several sites or ledges are available 
along a cliff, individual pairs may change about in different 
years or shift to different cliffs. If alternative sites or ledges 
are not available, pairs tend to stay put. Birds robbed of 
their eggs commonly move to alternate sites, and birds may 
move and re-lay after breeding failure. Use of particular 
sites may also alternate between Peregrines and other 
species of cliff-nesting raptors. Despite birds' switching to 
alternative sites, some Peregrine territories in Britain have 
been occupied at least since the 1 3th century. 

The Peregrine Falcon is a very fast, extremely agile, and 
versatile raptor specialized for capturing aerial prey. In level 
flight, Peregrines normally fly at 25 to 35 mph and usually 
do not exceed 60 mph; in vertical stoops, they usually make 
strikes at about 100 mph and from a height of 5000 feet 
would reach a terminal velocity of 230 to 240 mph (Palmer 
1988b). Breeding males may hunt throughout the day, but 
mosdy early in the morning and secondarily toward eve- 
ning. Timing of hunting may depend on the activity 
patterns or the size of prey. Peregrines living within colo- 
nies of nocturnal, burrowing seabirds hunt such prey (and 
bats) around their aeries in near darkness. Individuals 
specializing on waterfowl must hunt two times a day, and 
diose hunting small shorebirds must hunt three to four or 
even five to six times per day. Hunting demands peak 
when males are feeding both females and growing young. 

Often, probably when already satiated, Peregrines pur- 
sue flying birds seemingly for the pleasure of the chase 
(Palmer 1 988b) or perhaps to hone hunting skills or tactics 
(B.J. Walton pers. comm.). At other times, they turn 
deadly serious in their attempts. Hunting birds launch 



Falcons 



SPECIES ACCOUNTS 



Falcc 



attacks from a stationary position on a prominence or tall 
tree, or from flight. If the quarry is below them, they may 
make a shallow stoop or climb higher and stoop at tremen- 
dous speed. If their initial strikes fail, Peregrines rise 
rapidly and stoop again, making repeated attempts if nec- 
essary. When prey near cover, Peregrines sometimes get 
beneath them, turn over, and seize them from below. If 
quarry are initially high overhead or are forced upward, 
these falcons spiral upward spectacularly to get above 
them. Fast birds try to keep above falcons or to outdistance 
them in straight-away flight, whereas slower fliers try to 
keep above their pursuers. As a last resort, quarry may 
plummet earthward, but they are closely pursued and 
generally captured quickly. If the prey reaches ground but 
fails to find concealing cover, it may be killed. Peregrines 
may launch attacks from circling flight, stooping at lower 
targets or spiraling up after higher prey. Smaller prey are 
often grasped in the air and carried to earth. Larger victims 
are often struck with such speed (with either or both feet) 
that falcons slash the victim with the rear talon(s) and rip 
right through without holding on. Feathers fly, and the 
prey may be dead as it falls earthward. Peregrines then 
half-circle and alight, and whether the prey is alive or not 
they bite it near the base of the brain, breaking the neck 
with their specialized double-notched beak, adapted for 
this purpose. Mated pairs may hunt cooperatively and may 
stay together year round in Marin. There is some evidence 
that females may maneuver the quarry into a favorable 
position for the male to strike from a greater height, 
coming out of the sun (B.J. Walton pers. comm.). 

Peregrines also hunt flying low to the ground, keeping 
out of sight as best they can behind vegetation and irregu- 
larities of the terrain (Palmer 1988b). Some low flying is 
done deliberately to flush potential prey from vegetation. 
Peregrines also seek the shelter of waves when pursuing 
marine birds at sea. If the quarry escapes by diving, a falcon 
may wait overhead and pick the prey from the surface after 
it emerges exhausted after repeated dives. In forested 
regions, Peregrines take exposed prey crossing water or 
clearings. Peregrines usually flush standing or swimming 
birds before seizing them, but, very rarely, they do take 
some prey from or on the ground or water. Mammals 
(other than bats), usually found in Peregrine nests, are 
probably those pirated from other raptors (B.J. Walton 
pers. comm.). Relatively large mammals, such as full-grown 
ground squirrels, may be killed by a series of stoops at the 
head and neck that stun or blind the individual or fracture 
its skull. Lemmings are reportedly hunted afoot, but many 
are probably snatched by flying falcons. Ground hunting 
is more frequent in inexperienced young Peregrines, which 
catch insects, large slugs, fiddler crabs, and other suitable 
prey by this method. Adults sometimes hawk flying insects 
leisurely. These falcons also follow people, dogs, or harri- 
ers to catch birds they flush. Peregrines capture bats by 



cutting through flocks departing from caves or ambush 
individuals returning to them. They also catch fish break- 
ing the surface, pirate them from Ospreys, and perhaps 
take stranded or dead ones. Peregrines also pirate food 
from other raptors of Harrier to Kestrel size. Peregrines eat 
some smaller prey on the wing. They probably at least 
partially dismember, then eat during flight, prey such as 
bats, voles, and insects, which are unsuitable for plucking. 
Birds captured at sea may also be eaten in flight. These 
falcons generally partially pluck and feed on heavy prey 
where they strike them down, though typically they carry 
food to one or more plucking sites or perches. They pluck 
wings and tail and eat the head to make prey aerodynami- 
cally efficient for carrying (B.J. Walton pers. comm.). The 
beaks and feet of larger prey often are not eaten. 

Peregrines defend a small area around the aerie. Food 
supply probably determines the size of the much larger 
hunting range, which in the breeding season may be over 
40 square miles; the total feeding areas of breeding pairs 
frequendy overlap (Palmer 1 988b). Some foraging is done 
from die aerie or nearby perches, but foraging flights 
during breeding in California range up to 4 to 5 miles 
(mosdy 2 to 3 miles). The male feeds the female during 
prelaying and incubation. After hatching, he feeds both the 
brood and his mate, but the female hunts progressively 
more as her brooding declines. The male delivers prey 
(fresh or retrieved from a cache) to a nearby perch, and the 
females gets it there; or she may meet him in the air and 
obtain it in an aerial food pass or drop. Females may 
retrieve food cached by the male and again cache and 
retrieve any uneaten portion. Food caching also occurs at 
other seasons (B.J. Walton pers. comm.). Caching assures 
a supply of food during bad weather when hunting is 
difficult and at least sometimes is triggered by an abun- 
dance of prey. Sometimes the male delivers food to the 
young direcdy, especially when die female is absent. Avian 
prey are almost always, and mammals are frequendy, 
headless when delivered to the aerie. Males may subsist 
largely on heads at this time, but they apparendy consume 
some whole prey away from the breeding territory. 

Peregrines prey principally on fast-flying birds in the 
open and worldwide have captured well over 250 species 
(Palmer 1988b). These range in size, rarely, from prey 
heavier than the predator (loons, herons, cormorants, 
geese, and large gulls) down to very small passerines. Most 
important prey are usually small to medium-sized birds- 
blackbirds, swallows, shorebirds, seabirds, pigeons, doves, 
etc. The bulk of the prey taken in an area usually consists 
of a few common open-country species. Individuals some- 
times specialize on certain prey species, but less so when 
feeding a growing brood. In California, Peregrines usually 
do not deliver a predominance of any one prey species to 
a nest— prey remains usually mirror the array of the most 
numerous species of appropriate size recorded in the 



155 



Fake 



MARIN COUNTY BREEDING BIRD ATLAS 



Falc 



territory (B.J. Walton pers. comm.). About 25+ prey spe- 
cies is typical at California nests, and many prey items are 
represented at every nest. Coastal Peregrines often special- 
ize on seabirds during the breeding season and on shore- 
birds and waterfowl in winter (Palmer 1988b). Seasonal 
changes in diet likely reflect availability of common prey 
species. Rock Doves are a preferred food, especially in 
cities. Mammals are extremely rare fare, but a wide variety 
are consumed, including shrews, bats, rabbits and young 
hares, pikas, tree squirrels, chipmunks, ground squirrels, 
lemmings, rats, and voles. In California, mammal prey 
include only bats and very small mammals, often pirated 
from Black-shouldered Kites and Red-tailed Hawks (B.J. 
Walton pers. comm.). Extremely rarely, Peregrines eat 
invertebrates, insects, and carrion (not recorded in Califor- 
nia). Evidendy males and females eat prey of the same 
average size (Palmer 1988b). 

Historical Trends/Population Threats 

Peregrine Falcons formerly nested in Marin County on 
precipitous seacliffs (G6kW 1927, GckM 1944). Known 
former nesting sites include the cliffs between the Golden 
Gate Bridge and Point Bonita, the north side of Muir 
Beach, Bear Valley, Drake's Bay, the Point Reyes head- 
lands, Kehoe Beach, McClure's Beach, and Tomales Point 
(B.J. Walton pers. comm.). Peregrines also formerly nested 
on Red Rock in San Francisco Bay, but it is unclear if the 
aerie(s) were on the small Marin County portion of that 
island. In the 1930s, there were seven pairs of Peregrines 
nesting in Marin County in one year, but by the 1960s 
there were only one or two pairs (B.J. Walton pers. 
comm.). In the 1970s, there were no documented pairs; in 
the 1980s, circumstantial evidence suggested at least one 
pair may have nested here, though confirmation is lacking. 
In 1990, a pair began nesting on a cliff at the Marin 
Headlands, but the attempt failed soon after the young 
hatched (Golden Gate Raptor Observatory). In 1991, this 
same pair apparendy nested farther north along the soudv 
ern Marin coast and, unassisted by humans, successfully 
fledged four young. Breeding Peregrines may become re- 
established here via management efforts or from recruit- 
ment from natural population expansion/recovery. Young 
Peregrines were released at hack sites at Muir Beach from 
1983 to 1988 and at Tomales Point in 1987 and 1988 
(Linthicum 1988). One of the birds released at Muir Beach 
is now nesting on the San Francisco-Oakland Bay Bridge 
(B.J. Walton pers. comm.). 

Grinnell and Miller (1944) noted for California at the 
time that Peregrines were "fairly common for a hawk" and 
that, except locally, numbers were holding fairly constant. 
North American populations also were relatively stable up 
to that time, but rather suddenly, beginning in the late 
1940s, here and in Europe, Peregrine populations began 
to decline dramatically. This led to a severe population 

156 



crash by die early 1960s, which bottomed out by 1973 to 
1975 (Hickey 1969, Kiff 1988). Although other factors 
have affected Peregrines— mortality from collisions with 
wires, shooting on wintering grounds, habitat loss, egg 
collecting, taking young for falconry, disturbance at nest 
sites, and human encroachment affecting prey species— this 
decline was clearly linked to pesticide pollution. Because of 
their position high on the food chain, Peregrines readily 
concentrate contaminants from their prey. Population 
crashes in North America (and elsewhere) were primarily 
from reproductive failure caused by DDE-induced eggshell 
thinning (Cade et al. 1988, Peakall &. Kiff 1988, Rise- 
brough ek Peakall 1988), though adult mortality from 
dieldrin or other organochlorines may also have been an 
important factor (Cade et al. 1988, Nisbet 1988). In 
California, Peregrine eggs analyzed to date have extremely 
high levels of DDT, DDE, PCBs, and dioxin (B.J. Walton 
pers. comm.). 

The size of the historic Peregrine population in Califor- 
nia is unknown. Herman et al. (1970) estimated that 100 
California aeries were producing young until at least the 
mid-1 940s. The actual number of aeries active in a given 
year at the turn of the century may have been as many as 
300; extensive searching of historical records has now 
revealed about 200 pre-DDT era breeding sites, and many 
probably went unrecorded (B.J. Walton pers. comm.). 
From 1 946 to 1 950, there were definite signs of reproduc- 
tive problems, but no evidence of serious population 
decline; from 1950 to 1960, there was a precipitous decline 
in die number of pairs producing young; and from 1961 
to 1969, diere were sporadic nesting success and further 
declines, leaving fewer than 10 known breeding pairs in 
the state in 1969 (Herman et al. 1970). By 1970 the state 
breeding population did not exceed 5 successful pairs— a 
reduction of 95% from the estimated level in the mid- 
19405 (Herman 1971). Because of these declines, Pere- 
grines were placed on both federal and state Endangered 
species lists (USFWS 1989a, CDFG 1991a). Use of offend- 
ing pesticides was gready restricted in the United States in 
1972 (Kiff 1988). Since these bans, populations have 
expanded through natural recovery and management 
activities, which have included manipulation of eggs and 
young at nest sites in California beginning in 1977 (Wal- 
ton <St Thelander 1988). Recent increases in the number 
of known active nest sites in California are also, in part, a 
result of increased efforts to find them (Walton et al. 
1988). Since the 1970s, Peregrines have reoccupied only 
35 of California's 200 known historic (pre-DDT era) nest 
sites, though they also have occupied over 100 sites not 
previously known (B.J. Walton pers. comm.). Many seem- 
ingly suitable sites remain vacant, and the process of 
population expansion has been slow (Walton et al. 1988), 
increasing at an average of 8% (6.8 breeding pairs) per year 
since 1975 (Monk et al. 1989). As of 1989, researchers 



Falcons 



SPECIES ACCOUNTS 



Falcons 



knew of only 90 active nest sites in the state (Monk et al. 
1 989). The Coast Ranges north of San Francisco to the 
Oregon border currently support the largest number of 
nesting pairs in the western United States and about 
two-thirds of California's nesting population (Walton et al. 
1988, Monk et al. 1989). Between 1971 and the mid- 
1980s, no Peregrines were known to have nested along the 
coastline north of San Francisco, an area with over 30 
historic nest sites. In the late 1980s, Peregrine sightings 
along the north coast increased, and at least one pair 
nested successfully (B.J. Walton pers. comm.). With man- 
agement help, Peregrine populations on the central Cali- 
fornia coast (from San Francisco to Santa Barbara 
counties) have increased from 1 pair at Morro Rock in the 
1960s and 1970s to 12 pairs, many of which are located 
on the Big Sur coasdine. The central coast contains about 
65 historical nesting sites. The historical population of 
over 40 pairs on the southern California coast and Chan- 
nel Islands was extirpated but has since recovered to about 
8 pairs, 3 of which were established on large buildings or 
bridges in the Los Angeles basin by release programs; the 
island population is expanding rapidly (5 pairs in 1989). 
Interior populations have always been less densely spaced, 
and currendy there are fewer than 6 active sites in river 
canyons of the Cascades and Sierra Nevada, including 3 
pairs in Yosemite National Park. 

Although nesting productivity of wild pairs is improv- 
ing, it is still compromised by accumulations of pesticides 
remaining in the fatty tissues of these long-lived birds, 
augmented by further accumulations of pesticides still 
circulating in the environment (B.J. Walton pers. comm.). 
Eggshell thinning continues at critical or near critical levels 
throughout California (Walton et al. 1988), necessitating 
an aggressive management effort to maintain and increase 



the rate of recovery of the state's breeding population 
(Walton 6k Thelander 1988). In fact, one or both adults at 
nearly 50% of known nesting sites were born at the 
Peregrine Fund's facilities at Santa Cruz. There are three 
methods of "hands on" management of Peregrines: (1) 
fostering— placing in active Peregrine nests young produced 
from thin-shelled eggs removed from the wild and incu- 
bated in captivity, or from eggs laid and hatched by captive 
birds; (2) cross-fostering— placing captive reared young in 
the nests of Prairie Falcons, which raise them to indepen- 
dence; and (3) hacking— placing in a hack box at an 
appropriate nest site captive-reared young that are released, 
fed, and monitored through independence by concealed 
biologists. 

Possible sources of currendy accumulated DDE include 
(1) residues in soils, air, and water; (2) contaminants in 
other legal pesticides; (3) residues in migrant prey species; 
(4) illegal pesticide use; and (5) legal use for emergency 
applications (Walton ck Thelander 1988, Risebrough ck 
Monk 1989). Scientific studies indicate that Peregrines are 
accumulating pesticides mosdy from the United States 
rather than from Mexico and South America, where there 
is little restriction of pesticide use. Although chlorinated 
hydrocarbon pesticide use is restricted in this country, 
other toxins, such as dioxin, still pose serious threats to 
Peregrines (B.J. Walton pers. comm.). Whether Peregrines 
can ever again maintain large natural populations in Cali- 
fornia without human assistance is an acid test of our 
resolve and ability to cope with environmental degradation 
and, perhaps, of the chances of long-term survival of our 
species on the planet. Even with a complete solution to the 
pollution problem, Peregrines will never reach historical 
levels because of extensive habitat loss, particularly of 
wedands(Kiffl988). 



157 



Pheasants and Quail 



MARIN COUNTY BREEDING BIRD ATIAS 



Pheasants and Quail 



Pheasants and Quail 



Family Phasianidae 



RING-NECKED PHEASANT Phasianus colchicus 







ok 


A year-round resident, though popula- 
tions maintained by game farm releases. 

An uncommon, very local breeder; 
overall breeding population very small. 

Recorded in 9 (4.1%) of 221 blocks. 


\^>^^\^\^ 






O Possible = 5 (56%) 






\\^\ '" 


C Probable = 3 (33%) 
• Confirmed = 1 (11%) 

FSAR = 2 OPI = 18 CI = 1.56 








^^-%5?^ 





Ecological Requirements 

This resplendent native of eastern Asia has been intro- 
duced widely in California and has taken hold in fertile, 
irrigated agricultural lands. The most important Pheasant 
habitats are cultivated fields of grain, such as rice, barley, 
and wheat, but irrigated pastures and hayfields may also be 
prominent locally. In Marin County, Ring-necked Pheas- 
ants occupy areas dominated by hay and alfalfa crops, but 
their population here is apparendy not self-sustaining, as 
are those in similar habitat in nearby Sonoma, Napa, and 
Solano counties (Calif. Dept. Fish Game). Irrigated farm- 
land is crucial to Pheasants for food. It provides the best 
habitat for them when it is interspersed with vegetation 
that also meets their requirements for nesting and cover 
(Olsen 1977, Johnsgard 1986). 

In winter, birds form temporary mixed or unisexual 
flocks. For breeding, the polygynous males break off singly 
to attract harems of about three to ten females. The males 
select ill-defined crowing territories diat include patches of 
varied, interspersed habitat. They avoid all but the edges of 
large blocks of monotypic vegetation and prefer a blending 
of food patches with herbaceous, meadow, marsh, brush, 
and tree clump habitats. Pheasants use moderately dense 
herbaceous and brushy cover for roosting, loafing, and 

158 



nesting. For nesting early in spring, they prefer permanent 
residual cover of dry grasses and forbs persisting from the 
previous year; with continued plant growth, birds shift to 
farm crops and new spring vegetation. Nesting habitats 
include hayfields, small grain crops, strip cover, pastures, 
wetland and woodland edges, and, to a lesser extent, fallow 
fields, gravel pits, lakeshores, farmyards, stubble fields, 
plowed fields, and orchards. Strip cover, which is also 
important for roosting and loafing, generally consists of 
low-growing vegetation parallel to roadways, railroads, 
drainage ditches, irrigation canals, streams, dikes, and 
fence lines. Pheasants prefer wider strips to narrower ones, 
and moderate-sized fields to small, narrow, or large ones. 
Males apparendy adjust their crowing territories to include 
the nest sites of females. 

Pheasants place most nests in shallow depressions 
scooped out of the earth or in natural hollows. Occasion- 
ally, they raise their nests slighdy above the ground, for 
example, atop wind-drifted cornhusks or on tussocks in a 
marsh; rarely, diey locate nests well above ground in straw 
stacks or in the old nests of other birds or squirrels. Nests 
are lined with dried grasses or leaves, weed stalks, fine 
twigs, or cornhusks taken from the immediate vicinity of 



Pheasants and Quail 



SPECIES ACCOUNTS 



Pheasants and Quail 



the nest; a few feathers from the hen Pheasant may some- 
times be found in the nest. The nest site microhabitat 
generally has a greater density of vegetative cover (particu- 
larly to the side of the nest) than does the broader sur- 
rounding nesting habitat (Wood 6k Brotherson 1981). 
The height of vegetative cover (within a general range of 
about 10-100 in.) does not seem to be as important as 
vegetative density (Olsen 1977, Wood 6k Brotherson 
1981); but see Hanson (1970). Canopy cover can vary 
from leaving the nest completely exposed from above to 
providing partial or complete concealment. Although nest- 
ing cover appears to provide protection from predators, 
perhaps more importandy it furnishes a microclimate of 
lower temperature and higher humidity than is found at 
similar heights in surrounding vegetation (Francis 1968, 
Hanson 1970, Wood 6k Brotherson 1981). In California, 
brood cover consists primarily of grain, rice, natural cover 
areas, and plowed fields (Olsen 1977). In the fall, Pheas- 
ants tend to drift toward stream bottoms, swales, and 
swampy edges; ungrazed hayfields, wild grass, weeds, 
brushy woodlots, fencerows, and stubble fields provide 
additional cover. At all seasons, Pheasants require a source 
of water close by. 

Ring-necked Pheasants are primarily grain eaters, 
though leafage can be important in their diets seasonally; 
they take animal matter in small amounts from spring 
through fall. In California, the diet varies greatly among 
habitat types, locally as well as geographically and season- 
ally (Grenfell et al. 1980). In grain-growing regions of the 
Sacramento and San Joaquin valleys, cultivated grains, 
such as rice, barley, wheat, oats, and grain sorghums, are 
the most important foods year round and may constitute 
as much as 80% of the whole diet in a given season. In the 
Sacramento Valley grain regions, at least, Pheasants in- 
crease their use of green forb and grass leafage in spring 
and of forb and grass seeds in spring and summer; animal 
foods account for only about 3%-7% of the spring 
through fall diet. In contrast, in an area of extensive 
irrigated pastureland in the San Joaquin Valley, the spring 
to fall diet consists of about 60%-90% grass and forb 
seeds. The amount of green leafage in the diet there shifts 
dramatically to over 60% in winter and drops to slighdy 
over 25% in spring; cultivated crops reach a maximum of 
only about 1 7% of the diet in summer. Based on limited 
samples, the spring through fall diet in the Modoc Plateau 
and Great Basin regions varies seasonally from 75%-96% 
cultivated crops; winter diet (data available only from 
Modoc) is over 50% leafage, secondarily grass and forb 
seeds, and lasdy cultivated grains. In spring, birds in the 
Modoc region use over 85% cultivated grain, whereas 
those in the Great Basin rely on over 50% leafage and 
about 40% grass seed; animal foods in these areas account 
for as high as 23% of the diet in summer. Pheasants also 
eat limited amounts of corms and roots, fleshy fruits, 



flower buds and flowers, and mast; additionally, they do 
some damage to tomato, melon, and potato crops (Ferrel 
et al. 1949, Leach et al. 1953, Grenfell et al. 1980). Little 
appears to have been written on the foraging tactics of 
Pheasants, but they are primarily ground foragers and must 
uncover some items. Johnsgard (1 986) indicated that the 
birds' short toes, strong claws, and sharp bills are all well 
adapted for digging and scratching. Edminster (1954), on 
the other hand, reported that they pick and root in the 
ground better with their beaks and do not use their feet for 
scratching out food. Pheasants appear to take most culti- 
vated grain as waste grain from the ground, but they also 
take some from the plants before harvest (Ferrel et al. 
1949). The fact that Pheasants browse on flower buds, 
flowers, and leafage suggests they also procure some weed 
seeds direcdy from the plants. In the Sacramento Valley, 
Pheasants consume 18 orders of insects and other inverte- 
brates, grasshoppers being the most important; occasion- 
ally, they take mice (Ferrel et al. 1949). The diet of week-old 
chicks is exclusively insects; by 8 weeks of age it decreases 
to about 50% insects; and by 1 3 weeks it resembles that of 
adults. Chicks initially take small insects, such as beedes, 
and later larger ones, such as grasshoppers. Females 
appear to eat more insects than do males, and they fre- 
quendy take calcium-rich snails, which presumably are 
important for egg formation (Korschgen 1964). 

Marin Breeding Distribution 

Although there were a few adas sightings toward the coast, 
most were in the vicinity of agricultural hay and alfalfa 
fields near marshlands along San Pablo Bay near Novate 
The only confirmation of breeding was a hen with a 
half-grown chick in reclaimed marshlands east of Ignacio 
on 13 May 1979 (Anonymous fide DS). Despite such 
evidence, the Marin population is not viable but depends 
upon the release by hunt clubs of game farm-reared birds 
(G. Thomsen, J.R. Slosson pers. comm.). 

Historical Trends/Population Threats 

Ring-necked Pheasants were first introduced to California 
in Santa Cruz, San Mateo, and Marin counties prior to 
1889 by private parties (Grinnell et al. 1918; G6kM 1944; 
Hjersman 1947). Birds were first released by the Fish and 
Game Commission in 1889 in a number of counties, 
including coastal Marin and Monterey; a year or two after 
1894, they were also released in Humboldt and Santa 
Clara counties. Introductions began in earnest in 1908 
from breeding stock from the newly established State 
Game Farm. By 1916, Pheasants had been released in 37 
of the state's 58 counties, and they appeared to be estab- 
lished in the wild in coastal counties at Eureka and 
Fortuna, Humboldt County; Napa, Napa County; Wat- 
sonville, Santa Cruz County; Pacific Grove, Monterey 
County; and especially in the Santa Clara Valley north of 

159 



Pheasants and Quail 



MARIN COUNTY BREEDING BIRD ATlJ\S 



Pheasants and Quail 



San Jose, Santa Clara County. Based on the current 
distribution of Pheasants, it is unclear whether viable 
populations really existed in all these areas. Pheasants may 
have been extirpated locally on the coast by changing 
agricultural practices rather than by inherendy low fecun- 
dity. A hunting season was first opened in Inyo and Mono 
counties in 1925 and then statewide in 1933 (Hjersman 
1947, Nelson &. Hooper 1976, Mallette 6k Slosson 1987). 
By 1944, Pheasants had been planted in every county in 
the state except Alpine, and diey had been persistendy 
replanted in most of the seemingly "suitable" parts of the 
state (GckM 1944, Hjersman 1947). It appears that Pheas- 
ants never established themselves in Marin County 
(GckW 1927, SckP 1933, G&M 1944). 

Although Pheasants were initially successful along cer- 
tain sections of the coast, their most important populations 
there (in the Santa Clara Valley) declined in the early 
1920s because of changing farm practices (Hjersman 
1947). The heart of the state's Pheasant population 
switched to the Central Valley following the introduction 
of rice culture there during World War I. Nelson and 
Hooper (1976) and Mallette and Slosson (1987) reported 



that the state's Pheasant populations have declined in 
recent years from die advent of "clean farming" practices, 
which eliminate bordering weedy and brushy vegetation, 
and from the continued encroachment of urban-suburban 
sprawl. Although many naturalists might not mourn the 
decline of an alien species such as the Pheasant, it should 
be remembered that the Pheasant's demise would surely be 
paralleled by that of native species that also depend on the 
marginal refuges in large tracts of human-manipulated 
habitat. Breeding Bird Surveys, on the other hand, suggest 
that Pheasant numbers in California were relatively stable 
from 1968 to 1989 (USFWS unpubl. analyses). 

Remarks 

California birds were first obtained from China and Ore- 
gon (where they were first introduced in America in 1881; 
Bent 1932), but thereafter came largely from game farms 
(G&M 1944, Hjersman 1947). Although most California 
birds are of the race P. c. torquatus, other closely related 
races have been released, as well as crosses between some 
of them. 








#4 . # *^ m 



l\ 'Mf$# 





California Quail must be extreme!} vigilant if tkej are to raise all of their young to maturity. Draining by Keith Hansen, 1 989. 

160 



Pheasants and Quail 



SPECIES ACCOUNTS 



Pheasants and Quail 



CALIFORNIA QUAIL Callipepla californica 







A year-round resident. 






A fairly common, nearly ubiquitous 




breeder; overall breeding population 
large. 


^^^^k^k^^\^kky\ 




Recorded in 208 (94.1%) of 221 
blocks. 

O Possible = 31 (15%) 




^^^^k^0^^ 


^a 


© Probable = 92 (44%) 
• Confirmed = 85 (41%) 

FSAR = 3 OPI = 624 CI = 2.26 











Ecological Requirements 

From prominent lookouts, cocks stand sentry duty for wary 
coveys of "Valley Quail" before they break up into pairs in 
spring and scatter widely to breed. California Quail stick 
close to the brushy edges of, or openings in, all of Marin 
County's major scrub, woodland, and forest habitats 
where they border on pasturelands, weedy fields, mead- 
ows, and unkempt lawns and yards. In dense, extensive 
tracts of coastal scrub and chaparral, Quail frequent the 
edges or openings along trails, fireroads, localized burns, 
or other open disturbed sites. California Quail prefer edge 
situations with interspersed low protective cover, open- 
ground foraging areas, and water sources (Sumner 1935, 
Emlen ck Glading 1945, Leopold 1977). Cover provides 
shade, shelter from inclement weather, and, perhaps most 
importandy, refuge from predators for adults and broods. 
Cover can consist of small or extensive patches of bushes, 
bramble and briar thickets, trees, thick clumps of weed 
stalks, rough outcrops of rock, or brush piles that are dense 
at ground level and impervious to penetration by avian 
predators. Such cover provides a daytime loafing area and 
a sanctuary for escape. At night, California Quail normally 
roost off the ground in dense evergreen trees or tall shrubs 
out of reach of ground predators and concealed from 
hungry owls. Quail foraging areas ideally provide a sparse 
to moderately dense growth of annual grasses and particu- 
larly forbs, with a duff or litter layer that harbors fallen 
seeds. California Quail generally forage within about 50 
feet of escape cover. In dry areas in late summer and fall, 
they will venture several hundred yards into the open to 
forage until the arrival of the first migrant Cooper's Hawks, 



when they again retract their foraging radius to the close 
proximity of cover. For much of the year, Quail can obtain 
water from succulent greenery, but in the hot and dry 
months of summer and early autumn, they usually need a 
dependable source of water close to cover, be it only a drip, 
seep, or puddle. This is less of a summertime problem 
along the fog-shrouded coasdine. If hard pressed, Quail 
can live without drinking water, provided they have at 
hand heavy dew, berries, or succulent vegetation (Sumner 
1935). 

The absence of Mountain Quail in Marin County 
appears to be a fluke of geography, since suitable habitat is 
available here. They occur in coastal counties to the north 
and south and probably did not reach Marin because of 
habitat barriers to foot-powered dispersal. In coastal Cali- 
fornia, Mountain Quail inhabit coniferous forest with a 
shrubby understory, mixed evergreen forest, and chapar- 
ral. Although diey overlap in habitat somewhat with Cali- 
fornia Quail, Mountains generally occur at higher 
elevations, on steeper slopes, and spend most of their time 
inside chaparral thickets or beneath the forest canopy 
(Gutierrez 1980). 

California Quail generally prefer nest sites that are in 
the open away from a continuous canopy of shrubs or trees 
(Leopold 1977). Early-season nest sites may be far from 
water when green vegetation is at a peak; even chicks can 
derive their water needs from succulent vegetation. Quail 
may avoid nesting close to streams (fenced or unfenced) or 
water troughs where overgrazing has destroyed nesting 
cover. Widi the diminution or disappearance of greenery 

161 



Pheasants and Quail 



MARIN COUNTY BREEDING BIRD ATLAS 



Pheasants and Quail 



in midsummer, Quail must nest within about 400 yards 
(the cruising radius of chicks) from water (Sumner 1935, 
Leopold 1977). California Quail typically conceal their 
nests on die ground in dense clumps of grass or weeds, 
often sheltered at the base of bushes, fallen trees or limbs, 
vines, bnash piles, or fencerows. Other nests are sheltered 
beneath overhanging rocks, in rock crannies, in niches in 
narrow gullies, or under piles of scrap lumber (Grinnell et 
al. 1918, Dawson 1923, Bent 1932, Glading 1938, Leo- 
pold 1977). The nest is usually a hollow in the ground 
lined sparsely with grasses, leaves, weed stems, and per- 
haps a few feathers; usually nests are roofed over, at least 
during the early part of incubation, by vegetation, or 
sometimes by overhanging rocks. Occasionally, birds build 
a crude but more substantial nest of the same materials on 
a log or stump, in a brush pile, on a bale of hay or the side 
of a haystack, or among vines on a trellis. Rarely, they place 
nests up in trees at the broken or decayed ends of limbs or 
at an intersection of two large branches. One pair nested 
in a garden on a rooftop. Exceptionally, these Quail may 
lay eggs in the nests of other birds on or above ground. 

California Quail forage primarily on the ground by 
grazing on annual plants and by scratching in the soil and 
litter; occasionally, they jump to reach blossoms and climb 
into shrubbery. Mountain Quail also forage by these meth- 
ods, jump for flowers and seeds more frequendy, climb 
trees and shrubs to procure berries and fruits more readily, 
dig beneath the soil for bugs, and shell acorns (Gutierrez 
1980). Like other gallinaceous birds, both species pick up 
grit to grind seeds. California Quail generally forage mosdy 
right after dawn and during a second burst of activity in the 
evening (Sumner 1935, Leopold 1977). In hot weather, 
they usually go to water daily and generally lead chicks to 
drink after the morning feeding bout and often at other 
times. During breeding, adults subsist on short rations: the 
male because of time spent defending against intruders, 
guarding the female and nest, and protecting the young; 
the female because of intermittent feeding during incuba- 
tion and time spent tending the young. When Quail 
double brood, females desert their families after about 
three weeks to find new mates and nest again, leaving the 
original males to rear the first brood (Leopold 1977). 

California Quail eat primarily forb seeds and green 
leafage, but also fruits, berries, buds, blossoms, catkins, 
acorn fragments, plant galls, and waste grain (Browning 
1977, Leopold 1977, Grenfell et al. 1980). They consume 
about l%-7% animal matter only in spring and early 
summer and more in wet than dry years. Animal foods 
include insects, millipedes, mites, spiders, snails, and sow- 
bugs. The seeds of annual broadleaved forbs are a staple 
throughout the year and, along with minor amounts of 
grass seeds, constitute about 95% of the diet in summer, 
90% in fall, 70% in winter, and over 55% in spring 
(Leopold 1 977). In the Coast Ranges, bur clover is perhaps 

162 



the most important single food plant, though other 
legumes (various clovers, lotuses, and lupines) and filaree 
can be equally important locally. Other key seed-bearing 
forbs are fiddleneck, turkey mullein, geranium, vetch, 
various thistles, popcorn flower, chickweed, miners lettuce, 
red maids, buttercup, buckthorn weed, windmill pink, 
tarweeds, California poppy, vinegar weed, and gamble- 
weed (Browning 1977, Leopold 1977). Generally, the 
annual grasses that now dominate California's grasslands 
are not important Quail foods and in fact compete with and 
displace the more favored broadleaved annuals. Quail also 
browse extensively on green leafage, mosdy of the seed- 
bearing annual forbs mentioned above. They regularly eat 
small amounts of green grasses but feed on them exten- 
sively only when the seeds and green foliage of forbs are 
scarce or absent. Consumption of greens begins with the 
appearance of new growth after the first fall rains (even in 
years of seed abundance), peaks in winter and spring, and 
generally ceases when annual plants die in late spring and 
summer and the new crop of seeds becomes available. 
Usage of greenery climbs from a low of about 2% in 
summer and 10% in fall to a peak of about 30%-44% in 
winter and 35%-40% in spring (Browning 1977, Leopold 
1977). Use of greens in winter may vary between locations 
from as much as 75% to as little as 0%-3%. Acorn 
fragments can be an important component of the diet in 
fall or winter (as much as 35%) and even in spring, in 
some localities and in certain years (Browning 1977). In 
addition, Quail take limited amounts of the fruits of other 
woody perennials, such as poison oak, manzanita, cea- 
nothus, misdetoe, acacia, and black locust. In summer and 
fall, Quail may eat some waste grains left after harvest, 
especially wheat, oats, barley, corn, or even rice, kafir, or 
safflower. 

Quail chicks are born with a substantial residue of 
original egg yolk enclosed within the abdomen. This serves 
the vital function of tiding over the precocial young during 
the critical period when they are learning to feed, or during 
periods of rain or fog when foraging is difficult or impos- 
sible (Leopold 1977). Chicks a few days old live mosdy on 
insects (Grenfell et al. 1980). At one site, chicks one to 
three weeks of age ate mosdy seeds and only about 11% 
insects (n = 47) and at four to six weeks about 9% insects 
(n = 66); thereafter, insect consumption fell rapidly to a 
trace (Leopold 1977). Chicks start with small food items 
and, as they grow, graduate to larger ones (Sumner 1935, 
Grenfell et al. 1980). 

Marin Breeding Distribution 

The California Quail was one of the most widespread of 
Marin County's breeding birds, occurring in virtually every 
block during the adas period. Quail were absent locally 
only on the tops of some of the higher ridges, such as the 
peak of Mount Tamalpais, and in a few areas without 



Pheasants and Quail 



SPECIES ACCOUNTS 



Pheasants and Quail 



brushy cover. Representative breeding sites were Marshall 
(NE 6/15/76 -GJK); near Three Peaks (FL 6/17/82 
— DS); and Pine Gulch Creek, Bolinas Lagoon (FL 
6/14/80 -DS). 

Historical Trends/ Population Threats 

Grinnell and Miller (1944) reported that the California 
Quail had declined in the state in the previous 35 years, 
though it was still numerous in favorable territory where 
not "shot out." Many authors attributed this decline pri- 
marily to intense market hunting (e.g., Grinnell et al. 
1918). Settlers turned to Quail for food after the Gold 
Rush depleted supplies of edible large mammals (Leopold 
1977). Quail hunting for the San Francisco market began 
in Marin County in the 1860s (Welch 1928). During the 
late 1880s and 1890s, millions of Quail were shot, 
trapped, and sold in California. In 1895-96, hunters sold 
177,366 Quail in the open markets of San Francisco and 
Los Angeles alone. Because of decreases in the Quail 
population, laws were passed in 1880 that prohibited 
trapping and in 1901 that fixed a bag limit and oudawed 
the sale of Quail (Welch 1928, Leopold 1977). Boodeg 
operations continued to circumvent the laws, and by 1925 
only a pitiful remnant of the state's bountiful supply of 
Quail remained. 

Sumner (1935), on the other hand, concluded diat the 
decline was due to a combination of overhunting and 
various other factors, such as ' clean farming practices, 



that eliminate cover and increase erosion, overgrazing, 
man's usurpation of water sources, fire control that pro- 
motes growth of impenetrable brush that chokes out Quail 
food, and poisons set out for rodents. Despite the depre- 
dations of hunters, Leopold (1977) felt that the decline in 
Quail numbers was mainly from a decrease in the produc- 
tion of Quail food caused by changing land use. He 
envisioned a population peak at the start of the market 
hunting era that was greater than in presettlement times. 
Quail numbers may have been increased at the time by the 
breaking up of native bunch grasses allowing the intrusion 
of seed-bearing forbs; by the planting of grains and hedge- 
rows; and by the opening up of woods— factors that pro- 
vided a mosaic of Quail habitat and, along with the virgin 
fertility of the soils, fueled increased production of pre- 
ferred Quail foods. Leopold concluded that the subsequent 
decline in numbers was caused by the loss of soil fertility 
from intensive agriculture and overgrazing and by the 
invasion of alien annual grasses, which replaced the pre- 
ferred Quail foods of native and introduced forbs. The 
trend toward large landholdings and mechanized agricul- 
ture has eliminated much cover, leading to further habitat 
deterioration. Although clearing and logging have opened 
up some habitat in certain areas of California, the overall 
trend here has been toward a reduction in the extent and 
quality of Quail habitat (Sumner 1935). Quail numbers 
were relatively stable on Breeding Bird Surveys in Califor- 
nia from 1968 to 1989 (USFWS unpubl. analyses). 




163 



Rails 



MARIN COUNTY BREEDING BIRD ATLAS 



Rails 



Rails, Gallinules, and Coots 

Family Rallidae 



BLACK RAIL Laterallus jamaicensis 









A year-round resident; numbers swell 




^^^^^JOS 




from Sep through Mar. 


jC^^Q^c 




A fairly common, very local breeder; 




\"""\ J&^\ \^\\ \^\ \r\ \^\vy^. 


_ 


overall breeding population very small. 


^V*\ ^-Pc 


Pc^vV>\^^^Vv^^ 




Recorded in 6 (2.7%) of 221 blocks. 


\^\ 


w^cv -^c\ yc\ \^\ \^\ \^\ V--^ 




O Possible = 1 (17%) 


Vu 


^v\\\or \^\ vp^r>- \^^\-- \^^K^-"\^^\ 




© Probable = 4 (67%) 




f^\ i^^^-^-V^^A wV'X Jv^\ J^\ 3r"\ 




• Confirmed = 1 (17%) 




^5POf<i^^ 


V^Sp. T- 








^Vw-v 


FSAR = 3 OPI = 18 CI = 2.00 




J , jZ^"^ ^\^^\ \l^V^t^ \^^Y-^'V\^ 


^^5^^ 






>J^> \L_ / ^=>-=<5 







Ecological Requirements 

This furtive little rail inhabits tidal salt and brackish 
marshes and, to a lesser extent, freshwater marshes. Black 
Rails prefer areas of high marsh at the upper limit of tidal 
flooding with a dense cover, usually of pickleweed (Sali- 
cornia virginica) or sedges (Scirpus spp.) (Manolis 1978; 
Evens et al. 1989, 1991). The tidal marshes Black Rails 
inhabit in Marin County are either dominated by Sali- 
comia, have a preponderance of Salicornia with Scirpus on 
the borders or in small patches within the Salicornia 
matrix, or are a mosaic of Salicornia and Scirpus (Evens et 
al. 1989). At Marin County's largest freshwater marsh, 
Olema Marsh, Black Rails have occurred in stands of 
cattail (Typha) or bulrush (Scirpus) that mix with other 
freshwater marsh or coastal swale plant species and border 
on willow or alder riparian forest (D. Shuford pers. obs.). 
Black Rails are patchily distributed in tidal marshes. A 
suite of factors interact to determine the suitability of 
habitat, though no single factor or combination of factors 
is sufficient to predict Black Rail occurrence (Evens et al. 
1989, 1991). The most important factors influencing 
Black Rail distribution are tidal regime, marsh elevation, 
and freshwater influence; marsh age (maturity) and size, 
degree of channelization, soil and water salinity, and plant 

164 



composition may also bear on rail habitat preferences 
(Evens et al. 1989, 1991). Black Rails are more numerous 
in marshes with unrestricted tidal flow than with restricted 
tidal flow; they are extremely rare in diked marshes. They 
inhabit the upper reaches of marshes where there is emer- 
gent vegetation at high tides and avoid marshes dominated 
by salt grass (DisticKIis spicata). The importance of a mix of 
tidal and freshwater influence is suggested by the concen- 
tration of Black Rails in the northern sections of the greater 
San Francisco Bay estuary where freshwater inflow is 
greatest. There Black Rails are most plentiful in large, 
broad tidal marshes bordering major rivers. In bayshore 
marshes, these rails are most numerous at the mouths of 
sloughs, creeks, and rivers, and at some Sah'cornia-domi- 
nated marshes rails cluster at clumps of Scirpus, indicators 
of freshwater influence (often from seeps). Ultimately, 
Black Rail habitat must provide cover, nest sites, and 
suitable food. Since predation by natural enemies such as 
Great Egrets, Great Blue Herons, and Northern Harriers 
can be severe (Evens & Page 1986), dense cover is import- 
ant, especially during high tides. There is also speculation 
that Black Rails may avoid areas with large densities of 
Clapper Rails since the latter species may prey on small 



Raits 



SPECIES ACCOUNTS 



Rails 



birds QM. fide JGE). To support Black Rails, marshes 
should grade gradually into weedy or brushy upland vege- 
tation where the rails can retreat at extremely high tides. 

Black Rails usually conceal their nests completely from 
view in the dense growth of Salicomia, grasses, sedges, or 
other marsh vegetation, which they also use to construct 
their nests; rarely, they leave them open to view from above 
(Bent 1926). They place most nests from one to several 
inches above the mud, but sometimes nests are built on 
the ground or up to 10 to 12 inches above ground. Nests 
vary gready in structure from thin, frail platforms to, rarely, 
ground nests five inches thick; they may be flat or deeply 
cupped. Often the rails interweave surrounding vegetation 
over the nest to give it the appearance of a domed-over 
meadowlark nest with a side entrance. Thicker nests are 
usually built up from the ground, and thin ones are usually 
placed on convenient shelves of matted vegetation. Of six 
nests observed in the San Francisco Bay system, all were 
of the thin, raised type and lacked a dome of interwoven 
vegetation (J-G. Evens pers. comm.). 

Because of their elusive nature, very little is known of 
the food habits of Black Rails (Wilbur 1 974). Apparendy 
they feed on insects or other arthropods (Bent 1926); 
presumably they pick or glean dieir meals from the muddy 
substrate or from the surface of marsh plants. Black Rail 
occurrence is positively associated widi insect and spider 
abundance and negatively associated with amphipod abun- 
dance in marshes (Evens et al. 1986). It is unclear whether 
insects and spiders are a primary food source for Black 
Rails and whether these arthropods are more abundant in 
tidal marshes diluted by fresh water (Evens et al. 1989). 
What is clear is that much still needs to be learned of the 
habitat preferences and lifestyles of these secretive black 
gnomes. 

Marin Breeding Distribution 

In Marin County, Black Rails breed along the San Fran- 
cisco and San Pablo bayshores from Corte Madera Marsh 
north to the Petaluma marshes, and on the outer coast, 
along the fringes of Bolinas Lagoon, at the south end of 
Tomales Bay, and, perhaps sporadically, at Olema Marsh. 
We recorded Black Rails in five adas blocks on the bayside 
tidal marshes. The adas work and subsequent more inten- 
sive rail censuses (Evens et al. 1986, 1989, 1991) revealed 
breeding Black Rails at Corte Madera Marsh, China 
Camp, Gallinas Creek, Novato Creek, Day Island, Black 
John Slough, die Petaluma marshes, in addition to die 
spots on the outer coast mentioned above. During die adas 
period, breeding was confirmed only at China Camp. A 
nest was located at the upper edge of a salt marsh about 
eight inches above the mud in a patch of arrowgrass 
(Triglochin sp., presumably T. maritima) at a freshwater 
seep (NE 5/20/80 -ITa). In 1982, five additional nests 
were found in the same vicinity in Salicomia, again at the 



upper edge of the marsh near freshwater seeps (ITa, JGE, 
GWP). Subsequendy, during extensive surveys (Evens et 
al. 1986), Black Rails were confirmed breeding at Day 
Island near Novato with the observation of newly hatched 
chicks on 30 May 1986 (JGE) and at Black John Slough in 
the Petaluma marshes with the location of a nest with egg 
fragments on 8 May 1986 (GWP, NW). During the adas 
period, we recorded Black Rails away from the bayside 
marshes only at Olema Marsh (up to four calling 6/17- 
7/30/80 —JGE, DS). Birds have not been recorded there 
since the early 1980s, perhaps because of extensive silt- 
ation during the intense flooding of 1982. Previously, 
breeding had been confirmed at a brackish marsh along 
Tomales Bay south of Inverness with the observation of an 
adult and chicks in the summer of 1976 (GB). A high 
count of seven calling birds was recorded at the south end 
of Tomales Bay on 16 May 1986 (JGE). A record of a bird 
calling on the west shore of Bolinas Lagoon on 10 and 1 1 
March 1979 (RS et al.) may have been a breeder, a winter 
visitant, or a migrant. Probable evidence of breeding at 
Bolinas Lagoon is provided by records of one to two birds 
calling at the Pine Gulch Creek delta from 2 March to 20 
May 1 987 (DDeS, DAH) and one to two calling on the east 
shore on various dates from 29 March to 1 1 July, 1983 to 
1986 (CCu et al.). Surveys at Schooner Bay, Drake's 
Estero, in spring 1986 and 1988 and at Richardson Bay in 
spring 1988 did not reveal any Black Rails (Evens et al. 
1986, 1989). 

Historical Trends/Population Threats 

Until recently, the status of breeding Black Rails in north- 
ern California was clouded by the lack of adequate field 
work needed to detect such a secretive species. Early 
authorities had stated or implied that the species moved 
north to northern California in the fall after the breeding 
season (Bent 1926, AOU 1957). Incidental sightings from 
the late 1 950s to the present (ABN) and, particularly, recent 
thorough surveys (Jurek 1976; Manolis 1978; Evens et al. 
1 989, 1 991) have documented the occurrence of a substan- 
tial breeding population in tidal marshes around San 
Pablo and Suisun bays and smaller numbers in the Sacra- 
mento-San Joaquin Delta as far inland as Lodi, San 
Joaquin County. Kiff (1978) documented the first breeding 
record for northern California from the reidentification of 
an egg set collected on 10 April 191 1 at Newark, Alameda 
County, in south San Francisco Bay, where today the 
species is a very rare or sporadic breeder. The greater San 
Francisco Bay estuary supports the bulk (80%) of the Black 
Rail population of the West, with the remainder along the 
lower Colorado River (<1 50 individuals); at the Salton Sea, 
canals, and lakes of die Salton Trough (<50 individuals); 
at Morro Bay; and at the Point Reyes sites described above 
(Evens et al. 1991). 

165 



Rails 



MARIN COUNTY BREEDING BIRD ATLAS 



Rails 



Because tidal marshes in the San Francisco Bay system 
have been reduced by 60%-95% (Nichols 6k Wright 
1971, Josselyn 1983), Black Rail populations there must 
also have declined drastically (Evens et al. 1989, 1991). 
Black Rails no longer breed in salt marshes on the Califor- 
nia coast south of Morro Bay (Garrett &. Dunn 1981, 
Unitt 1984). The California Black Rail (L. j. coturniculus) 
is currently listed as Threatened in California (CDFG 
1991a) and is a Candidate (Category 2) for listing as 
federally Threatened or Endangered (USFWS 1991). 
Saltmarsh habitat was lost through diking, filling, and 
conversion to agricultural lands, salt ponds, and urban 
development. Because of its proximity to human habita- 
tion and agricultural land, high marsh habitat is most 
susceptible to conversion. Intensive alteration of high 
marsh habitat in south San Francisco Bay and apparent 
subsidence of remaining marshes may explain the near 



absence of breeding Black Rails there (Manolis 1978; 
Evens et al. 1989, 1991). Most marsh habitat in the South 
Bay is completely flooded at high tide since remaining 
marsh abuts directly on salt pond dikes and roadways; 
formerly the marsh graded gradually into upland habitat. 
Further loss or degradation of marshes, possible rising sea 
level, and diversion of freshwater inflow to the North Bay 
potentially pose ongoing threats to Black Rail habitat 
(Evens et al. 1989, 1991). In addition, these rails face 
higher predation pressures from the lack of transitional 
upland vegetation in many marshes and possibly from 
introduced Norway rats, which are known to a be serious 
threat to Clapper Rail nests (Harvey 1 980a). It is unknown 
whether toxic substances are affecting Black Rails, but 
there is increasing evidence of their effects on other birds 
in San Francisco Bay (Ohlendorf 6k Fleming 1988). 



CLAPPER RAIL Rallus longirostris 



"Xv3rv 


>p^^^ \ jTV 




A year-round resident. 

A fairly common, very local breeder; 
overall breeding population very small. 

Recorded in 5 (2.3%) of 221 blocks. 


^\^ 






O Possible = 1 (20%) 
€ Probable = 2 (40%) 
• Confirmed = 2 (40%) 






__■• r- 


FSAR = 3 OPI = 15 CI = 2.20 






Vo 




t 


■J^* X1-^j ^~^"^<^\^\ 







Ecological Requirements 

California Clapper Rails (Rallus longirostris obsoletus) are at 
home in the ebb and flow of salt marshes intersected by 
numerous tidal channels. The vegetation of these marshes 
is usually dominated by cord grass (Spartina joliosa), pickle- 
weed (Salicomia spp.), and salt grass (Distichlis); gumplant 
(Grindelia) provides important cover in some marshes 
(Gill 1979, Evens 6k Page 1984). During the breeding 
season, Clapper Rails also occupy brackish tidal marshes 
in those parts of south San Francisco, San Pablo, and 
Suisun bays that are dominated by bulrushes (Scirpus spp.) 



and other low, salt-tolerant marsh plants (Gill 1979, Har- 
vey 1980b). During die winter, Clapper Rails leave brack- 
ish marshes and contract into favored areas of Spartina salt 
marsh, further suggesting that brackish marshes may be 
suboptimal breeding habitat (P.R. Kelly pers. comm.). 
Their concentration in south San Francisco Bay (see 
below) also suggests they can tolerate higher salinities and 
a wider tidal range than Black Rails. Perhaps salinities and 
the degree of tidal inundation affect the distribution of 
these rails via vegetation structure and/or food resources. 



166 



Rails 



SPECIES ACCOUNTS 



Rails 



Important factors for breeding Clapper Rails are (1) 
well-developed sloughs and secondary tidal channels; (2) 
extensive (dense, tall, lush) cord grass stands (though not 
all are used); (3) dense saltmarsh vegetation for cover, nest 
sites, and brooding areas; (4) intertidal mudflats, gradually 
sloping banks of tidal channels, and cord grass beds for 
foraging; (5) abundant invertebrate food resources; and (6) 
transitional vegetation at the upland edge of the salt marsh 
as a refuge during high tides (Evens & Page 1984, Harvey 
1987, P.R. Kelly pers. comm.). These rails do occur where 
there are few stands of cord grass, as at Corte Madera 
Marsh in Marin County (J.G. Evens pers. comm.). At 
Corte Madera, the minimum channel size used by Clapper 
Rails averaged about 14 inches deep by 25 inches wide 
(Evens ck Page 1983, n = 8). They avoid large channels 
that have undercut banks and small channels that are 
overgrown with vegetation. Clapper Rails may use all parts 
of the marsh during the year, but in winter, at least in the 
South Bay, they tend not to use extensive areas of pickle- 
weed and instead concentrate then in cord grass in the 
lower marsh (P.R. Kelly pers. comm.). Unlike Black Rails, 
they are not restricted to high upper marsh for breeding. 
In Corte Madera Marsh, all nests found were in the upper 
marsh Q.G. Evens pers. comm.); in the South Bay, many 
nests are located in both the lower and higher marsh 
(Harvey 1980a). 

Although there is much intersite variation in nest place- 
ment, Clapper Rails generally tend to locate their nests on 
raised ground near the tidal sloughs that intersect marshes 
(Harvey 1980a, 1987; Evens 6k Page 1984). Birds usually 
build their nests under the dense, sheltering vegetation of 
pickleweed, cord grass, gumplant, or salt grass; on occa- 
sion, or sometimes commonly (in the Soudi Bay), they nest 
under a canopy of wrack or debris stranded on top of the 
marsh vegetation. At Corte Madera Marsh, 14 nests were 
under dense gumplant bushes on the elevated natural 
levees of tidal sloughs, and 3 late-season nests were under 
dense clumps of pickleweed (Evens ck Page 1983). In the 
South Bay, nest concentrations vary from areas dominated 
by pickleweed and tidal wrack to those dominated by cord 
grass; nests are sheltered by pickleweed, cord grass, gum- 
plant, salt grass, or tidal debris; and generally nests are 
associated with sloughs or the open bay edge (Harvey 
1980a, 1987). Mean canopy height of nesting cover in the 
South Bay is 22 to 23 inches (Harvey 1980a). At Corte 
Madera Marsh, most nests are within 5 feet, and none 
were more than 10 feet, from a tidal channel (Evens ck 
Page 1983, n = 17). In the South Bay, mean distances of 
nests to the nearest channel range from about 20 to 36 feet 
(Harvey 1980a, n = 50). Clapper Rails may prefer nesting 
in the cord grass zone of the lower marsh because nests 
there are better protected from terrestrial predators and 
because nests made of Spartina float during high tide. 
Variations in nest site distribution may reflect intersite 



differences in plant succession and abundance, tidal gradi- 
ents, or predation pressures. Nest placement may also vary 
seasonally (more use of wrack or gumplant early in the 
season before Spartina growth occurs) and yearly (more use 
of Salicornia nest sites in years of severe high tides). 

Clapper Rails lay their eggs in the hollow of a bulky 
platform of dry cord grass, pickleweed, salt grass, or other 
marsh vegetation built up three to six inches above the 
sodden ground (Dawson 1923, Bent 1926, Harvey 1980a, 
Evens ck Page 1984). Cord grass is the preferred nest 
material in the South Bay, perhaps because the hollow 
stems float and thus provide better protection for eggs 
during high tides (Harvey 1980a). Rails usually approach 
the nest site via a runway or tunnel through the matted 
vegetation (Dawson 1923). While one parent broods 
newly hatched chicks and incubates the remaining eggs, 
the other sometimes leads the first-hatched chicks up to 50 
feet from the nest (Adams ck Quay 1958). During their 
first few days, chicks are brooded almost continually either 
on the original nest, on brood nests, or on floating drift. 
In North Carolina, adults construct up to six brood nests 
of buoyant Spartina that float with the rising tide and 
provide dry places for refuge; brood nests there are like egg 
nests without canopies. 

Clapper Rails forage mosdy when tidal flats and channel 
banks are exposed. From widiin or near cover, they obtain 
food by probing and digging in the mud, picking from the 
surface, or rapidly seizing or chasing down more mobile 
prey. They often wash their food before eating it, peck open 
or dismember larger items before consuming them, and 
usually swallow small items whole (Williams 1929). The 
diet is predominandy invertebrates and small amounts of 
cord grass seeds. The main animal foods are ribbed mus- 
sels, clams, amphipods, shore crabs, spiders, along with 
occasional snails, nereid worms, and insects; minor items 
are small vertebrates, such as mice, fish, frogs, brush 
rabbits (probably carrion), and (for other races besides 
obsoletus) birds (Moffitt 1941, Evens ck Page 1984). Stom- 
ach samples of 18 birds collected near Palo Alto, Santa 
Clara County, on 4 February 1939 were composed (by 
volume) of 85% animal matter and 14.5% cord grass seeds 
and hull fragments (Moffitt 1941). The summer diet may 
include more insects and less vegetable matter; vegetable 
foods at any season may be more important during tidal 
regimes where the mudflats are exposed for only limited 
periods. Parental care (including feeding of young) extends 
into the fifth or sixth week after hatching (Adams ck Quay 
1958). 

Marin Breeding Distribution 

In Marin County, breeding Clapper Rails are restricted to 
salt marshes along the shorelines of San Francisco and San 
Pablo bays. During the adas period, Clapper Rails were 
confirmed breeding at Corte Madera Ecological Reserve, 



167 



Rai 



MARIN COUNTY BREEDING BIRD ATIAS 



Rails 



Greenbrae (NE 3/25-6/30/82 -JGE, GWP). Other 
recent breeding records exist for Richardson Bay in 1972 
(SFBCDC & Harvey 1983) and for Creekside Park along 
Corte Madera Creek, Greenbrae (FL 8/23/84 — HoP). 
Historical breeding records exist for Manzanita in 1930 
and near Greenbrae in 1931 (Gill 1979). Other sightings 
for Marin are from Muzzi Marsh, Triangle Marsh, and San 
Clemente Creek, Corte Madera; Tiscornia Marsh, San 
Rafael; from the mouth of San Rafael Creek upstream 
approximately 0.6 miles; China Camp SP; Santa Venetia 
marshes; Las Gallinas Creek to the mouth of Novato 
Creek; Novato Creek upstream to 1 .2 miles north of Hwy. 
37; Day Island; and Black John Slough (Evens 6k Page 
1984, JG. Evens pers. comm.). 

Gill (1979) estimated that for the period 1971 to 1975, 
the average yearly population in Marin was 153 birds 
(range 102-204), though his estimates for the entire San 
Francisco Bay system at that time were probably much too 
high (see below). Page and Evens (1987) estimated a 
population of about 40 birds at Corte Madera Ecological 
Reserve in both 1982 and 1987. Estimates from censuses 
at other Corte Madera marshes in 1987 were 15 birds at 
Muzzi Marsh, 3 birds at San Clemente Creek, and 1 bird 
at Triangle Marsh. The 59 birds inhabiting die Corte 
Madera marshes represents an unknown portion of the 
entire Marin population. Because of continuing dramatic 
declines in the South Bay and threats to the entire San 
Francisco Bay system population (see below), periodic 
censuses should be made of all Clapper Rail habitat in 
Marin County. With these declines, the Corte Madera and 
Gallinas Creek populations, in particular, are taking on 
increasing importance to the California Clapper Rail popu- 
lation as a whole (P.R. Kelly pers. comm.). 

Historical Trends/ Population Threats 

The California Clapper Rail was formerly a resident of 
coastal salt marshes from Humboldt Bay south to Morro 
Bay, with the greatest population in San Francisco Bay 
(G6kM 1944, Gill 1979, Evens & Page 1984). Populations 
have declined drastically since the late nineteenth century. 
The California Clapper Rail is now restricted to San 
Francisco, San Pablo, and Suisun bays and is listed as 
Endangered by both state (CDFG 1991a) and federal gov- 
ernments (USFWS 1 989a). Although formerly recorded in 
Marin County at Tomales Bay, it is unclear if it ever bred 
there (Evens 6k Page 1984, Shuford et al. 1989). Intensive 
rail surveys from 1984 to 1986 at the south end of Tomales 
Bay failed to reveal any birds (JGE, GWP). Marin popula- 
tions along San Francisco and San Pablo bayshores have 
surely declined over historical levels. 

Gill (1979) estimated that 4200 to 6000 birds inhabited 
greater San Francisco Bay, with 55% of the population in 
the South Bay and 38% in the Napa Marsh. Logistical 
restraints limited Gill's ability to census many of the bay's 

168 



marshes, and his use of extrapolations probably greatly 
overestimated the size of the population (I larvey 1987, P.R 
Kelly pers. comm.). Based on more thorough high-tide 
censuses in winter, die population of the entire San Fran- 
cisco Bay system from the time of Gill's work up until 
about 1985 was about 700 Clapper Rails, with approxi- 
mately 90% in the Soudi Bay (P.R. Kelly pers. comm.). As 
of the winters of 1988-89 and 1989-90, the population 
had declined dramatically to under 500 birds, apparendy 
mosdy because of predation by introduced red foxes that 
appeared in South Bay marshes by at least 1983 (P.R. Kelly 
pers. comm.). Winter censuses in 1990-91 revealed con- 
tinuing declines that leave only 300 to 400 rails still 
inhabiting the Bay (P.R. Kelly pers. comm.). Surely histori- 
cal numbers gready exceeded these recent population esti- 
mates. 

Gill (1979) speculated that Clapper Rails had colonized 
the Napa Marsh in numbers since the 1930s because of 
increasing salinity there from reduced freshwater inflow 
into the Bay. The lack of many Clapper Rail sightings in 
the Napa Marsh before 1940 parallels the absence of 
records of breeding Black Rails in San Pablo Bay prior to 
the late 1950s. This is probably from very limited observer 
coverage in these areas radier than recent dramatic changes 
in these populations, which is further supported by the 
small numbers of Clapper Rails found in the Napa Marsh 
in die 1980s (P.R. Kelly pers. comm.). Gill (1979) sug- 
gested that local population fluctuations were from changes 
in production and biomass of Spartina— in dry years, popu- 
lations were reduced because of increasing intraspecific 
competition for nesting territories. Greater use of 
Salicornia for nest sites in die South Bay in recent years, 
perhaps because of successional changes in marshes, sug- 
gest other factors may be responsible for population fluctu- 
ations (Harvey 1980a). 

After, and in concert with, initial reductions from mar- 
ket hunting (Grinnell et al. 1918), habitat loss has been the 
primary cause of decline and failure to return to historical 
levels (Gill 1979, Evens 6k Page 1984). Gill (1979) sum- 
marized data on market hunting in the South Bay around 
the turn of the century. At that time, it was not uncommon 
for individual hunters to kill 30 to 50 rails a day. One 
newspaper account referred to 5000 rails killed in a one- 
week period alone in 1897— ten times today's total popula- 
tion in greater San Francisco Bay! Outcries led to passage 
of protective laws in 191 3, and numbers began to rebound 
with recolonization of areas of local extirpation (G6kM 
1944). This population resurgence was short lived. Steady 
habitat destruction from human development has resulted 
in a loss of 60%-95% of the former saltmarsh habitat 
around the San Francisco Bay estuary (Nichols 6k Wright 
1971, Josselyn 1983) and about 90% around Humboldt 
Bay (MacDonald 1977). Clapper Rails face a host of 
problems, including further habitat loss and degradation 



Rails 



SPECIES ACCOUNTS 



Rails 



from human encroachment as summarized for the Black 
Rail (see account). Additional threats are recent losses of 
potential habitat formerly suitable for restoration, an 
increase in brackish marsh at the expense of salt marsh in 
the South Bay from dramatic increases in sewage outfall 



since the early 1970s, and the introduction of red foxes as 
noted above (Harvey 1983, 1987; P.R. Kelly pers. comm.). 
Restoration of large tracts of former salt marsh offers the 
best hope for offsetting other immediate threats. 



VIRGINIA RAIL Rallus limicola 







A year-round resident; numbers swell 




^t^\ ^ ^Cb<u^ 


from Sep through Mar. 


■i^o^Or 


An uncommon, very local breeder; 


^v \><\\ y^ 


\^^\ V^**v^ \ ^\ \ i-*V*^\ ^-"V^A ^\^~^x — - 


overall breeding population very small. 


<^\ \^>\ 


{\j^Kjy^fj^ 


Recorded in 17 (7.7%) of 221 blocks. 


V^C^ 


vOt^V^rx \^\J^\^Jk^\\^\ \ 


O Possible = 8 (47%) 


yrt 


^^^^S^X^^S^^^^X^^ 


© Probable = 5 (29%) 




#^\ !j(^^V^yV\ 3s*^\ J^c\ \^\ \"v 


• Confirmed = 4 (24%) 






FSAR = 2 OPI = 34 CI = 1.76 




^^^^^^k^^^f^C^^c^ 






'>[ ^-^cTv'5< ! ^ J4r\J5?\ 




t 


^ "w'^^oW 





Ecological Requirements 

Eerie cackling or hiccupping calls betray die presence of 
Virginia Rails in their Marin County breeding haunts of 
freshwater marshes, coastal swales, wet meadows, and, 
perhaps, brackish marshes. In the winter, some Virginias 
disperse to tidal salt marshes, which they shun during the 
breeding season here, but not in some other parts of the 
range (Zimmerman 1977). Important needs of breeding 
birds are shallow standing water; dense marsh vegetation 
for cover, nest sites, and brooding areas; and a suitable 
supply of invertebrate food. Virginia Rails overlap consid- 
erably in habitat preference with Soras. Virginias are more 
widespread here and appear to have less stringent nesting 
requirements, though the differences in habitat needs 
between the two species are not easily explained. In Colo- 
rado, both Virginia Rails and Soras prefer breeding 
marshes widi water less than 6 inches in depdi (Griese et 
al. 1980). In Missouri, migrant Virginias, at least, inhabit 
areas of marsh with shallower water than Soras, though 
both species prefer similar vegetation density and height 
(Sayre 6k Rundle 1984). Migrant Virginias in Missouri 
prefer saturated to shallowly flooded sites less than 2 
inches deep and are usually flushed near the interface of 
standing water and soil; Soras prefer water depths from 2 



to 6 inches but use saturated sites more often than those 
with 18 inches of water. In Iowa, breeding Virginia Rails 
and Soras show little evidence of distinct niche segregation. 
Soras diere are most numerous at nearshore sites with 
relatively shallow water, diverse vegetation, and many seed- 
producing plants, while Virginias are more evenly distrib- 
uted across various marsh types (Johnson & Dinsmore 
1986). Marsh areas with floating residual plants may be 
important to Soras because such cover provides good 
substrate for invertebrates that are kept near the surface 
within reach of these short-billed rails. The availability of 
preferred foods and the species' ability to exploit them, as 
reflected by bill shape, may be more important than water 
depdi per se in explaining the difference in habitat prefer- 
ences of these two species. 

Virginia Rails conceal their nests in dense marsh vege- 
tation where they usually suspend them in, and intertwine 
them with, emergent plant stalks (Townsend 1926, 
Walkinshaw 1937). Water depths around nest sites at the 
beginning of incubation range from about 3 to 10 inches 
(Walkinshaw 1937, Berger 1951, Zimmerman 1977, Gri- 
ese et al. 1980). Sometimes Virginias build nests from the 
ground up that may reach seven or eight inches in height 



169 



Rai 



MARIN COUNTY BRFHDING BIRD ATlJ\S 



Rails 



(Townsend 1926). Glahn (1974) reported that Virginias 
locate most of their nests within tall vegetation near dis- 
continuities with shorter vegetation, water, or mud along 
territory boundaries. Virginia Rails usually build their 
shallow platform nests from pieces of coarse rushes, 
sedges, or grasses and line diem with finer materials of the 
same types (Townsend 1926, Walkinshaw 1937). When 
die vegetation allows, die nest is usually arched over widi 
a canopy of rushes and sedges. Birds approach their nests 
via a sloping runway. At die first sign of rising water, these 
rails add material to raise die eggs above harm's way. The 
eggs hatch asynchronously, and one parent leads die first 
chicks away from the nest while the other continues to 
incubate the remaining eggs. Young feed with parental 
assistance the first day but are self-sufficient foragers by the 
end of die first week of life (Zimmerman 1977). 

Virginia Rails use their slender curved beaks to probe 
and, presumably, to peck and glean for food along the 
muddy interface of standing water and marsh vegetation 
and in openings between plants. Breeding birds in Iowa 
consume 84.6% animal matter and 1 5.4% vegetable fare 
(Horak 1970, n = 37). Continentwide, die diet ranges 
from 97% animal matter in summer (n = 20) to 68% in 
fall (n = 69) (Martin et al. 1 951 ). Animal matter is predom- 



inantly insects, along widi spiders, snails, crayfish, bryozo- 
ans, slugs, small fish, frogs, and small snakes (Townsend 
1926, Martin et al. 1951, Horak 1970). Seeds of marsh 
plants and duckweed are important plant foods. 

Marin Breeding Distribution 

Virginia Rails are patchily distributed in Marin County, 
reflecting the availability of suitable marshes, found mosdy 
near the outer coast. Representative nesting sites were 
Olema Marsh (FL 5/1/80 — DS) and Cypress Grove, near 
Marshall, Tomales Bay (FL 5/23 6k 6/6/78 -FMa, BTy). 

Historical Trends/ Population Threats 

Grinnell and Miller (1944) noted a decline in Virginia Rail 
numbers in California from habitat loss. Subsequendy, 
numbers must have continued to decline for the same 
reason. Because of its proximity to human endeavors, 
freshwater marsh habitat must have decreased to an even 
greater degree than tidal marsh habitat, which has been 
reduced in the San Francisco Bay system by 60%-95% 
from historical levels (Nichols 6k Wright 1971, Josselyn 
1983). 




170 



Rails 



SPECIES ACCOUNTS 



Rails 



SORA Porzana Carolina 



^<\3cC 


?^^-^ \ 


^P*0f\^rC^ 




A year-round resident; numbers swell 
from Sep through Apr. 

A rare, very local breeder; overall breed- 
ing population very small. 

Recorded in 10 (4-5%) of 221 blocks. 

O Possible 3 (30%) 




V?\^' V"\ 3^v^ 


zkrZ \^X^ \^K-^\^*K 




© Probable = 6 (60%) 








w2?° 


• Confirmed = 1 (10%) 
FSAR=1 OPI = 10 CI = 1.80 



Ecological Requirements 

The Sora is another shadowy recluse of the moist soils and 
shallow waters of freshwater marshes, coastal swales, wet 
meadows, and, perhaps, brackish marshes. Some Soras 
disperse to winter in tidal salt marshes, from which they 
are absent during the breeding season. Important needs of 
breeding birds are standing fresh water; dense marsh 
vegetation for cover, nest sites, and brooding areas; and a 
suitable supply of seeds and invertebrate foods. Differences 
in habitat preferences between Soras and Virginia Rails are 
subde, though Soras are less widespread here and appear 
to have more stringent requirements (see Virginia Rail 
account). 

Soras usually nest in dense marsh vegetation and, occa- 
sionally, in somewhat open surroundings. Whether in tall 
or short vegetation, Soras tend to locate their nests near 
discontinuities with vegetational borders, water, or mud 
along territory boundaries (Walkinshaw 1940, Glahn 
1974). They build nest platforms or well-built baskets of 
rushes, sedges, or grasses raised up from the marsh floor 
or suspended and interwoven with surrounding vegetation 
(Bent 1926, Walkinshaw 1940). Nests are lined widi finer 
marsh vegetation and are usually more or less concealed 
with an arched-over canopy of grasses, reeds, or cattails. 
There is often a slanting runway of nest materials leading 
to and from the nest. Nests are generally raised an average 
of about three to seven inches above the water (Bent 1 926; 
Walkinshaw 1937, 1940; Berger 1951; Griese et al. 1980). 
Birds usually complete the nest after they start laying and 
will add materials to raise the nest when water levels rise 



(Walkinshaw 1940). Eggs hatch asynchronously and one 
parent leads the first chicks away while the other incubates 
the remaining eggs. 

With their stubby bills, Soras pick or glean from the 
marsh substrate or water's surface and strip seed heads 
from marsh plants (Bent 1926, Walkinshaw 1940, Sayre 
ck Rundle 1984). Breeding birds in Iowa consume 80.8% 
vegetable matter, mostly the seeds of aquatic plants (Horak 
1970, n = 19). Continentwide the diet ranges from 60% 
(n = 109) animal matter in spring and summer to 31%- 
32% (n = 223) animal matter in fall and winter (Martin et 
al. 1951). In Missouri, spring and fall migrants consumed 
63% (n = 18) and 82.5% (n = 20) vegetable matter, respec- 
tively, with sedge seeds predominant in spring and grass 
seeds in fall (Rundle <St Sayre 1983). In Connecticut, the 
fall diet was 98% seeds in freshwater marshes and 91% 
insects in brackish marshes (Webster 1964 fide Odum 
1977). Animal foods are various aquatic insects, snails, 
crustaceans, and small tadpoles or fish (Bent 1926, Martin 
et al. 1951, Horak 1970, Odum 1977). 

Marin Breeding Distribution 

During the adas period, Soras were found breeding at only 
scattered sites in Marin County. Their distribution was 
similar to, but more restricted than, that of Virginia Rails. 
Most Soras were found on the outer coast. Nevertheless, 
the only confirmed breeding record was of a nest (with ten 
eggs) on the marshy edge of a fish-breeding pond inland in 
Hicks Valley from 1 to 21 May 1982 (ScC, ITa). 



171 



Rails 



MARIN COUNTY BREEDING BIRD ATLAS 



Rails 



Historical Trends/ Population Threats 

Grinnell and Miller (1944) reported no change in the 
aggregate numbers of Soras in California except as caused 
by reclamation of marshes, which up to that time and since 
must have been great (see Virginia Rail account). 



COMMON MOORHEN Gallinula chloropus 







Generally of irregular occurrence year 


^JpOc^>r-^ N ^O 




round; slightly more numerous from Sep 




through early Mar. 


^K\ >V^\ Jrv ^V\ %^\J^C\Jk 


3r\iX" 


A very rare (perhaps rare), very local 
breeder; overall breeding population very 


\§^^VvVvV\>V^V 


^rC^X^I 


small. 


VO\L^ W) 3r\ ie^\ \^\ 


V-^\ ®\^ \ 


Recorded in 2 (0.9%) of 221 blocks. 




^^r^^cyp^) 


O Possible = (0%) 


xpV^a^A^v^^a^-Vv-A^^ 


J\^\ S-^C\ '^<\^^— -r" 


€ Probable = 1 (50%) 


7 \MVurA0r^\^^\3^ 


\^^X \^cS\\li^\ 


• Confirmed = 1 (50%) 




• i ^"^-^^\ ^ 


^$^^5?^ 


FSAR =1 OPI = 2 CI = 2.50 



Ecological Requirements 

In both appearance and habits, the Common Moorhen 
bridges the gap between our typical small rails and their 
cousin the American Coot. In Marin County, breeding 
Moorhens inhabit freshwater marshes with some open 
water, natural or artificial ponds, and drainage or irrigation 
ditches, though lakes or slow-flowing streams edged with 
emergent vegetation suffice elsewhere in California. The 
main breeding requirements seem to be robust emergent 
vegetation for cover, nest sites, and brood rearing; some 
open standing or slow-moving fresh water; and an ade- 
quate food supply. Moorhens typically use deeper-water 
marshes and prefer centrally placed emergents rather than 
the bordering vegetation of shallow-water marshes used by 
Virginia Rails and Soras (Weller 6k Fredrickson 1973, 
Byrd & Zeillemaker 1981). Moorhens keep more to cover 
and less to open water than do Coots. 

In marshes of variable water depth, Moorhens select 
nest sites in areas of deeper water (Byrd 6k Zeillemaker 
1981). In the managed shallow water of irrigated taro fields 
in Hawaii, Moorhens choose nest sites where the water 
depth averages 2.6 inches; elsewhere in the breeding range, 
they may nest over waters 1 to 6.5 feet deep. Moorhens 

172 



usually attach their nests to emergent or broken-down 
aquatic vegetation; nests range from near floating up to six 
inches, or, rarely, two or three feet above water (Dawson 
1923, Bent 1926, Fredrickson 1971, Strohmeyer 1977, 
Roselaar 1980, Byrd 6k Zeillemaker 1981). They may build 
more nests than they use for egg laying (Bent 1926). Less 
frequendy used nest sites include among scattered sub- 
merged bushes; under weeds on a floating island; on 
mobile, semisubmerged logs; high on a mass of cattails 
tangled by the wind; on top of a dead stump two feet from 
the water and surrounded by willows; on banks amid thick 
tangles of briars, grasses, and vines within a few feet of 
water; and in a low crotch of a bush near water (Bent 
1926). Birds usually conceal their nests within stands of 
vegetation and only rarely nest in exposed situations. In 
Iowa, nests averaged 1 1 feet (range 0-59 ft.) from an edge 
of vegetative change or water and 15 feet (range 7-92 ft.) 
from open water (Fredrickson 1971); easy access to open 
water is probably important. 

Moorhens build their shallow nest baskets from the 
dead leaves and stalks of cattails, bulrushes, grasses, or 
other coarse aquatic plants. They may preferentially select 



Rails 



SPECIES ACCOUNTS 



Rails 



for nest building certain plants among those available 
(Byrd & Zeillemaker 1981). Often the nest is screened 
from above by a canopy of aquatic plants. It usually has an 
inclined runway of nest material used to leave or enter the 
nest (Bent 1926). Nest building continues through egg 
laying and incubation, and birds will add additional nest 
material to raise the nest above rising water (Bent 1926, 
Fredrickson 1971). The first clutch normally hatches syn- 
chronously; replacement and second clutches hatch asyn- 
chronously (Roselaar 1980). Within hours after hatching, 
the young are fed by the parents (Fredrickson 1971). 
Moorhens brood their young on brood platforms (with 
ramps) they build or on platforms built by Coots or 
muskrats. One adult brings food to die brooding mate, 
which transfers it to the young. Groups of Moorhens 
occasionally contain two adults along with both downy and 
nearly fledged young (Roselaar 1 980, Byrd ck Zeillemaker 
1981). The older chicks of these "multiple brood family 
units" will sometimes feed and brood younger chicks, 
incubate eggs if the pair has not yet hatched die second 
clutch, and make nest repairs. 

These aquatic rails feed while swimming, walking on 
floating vegetation, or walking on land, nodding their 
heads and perking their tails as they go (Dawson 1923, 
Bent 1926, Roselaar 1980). They obtain food by dipping 
the head underwater, by surface picking or sifting, and, 
rarely, by diving. Birds also up-end and snatch food from 
other birds. In addition, they pick food items off the 
ground and off plants, often by clambering over leaves and 
balancing on stems by flapping their wings. Moorhens 
often feed in dense cover, threading their way through the 
reeds, and also range onto grass on the dry borders of 
marshes, though seldom beyond easy reach of cover. The 
diet is omnivorous, with varying proportions of plant and 
animal matter; little quantitative work has been done, so 
changes in diet with season and locality are unknown 
(Bent 1926, Roselaar 1980). Animal matter in the diet 
consists of snails and other small mollusks, adult and 



larval insects, spiders and harvestmen, earthworms, 
amphibian tadpoles, and, rarely, small fish. Moorhens also 
eat carrion and birds' eggs up to those of Mallards' in size. 
Vegetable foods include die seeds, roots, and soft parts of 
succulent aquatic plants, and berries and fruits. Birds also 
feed on vegetable peelings and scraps. Initially, parents 
apparendy feed chicks mosdy aquatic insects, such as 
dragonfly and mayfly nymphs (Fredrickson 1971). 

Marin Breeding Distribution 

During the adas period, Moorhens were recorded in the 
breeding season in small marshes in only two adas blocks 
near Novate Breeding was confirmed at a pond off High- 
way 37 with the presence of adults with small young on 2 
June 1977 (RMS). There is a prior breeding record from 
Olema Marsh (FL 8/31/67 — CJR) and more recent ones 
from the Bolinas sewage ponds (FL 7/12/83 — JGE) and 
from a pond near Rush Creek, Novato (FL 7/21/84 
— HoP). See Shuford et al. (1989) for additional records 
suggestive of breeding elsewhere on Point Reyes. 

Historical Trends/ Population Threats 

Earlier avifaunal accounts lacked records of Moorhens 
breeding in Marin County (G&W 1927, S&P 1933) or, 
for that matter, along the coast north of the Golden Gate 
(G&M 1944). The recent breeding records in Marin 
County probably reflect greater observer coverage rather 
dian any recent expansion of the breeding range. Grinnell 
and Miller (1944) felt that numbers had declined because 
of habitat loss, which was offset somewhat by the develop- 
ment of irrigation. Continued destruction of marshland 
since that time has surely reduced numbers substantially 
(see other rail accounts). Numbers of Common Moorhens 
were relatively stable on Breeding Bird Surveys in Califor- 
nia from 1968 to 1989, a period after most habitat destruc- 
tion had occurred, though numbers increased from 1980 
to 1989 (USFWS unpubl. analyses). 



173 



Rai 



MARIN COUNTY BREEDING BIRD ATLAS 



Rails 



AMERICAN COOT Fulica americana 




Occurs year round, though primarily as a 
winter resident from mid-Sep through 
May. 

A fairly common, local breeder; overall 
breeding population small. 

Recorded in 58 (26.2%) of 221 blocks. 

O Possible 25 (43%) 

C Probable 1 (2%) 

• Confirmed = 32 (55%) 

FSAR = 3 OPI = 174 CI = 2.12 



Ecological Requirements 

The "Mud-hen," though of humble mien, is perhaps our 
most adaptable nesting waterbird. Breeding Coots inhabit 
Marin County's freshwater ponds, lakes, reservoirs, 
marshes, sewage ponds, and irrigation channels. Prime 
breeding habitat has a good mix of open water and dense 
emergent vegetation, particularly tules or cattails. Robust 
emergents provide nest sites, nest materials, and cover for 
adults and broods; open water provides foraging habitat. 
Coots will nest where open water is patchily or continu- 
ously edged with emergent vegetation, but in Iowa, at least, 
Coots reach highest densities where there is a 50:50 mix 
of open water and emergent cover (Weller &. Fredrickson 
1973). Coots tend to occupy microhabitats with shallower 
water than do Pied-billed Grebes (Nudd 1 982). 

In the San Francisco Bay Area, breeding Coots build 
from seven to nine structures associated with nesting- 
including display platforms, egg nests, and brood nests 
(Gullion 1954). Elsewhere, Coots build fewer structures 
(except egg nests) when naturally occurring platforms, such 
as those built by muskrats, are available (Fredrickson 
1970). Display platforms are composed of coarse cattail 
stems and leaves and are built on a foundation such as a 
floating board or stick or a heap of broken-down cattails 
(Gullion 1954). Display platforms are usually thoroughly 
water soaked and after the copulation period are allowed 
to disintegrate, though up to that point nonincubating 
birds may use them as roost sites. Egg nests are shallow 
baskets, usually with a floating foundation, attached to 
emergent vegetation over various depths of water (Bent 
1926, Gullion 1954, Fredrickson 1970, Fredrickson et al. 

174 



1977). Most nests are partially or well hidden in emergent 
vegetation, though sometimes they are in plain sight at the 
edge of vegetation, in an isolated clump, or in an entirely 
open situation without concealment Usually overhead 
cover is naturally provided. Coots do not build canopies 
above the nest (Gullion 1 954). In the Bay Area, all struc- 
tures are located close (usually 2 to 3 ft., or a maximum of 
4 ft.) to open water. Throughout the range, exceptional 
nest sites are on dry ground; two feet above ground on a 
platform of dead cattails; and four feet above ground in the 
lower branches of an apple tree following receding flood- 
waters (Bent 1926). These odd records suggest nests were 
built at times of unusual water conditions (Gullion 1954). 
Coots often build more than one nest structure before 
finally selecting one in which to lay the eggs (Gullion 
1954). Coots usually build their nests— starting with coarse 
materials and progressively adding finer materials— from 
dry or green stems and leaves of cattails or other marsh 
plants; they sometimes also use willow leaves and small 
sticks. The cup is composed of fine dry leaves. Coots enter 
and leave the nest via a ramp of coarse stems laid length- 
wise and incorporated into the side of the nest They may 
begin laying eggs while the nest is still under construction 
but finish the nest and line it before the clutch is complete 
(Bent 1926, Gullion 1954). Since egg nests are actually 
elaborate rafts, Coots must constandy add material to them 
to keep them from settling below the surface (Gullion 
1954). Pairs of Coots may either build new egg nests or 
reuse display platforms or brood nests used previously by 
them or other pairs. When the eggs hatch, Coots usually 



Rails 



SPECIES ACCOUNTS 



Rails 



construct a new, larger brood nest, or convert an egg nest 
to a brood nest Brood nests frequendy lack a cup, or, if 
present, it is usually obscured in the restructuring process. 
Brood nests are distinguished from egg nests by the wet 
materials worked into the final lining and by the presence 
of feces in and about the nest. 

Since eggs hatch asynchronously, one parent takes over 
the major share of incubation while the other seeks food 
for the young already hatched (Gullion 1954). Initially one 
adult collects food and presents it to the incubating bird, 
which in turn feeds the chicks (Fredrickson 1970). Usually 
the male incubates during this period and also broods the 
young not feeding with the female (Gullion 1954). The 
female returns frequendy to the nest widi food. Two- and 
three-day-old chicks leave the nest, swim out to be fed by 
the female, then return to the nest and the protection of 
the incubating male. Three or four days after hatching, the 
female broods the older, more active young at night on the 
brood nest while the male incubates the remaining eggs 
and the newly hatched young on the egg nest. When a 
sufficient number (about eight) of young have hatched, 
both parents turn their attention to the care of the young 
and either desert the remaining eggs or dump them out of 
the nest. At about five days of age, the young begin to 
spend most of the day following their parents on foraging 
excursions in the emergent vegetation and later in open 
water. At dusk, when broods move to platforms, the male 
appears to assume the responsibility of caring for most 
young (Fredrickson 1970). 

Coots obtain their food by dabbling and grazing from 
or near the surface of the water, by grazing or picking items 
from the surface of moist or dry land along or well back 
from the shore, and by diving to moderate depths (Bent 
1926). While swimming or walking, they nod their heads 
in rhythm to their foot movements. Coots clip off green 
grasses in meadows, pastures, or lawns and sometimes 
grain (particularly rice) in cultivated land. They also steal 
some aquatic plant food from ducks and eat grain set out 
to attract ducks. Continentwide, the annual diet is about 
89.4% plant matter and 10.6% animal matter and varies 
little by region (Jones 1940, n = 792). Plant foods make up 
about 98% of the diet in fall and winter (n = 658), 84% in 
spring (n = 82), and 56% in summer (n = 36) (Martin et 
al. 1951). Coots prefer the foliage, stems, fleshy rootstalks, 
and, secondarily, seeds of a wide variety of marsh and 



aquatic plants, especially pondweeds, sedges, algae, and 
grasses (Jones 1940, Martin et al. 1951). The main animal 
foods are insects (especially beedes, true bugs, and dragon- 
fly and damselfly larvae and nymphs), mussels, and snails; 
infrequent items are spiders, crustaceans, and, rarely, small 
fish and amphibians. Initially, the young are fed exclusively 
animal matter, mosdy in the form of aquatic insect larvae 
of dragonflies and damselflies; by the time they are eight 
days old, young Coots consume considerable quantities of 
vegetable food (Gullion 1 954). 

Marin Breeding Distribution 

During the adas period, Coots bred widely, though patch- 
ily, throughout the lowlands of Marin County. They were 
somewhat more prevalent breeders on the outer coast and 
along the San Pablo and San Francisco bayshores. Repre- 
sentative nesting locations were Olema Valley (FL 6/1 1/82 
— BiL); Bolinas sewage ponds (FL 5/5/80 — DS); pond at 
ocean end of Tennessee Valley (FL 9/1 1/82 — DS); Hicks 
Valley (FL 6/21/82 -DS); and McGinnis Park, San Rafael 
(FL 6/3/80 -DS). 

Historical Trends/Population Threats 

Grinnell and Miller (1944) reported that Coot numbers 
had held up over the years, but historically they must have 
declined dramatically with the drainage of most of the 
state's wedands. Marin County has few natural lakes and 
ponds, so Coot numbers may have increased here over the 
years because of the impoundment of water in municipal 
reservoirs, stock ponds, and sewage ponds. On the other 
hand, some marshes have been drained; others, such as 
Olema Marsh, have been lost to Coots as they have choked 
with emergent vegetation from poor drainage, apparendy 
caused by road building and diking. Numbers of Coots 
wintering on Pt. Reyes have declined since 1976, appar- 
endy because of two periods of widespread drought in 
California (Shuford et al. 1989). On the whole, Coot 
numbers were relatively stable on Breeding Bird Surveys 
in California from 1968 to 1989 (USFWS unpubl. analy- 
ses), a period after most habitat loss had occurred. Num- 
bers decreased from 1980 to 1989 (USFWS unpubl. 
analyses), perhaps because much habitat dried up during 
the three years of drought at the end of that period. 



175 



Plovers 



MARIN COUNTY BREEDING BIRD ATLAS 



n 



overs 



Plovers 

Family Charadriidae 



SNOWY PLOVER Charadrius alexandrinus 









Occurs year round, though numbers 




Ap^s^. N Jf 




swell considerably from Jul through early 






Apr. 






\^\, \^\—^- ~ 


A fairly common, very local breeder; 


C\9\V 


-^V>A^^V\^O^W 




overall breeding population very small. 




^\\yf^%^&^. 




Recorded in 11 (5.0%) of 221 blocks. 








O Possible = 1 (9%) 




\ L#F^A ^t^-^-V^A X^i\ J*?' 




C Probable = 4 (36%) 




\*J^T\ ^C\"^^\-^\\'^ 


Jk^Kj^K^k^^ 


• Confirmed = 6 (55%) 






^^¥^^w^?a 


FSAR = 3 OPI = 33 CI = 2.45 






x^'^^^^c^V^^L 





Ecological Requirements 

An illusory movement is all that betrays a tiny sand-colored 
plover as it stealthily slips off its nest amid coastal dunes. 
On the mainland California coast, Snowy Plovers scatter 
widely to breed on sandspits, dune-backed beaches, lagoon 
and estuarine margins, around salt evaporators, and on 
small pocket beaches (Stenzel et al. 1981). Sandspits and 
bars, with their low topographic relief of well-developed 
hummocks and dunes, separate the ocean from coastal 
wedands. Bordering wedands enhance the spits as plover 
habitat. They provide tidal flats and other barren open 
areas in marshes that afford alternate, often productive, 
feeding areas and also refuge from human disturbance on 
the beaches. Frequent plover movements between beaches, 
tidal flats, and salt pans illustrate the attractiveness of a 
suite of favorable habitats close at hand. Dune-backed 
beaches are usually interrupted by rivers, creeks, ponds, 
lagoons, or salt pans, and the sections of these beaches that 
abut such wedands hold disproportionate numbers of 
nesting plovers relative to the availability of the habitat. In 
Marin County, Snowy Plovers nest primarily on spits or 
on dune-backed beaches, but also on the margins of 
Abbott's Lagoon and occasionally on bluff-backed beaches 
(i.e., soudi end of Pt. Reyes Beach). Elsewhere on the coast, 

176 



plovers also nest in or on the margins of estuaries and 
lagoons, in naturally open or disturbed areas, such as salt 
pans in salt marshes or bay fill. At a few sites, plovers breed 
commonly at commercial or abandoned salt evaporators, 
where they nest on low dikes separating evaporator ponds 
and on the floors of dried ponds. Only small numbers of 
plovers nest at the mouths of coastal creeks and lagoons— 
on small pocket beaches that are set off by high bluffs or 
rocky points and usually lacking in dunes or hummocks. 
Snowies prefer to nest in flat or gendy undulating open 
areas devoid of, or sparsely covered with, low-growing 
vegetation, driftwood, or odier debris (Stenzel & Peaslee 
1979, Stenzel et al. 1981). In coastal areas, these may 
include sand beaches; sand flats among the dunes; salt or 
alkali flats in marshes, on lagoon or estuarine margins, or 
in evaporator ponds; low, unvegetated dikes; or, rarely, 
wind-eroded sandy bluffs or sandy dredge islands (Stenzel 
et al. 1981, D. Shuford pers. obs.). Coastal nest sites are 
usually within 100 yards of water but occasionally, when 
there is not a formidable barrier between the nest and 
water, are several hundred yards away from it (Stenzel et al. 
1981). Openness of nesting habitat seems a key require- 
ment— for visual security from predators while the birds are 



P!c 



SPECIES ACCOUNTS 



Plovers 



incubating eggs, for foraging, and for leading plover chicks 
from nesting sites down to shoreline foraging areas. For 
breeding, Snowies eschew beach areas that are heavily 
littered with driftwood, are back in dunes with steep dune 
faces perpendicular to the shoreline, or where there is 
moderate to dense vegetation, since any of these hinder 
movement of chicks to low-lying foraging grounds. Snow- 
ies also avoid breeding in areas very heavily used by 
humans. 

During courtship, a male uses his belly and feet to make 
a series of shallow nest scrapes in soft substrate, only one 
of which the female selects for egg laying (Page et al. 1985). 
The nest scape may or may not be lined with bits of broken 
shell, fish bones, small stones, salt crystals, bits of wood, 
or other debris (Bent 1929, D. Shuford pers. obs.). Snow- 
ies typically continue to add nest material to the scrape 
throughout incubation (G.W. Page pers. comm.). They 
often select nest sites next to an object, such as driftwood, 
kelp, other stranded flotsam or jetsam, or a clump of 
vegetation (Stenzel 6k Peaslee 1979, Stenzel et al. 1981, 
Page et al. 1985). In California, 68% of 136 nests were 
within six inches of such an object (Page et al. 1985); 
objects selected are usually small radier than large ones 
(G.W. Page pers. comm.). Nearby objects possibly serve as 
concealment for incubating plovers, or they may function 
in unknown ways in courtship activities; observations 
suggest that objects do not function as windbreaks, to 
provide shade, as cues to nest location, or to conceal eggs 
or adults coming from or going to them (G.W. Page pers. 
comm.). At Mono Lake, the disruptive effects of nearby 
objects did not reduce predation, as did overhead cover 
(Page et al. 1985). The situation there may be anomalous, 
because objects are not numerous as they are at coastal 
sites; at the latter, searches by predators for plovers' nests 
beside objects might prove fruidess. In California, the 
Snowy Plover breeding system is one of serial polyandry— 
that is, in the same season, females nest in succession with 
different males (Warriner et al. 1986). A few days after the 
hatching of the first clutch, the female leaves the precocial 
young to the care of the male and departs to search for a 
new mate. 

Snowy Plovers forage in open areas on outer-coast 
beaches, from the water's edge back to the fore edge of die 
dunes, and on tidal, sand, or alkali flats of estuaries, 
lagoons, salt marshes, river mouths, and evaporator 
ponds. They forage on beaches, mosdy on wet sand or 
higher up where invertebrates concentrate around cast-up 
wrack. Like most plovers, Snowies are visual predators that 
primarily search for prey in a robinlike style— by walking or 
running several steps, peering down, and then picking 
items from the surface. Foraging birds run along in a 
halting zigzag fashion, stopping frequendy to peck at prey 
items (Swarth 1983). Snowies also probe into the surface 
of the mud for wriggling prey they detect visually. They 



commonly charge, open mouthed, into mats of brine flies 
and twist their heads and snap at airborne flies. Infre- 
quendy, they wade into shallow water to feed on inverte- 
brates. Rarely, they vibrate one foot on a solid surface to 
make prey move and reveal themselves (Johnsgard 1981). 
Snowy Plovers breeding at inland sites feed on a great 
variety of ground-dwelling arthropods, primarily flies and 
beedes (Swarth 1983). At Mono Lake, California, they feed 
primarily on brine flies and a species of carabid beede; 
brine flies are also important at coastal salt evaporation 
ponds. A small stomach-pumped sample (n = 3) from 
birds at Limantour Estero, Point Reyes, indicates that 
coastal birds eat polychaete worms, insects, various small 
crustaceans, and an occasional clam (G.W. Page unpubl. 
data). 

Marin Breeding Distribution 

During the adas period, Snowy Plovers bred on the outer 
coast of Point Reyes at the Seadrift/Stinson Beach spit at 
Bolinas Lagoon, Limantour Estero spit, Drake's Beach 
spit, Point Reyes Beach, and Abbott's Lagoon (Stenzel et 
al. 1981). Occasionally they may also breed at the mouth 
of Tomales Bay, at Sand Point at the south end of Dillon 
Beach, though documentation is lacking for that site. 

Single-day surveys in the breeding season revealed 40 
Snowies on Point Reyes in 1977 (Stenzel et al. 1981), 24 
in 1989, and 25 in 1991 (PRBO unpubl. data). These are 
low estimates of the total breeding population as intensive 
studies of color-banded plovers at Point Reyes in 1989 
documented that there were at least 32 adult Snowies 
breeding that year (Page et al. 1991). Representative breed- 
ing locations during the adas period were the spit at 
Bolinas Lagoon (NE 5/11 6k 12/77 -GWP et al.); Ab- 
bott's Lagoon (NE 4/14-27/77 -LES et al.); and Liman- 
tour Estero (NE 4/20-30/77 -SCP et al.). 

Historical Trends/ Population Threats 

Grinnell and Miller (1944) noted declines in Snowy Plover 
numbers in the southern portion of the state. Page and 
Stenzel (1981) documented the decline in California's 
coastal breeding population. During statewide surveys 
from 1977 to 1980, plovers were not found breeding at 33 
of 53 coastal sites with breeding records prior to 1970. It 
seems unlikely diat plovers will breed again at 28 of the 33 
sites because of habitat destruction or intense human use. 
The greatest losses are along the heavily urbanized south- 
ern California coast; this region, if left undisturbed, pro- 
vides the best coastal breeding habitat. The coastal Oregon 
breeding population has also declined since 1979, and 
surveys in 1989 suggest that California's coastal popula- 
tion may still be declining (Page et al. 1991). Interior 
breeding populations may be declining as well, but popu- 
lation trend data for that region are difficult to interpret. 

177 



Pic 



MARIN COUNTY BREEDING BIRD ATLAS 



Plovers 



Numbers of Snowy Plovers wintering on the southern 
California coast also appear to have decreased since at least 
1961 (Pageetal. 1986). 

Plover habitat has been degraded by industrial and 
residential development, intense recreational use by 
humans and their animals, off-road vehicle use, and 
grooming of beaches. Less noticeable but perhaps equally 
important are the indirect effects resulting from the plant- 
ing for dune stabilization, and subsequent naturalization, 
of the introduced European beachgrass or marram grass 
(Ammophila arenaria), which is now well established north 
of Point Conception. Natural dune systems along our 
coasdine have a series of dunes that run perpendicular to 
the shoreline and are frequently interspersed with 
expanses of flat sand that extend back from the beach and 
provide excellent plover nesting habitat (Page &. Stenzel 
1981). Ammo^hila-dominated dunes usually have a 
continuous foredune running parallel with die shoreline 
diat restricts access to the interdune sand flats. Addition- 
ally, Ainmophila reduces the species diversity of native 
plants, increases plant cover, steepens the dunes (Barbour 
et al. 1976), and markedly depresses the abundance and 
diversity of sand dune arthropods (Slobodchikoff &. 



Doyen 1977). This reduction in potential prey may 
adversely affect the plovers since they frequendy feed on 
insects well above the tide line (Stenzel et al. 1981). In 
northern California, degradation of plover habitat has 
been balanced to a large degree by the creation inside San 
Francisco Bay of salt evaporation ponds, which breeding 
plovers have used since at least 1918. For the whole coast, 
habitat degradation has far outweighed such enhancement 
(Page ck Stenzel 1981). 

Federal and state agencies and private conservation 
groups have expressed concern over the plover's declining 
populations. The Snowy Plover was on the Audubon 
Society's Blue List from its inception in 1972 to 1982 and 
on its list of Species with Special Concerns in 1986 (Tate 
1981, 1986; Tate ck Tate 1982). In California, this plover 
is currendy a Bird Species of Special Concern (Remsen 
1978, CDFG 1991b). Spurred by a petition by Page and 
Walter (1988), the coastal population of the Western 
Snowy Plover (C. a. nivosus) was finally listed by the U.S. 
Fish and Wildlife Service as federally Threatened in March 
1993. Techniques to successfully hand-rear Snowy Plovers 
have been developed should the need arise (Page et al. 
1989). 




178 



Plovers 



SPECIES ACCOUNTS 



Pic 



KILLDEER Charadrius vociferus 







Occurs year round, though numbers 


^^^^rOw^^ci^^ 




swell gready from Sep through Mar. 




A fairly common, widespread breeder; 


^^^k^^^^^^- 




overall breeding population fairly large. 


j?ZS «JV<\ °3r<\ °J^\ *K^\o V^A* V>^Co V>A 




Recorded in 146 (66.1%) of 221 


V5o\V\**\ "->V\ *tVo\ °A :> \ 3r-"\ Jv"\ V^l 




blocks. 

O Possible = 51 (35%) 
€ Probable = 31 (21%) 
• Confirmed = 64 (44%) 




,-Pvv Jk^kX V\ V^v V-^^M^OfV^C V^A*^ 
Pw^JkOiAS — X_ \^\ w^a V-^\ lkZ^\- \^\ * V^: 




FSAR = 3 OPI = 438 CI = 2.09 






^o 





Ecological Requirements 

Whether described as plaintive or petulant, sonorous or 
strident, vociferus is apdy named. Killdeer are the first and 
most persistent of birds to raise the hue and cry when 
intruders invade their nesting haunts. Scantily vegetated 
open terrain that provides clear vistas at ground level 
characterizes both nesting and foraging areas. These 
include pastures; plowed or uncultivated fields; roadside 
margins; lawns and playing fields; the fringes of quiet 
shallow waters, such as stock or sewage ponds, lakes, 
lagoons, estuaries, and streams; and other disturbed sites. 
Killdeer select barren or sparsely vegetated, often gravelly 
nesting sites with an ample supply of nest material at hand 
(Bunni 1959). Lawns, even if close cropped, are rarely used 
for nesting unless they offer bare spots. Preferred gravelly 
areas often arise from erosion or stream flooding or from 
construction of roadsides, dikes, or railroad right-of-ways. 
Nest sites are usually in close proximity to water used for 
bathing, cooling in hot weather, and foraging. Rarely, nest 
sites may be 1.5 to 2 miles from water; irrigated lawns or 
fields may sometimes substitute for standing shallow water 
(Townsend 1929, GckM 1944, Bunni 1959). Nest sites 
frequendy are elevated slighdy on mounds, knolls, hills, or 
slopes, giving incubating birds a wide view of their sur- 
roundings and affording protection from flooding by rain- 
water (Bunni 1959). Nest scrapes and nests are often 
placed beside an object, such as a stone, plant, log, manure 
pile, or dirt mound, which perhaps provides concealment 
from predators. In the absence of such an object, the 
cryptically colored eggs blend with uneven surfaces or the 



coloration or texture of the substrate to afford protection. 
If these methods fail, Killdeer often resort to their classic 
broken-wing displays to lure potential predators away from 
nests or vulnerable young. 

During courtship ceremonies, males construct with 
their feet several shallow scrapes, into the last of which 
their mates lay eggs; additional scrapes sometimes serve as 
reserve nests when eggs are lost (Bunni 1959). Killdeer 
avoid digging scrapes in hard substrates or in soft or 
muddy substrates where the pits might collapse. Availabil- 
ity close at hand of small, loose objects for nest building is 
important, since Killdeer obtain materials by tossing them 
with their bills toward the nest, usually from less than three 
feet. Typically, the bottom of the nest scrape is formed of 
stones that protect the nest from flooding by allowing water 
to percolate down; a fringe of nest material outside the nest 
protects from erosion. Killdeer prefer flat or angular stones 
from 0.2 to 0.4 inches (secondarily up to 0.6 in.) in length 
that are easily flipped. Birds select weed stems, twigs, and 
wood or bark chips in the 0.2- to 0.8-inch (secondarily up 
to 2.4-in.) size range over stones for lining the nest. White 
objects are preferred over black ones, though size and 
shape considerations will override color in the selection 
process. Almost any flat, lightweight object will do for nest 
material; these can include shells, lichens, manure, small 
bones, crayfish armor, dry shells of melon seeds, dead 
leaves, cornhusks, and rubbish such as peanut shells, 
paper matches, cigarette butts, dry chewing gum, bits of 
paper, plaster chips, or charcoal. Rarely, a nest on an area 

179 



Plovers 



MARIN COUNTY BREEDING BIRD ATLAS 



Plovers 



of sparse grass on a lawn is made mostly of grass, and 
sometimes eggs are laid in a bare scrape without nest 
materials. Killdeer sometimes nest on the gravel or crushed 
stone roofs of one- to four-story buildings (Bunni 1959, 
Demaree 1975). Chicks that don't succeed in tumbling 
safely from the roofs to the ground often succumb to 
dehydration. Killdeer have also nested three feet off the 
ground in a depression of semidecayed wood on a log 
(Bunni 1959). Killdeer sometimes begin to lay before the 
addition of much nest material, and birds typically con- 
tinue to add nest material throughout incubation. During 
one nesting season, females will sometimes lay successive 
clutches successfully in the same scrape. 

Killdeer forage in a variety of open habitats, particularly 
in pastures, plowed fields, lawns, and on the muddy, 
sandy, or marshy margins of shallow still or slow-moving 
water. Like most plovers, Killdeer are visual predators that 
forage robinlike by walking or running in a zigzag pattern, 
then stopping and peering intendy in search of hidden 
prey, which they pick from the ground (Bunni 1959). They 
also pull earthworms from the ground and wade into 
shallow water to feed. A Killdeer will sometimes cleanse its 
prey by dropping it into water several times and picking it 
up, or by rinsing it with sideways movements of the bill. 
After the precocial young hatch, adults lead them to the 
nearest feeding area. The Killdeer diet is about 97.7% 
insects and other animal matter and 2.3% vegetable mat- 
ter, chiefly weed seeds (McAtee &. Beal 1912, n = 228). 
Important animal foods are beedes, grasshoppers, caterpil- 
lars, ants, true bugs, caddisflies, dragonflies, flies, centi- 



pedes, spiders, ticks, nereid worms, earthworms, snails, 
crabs, and other crustaceans. The diet of upland-feeding 
birds undoubtedly varies considerably from that of birds 
foraging on freshwater margins or tidal flats. 

Marin Breeding Distribution 

During the adas period, Killdeer bred widely throughout 
the lowlands of Marin County. Representative breeding 
locations were the head of Schooner Bay (NE 3/22- 
4/7/81 — DS); near the mouth of Estero Americano 
(NE/NY 7/12/82 -DS); near Chileno Creek, Chileno 
Valley, on a gravel wash formed by the 1982 flood (NE 
5/5/82 —PA); and Bahia Drive ponds near the Petaluma 
River moudi, Novato (NE 5/7/78 — DS). Of interest was 
an earlier sighting of a pair that successfully raised two or 
three broods on the roof of the Inverness Motel (NE-FL 
3/29-8/3/67 -PL). 

Historical Trends/Population Threats 

Grinnell and Miller (1944) felt that the augmentation of 
favorable Killdeer habitat in California because of irriga- 
tion had more than compensated for reductions in "natu- 
ral" territory. This may be the case, but it is difficult to 
judge the skimpy facts available regarding the effects on 
wildlife of the tremendous changes brought by the state's 
extensive agricultural and urban development. Killdeer 
numbers decreased on Breeding Bird Surveys in Califor- 
nia from 1968 to 1989 but were relatively stable from 1980 
to 1989 (USFWS unpubl. analyses). 







180 



Oystercatchers 



SPECIES ACCOUNTS 



Oystercatchers 



Oystercatchers 

Family Haematopodidae 



BLACK OYSTERCATCHER Haematopus bachmani 











A year-round resident. 


/OaVv\ 




\ ^jCV-. 




An uncommon, very local breeder; 


-C^V^Cip^V^ 








overall breeding population very small. 


"x^i \^*\\ \^ 




^f>A3^^CVTV 




Recorded in 10 (4.5%) of 221 blocks. 
O Possible 4 (40%) 


VV^ 


^C\^ 


i^A \^\ \-- — \ V^\ \^^\ V- 




€ Probable = (0%) 


\ Je* 


vJV<o> 


^^-^A^^A ^\^\ 3s^\ ^-V^\ ^\^ 




• Confirmed = 6 (60%) 






i V^\^ V^\ -V^^x ^^^\ \^c\\ 


Oi^^V- — "" s " 


FSAR =2 OPI = 20 CI = 2.20 




"TUf/ v 




^^^^ 






JU 


^-->C^\^^^-^fe^o 







Ecological Requirements 

Boisterous, effusive piping greets any meddler in Oyster- 
catcher affairs along the wave-battered, sea-sprayed rocky 
shoreline of Marin County's outer coast. Black Oyster- 
catchers inhabit rocky reefs, offshore islets, and sea stacks 
on promontories and stretches of exposed coasdine. Their 
breeding requisites include nest sites sheltered from high 
tides, spray from crashing waves, prevailing winds or 
storms, and mainland ground predators; and suitable 
rocky intertidal foraging grounds (Webster 1941a, Hart- 
wick 1974). Oystercatchers avoid nesting near high densi- 
ties of gulls (Hartwick 1974). In British Columbia, the 
distance from nest sites to foraging areas varies from about 
40 to 200 feet (Hartwick 1974). Nest sites and feeding 
territories are usually contiguous, but sometimes a nest site 
is isolated from the feeding area by water or the territories 
of other birds. Nest sites are often on shelves just above the 
sea, but they may range up to 90 feet or more above the 
tide line on the exposed shoulders of great rocks (Dawson 
1923, Webster 1941a). 

During the ten days to two weeks before egg laying, 
Oystercatchers build several "play nests," which are usually 
inferior in construction to the one finally chosen for the 
eggs (Webster 1941a). A typical egg nest is a platform or 



bowl of rock flakes, rounded pebbles, or bits of shell placed 
in a cranny of bare rock (Dawson 1923, Bent 1929, 
Webster 1941a, Hartwick 1974). Oystercatchers also lay 
their eggs in hollows scraped from soil pockets in irregu- 
larities of the rock or from weedy turf on ledges, lining 
these nests with similar materials to those in bare rock. 
Exceptionally, they build bowls of grass or dried moss 
similar to a gull's nest or lay their eggs in a hollow scooped 
in a gravel or shell beach. Birds add nest material through- 
out incubation (Webster 1941a). When the nonincubating 
bird is not foraging, it stands on lookout on a rock 
eminence, usually at or below nest level and within about 
5 to 25 yards of the nest. After chick hatching, adults make 
initial foraging trips by flying, but after a few days they 
begin to walk; occasionally they continue to fly to feeding 
grounds distant from the defended territory (Webster 
1941a, Hartwick 1974). 

Black Oystercatchers forage mosdy in the rocky inter- 
tidal zone. There they hop about rocks and wade in tide 
pools, pounding, prying, and cutting mollusks from their 
protective armor and anchorages and picking or probing 
in hiding places for unshelled prey (Webster 1941b, Hart- 
wick 1976). At the Farallon Islands, 70%-95% of feeding 

181 



Oystercatchers 



MARIN COUNTY BREEDING BIRD ATEAS 



Oystercatchers 



activity occurs during the lower half of the tidal cycle, 
depending on shoreline exposure and swell height (Mor- 
rell et al. 1979). Oystercatchers there feed during the high 
half of the tidal cycle, mosdy in late summer, when adults 
are feeding large, rapidly growing chicks, and in fall, when 
fledglings require more time to feed themselves until they 
master dieir apprenticeship. Adults may concentrate in 
particular areas of die intertidal. For example, adults feed- 
ing chicks may spend considerahle time foraging at the 
bottom edge of mussel beds, where a certain species of crab 
lives (Harfwick 1976). At the Earallon Islands, Oyster- 
catchers also probe in the soil of a nontidal marine terrace 
for tenebrionid beede larvae (Morrell et al. 1979), and at 
Vancouver Island they also feed in mussel beds on tidal 
mudflats, particularly in winter (Hartwick ck Blaylock 

1979). 

An adult Oystercatcher's sturdy, bright vermillion, later- 
ally compressed bill is an essential tool for mining the 
abundant (though tighdy fastened and armored) inverte- 
brate fauna of rocky reefs. Birds seek recendy exposed or 
slighdy inundated, hence partially relaxed, mollusks since 
these are the only ones they have a reasonable chance of 
opening (Webster 1941b, Hartwick 1976). Because of the 
short period when gaping mussels are available, Oyster- 
catchers move quickly, often pass over small food items, 
and appear to cover greater distances while foraging on 
mussels than when seeking a greater array of prey (Hart- 
wick 1976). In search of gaping mussels, Oystercatchers 
walk over reefs with their heads directed forward and their 
"chisel-tipped" bills poised to strike a sharp blow on the 
dorsal border oblique to the long axis of die mussel 
(Webster 1941b, Hartwick 1976). Such a blow depresses 
die valve, forming an abnormal gap diat will admit the tip 
of the bird's bill. Birds also search for mussels already 
opened wide enough to permit entrance of dieir bills. In 
either case, the bill is then stabbed down into the mussel 
with a number of forceful and rapid jerks until the deepest 
part of the bill lies lengthwise between the margins of the 
valves. Next the mussel is opened by rapid levering and 
biting that severs the adductor muscle; sometimes the shell 
is fractured in the process. In rare instances, when a 
mussel sits with its ventral byssal fissure exposed, Oyster- 
catchers work from that juncture to open the mussel by the 
above methods. Birds sometimes detach the mussel after 
opening it and carry it to a more convenient location to 
remove the meat. Many mussels are also located by prob- 
ing in die mud and dien usually are opened from the 
vulnerable ventral side. Oystercatchers remove the flesh by 
first tearing larger pieces and then laying the bill flat on the 
shell like scissors and pushing it forward as the points snip 
away the adherent flesh. 

Oystercatchers loosen limpets from their moorings by 
first delivering one or more sharp strokes of die bill from 
a low angle (Webster 1941b, Hartwick 1976). This will 



remove small limpets but only weakens the grip of larger 
ones and sometimes chips their shells. As needed, this is 
followed by firm pushing, lateral head swaying, or to-and- 
fro rotation of the bill. If diis does not complete the job, 
die bird forces the bill under the shell and levers it free. 
The Oystercatcher dien seizes the limpet and carries it to a 
niche or rock crevice to remove the meat. Placing the 
limpet shell down, the bird rapidly bites around die edge 
of the shell and finally picks the body up, shakes the shell 
off, and swallows the meat in one gulp. Oystercatchers 
attack chitons in the same way as limpets, but unless they 
topple small ones with the first stroke, further quick work 
is needed (Webster 1941b). Birds push the tip of the bill 
under one corner of die leathery shell, breaking the vac- 
uum set up by the muscular foot of the mollusk. Then they 
usually slip the bill under, flat side against the rock, and 
cut the animal loose by sawing strokes of the bill. Oyster- 
catchers obtain barnacles by sharply tapping one valve, 
levering the valves apart by circular leverage, and then 
pulling out the bite-sized body whole. Oystercatchers 
obtain many smaller prey, such as small limpets and sea 
cucumbers, by probing and moving aside seaweed (Hart- 
wick 1976). 

The Black Oystercatcher diet consists primarily of mus- 
sels, limpets, and chitons, along with smaller amounts of 
barnacles, marine worms (annelids, nemerteans, and 
sipunculids), crabs, snails, young abalone, isopods, echino- 
derms, and sometimes insects (Webster 1941b, Hartwick 
1976, Morrell et al. 1979). At the Farallon Islands, the diet 
is primarily the California mussel fM^tilus californianus), 
several species of limpets, beede larvae, and marine (ne- 
mertean and polychaete) worms; crabs are also taken 
(Morrell et al. 1979). The diet at the Farallones varies 
among territories, depending on the topography of the 
shoreline— sloping shoreline supports mussel beds, where- 
as steep shoreline does not. At nests where mussels com- 
prise 40% or more of the prey remains, the diet is more 
varied than at nests where mussels comprise 30% or less 
of the diet. Presumably this reflects a greater diversity of 
prey in mussel beds. In any case, when mussels are 
available there, they are preferred over limpets as a food for 
chicks (Morrell et al. 1979). 

Unlike most precocial shorebirds, Oystercatcher young 
initially are not able to feed themselves because their 
underdeveloped bills and feeding skills are no match for 
armored prey. Very young chicks usually remain close to 
the nest, and the parents take turns guarding the chicks 
and carrying food items singly to them from the intertidal 
zone (Webster 1941a, Hartwick 1974, Groves 1984). 
Adults may hold food in the bill before chicks or drop 
items and point to them on the ground (Hartwick 1976). 
One adult may also pass the food to the other adult, who 
prepares it and presents it to the chick while the first adult 
returns to foraging duties. In British Columbia, adults feed 



182 



Oystercatchers 



SPECIES ACCOUNTS 



Oystercatchers 



chicks at nests mostly mussels (larger than average sized) 
and limpets of large size or species (Hartwick 1976). 
Chicks are also fed quite large chitons and crabs. Crabs 
may be a special part of the chick diet since adults there do 
not prey on them until they begin feeding their chicks. In 
contrast to chicks, adults then are eating less profitable 
items— small to medium-sized limpets, smaller mussels, 
and a lot of small unshelled food items. Chicks move with 
foraging parents to the littoral zone in stages (Hartwick 
1974). Young may reach the feeding area as early as two 
days after hatching but usually not until the third to fifth 
week (Webster 1941a, Hartwick 1974), depending on the 
difficulty of descent to feeding areas (Webster 1941a). 
Sometimes gulls prevent adults from moving their chicks 
to the intertidal zone, and so the young remain near the 
nest site until fledging (Hartwick 1974). Chicks that 
remain at the nest for long periods are fed an increasing 
proportion of mussels (Hartwick 1976). Although prefledg- 
ing chicks that move to the littoral zone are able to capture 
small prey items, they still depend on adults for the most 
profitable ones (Hartwick 1976, Groves 1984). When 
young begin to move to foraging areas with parents, one 
adult often remains higher in the intertidal guarding the 
chicks and feeding them smaller items, while the other 
adult hunts farther down and carries mussel meat and 
other larger items to the chicks. It does appear, diough, 
that feeding becomes a teaching process, as progressively 
more and more work is left to the chicks. The diet during 
this period shifts away from a dependence on mussels to 
more limpets and smaller unshelled items. At the Farallon 
Islands, marine worms and beede larvae make up as much 
as 57% (by number) of the diet fed to 1- to 40-dayold 
chicks (Morrell et al. 1979). When chicks there are old 
enough (67-100 days) to forage with their parents in the 
littoral zone, limpets are the major prey item (60%-85%), 
whether fed to a chick by a parent or captured by a chick 
itself. The diet of newly independent chicks is mainly 
limpets, marine worms, and beede larvae, since the young 
birds have not yet developed the skill to open mussels. 
Adults have been observed feeding adult-sized young at 
Point Lobos, Monterey County, as late as 3 November 
(Williams 1927), and fledglings are not fully adept at 
opening mussels or prying barnacles or chitons from rocks 
until they are three to four months old (Webster 1941a). 

Marin Breeding Distribution 

During the adas period, Black Oystercatchers bred at a 
number of spots at irregular intervals along Marin Coun- 
ty's outer coast where suitable stretches of rocky shoreline 



exist (Table 14, Figure 14). Representative breeding loca- 
tions were Double Point, PRNS (NE summer 1978 — SGA); 
about V 2 mi. NW of the mouth of Bear Valley, PRNS (FL 
7/3/80 — DS); and near the mouth of Estero de San 
Antonio (FL 6/24/82 -DS). In 1988, Rauzon and Carter 
(1988) documented Black Oystercatchers breeding on 
West Marin Island inside San Francisco Bay; two Oyster- 
catchers were also present there during USFWS surveys in 
1990 (Carter et al. 1992). 

Historical Trends/ Population Threats 

In surveys of selected sites in 1969 to 1972, Ainley and 
Whitt (1973) estimated 14 Oystercatchers were breeding at 
three sites on the Marin County coast. From complete 
surveys, Sowls et al. (1980) estimated 30 birds were breed- 
ing at eight sites on Marin's outer coast in 1979, and 
Carter et al. (1992) estimated 27 birds were breeding at 
nine sites there in 1989. Even numbers from the more 
recent surveys are probably low, considering die difficulty 
of counting this scattered, solitary-nesting species from 
boats. 

From surveys in 1979 to 1980, Sowls et al. (1980) 
estimated a total of 462 Oystercatchers were breeding on 
the central and northern California coast. In 1989, Carter 
et al. (1992) recorded 575 birds in surveys of the same 
region (plus 6 in S.F. Bay in 1990); their numbers would 
have been even greater than those of the prior survey if they 
had rounded numbers to represent breeding pairs as was 
done previously. They concluded that the higher 1989 
figures (rounded or not) may be indicative of better viewing 
conditions during die later survey and slighdy different 
definitions of breeding birds between the surveys, rather 
than a true population increase. 

Oystercatchers have held up well to human pressures in 
California, except locally at the Farallon Islands and at San 
Pedro, Los Angeles County (GckM 1944). Ainley and 
Lewis (1974) claimed that Oystercatchers disappeared from 
the Farallones in die 1860s, probably because of too much 
disturbance from humans and domestic animals. That 
population has subsequendy recovered. Although it seems 
clear that the Farallon population did decline, it is hard to 
imagine the species totally disappearing there because the 
island harbors inaccessible intertidal areas where Oyster- 
catchers could have taken refuge from disturbance. Near- 
shore oil spills potentially could decimate the littoral food 
resources upon which Oystercatchers depend (Sowls et al. 
1980). 



183 



Stilts and Avocets 



MARIN COUNTY BREEDING BIRD ATLAS 



Stilts and Avocets 



Stilts and Avocets 

Family Recurvirostridae 



BLACK-NECKED STILT Himantopus mexicanus 



l 






A year-round resident; numbers 


A/Sp%S?^>^^ N 






depressed from Oct through mid-Mar. 




^-V >< AQ^x 




A fairly common, very local breeder; 








overall breeding population very small. 




XAa^Vsa 




Recorded in 9 (4.1%) of 221 blocks. 


v^^A^Aj^VA-Air^?^ 


^\^OoC-i 






\^ < S^S^tv\^J^\'^ 






O Possible 4 (44%) 


\$P^H^rv^^^ 


\ ~^k"^ \ ^-V^^A ^L-'t'-^ 




C Probable (0%) 


\Jl^v*^\ '•^\ < ^^^ l ^^i\L\^^\ 3* 






• Confirmed = 5 (56%) 








— T- 






^wPsV^P^^^Vv 






FSAR = 3 OPI = 27 CI = 2.11 








Vo 





Ecological Requirements 

These dainty black and white shorebirds perched on out- 
landishly long pink legs cast surreal reflections in the 
waters of a variety of shallow freshwater, brackish, and 
alkaline wedands. In Marin County, Stilts breed in the 
reclaimed or altered bayshore saltmarsh habitats of diked 
(tidal and nontidal) brackish ponds, sewage ponds, and 
ephemeral freshwater ponds. Although they often overlap 
in habitat preferences with American Avocets, Stilts tend 
to prefer fresher water and more emergent vegetation, and 
they feed more in marshes, than do Avocets (Hamilton 
1975). Observations at agricultural evaporation ponds in 
the San Joaquin Valley suggest that prey availability is die 
primary limiting factor for both of these shorebirds. 
Regardless of salinity and alkalinity, both Stilts and Avo- 
cets are numerous in evaporation ponds when suitable 
invertebrate prey are abundant and available to them via 
their respective foraging techniques (G. Gerstenberg pers. 
comm.). Hence, the habitat choices of these species seem 
to reflect the likelihood of finding suitable prey resources 
rather than a selection for particular water chemistry. 

Stilts generally nest as close as possible to accessible 
feeding areas. They prefer rather open habitat, where they 
generally congregate in small, loose colonies, often mixing 

184 



with Avocets. Birds choose nest sites on bare to moderately 
vegetated flat terrain; on the flat or irregular surfaces of low 
rises on dry land; at the water's edge; or, less frequendy, 
built up in shallow water (Bent 1927, Palmer 1967, Ham- 
ilton 1975). When Stilts nest in vegetated areas, they tend 
to select sites on the edge of the vegetation with good 
visibility of their surroundings (Hamilton 1975). In the 
Bay Area, Stilts most often nest on unvegetated or sparsely 
vegetated dikes and levees of salt ponds, and occasionally 
in openings in salt marsh (Gill 1972, 1977; Hamilton 
1975; Rigney ek Rigney 1981). At San Francisco Bay salt 
evaporators, Stilts concentrate in die interior of pond 
complexes away from the bayshore. Most nests there are 
on discontinuous levees, insular levee fragments, or small 
dirt mound islands; few nests are on continuous or well- 
traveled dikes. Hamilton (1975) noted that Stilt nests 
tended to be located on the leeward side of saltpond dikes 
and on the side toward favored feeding areas; birds may 
also select nest sites with reference to the direction of 
human approach. Nests located on the centers of dikes 
usually are constructed downwind of small natural objects, 
which may serve as windbreaks. 



Stilts and Avocets 



SPECIES ACCOUNTS 



Stilts and Avocets 



Depending on the availability of materials and the nest 
location, Stilt nests vary from shallow scrapes in bare earth 
with hardly a vestige of nest material to elaborate raised 
platforms (Dawson 1923, Bent 1927, Palmer 1967, Ham- 
ilton 1975). Nest materials include pebbles, shells, plant 
stems, twigs, large feathers or dried bird carcasses, dry mud 
chips, bones, cow dung, and grasses. Sometimes Stilts add 
substantial amounts of material to nests to avoid rising 
waters. Normally, birds continue to occasionally add small 
amounts of nest material throughout incubation (Hamil- 
ton 1975). After the newly hatched precocial young obtain 
mobility, they are led to shallow-water feeding areas, pref- 
erably with low vegetative escape cover. 

The extremely long legs of Black-necked Stilts are per- 
fecdy suited for wading well out into shallow water. Stilts 
are primarily visual foragers that obtain most of their prey 
from an immobile stance or while slowly walking (Hamil- 
ton 1975). Their main mode of prey capture is by pecking 
at the mud or from or near the water's surface without 
immersing the head. Stilts also capture terrestrial insects by 
pecking at the ground or vegetation. Less frequendy, they 
plunge their heads or even necks or upper breast into water 
in pursuit of items, but apparendy their bills do not reach 
bottom. Stilts also snatch flying insects from the air with 
their bills after running or fluttering toward them. Only 
very rarely do they forage by back-and-forth scythelike 
motions of the bill as Avocets commonly do (see account). 
On San Francisco Bay, Stilts often feed in marshes after 
their chicks hatch. Unlike Avocets, they use grass-bordered 
ephemeral pools, and in the rare instances when they feed 
on tidal mudflats, they do so close to shore. Hamilton 
(1975) compared the foraging niches between the sexes 
and between Stilts and Avocets. Male Stilts, with their 
longer legs, tend to feed in deeper water than do females. 
Although male Stilts can feed in deeper water than do 
Avocets, both species concentrate their foraging in "ankle- 
deep" water. On the other hand, female Stilts do tend to 
feed in shallower water than do Avocets. The niches are 
further separated by Avocets feeding more commonly by 
plunging below the surface than do Stilts; by feeding at 
greater depths; and by Avocets using a number of tactile 
foraging methods (see account). 

The Black-necked Stilt diet is about 98.9% animal 
matter and only 1.1% vegetable matter in the form of a few 
seeds of aquatic or marsh plants (Wetmore 1925, n = 80). 
The animal foods are primarily aquatic forms of true bugs 
(water boatmen, water striders, waterbugs, and backswim- 
mers) and beedes, along with lesser amounts of flies (types 
with aquatic larvae), snails, caddisflies, small fish, dragon- 
fly and mayfly nymphs, crustaceans, miscellaneous insects, 
and, rarely, small frogs. Brine shrimp and brine flies are 
important food in south San Francisco Bay salt ponds 
(Hamilton 1975), as they may also be at alkali lakes in the 
interior. 



Marin Breeding Distribution 

There is no evidence that Stilts occurred in Marin County 
through the breeding season prior to the adas period. 
During the adas years, they bred at a few scattered diked 
wedands along the San Pablo and San Francisco bay- 
shores, with the stronghold of the population in the Rush 
Creek/Bahia Drive ponds area of Novate Probably fewer 
than 50 pairs have nested recendy in Marin each year, even 
under the most favorable conditions. Stilts were first dis- 
covered nesting in Marin County in 1978 at the Spinnaker 
wedands, San Rafael (FL 7/2/78 — ARa); a high count of 
five nests was recorded there in 1983 (NE-FL May ck Jun 
1983 -HoP). In 1980 at least 32 pairs nested at the Bahia 
Drive ponds near the Petaluma River mouth, Novato 
(NE-FL 6/7-26/80 — DS), and 7 pairs attempted nesting 
at McGinnis Park, San Rafael, in an ephemeral wedand 
created by a broken dike (DD 5/10 ck 6/3/80 -DS). 
Nesting failed at the latter site because it had dried up by 
late June. Since 1985, Stilts have bred each year at the Las 
Gallinas sewage ponds (ABN:DAH et al.) just north of 
McGinnis Park. Stilts also occasionally have bred at the 
Shorebird Marsh by the Village Shopping Center, Corte 
Madera (DD 3/17/87 -JGE, FL 8/3/89 -LES). 

Historical Trends/ Population Threats 

Stephens and Pringle (1933) considered the Black-necked 
Stilt a rare winter visitant in Marin County based on a 
record in Grinnell and Wythe (1927). The latter authors 
reported that Stilts at that time occurred in limited num- 
bers in the immediate vicinity of San Francisco Bay in 
summer and fall, and that stragglers were present only 
rarely in winter. Sibley (1952) considered the species an 
uncommon summer resident and a rare winter visitant to 
the south San Francisco Bay region. The Black-necked Stilt 
increased steadily in saltpond habitat in south San Fran- 
cisco Bay in the 1950s and 1960s, and the species is now 
considered a common breeder and a regular winter visitor 
there (Gill 1972, 1977). Breeding probably did not begin 
in the North Bay until after the mid-1960s (RE. Gill, Jr., 
fide GWP), and the number of breeders there still appears 
to be increasing (ABN). An increase in the estimated 
number of Stilts breeding in south San Francisco Bay, 
from 400 to 500 pairs in 1971 (Gill 1977) to 600 to 650 
pairs in 1981, is attributed to more extensive observer 
coverage in the more recent survey (Rigney ck Rigney 
1981). In the Monterey Bay area, the wintering population 
has increased at least since 1959 (Roberson 1985). The 
coastal breeding population is still increasing, as indicated 
by the recent breeding records in Marin, the first breeding 
records for Sonoma County and coastal San Mateo 
County in 1977 (AB 31:1184), and the extension of the 
breeding range nordi to near Humboldt Bay in 1985 (AB 
39:958). 



185 



Stilts and Avocets 



MARIN COUNTY BRHHDING BIRD ATIAS 



Stilts and Avocets 



It is not clear what is fueling the increase of coastal Stilt 
populations. Although evaporation ponds support the 
hulk of the Bay Area breeding population, these ponds 
have been in operation and available to shorebirds starting 
in the 1860s, suggesting that odier factors are responsible 
for recent increases. Perhaps the coastal population is 
being augmented by recruitment of young from productive 
interior populations, or by adults fleeing from drought- 
diminished interior wetlands. Although the increase of die 
coastal Stilt population is encouraging, it should be 
remembered that populations in the interior, particularly 
in die Central Valley, have been reduced drastically in 
historical times by the draining of wedands (G&.M 1944, 
Cogswell 1977). These losses have been at least partially 



offset by irrigation (especially rice fields), agricultural drain- 
age ponds, and sewage ponds. On the other hand, concen- 
trations of pesticides, heavy metals, and other 
contaminants in agricultural and urban waters may ulti- 
mately have severe adverse effects on shorebirds. Selenium 
from agricultural drainage waters that concentrated in die 
food chain was the most likely cause of complete reproduc- 
tive failures of Stilts and Avocets at Kesterson Reservoir, 
Merced County, in 1984 and 1985 (Williams et al. 1989). 
On the whole, Stilt populations were relatively stable on 
Breeding Bird Surveys in California from 1968 to 1989, 
despite an increase in numbers from 1980 to 1989 
(USFWS unpubl. analyses). 




An adult Black Oystercatcher solemnly surveys its rocky realm. Photograph by Ian Tait. 



Stilts and Avocets 



SPECIES ACCOUNTS 



Stilts and Avocets 



AMERICAN AVOCET Recurvirostra americana 



Occurs year round, though almost exclusively as a winter resident from late Aug through late Apr. A rare breeder 
(postadas) only since 1984- 



Ecological Requirements 

Although the hues of their burnt orange heads and necks 
and pastel blue legs bear little resemblance to the attire of 
Black-necked Stilts, Avocets inhabit a similar variety of 
shallow freshwater, saline, brackish, and alkaline wedands. 
Despite the tendency for Avocets to be more partial to 
saline and alkaline (versus freshwater) habitats than are 
Stilts (Hamilton 1975), prey availability may be the factor 
most direcdy affecting distribution of these shorebirds (see 
Stilt account). Avocets avoid grass-bordered ephemeral 
freshwater ponds, where Stilts often feed and breed, and 
because of their affinity for barren habitat, Avocets feed less 
in salt marshes (but do so on exposed mud) than do Stilts. 
Avocets generally breed in loose colonies, often with Stilts, 
in proximity to suitable foraging areas (Gibson 1971, 
Hamilton 1975). Along the coast, Avocets breed primarily 
around diked brackish ponds, especially salt evaporators, 
and at sewage ponds. Although both Avocets and Stilts 
feed in these ponds, only Avocets also feed extensively on 
bay mudflats. There they feed mosdy on the water's edge 
far from shore or in supersaturated mud or in shallow 
saltwater puddles left on the flats (Hamilton 1975, D. 
Shuford pers. obs.). 

The type and location of territories that Avocets defend 
change with the stage of the nesting cycle (Gibson 1971). 
Prior to egg laying, pairs defend a feeding area. During 
incubation, pairs defend the nest site and either a contigu- 
ous or distant feeding area. Birds whose nests are close to 
the primary feeding area may also defend a more distant 
secondary feeding area, whereas birds with separate feed- 
ing areas and nest sites defend only one feeding area. After 
the chicks hatch, the territory becomes chick centered and 
mobile. 

Other than a tendency to distribute dieir nests more 
irregularly than Stilts do (Hamilton 1975), Avocets build 
their ground nests of similar materials and in nearly 
identical locations to those of Stilts (see account and 
references). Gibson (1971) noted that Avocets choose nest 
materials available in the immediate vicinity of the nest. 
Avocet nests are distinguished from those of Black-necked 
Stilts by the larger size of Avocet eggs. 

Like Stilts, Avocets also feed extensively by pecking at or 
just below the water's surface (and at mud), but Avocets 
plunge their heads or necks below the water much more 
frequendy than do Stilts (Hamilton 1975). Avocets snatch 
flying insects from the air widi their bills after running or 



fluttering in pursuit of their prey. They also run up hastily 
and strike at mats of brine flies on the mud with lateral 
sweeping motions of their bills (Wetmore 1925). Another 
rarely used method of visual feeding is for birds to rapidly 
open and close their bills while simultaneously moving 
them erratically along the water's surface (Hamilton 1975). 
Unlike Stilts, Avocets make extensive use of tactile meth- 
ods of foraging. In shallow pools over mudflats, Avocets 
fdter mud by rapidly opening and closing their bills slightly 
while at the same time moving them apparendy at random 
over the mud. Birds also scrape mudflats by placing the 
recurved lower tip of die bill on the mud direcdy in front 
of them and then moving it forward and backward by 
stretching the neck. Avocets are perhaps best known for 
their scythelike, side-to-side feeding movements in mud or 
water. Leaning down, birds progress forward a step at a 
time, placing the slighdy opened, recurved tip of the bill 
flat on the substrate to one side and rapidly moving it to 
the other side. They pause to raise the head and swallow 
and then repeat the process on the other side with the next 
step; infrequendy, birds make multiple side-to-side move- 
ments (the first is longest) before pausing to swallow. 
Mosdy outside the breeding season, birds sometimes con- 
gregate in large groups to forage in this manner, swaying 
back and forth as they progress forward in unison, appar- 
endy herding prey in front of them. From a swimming or 
breast-wading position, Avocets will tip up like dabbling 
ducks and make similar sideswipes of their bills on the 
mud below the surface while maintaining their position 
with backward-kicking legs. See the Black-necked Stilt 
account (above) for other differences in the foraging niche 
of these two species. Hamilton (1975) compared foraging 
between male and female Avocets at an inland site where 
they only used pecking and plunging. Males there had a 
much greater tendency to plunge-feed than females did, 
perhaps because of their longer but less curved bills. 

Overall, the Avocet diet is about 65.1% animal matter 
(aquatic and shoreline forms) and 34-9% vegetable matter 
(Wetmore 1925, n = 67), die latter a very high percentage 
for a shorebird. Vegetable matter is primarily the seeds but 
also die leaves and stems of aquatic and marsh plants. 
Animal fare is primarily flies, beetles, crustaceans, true 
bugs, miscellaneous insects, and, rarely, snails, small fish, 
and salamanders. In fall and winter at south San Francisco 
Bay salt ponds, Avocets feed on brine flies, brine shrimp, 

187 



Stilts and Avocets 



MARIN COUNTY BREEDING BIRD ATLAS 



Stilts and Avocets 



water boatman beetles, polychaete worms, plant stems, and 
a few small mollusks (Anderson 1970). Martin et al. 
(1951) reported little variation in the proportion of animal 
and vegetable matter in the diet between spring and fall. 

Marin Breeding Distribution 

There was no evidence of Avocets breeding in Marin 
County during, or prior to, the adas period. Subsequendy, 
the progress of a pair nesting on a tiny island at Spinnaker 
Lagoon, San Rafael, was followed during the breeding 
season of 1983 (NE-FL 5/18-6/12/83 -HoP). Avocets 
also attempted to nest at the Las Gallinas sewage ponds in 
1987 but were unsuccessful (ABN:DAH, CLF). 

Historical Trends/Population Threats 

Both coastal wintering and breeding populations of Avo- 
cets have increased historically. Formerly, the American 
Avocet was considered an irregularly common fall and 
winter visitor to San Francisco Bay (GckW 1927), where 
nesting was first recorded in 1926 (Gill 1977). Although 
Avocets began to use Bay Area salt ponds by at least 1 899 
(Grinnell et al. 1918), not until the early 1940s did their 
population begin to expand there to include large numbers 
of wintering and breeding birds (Storer 1951; Gill 1972, 
1977). By 1952, the Avocet was considered a common 
resident (more numerous in winter) of the south San 
Francisco Bay region (Sibley 1952). A decrease in the 
estimated number of Avocets breeding in soudi San Fran- 



cisco Bay, from 1800 pairs in 1971 (Gill 1977) to 650 pairs 
in 1981, apparendy represents different calculation meth- 
ods rather than an actual population decline (Rigney ck 
Rigney 1981). Winter numbers have also increased in 
Humboldt Bay since 1958, especially from 1961 to 1968 
(Gerstenberg 1972), and at Bolinas Lagoon, particularly 
since 1974 (Shuford et al. 1989). First breeding records for 
Sonoma County in 1981 (AB 35:974) and Marin County 
in 1984 (above) document continued range expansion of 
breeding Avocets in the San Francisco Bay Area. See 
Black-necked Stilt account for comments on the increase 
of coastal breeding populations, which also apply to Avo- 
cets. Avocet populations in California's interior decreased 
early in this century because of extensive loss of marsh- 
lands (G6kM 1944). This habitat loss has been offset at 
least partially by the addition of irrigation ponds, agricul- 
tural drainage ponds, and sewage ponds. On the other 
hand, these waters may be very harmful to shorebirds 
because they concentrate pesticides, heavy metals, and 
other contaminants. Selenium from agricultural drainage 
waters that concentrated in the food chain was the most 
likely cause of complete reproductive failures of Avocets 
and Stilts at Kesterson Reservoir, Merced County, in 1984 
and 1985 (Williams et al. 1989). For California as a whole, 
numbers of Avocets appeared to decrease slighdy on Breed- 
ing Bird Surveys from 1968 to 1989 (USFWS unpubl. 
analyses). 




Sandpipers 



SPECIES ACCOUNTS 



Sandpipers 



Sandpipers 

Family Scolopacidae 



SPOTTED SANDPIPER Actitis macularia 



Occurs year round, though primarily as a winter resident and transient from Jul through May. Only two (nonadas) breed- 
ing records. 



Ecological Requirements 

Wasting no time in taking on the airs of their elders, fluffy 
newborn Spotted Sandpipers teeter and bob as they test 
their spindly legs on the shifting sands and gravels of their 
breeding haunts. In California, Spotted Sandpipers nest 
from sea level to 1 1 ,000 feet— widely in higher and moister 
mountains and less extensively in coastal lowlands. Breed- 
ing habitats in the state include slow-moving streams and 
rivers, freshwater lakes, tarns, saline and alkaline lakes, 
and, rarely, coastal lagoons (G6kM 1944, D. Shuford pers. 
obs.). Because Spotted Sandpipers have bred only twice in 
Marin County (see below), there is little to say about 
habitat preferences here. 

Throughout the range, nest sites are usually on open or 
semiopen shoreline beaches or on offshore islands or 
gravel or sandbars. Nests are usually concealed in grassy or 
herbaceous cover and sometimes are placed next to or 
under logs, driftwood branches, rocks, bushes, or trees 
(Dawson 1923, Tyler 1929, Grinnell et al. 1930, Miller 6k 
Miller 1948). Where the vegetation is low and open, these 
sandpipers often nest well back from the shoreline, but 
otherwise they often nest just above the wave-cast debris 
line. One nest in the mountains of California was on the 
floor of an open yellow pine woodland over 1 50 feet back 
from and about 50 feet above a beachless lakeshore (Grin- 
nell et al. 1930). On one island on a lake in Minnesota, a 
number of nests were from 65 to 165 feet back from the 
shoreline under a dense canopy of bushes and trees (Oring 
6k Knudson 1972). Nests are saucer-shaped depressions in 
sand, gravel, or turf that are thinly to well lined with 
grasses, pine needles, leaves, twigs, bits of wood, or feath- 
ers (Dawson 1923, Tyler 1929, Grinnell et al. 1930). A 
frequent mating strategy of Spotted Sandpipers is serial 
polyandry— females nest successively with up to four males, 
leave the care of the precocial young to each male, and help 
incubate only the last clutch of eggs (Hays 1972, Oring 6k 
Knudson 1972, Oring et al. 1983). Sometimes females are 
monogamous, and, rarely, they pair with two males at the 
same time (Oring 6k Maxson 1978). 



No detailed work apparendy has been done on the diet 
of Spotted Sandpipers, but birds are known to prey on 
larval and adult forms of a variety of land and aquatic 
insects, assorted invertebrates, and, occasionally, on fish 
(Tyler 1929, Palmer 1967, Kuenzel 6k Wiegert 1973). 
Spotteds forage on aquatic shorelines and also in adjacent 
open upland grass and sedge covered beaches and mead- 
ows. While walking along, they capture prey by rapid 
downward pecks; by forward horizontal thrusts of the bill 
from a slow, crouched approach; or by snapping insects 
from the air. Also, they wade in the water and jump into 
deeper water to seize floating prey (Palmer 1967). Spotted 
Sandpipers sometimes immerse insects several times in 
water before swallowing them (Tyler 1929). 

Marin Breeding Distribution 

Bracketing the adas period, there were two breeding 
records for Spotted Sandpipers in Marin County: an adult 
with three downy young at the "Canal Street Pond," San 
Rafael, on 9 July 1971 (WMP, ABu); and an adult with 
young at Rush Creek marsh off Binford Rd., Novato, on 
21 June 1985 (GWP, BHe). No breeding confirmations or 
strongly suggestive breeding evidence was recorded during 
the adas period. The loosely overlapping spring and fall 
migration periods of Spotted Sandpipers and the presence 
of occasional oversummering individuals here, patterns 
typical of many shorebirds, make it difficult to determine 
breeding status unless direct evidence of nesting is 
observed. 

Historical Trends/Population Threats 

Formerly, Spotted Sandpipers were not known to breed in 
Marin County (G6kW 1927, SckP 1933, G6kM 1944). It 
seems best to attribute the recent breeding records to 
increased observer coverage detecting irregular breeding 
rather than to a range expansion. Spotted Sandpiper num- 
bers appear to have changed little in California in historical 
times (G6kM 1944). Hydraulic mining on rivers may have 

189 



Sandpipers 



MARIN COUNTY BREEDING BIRD ATLAS 



Sandpipers 



scoured out some nesting habitat, but then again, to the 
species advantage, it must have deposited much silt and 
gravel on beaches and bars. Increasing recreational use of 
rivers and lakes must displace some breeding sandpipers 



but does not appear to have had a marked effect on the 
population. Numbers of Spotted Sandpipers were rela- 
tively stable on Breeding Bird Surveys in California from 
1968 to 1989 (USFWS unpubl. analyses). 




The prim and proper appearance of a Western Gull on its nest belies its otherwise piratical, aggressive, and resourceful 
tendencies when in pursuit of food, any food. Photograph by Ian Tait. 



190 



Gulls 



SPECIES ACCOUNTS 



Gulls 



Gull: 



Family Laridae 



WESTERN GULL Lams occidentalis 









A year-round resident; gulls occupy Faral- 




r^^-^ \ JC\~. 




lon Island (and probably Marin) breed- 


J5k^X±B**<? 




ing colonies almost year round, except 


X^\J s rTAL^ 


\^\^%^vX^\ \ j-V\l-A^\ J^\^-^ 


{- - 


the short period from mid-Sep until late 
Oct or early Nov. 




^Ck^c^c^c^^^^C 




A common, very local breeder; overall 


v\> 


^s^^ 




breeding population very small. 
Recorded in 14 (6.3%) of 221 blocks 




jk^s\ ', i^^^^^-^V^A " v lV**'\ >f^\ \*^\ \^" 




(see Methods). 






t- <r -CT*^' — 


O Possible = (0%) 




l^^^^v^V^A^^CAt^- 


\\> s rx^ 


€ Probable = 1 (7%) 






p^?° 


• Confirmed = 13 (93%) 






"^^r\>-^ 


FSAR = 4 OPI = 56 CI = 2.93 



Ecological Requirements 

What better avian symbol of seaside existence dian the 
graceful, versatile, yet rapacious Western Gull. Our only 
breeding gull inhabits a wide variety of habitats ranging 
from the open sea, to coastal bays, estuaries, lagoons, tidal 
reefs, and beaches— it exploits more of the coastal marine 
environment than any other local breeding seabird (Ainley 
& Boekelheide 1990, Chap. 3). At sea, Western Gulls are 
confined largely to waters over the continental shelf and 
are rare more than 1 5 miles seaward of the edge of the shelf 
break (Briggs et al. 1987). Foraging areas for Farallon 
Island breeders vary yearly with ocean conditions (Ainley 
& Boekelheide 1990, Chap. 3). In years of high food 
availability in the ocean, birds forage mosdy offshore near 
the Farallon colony. In years of reduced food availability, 
birds forage more in inshore waters, and they may fly 60 
miles or more to feed at garbage dumps. Western Gulls are 
very rare inland except within a mile or two of the coast- 
line, where they bathe at freshwater ponds or lakes or 
forage at dumps. 

Western Gulls nest in large colonies, scattered aggrega- 
tions, or, occasionally, isolated pairs on islands, offshore 
rocks, inaccessible mainland cliffs, and human structures, 
such as bridges and pilings. At the Farallon Islands, they 



exploit a greater range of nesting habitat than any other 
surface-nesting seabirds (i.e., cormorants and murres) and 
are the most willing to nest near people (Ainley &. 
Boekelheide 1990, Chap. 7). Western Gulls generally 
favor moderate slopes with rocky cover but also nest on 
open island terraces, steep hillsides, rocky slopes, ledges of 
cliff faces, and exposed summits of rocks (Bent 1921; 
Dawson 1923; Schreiber 1970; Harper 1971; Coulter 
1973; Ainley 6k Boekelheide 1990, Chap. 7). Nests are 
usually situated in depressions on the ground out in the 
open on bare soil, grass, or low matted vegetation; near 
sparse low bushes; nesded in natural hollows among 
rocks; in niches in bare rock; or on human structures. 
Western Gulls construct small to bulky nest cups of 
grasses, weeds, seaweed, or other natural debris, including 
carcasses of dead birds or small mammals. In addition, 
they may incorporate into or "decorate" the nest with 
plastic or other refuse from our throwaway society. Some- 
times nests are just a scrape in soil and accumulated guano 
with a few feathers or pebbles scattered about (Harper 
1971). The young are semi precocial— wandering from the 
nest after a few days— and are fed by regurgitation. 

191 



Gulls 



MARIN COUNTY BREEDING BIRD ATIAS 



Gulls 



Western Gulls are consummate generalists, opportunis- 
tic foragers capable of searching large areas for mobile prey. 
Although well known for their scavenging habits, Farallon 
breeders apparently exploit primarily live marine prey, 
principally fish, from the ocean's surface (Ainley 6k Boekel- 
heide 1990, Chap. 3). They apparently forage mostly early 
and late in the day and little at midday. Western Gulls 
usually feed in conspecific flocks, especially during warm- 
water years when food presumably is less available. They 
also commonly forage in mixed-species flocks (usually with 
Brandt's Cormorants), where they can play a catalyst role, 
attracting other birds to the site. Western Gulls are accom- 
plished kleptoparasites, stealing prey from odier successful 
birds of any species in a variety of settings (see below). On 
their own, they apparendy follow schools of fish, hovering 
and screaming, and feed at the surface by dipping, seizing, 
or making shallow plunges, usually barely immersing 
themselves. In addition, they scavenge on beaches and 
mudflats; pick prey from tide pools; follow boats or wait at 
wharfs and seaside restaurants for cast-off fish offal, hand- 
outs, or garbage; pick through refuse at garbage dumps; 
and eat the eggs and young of other seabirds or their own 
species (Bent 1921, Pierotti 1981). Along the shoreline, it 
is not uncommon to watch a Western Gull sauntering 
along with a starfish protruding from its overstuffed maw. 
At seabird colonies, one can see a single gull with the feet 
of a seabird chick, about to be swallowed whole, pointing 
skyward from its open beak; several gulls tearing apart a 
chick or a small seabird, such as a Cassin s Auklet, in a 
merciless tug of war is a frequent occurrence as well. 
Western Gulls also carry clams, mussels, or sea urchins 
aloft to drop them on hard ground or rocks to break them 
open. They also detach morsels of carrion by seizing the 
body in die bill, dragging it away, and shaking their heads 
until a piece breaks off. 

Although Western Gulls consume a great diversity of 
food items and will capitalize on irregularly abundant food 
sources, certain marine organisms predominate in the diet 
at the Farallon Islands, where chicks are fed about 60%- 
80% fish, 15%-20% garbage, 5%-7% marine inverte- 
brates, and l%-2% birds (Ainley 6k Boekelheide 1990, 
Chap. 3). In warm-water years, the diet diversifies, and 
adults eat (and feed to young) more garbage. Hence, in 
those years, they exploit the same resources as roosting, 
nonbreeding gulls normally do. Juvenile rockfish are the 
dominant fish prey, though cusk-eels, Pacific hake (as offal 
from fishing boats), and midshipmen are also important. 
Since many of the fish in the gulls' diet occur in waters 
deeper than they can exploit, they may obtain them from 
scavenging fish that cormorants regurgitate at sea, by 
exploiting fish driven to the surface by cormorants or other 
predators (Ainley 6k Boekelheide 1990, Chap. 3), or by 
stealing fish from cormorants or other seabirds at dieir nest 
sites (Bent 1921, LB. Spear pers. comm.). The main 

192 



invertebrate foods at the Farallones are barnacles and 
euphausiids, along with lesser quantities of mysid shrimp, 
miscellaneous decapods, squid, limpets, mussels, and a 
few terrestrial invertebrates (primarily beedes). 

Western Gulls sometimes feed on sea lion placentae 
and fecal matter (Schreiber 1970) and on other birds, such 
as Rock Doves, or small mammals (Annett 6k Pierotti 
1989). Western Gulls at Santa Barbara Island in the 
Channel Islands feed their chicks mosdy anchovies, other 
schooling fish, and squid, and a few intertidal invertebrates 
(Hunt 6k Hunt 1976). During the period of extremely 
warm ocean waters in the winter and early spring of the 
1982-83 El Nino, Western Gulls at the Channel Islands 
were apparendy feeding primarily, or perhaps solely, on 
pelagic red crabs (Stewart et al. 1984), as they also were 
then at the Farallon Islands (PRBO unpubl. data). Early in 
the nesting season, Western Gulls at Alcatraz Island in 
San Francisco Bay feed predominately on garbage (mosdy 
chicken). They switch mosdy to fish to feed their young at 
die time they hatch, not when fish are first available (Annett 
6k Pierotti 1989). Adult males feed on larger food items 
and apparendy travel longer distances to forage than do 
females (Pierotti 1981). Only males pirate food from neigh- 
boring gulls trying to feed their young, and they do so more 
in years of poor oceanic productivity. Preying on the eggs 
and young of other seabirds and neighboring gulls also 
increases in years of poor ocean food supplies (Ainley 6k 
Boekelheide 1990, Chap. 8; PRBO unpubl. data). 

Marin Breeding Distribution 

During the adas period, Western Gulls were breeding at 
13 sites along the outer coast of Marin County (Sowls et 
al. 1980; Table 14, Figure 14). At that time, they were also 
nesting at at least four sites in San Francisco or San Pablo 
bays, but bay waters were incompletely surveyed (Sowls et 
al. 1980; Marin adas map). Representative breeding loca- 
tions during the adas period were the rocks near the mouth 
of Bear Valley (FL 7/3/80 -DS); Point Reyes Lighthouse 
(FL 6/21/81 -DS, RSt); Bird Rock off Tomales Point (FL 
6/15/82 — DS); both West and East Marin islands off San 
Rafael (NE 5/22/82 — HPr); and a channel marker in 
Richardson Bay between Sausalito and Belvedere (NE 
5/22/82 -HPr). 

In 1989, Carter et al. (1992) found Western Gulls 
nesting at 16 sites on the outer coast of Marin County, 
including all 13 sites where they had been nesting in 1979 
and 3 new sites (Table 14, Figure 14). Their baywide 
surveys in 1990 revealed about 230 Western Gulls nesting 
at 12 sites in the Marin County portions of San Francisco 
and San Pablo bays: Yellow Bluff (2), Sausalito Point Area 
(4), Peninsula Point and Cone Rock (6), Angel Island (6), 
Bluff Point to Paradise Cay (4), Point San Quentin (6), 
Marin Islands (64), The Sisters and Point San Pedro (96), 
Rat Rock (2), Southwest San Pablo Bay Duck Blinds (16), 



Gulls 



SPECIES ACCOUNTS 



Gulls 



Marin County- West San Pablo Bay Ship Channel (14), 
and Richmond-San Rafael Bridge (10). Boat counts tallied 
45 gull nests on West Marin Island in 1990 and 48 in 
1991 (H.M. Pratt pers. comm.) and on-site counts in the 
same years revealed 50 and 47 nests, respectively (R.L 
Hothem/USFWS pers. comm.). 

Historical Trends/Population Threats 

Ainley and Whitt's (1973) preliminary surveys of selected 
sites along the Marin coast probably underestimated the 
county's Western Gull population at 186 breeding birds. 
Sowls et al. (1980) estimated 426 Western Gulls were 
nesting on the outer coast of Marin County in 1979 to 
1980, and Carter et al. (1992) estimated 590 were nesting 
there in 1989. Carter et al.'s (1992) surveys of seabirds 
inside the San Francisco Bay estuary provided the first 
estimates of die number of Western Gulls breeding in that 
region of Marin County (see above); prior information was 
fragmentary (Sowls et al. 1 980). 

In historic times, numbers of Western Gulls in Califor- 
nia have changed most dramatically at the Farallon Islands. 
In the mid-1 800s, the population was about 20,000, close 
to today's size, but earlier had probably been smaller before 
the reduction of pinniped populations provided additional 
gull nesting habitat (Ainley 6k Boekelheide 1990, Chap. 
7). The Farallon gull population was reduced by distur- 
bance from humans and domestic animals and reached a 
low ebb around the turn of the century (Ainley 6k Lewis 
1974). The greatest impact on the gull population was 
caused by commercial egg collectors gathering Common 
Murre eggs from 1848 to the early 1900s. Fearing compe- 



tition from the gulls for murre eggs, collectors stepped on 
gull eggs and young. It is unknown if the disturbance 
caused many gulls to abandon breeding on the Farallones 
and, if so, if diis affected nearby mainland breeding popu- 
lations of gulls by increasing competition for nest sites. 
The Farallon population rebounded from a low of at most 
6000 gulls to reach a plateau by 1959 and from then until 
the present has ranged from 22,000 to 25,500 breeding 
birds (Ainley 6k Lewis 1974; Ainley 6k Boekelheide 1990, 
Chap. 7). The 1979 PRBO estimate of 32,000 breeding 
gulls reported by Sowls et al. (1980) was subsequendy 
revised to 25,500 by Ainley and Boekelheide (1990). 

Sowls et al. (1980) compared their data with earlier 
surveys along the California coast and suggested that 
Western Gulls had been increasing since at least about 
1970. Like many species of gulls, Westerns may have 
increased and reached all-time-high population levels by 
taking advantage of garbage and fish offal produced by an 
expanding human population, thus enhancing the gulls' 
winter survival (Ainley 6k Lewis 1974, Sowls et al. 1980, 
Spear et al. 1987). The recent closing of many San Fran- 
cisco Bay Area dumps may reverse this trend. Estimates of 
the number of gulls breeding on the outer coast of central 
and nordiern California range from about 39,202 birds at 
147 colonies in 1979 to 1980 (Sowls etal. 1980) to 30,534 
birds at 205 colonies in 1989 (Carter et al. 1992). The 
estimate of 3270 Western Gulls nesting in the San Fran- 
cisco Bay estuary in 1990 is 10% of the total of 33,804 
birds nesting in die entire region in 1989 to 1990; the 
estimate of die Farallon population at 22,278 birds is 66% 
of the regional total. 




193 



Ale ids 



MARIN COUNTY BREEDING BIRD ATLAS 



Ale ids 



Auks, Murres, and Puffins 



Family Alcidae 



COMMON MURRE Uria aalge 





A year-round resident; numbers on 




ocean waters swell from Oct through 


A^^P^?^^-^ s ^o^. 


Mar. Murres occupy Farallon Island 


~^^v-^5^ 


(and probably Marin) breeding colonies 


^C\ J<\\ ^\^\ 3r""\ %t^\ \^\ \^\ V-^s 


irregularly starting in late Oct (rarely start- 


^^^^^j^j^j^^^c^P^Ca 


ing mid-Dec) and regularly from late Dec 


through departure from Jul to mid-Aug. 


Yv^^^ 


Fewer murres nest, and some desert colo- 


X}^<\^>£}^& 


nies, in extremely poor food years. 


\ \-ef^\ i^\ < ^<£-V''^A \l\^\ j*r\ Jk^\ \-"^\ >-7 


A very abundant, very local breeder; 


1 /\i^\ \^\"~ X Jr*K \ ^-*c^w I \ ^\ \ ^\ V -*T ! 


overall breeding population very small. 




jfi^fjcIV^ 


Recorded in 3 (1.4%) of 221 blocks 
(see Methods). 






O Possible (0%) 




ii^, ^"^ < r^/'^"^ i< ^^C^\-^Vv>^ 


© Probable = (0%) 




• Confirmed = 3 (100%) 




FSAR = 7 OPI = 21 CI = 3.00 



Ecological Requirements 

These penguin look-alikes are our most numerous breed- 
ing seabird. At sea, Common Murres primarily inhabit 
inshore and offshore ocean waters out to the edge of the 
continental shelf. Most birds concentrate over the inner 
shelf, usually in waters from about 200 to 500 feet deep 
(Wahl 1975, Briggs et al. 1987). During nesting, over 75% 
of the Murres at sea off California are within 25 miles of a 
colony, though they may range to 45 miles or more during 
warm-water years. Areas of concentration in over-shelf 
waters vary both seasonally and annually, with changes in 
food supply related to water temperatures (Ainley 6k 
Boekelheide 1990, Chap. 3). During early spring, Farallon 
Island breeders frequent deep waters along the continental 
shelf near the islands. Some birds range north along the 
continental slope almost 40 miles to the Cordell Bank, 
though most feed much closer. During May and June in 
cold-water years, they contract closer to the islands (and 
mainland colonies); in warm-water years, they spread out, 
especially over the shelf toward the mainland. By June in 
cold-water years, they occupy waters of a variety of temper- 
atures and salinities but prefer turbid waters near colonies 
(Briggs et al. 1988). Perhaps Murres are better able to 
approach and prey upon relatively mobile fish when water 
clarity is low, or perhaps the turbid waters have the highest 

194 



abundance offish prey (Briggs et al. 1988). In July in many 
years, they begin to exploit nearshore waters along the 
Marin and San Francisco shorelines (Ainley 6k Boekel- 
heide 1990, Chap. 3). This shift occurs earlier in warm- 
water years. Inshore movement in July and August is 
evident by the distribution of dependent young and their 
parents— mosdy inside the 90-foot depth contour (Ainley 
6k Boekelheide 1990, Chap. 8). Inshore, Murres feed 
where the surface estuarine outflow of San Francisco Bay 
is underlain by cold, salty water, probably upwelled along 
the outer continental shelf (Briggs et al. 1988). By Septem- 
ber, Murres, including fathers with chicks, have spread out 
south along the coast 60 miles or more to Monterey and 
beyond (Ainley 6k Boekelheide 1990, Chap. 3). Little is 
known of the seasonal habitat shifts of birds breeding at 
mainland colonies on Point Reyes. 

Murres are highly gregarious, nesting shoulder to shoul- 
der on islands, offshore rocks, and inaccessible mainland 
cliffs. The densely packed colonies usually range in size 
from groups of tens to thousands of pairs. Murres select 
nesting terrain varying from terraces, gradual slopes, and 
shoreline promontories to the narrow ledges and shelves 
of cliffs, steep slopes, grottoes, and sea caves (Bent 1919; 
Ainley 6k Boekelheide 1990, Chap. 8). The majority of 



Alcids 



SPECIES ACCOUNTS 



Alcids 



Murres in California nest on gentle slopes and flat areas at 
the base of slopes or on rounded hilltops (Takekawa et al. 
1990). Murres lay a single egg on bare rock or soil, 
occasionally placing a few small stones around the egg. 
They almost always nest facing a vertical face, which they 
lean against while incubating (Ainley & Boekelheide 
1990, Chap. 8). Lacking true nests, Murres lay pyriform 
(pear-shaped) eggs that roll in a circle, an adaptation that 
keeps eggs from easily plummeting off precipices to smash 
on rocks or fall into the sea. Unlike other alcids at the 
Farallon Islands, the young depart from the nesting ledge 
when they attain only 20%- 25% of adult weight at about 
22 to 25 days of age. Depending on the terrain near the 
nest site, chicks waddle to the shoreline or jump off cliffs 
to the sea below (W.J. Sydeman pers. comm.). Most 
departure occurs in the evening or just after nightfall on 
calm days (Ainley & Boekelheide 1990, Chap. 8). Unable 
to fly, chicks swim with one parent (the male) to feeding 
areas some distance from the breeding colony. The male 
accompanies and feeds the chick until it is fully grown and 
capable of flight. 

Like other alcids, Murres pursue their prey underwater, 
propelled by partly folded wings. They apparendy forage 
mosdy early and late in the day and little during midday. 
Murres most often feed in large single-species flocks or 
occasionally in mixed flocks, especially with Brandt's Cor- 
morants and Western Gulls (Ainley ck Boekelheide 1 990, 
Chap. 3). They are very efficient divers and can descend to 
depths of almost 600 feet, or perhaps even deeper (Piatt 6k 
Nettleship 1985). Early in the nesting season, prior to the 
period of intense coastal upwelling, adult Murres foraging 
on the outer continental shelf near the Farallones feed on 
euphausiids (Briggs et al. 1988; Ainley 6k Boekelheide 
1990, Chap. 3). Chicks are not fed euphausiids, presum- 
ably because they provide insufficient nutrition relative to 
the high cost to adults of delivering them one by one to 
young. Generally Murres feed their young food items of 
high caloric value that can be easily carried lengthwise in 
the bill— these are usually midwater schooling organisms 
(fish and squid) about 1.5 to 6 inches long. Farallon 
breeders feed their chicks predominantly juvenile rockfish, 
anchovies, nightsmelt, and market squid, though the diet 
varies both seasonally and annually (Ainley 6k Boekelheide 
1990, Chap. 3, n = 20,427). In cold-water years, rockfish 
account for 85% and anchovies for 7% of the early-season 
diet, but the diet diversifies later as Murres move to inshore 
feeding areas. In warm-water years, prey diversity is greater, 
and the percentage of rockfish in the diet declines (from 
20% to 13% of the diet) from early to late season, while 
the percentage of anchovies increases (from 40% to 70%). 
The greater seasonal change in diet in warm-water years 
reflects the early movement of Murres inshore. Croll 
(1990) also documented seasonal and annual shifts in the 
Murre diet in Monterey Bay. It is likely that seasonal 



movements and diet of Murres breeding on the mainland 
at Point Reyes may differ from that of Farallon breeders, as 
suggested by dietary differences between offshore and 
inshore colonies of Rhinoceros Auklets in British Colum- 
bia and Washington (Vermeer 6k Westrheim 1984, Wil- 
son 6k Manuwal 1986). See Ainley 6k Boekelheide (1990, 
Chap. 3), for possible size differences in prey selected by 
Murres, Pigeon Guillemots, and Tufted Puffins at the 
Farallon Islands. Elsewhere Murres eat mosdy fish of a 
variety of species and euphausiids; they eat minor amounts 
of amphipods, isopods, decapods, cephalopods, and poly- 
chaete worms (Johnsgard 1987). 

Marin Breeding Distribution 

During the adas period, Common Murres were breeding 
at three large colonies along the Marin County coast: Point 
Reyes Lighthouse, Point Resistance, and Double Point 
Rocks (Sowls et al. 1980; Table 14, Figure 14). In 1989, 
these three colonies were still active, as was a small one 
established at Millers Point Rocks at least by 1982 (Carter 
etal. 1992). 

Historical Trends/ Population Threats 

Preliminary surveys in 1969 to 1972 estimated 9440 Com- 
mon Murres were breeding at the three sites where the 
bulk of the Marin County population has subsequendy 
bred (Ainley 6k Whitt 1973); these estimates were prob- 
ably low because the counts were not all taken from boats, 
as recent counts have been. Sowls et al. (1980) estimated 
that a total of about 37,000 Murres were breeding at three 
Marin County colonies in 1979 to 1980. Takekawa et al. 
(1990) documented declines from 1980-1982 to 1986 in 
numbers of breeding Murres in central California, includ- 
ing Marin County; total numbers in Marin fell to about 
26,900 birds. Numbers dropped at Point Reyes Lighthouse 
by 53.5% (from 44,250 in 1982 to 20,590 in 1986), at 
Point Resistance by 59.8% (from 7540 in 1980 to 3030 in 
1986), and at Double Point Rocks by 77.9% (from 14,870 
in 1980 to 3280 in 1986). The declines had multiple 
causes (see below). In 1989, Carter et al. (1992) estimated 
that 23,495 Murres were breeding at Marin County colo- 
nies. A pattern of declines of this magnitude in a local area 
is best interpreted in a regional and historical context. 

The history of the Farallon Murre colony is the best 
known of any in California. From a peak of 400,000 
Murres in the 1850s, the Farallon population declined 
precipitously to about 60,000 birds at the end of commer- 
cial egg collecting just after 1900 (Ainley 6k Lewis 1974). 
By 1959, continued human disturbance and chronic oil 
pollution had further depressed numbers to a low of 6000 
to 7000 birds. With protection and diminished oil pollu- 
tion, the colony rebounded to 20,500 birds in 1972 
(Ainley 6k Lewis 1974), peaked at 102,110 birds in 1982 
(Takekawa et al. 1990), and dropped to 68,168 by 1989 



195 



Alcids 



MARIN COUNTY BRFFDING BIRD ATIAS 



Alcids 



(Carter et al. 1992). Despite such fluctuations, apparently 
numbers of breeding Murres in general have increased 
along the entire California coast throughout much of this 
century (Osborne & Reynolds 1971, Osborne 1972, 
Sowls et al. 1980, Briggs et al. 1987). 

The entire population of Common Murres in Califor- 
nia breeds along the state's central and northern coast. 
Sowls et al. (1980) estimated diat 363,154 Murres bred at 
19 sites in that region in 1979 and 1980; they did not 
include an estimate for the North Farallon Islands which 
then held about 51 ,540 birds (Carter et al. 1992). The state 
population peaked at about 520,000 in 1982 (Briggs et al. 
1987). Between 1980-1982 and 1986, the central Califor- 
nia breeding population declined overall by 52.6% (from 
229,080 to 108,530 birds), while the northern California 
population remained relatively unchanged (Takekawa et al. 
1990). The decline in the central coast population was 
caused by a high rate of incidental take of Murres during 
an intensive nearshore gill-net fishery, compounded by 
mortality from two major oil spills and a severe El Nino 
event in 1982-83. Declines at individual colonies ranged 
from 46%- 100% and were most severe at colonies located 
nearest to areas of highest gill-net-fishing mortality. 
Declines along the Marin County coast, as detailed above, 
were among the most severe. From 1979 to 1987, gill-net 



fishing in central California waters killed roughly 70,000- 
75,000 Murres. Takekawa et al. (1990) estimated that 
gill-net mortality accounted for roughly 40%-45% of the 
120,550 Murres lost from the central California popula- 
tion during this period. An estimated 4500 Common 
Murres were killed or debilitated in the Gulf of the Faral- 
lones in die November 1 984 Puerto Rican oil spill (PRBO 
1985) and about 7500 along die central California coast in 
die February 1986 Apex Houston oil spill (Page et al. 1990); 
a minimum (probably much higher) of 1 100 Murres met 
a similar fate in die 1971 San Francisco oil spill (Smail et 
al. 1972). Carter et al. (1992) estimated 351,336 Murres 
were breeding at 23 sites in California in 1989; this 
number is only 3% fewer dian die population estimate for 
1979 to 1980, and die proportions of the population 
breeding in northern and central California were similar 
in bodi periods. 

Murres at the Farallon Islands have also experienced 
eggshell diinning from pesticide residues (Gress et al. 
1971), but no adverse effects on reproductive success have 
been detected (Ainley <Sl Boekelheide 1990, Chap. 8). It is 
clear, diough, diat a species widi such a history of popula- 
tion declines attributable to human causes should continue 
to be carefully monitored. 




Subdued Kissing sounds issue from the carmine mouths of Pigeon Guillemots scattered widely on rocky coastal sea cliffs. 

Photograph b> Ian Tail. 



196 



Alcids 



SPECIES ACCOUNTS 



Alcids 



PIGEON GUILLEMOT Cepphus columba 




Occurs year round, though primarily as a 
summer resident from mid-Mar through 
Aug (winter lows reached by mid-Oct). 

A common, very local breeder; overall 
breeding population very small. 

Recorded in 10 (4.5%) of 221 blocks. 

O Possible = 2 (20%) 
C Probable = 1 (10%) 
• Confirmed = 7 (70%) 

FSAR = 4 OPI = 40 CI = 2.50 



Ecological Requirements 

Our dapper "Sea Pigeons" inhabit nearshore ocean waters, 
over rocky substrate, usually within sight of land; some- 
times they range up to three to (rarely) nine miles from 
mainland or island shores where they nest (Briggs et al. 
1987; Ainley 6k Boekelheide 1990, Chap. 3). Guillemots 
nest solitarily or in small, loose colonies when suitable nest 
sites are clustered. They typically lay their eggs in shallow 
natural rock cavities in talus slopes, crevices in solid rocks 
of cliffs or crests of ridges, in sea caves, or among boulders 
on beaches (Bent 1919; Dawson 1923; Sowls et al. 1980; 
Ainley & Boekelheide 1990, Chap. 9). Nest sites range 
from the rocky shores at the water's edge up to the summit 
of precipitous cliffs. Guillemots also sometimes nest in 
burrows dug into clay or loose conglomerate rock, aban- 
doned rabbit burrows, and artificial structures, such as 
rock walls, bridges, suitable wharf timbers, drainpipes, 
tires hung from pilings, or artificial nest boxes (Bent 1919; 
Sowls et al. 1980; Johnsgard 1987; Ainley & Boekelheide 
1990, Chap. 9). The principal nest site requirement seems 
to be "a roof over their head" (Ainley & Boekelheide 1 990, 
Chap. 9). Most cavities are slighdy deeper than die length 
of an adult, thus allowing chicks refuge from predatory 
gulls. Even in deep cavities, suitable for Tufted Puffins or 
Rhinoceros Auklets (see below), Guillemots lay eggs and 
incubate near the entrance. Sometimes eggs laid under 
projecting ledges, boulders, or in spaces between piles of 
rocks can be seen from above (Bent 1919). In water-worn 
sea caves, Guillemots lay their eggs in various nooks and 
crannies about the walls or roof, in cavities under loose 
rock in the talus of crumbling walls, in open situations on 



flat rocks or shelfs, or even on the sandy floor of the cave, 
if beyond reach of daylight and water (Bent 1919, Dawson 
1923). 

Guillemots do not construct a true nest. Frequendy the 
eggs repose upon a collection of small granite chips, 
pebbles, shell fragments, bones, or other natural or human 
debris, or sometimes on bare rock, gravel, or soil. It is 
unclear whether Guillemots just select hollows with accu- 
mulated debris or actually gather nest material, but at most 
sites they likely just scrape together whatever material is 
available within easy reach (Bent 1919). Availability of nest 
sites may limit Guillemot populations at some sites (Ainley 
6k Boekelheide 1990, Chap. 9), but not at others (Kuletz 
1983). At the Farallon Islands, some pairs may control 
more than one nest site within a few yards of the site being 
used and may perhaps alternate irregularly among them 
(Ainley ck Boekelheide 1990, Chap. 9). 

Pigeon Guillemots use wing-propelled dives to search 
for prey, primarily on or over rocky substrate of the 
subtidal zone (Follett ck Ainley 1976; Ainley 6k Boekel- 
heide 1990, Chap. 3). Guillemots are deep divers and 
probably descend to 330 feet or more (Ainley 6k Boekel- 
heide 1990, Chap. 3). Once they reach the bottom, they 
"hover" by paddling their feet as they probe nooks and 
crannies for prey (Johnsgard 1987). Birds apparendy for- 
age mosdy early and late in the day (Ainley 6k Boekelheide 
1990, Chap. 3). In years of superabundant prey at the 
Farallones, small groups of Guillemots participate in multi- 
species foraging flocks preying on midwater-schooling 
rockfish, but otherwise they feed alone at submerged reefs 

197 



Alcids 



MARIN COUNTY BREEDING BIRD ATLAS 



Alcids 



and hunt for solitary prey that hide in the rocks. In years 
of poor prey availability in rocky habitat, Guillemots appar- 
endy feed more on sandy bottoms, diough probably near 
rocks. 

Farallon breeders feed dieir chicks primarily juvenile 
rockfish and sculpins, along with small numbers of a 
variety of other fish, octopuses, and mysid shrimp (Ainley 
6k Boekelheide 1990, Chap. 3, n = 2586). Adults bring 
food items one at a time, and prey are held crosswise in the 
bill, grasped by the head widi the tail dangling out of one 
side (J onns g ar d 1987). During cold-water years at the 
Farallones, rockfish are the principal prey, whereas in 
warm-water years, sculpins and other bottom-dwelling fish 
are more prevalent and dietary diversity is higher (Ainley 
6k Boekelheide 1990, Chap. 3). Dietary overlap is high 
between Guillemots and Pelagic Cormorants, especially in 
years of rockfish abundance. In all years, the Guillemot 
diet diversifies toward die end of die chick-rearing period, 
when rockfish decrease and other species increase in 
importance. In warm-water years, die proportion of rock- 
fish in the diet decreases from about 50% to 9% during 
the course of the nestling period, and in other years it 
decreases from about 80% to 50% during that period. In 
Alaska, individual Guillemots may display foraging site 
and prey preferences that are generally maintained widiin 
a season and between years; differences in diet between 
individuals appear to be correlated with laying date, habitat 
use, and possibly an acquired search image (Kuletz 1983). 
At the Farallones, individual Guillemots would more likely 
specialize in different prey in warm-water years, when 
overall dietary diversity is greater, than in cold-water years, 
when most individuals concentrate on rockfish (Ainley 6k 
Boekelheide 1990, Chap. 3). Apparendy adults feed to a 
greater extent on invertebrates, such as crustaceans, mol- 
lusks, and marine worms, than is indicated by the food fed 
to chicks (Lewis 6k Briggs 1985). 

Marin Breeding Distribution 

During the adas period, Pigeon Guillemots nested at 
scattered sites along the outer coast of Marin County 
endowed with rocky bluffs and sea stacks (Table 14, Figure 



14). Representative breeding locations were Double 
Point/Allamere Falls (ON/FY 7/5/80 -DS, ITi) and the 
mouth of Cold Stream, just N of Slide Ranch (ON 
7/26/82 -DS). 

Historical Trends/ Population Threats 

In surveys of only selected portions of the Marin County 
coast, Ainley and Whitt (1973) estimated that 82 Pigeon 
Guillemots were breeding at four sites. In 1979, Sowls et 
al. (1980) estimated that 366 Guillemots were nesting 
along 1 5 stretches of the Marin coasdine; this estimate is 
low because they only noted the presence of Guillemots, 
without reporting numbers, at four sites. In complete 
surveys, Carter et al. (1992) estimated that 1108 Guille- 
mots were breeding along 14 stretches of the Marin coast. 
The most notable increase from 1979 to 1989 was from 
120 to 616 birds at the Point Reyes headlands. Estimates 
for the entire Guillemot population breeding on the cen- 
tral and northern California coast ranged from 13,814 in 
1 979 to 1 980 (Sowls et al. 1 980) to 1 2,252 in 1 989 (Carter 
etal. 1992). 

Pigeon Guillemots declined on the Farallon Islands 
early in die 1900s because of oil pollution and disturbance 
from humans and domestic livestock (Ainley 6k Lewis 
1974; Ainley 6k Boekelheide 1990, Chap. 9). Numbers 
have since increased, and the population estimate for most 
of the 1970s and early 1980s of 2000 to 2200 birds may 
be an all-time high because of the current availability of 
nest burrows formerly occupied by Tufted Puffins during 
their population peak on the islands early in this century. 
The population of Guillemots breeding at Southeast Faral- 
lon Island dropped to less than 100 birds during the 
1982-83 El Nino event, but subsequendy increased to 
1867 birds in 1989 (Ainley 6k Boekelheide 1990, Chap. 
9; Carter et al. 1992). 

Oil pollution and disturbance continue to pose threats 
to Guillemot populations (Sowls et al. 1980). In the 1980s, 
many were also killed in gill nets along with Murres (H.R. 
Carter pers. comm.). 



198 



Aicids 



SPECIES ACCOUNTS 



Aicids 



RHINOCEROS AUKLET Cerorhinca monocerata 





Occurs year round, though primarily as a 
winter resident from mid-Oct to mid- 
Apr; occupies Farallon Island (and per- 
haps Marin) breeding colonies mosdy 
from Apr through Aug (extremes Feb- 
Sep). 


\ \ \^\ -^^S^i >r<3v V-^^v\^-A^\^^A\ --^V'X ^-1 


An uncommon, very local breeder; 
overall breeding population very small. 
Recorded in 1 (0.4%) of 221 blocks. 






O Possible = 1 (100%) 
€ Probable = (0%) 
• Confirmed = (0%) 




FSAR = 2 OPI = 2 CI = 1 .00 



Ecological Requirements 

Despite a horny protuberance on its bill suggesting a 
superficial likeness to a rhinoceros and a surname claiming 
affinity to the auklets, this species is in fact a puffin. 
Nevertheless, by habit and fondness we are perhaps for- 
ever doomed to use the nickname "Rhino." Off California, 
these puffins inhabit inshore and offshore ocean waters 
over the continental shelf, but they are most numerous in 
deeper waters over the continental slope (Briggs et al. 
1987). During the breeding season, most birds in Califor- 
nia are found within about 30 to 35 miles of nesting 
colonies (Briggs et al. 1987; Ainley &. Boekelheide 1990, 
Chap. 3). Birds disperse farther from colonies during 
warm-water years, when food is scarcer (Ainley & Boekel- 
heide 1990, Chap. 3). In Marin County, Rhinoceros 
Auklets are seen in season on the water below die cliffs at 
the Point Reyes headlands and near Bird Rock— sites where 
they apparendy breed— but the foraging range of diese birds 
is unknown. 

The generally small, loose breeding colonies on Cali- 
fornia s islands and steep mainland slopes (Sowls et al. 
1980, Carter et al. 1992) contrast with the dense colonies 
of thousands of birds prevalent from Washington north to 
Alaska (Bent 1919, Sowls et al. 1978). Rhinoceros Auklets 
initially recolonizing the Farallon Islands were nesting in 
deep natural cavities or clefts in rocks, in caves, or in 
artificial nest boxes (Sowls et al. 1 980; Sander 1 986; Ainley 
&. Boekelheide 1990, Chap. 11). As die population has 
continued to increase, some birds are now digging earthen 
nesting burrows under boulders in deep soil areas (W.J. 



Sydeman pers. comm.). Rhinos also dig earthen burrows 
at Afio Nuevo Island, San Mateo County, and Casde 
Rock, Del Norte County (H.R. Carter pers. comm.). Nest 
cavities on the Farallones are usually deeper than the 
average person's reach (Ainley ck Boekelheide 1990, 
Chap. 11). In one Farallon cave, nests were found up to 
about 65 feet back from the opening (Sander 1986). Six of 
eleven nests inside the cave were located in one- to two- 
foot-deep rock crevices, while five were exposed on the 
surface of the cave floor; seven nests were lined with grass, 
and the remainder had no nesting material. Throughout 
most of the range, Rhinos visit nest sites only in the dark 
of night, but to a limited extent in California and Oregon, 
diey move about dieir burrow entrances during daylight 
(Sowls et al. 1 980; Ainley ck Boekelheide 1990, Chap. 1 1). 
At the Farallones, their activities are largely crepuscular 
and nocturnal (Sander 1986). Nocturnal visitation may be 
an adaptation to reduce predation, to avoid kleptoparasi- 
tism by gulls while feeding chicks, or, perhaps more likely, 
to reduce interference by Tufted Puffins, with which Rhi- 
nos apparendy compete for limited nest sites (Ainley &. 
Boekelheide 1990, Chap. 11). 

To the north in the heart of the breeding range, Rhinos 
nest primarily in earthen burrows on both forested and 
unforested islands (Bent 1919, Richardson 1961, Sowls et 
al. 1980, Wilson & Manuwal 1986). They select nest sites 
there on sea-facing slopes, cliffs, or level areas adjacent to 
edges of islands in terrain varying from forests with open 
understories to dense shrubbery, grassy slopes, or bare 

199 



Alcids 



MARIN COUNTY BREEDING BIRD ATIAS 



Alcids 



earth. With their feet and bills, Rhinos excavate or reno- 
vate burrows from about 4 to 25 feet deep; average burrow 
lengths at different locations vary from 6-8 to 10-15 feet 
(Richardson 1961). Soil conditions, slope, and vegetation 
are important determinants of "Auklet" occupation and 
apparendy burrow length. Rhinos prefer rather firm sandy 
soil held together by roots at die surface and avoid soil that 
is too loose and sandy or too hard and rocky. Rhinos 
usually dig roughly horizontal burrows into steep slopes or 
into the base of small banks in flatter areas. Burrows 
sometimes have one or more spurs or blind alleys, and the 
last few feet of the main passage tend to slope down, often 
dropping off an inch or two, to terminate in an enlarged 
dome-shaped nest chamber. Rhinos lay their single egg in 
a nest that varies from a depression in the dirt (with or 
without a little dried or green grass) to a shallow saucer- 
shaped nest of grass, leaves, twigs, moss, ferns, feathers, or 
other available material (Bent 1919, Richardson 1961). 
Dry grass often is added to the nest or is found in the 
burrow during the nesding stage. In all areas, Rhinos 
prefer nest sites with some slope or altitude close at hand, 
presumably to aid in both takeoffs and landings (Richard- 
son 1961, Wilson & Manuwal 1986). In general, Rhinos 
favor somewhat more gende slopes and tolerate taller 
vegetation at nest sites than do Tufted Puffins (Vermeer 
1979). In some areas, Rhinos make runways through 
dense vegetation from open takeoff and landing spots to 
the mouths of their burrows (Richardson 1961). Burrows 
may be used year after year, often by die same pairs. Chicks 
fledge at about 50%-70% of adult weight (Vermeer 6k 
Cullen 1982) and are barely able to fly from their burrows 
down to the water (Richardson 1961). 

Rhinos' wing-propelled dives apparendy carry them to 
intermediate depths— 65 to 260 feet— in their pursuit of 
prey (Ainley & Boekelheide 1990, Chap. 3). They appar- 
endy feed mosdy early and late in the day, alone or 
occasionally in small flocks. The diet fed to chicks at the 
Farallon Islands in 1986 (a warm-water year) was 74% 
juvenile rockfish, 22% Pacific saury, and 4% black cod (n = 
27, Sander 1986; Ainley 6k Boekelheide 1990, Chap. 3). 
Ongoing diet studies at the Farallones from 1987 to 1989 
indicate that rockfish, anchovies, sablefish, and salmon are 
in fact the main prey items and that the diet varies consid- 
erably with season and year (PRBO unpubl. data). Through- 
out the range, the diet fed to chicks is almost exclusively 
small fish (1 .5 to 9.4 inches long)— particularly sandlance, 
anchovies, smelt, herring, rockfish, capelin, and Pacific 
saury— and sometimes small amounts of squid and octopus 
(Vermeer 1980, Hatch 1984, Vermeer 6k Westrheim 
1984, Wilson 6k Manuwal 1986). Adults often carry many 
fish at a time crosswise in their bills to their chicks 
(Vermeer 6k Cullen 1982); later in the season they deliver 
one fish at a time (S.D. Emslie and W.J. Sydeman pers. 
comm.). The chick diet varies between nearby offshore and 



inshore colonies, latitudinally, and both annually and 
seasonally, apparently as affected by changes in the distri- 
bution and abundance of prey. At least in winter, adults 
also eat euphausiids (Ainley 6k Sanger 1979) and greater 
quantities of nonfish prey. In Monterey Bay, the winter 
diet can be as much as 70% market squid (Baltz 6k 
Morejohn 1977, n = 26). 

Marin Breeding Distribution 

Since at least 1977, up to 1 1 Rhinoceros Auklets at a time 
have been observed on the water below the Point Reyes 
headlands in May and June (ABN). These birds have been 
observed "billing" and in "passing flights," which suggest 
local breeding. Three of these "Auklets" were seen off Bird 
Rock, Tomales Point, on 5 June 1989 (Carter et al. 1992). 
Representative records during the adas period included 
one to three birds just off the seabird nesting cliffs by the 
Point Reyes Lighthouse (6/1 1 -24/80 -DS, JGE; 6/21/81 
-DS, RSt) and four birds off Chimney Rock (6/20- 
7/16/77 —PRBO, JMR). The apparent establishment of 
breeding at Point Reyes is part of a much wider increase 
and range expansion of the breeding population described 
below. 

Historical Trends/ Population Threats 

Rhinoceros Auklets were absent from the Farallon Islands 
from the 1860s (when perhaps eliminated by overzealous 
collectors) until die early 1970s (Ainley 6k Lewis 1974; 
Ainley 6k Boekelheide 1990, Chap. 3). Perhaps aided by 
die elimination of burrow-competing rabbits, by 1989 the 
Farallon population numbered 516 birds (Carter ct al. 
1992). Coincident widi this recolonization, the population 
was increasing within the portion of the historic range 
from British Columbia to California and the breeding 
range expanded south to Point Arguello, Santa Barbara 
County (Scott et al. 1974, Sowls et al. 1980, Briggs et al. 
1987, ABN), and recendy to the San Miguel Island area of 
the Channel Islands (Carter et al. 1992). From surveys in 
1979 to 1980, Sowls et al. (1980) estimated 362 Rhinoc- 
eros Auklets were breeding at six sites along the central and 
northern California coast. Continued rapid expansion of 
the population in that region is documented by estimates 
of 1750 breeding birds at 29 colonies in 1989 (Carter et 
al. 1992). The reasons for this range expansion and popu- 
lation increase appear to be unknown. Despite this 
upswing in the breeding population, the Rhinoceros Auk- 
let is still considered a Bird Species of Special Concern in 
California (Remsen 1978, CDFG 1991b). 

Like other alcids, Rhinos are susceptible to oil pollution 
and human disturbance at nesting sites. About 1600 
Rhinoceros Auklets were killed or debilitated along the 
central California coast in die Apex Houston oil spill in 
February 1986 (Page et al. 1990). 



200 



Alcids 



SPECIES ACCOUNTS 



Alcids 



TUFTED PUFFIN Fratercula cirrhata 



-m^ 




Occurs year round, though primarily as a 




?^5^\ ^ yC\> 


winter resident from Oct through Apr. 


'xAji^v' 




Puffins occupy Farallon Island (and per- 


<r\X^h 


-\j^\& 


haps Marin) nesting sites from mid-Mar 
(rarely starting early Apr) through Aug or 




%\\\jC\)^ 


early Sep. 


V^ 


y$$<Kr^^^ 


An uncommon, very local breeder; 






overall breeding population very small. 




^T^V^^^f^S\%>^ 


Recorded in 1 (0.4%) of 221 blocks.- 




"^i^Or^v^^^^V^^^ '" 


O Possible = (0%) 




2^s\/^cV\\^A^v-A^\3^^r y^\yc\\x — v 


C Probable = 1 (100%) 






• Confirmed = (0%) 




FSAR = 2 OPI = 2 CI = 2.00 



Ecological Requirements 

At sea, the oudandish headgear of the solemn "Sea Parrot" 
enlivens the deep waters over the continental slope (Briggs 
et al. 1987; Ainley 6k Boekelheide 1990, Chap. 3). Near 
the Farallon Island breeding colony, Tufted Puffins most 
frequendy forage in waters deeper than 260 feet between 
30 miles south of the island to the Cordell Bank about 35 
miles to the north (Ainley 6k Boekelheide 1990, Chap. 3). 
In warm-water years, breeders disperse farther from the 
island to feed. The foraging haunts of Point Reyes breeders 
are unknown but probably are similar to those of Farallon 
birds and probably involve longer commutes to and from 
nest sites. 

Tufted Puffins breed on treeless islands, large offshore 
rocks and sea stacks, and steep mainland cliffs. In Califor- 
nia, Tufted Puffins nest in small, loose colonies of up to 
100 birds (Sowls et al. 1980), unlike the dense colonies of 
thousands of birds in Alaska (Sowls et al. 1978). Because 
the soil is generally shallow at the Farallones, Puffins diere 
do not dig earthen nest burrows, as they do extensively 
elsewhere, but instead lay their single eggs in natural 
cavities or clefts in rock of steep terrain high on the island; 
some nest in artificial cavities of tile pipe and rock (Ainley 
6k Boekelheide 1990, Chap. 11). Puffins probably breed 
in rock cavities at Point Reyes and other sites on the 
California coast, though at Castle Rock, Del Norte 
County, some birds dig burrows in hard soil (H.R. Carter 
pers. comm.). Lack of adequate nesting habitat may limit 
Tufted Puffins in California. Recently at the Farallones, 
nesting cavities are usually deeper than an average person's 
reach (Ainley 6k Boekelheide 1990, Chap. 11), but for- 



merly, perhaps when nest sites were more limiting, many 
sitting birds and eggs were visible from outside (Dawson 
1923). Some nesting sites then were "nothing more than 
the innermost recesses of niches and caves occupied by 
Murres." Throughout much of their breeding range, Tuf- 
ted Puffins prefer grassy slopes, rocky slopes, boulder 
rubble, and cliff faces and edges as nesting habitat. There 
they nest mosdy in earthen burrows dug with feet and bills 
three or four feet into soil or conglomerate rock; less 
frequendy they use rock cavities (Bent 1919, Dawson 
1923, Vermeer 1979, Johnsgard 1987). They prefer cliff 
tops and steep slopes clothed with low-growing or sparse 
vegetation (Vermeer 1979, Johnsgard 1987). Rarely, they 
drive tunnels dirough matted vegetation and deposit their 
eggs in die shade on the surface of the ground (Dawson 
1923). Generally, Tufted Puffins favor steeper, more open 
terrain for nesting than do Rhinoceros Auklets; small 
numbers of Tufted Puffins nest on flat islands and run to 
take off on beaches below the vegetation (Vermeer 1979). 
Tufted Puffins may or may not line their nests scantily with 
leaves, grass, seaweed, or feathers (Bent 1919, Dawson 
1923, Johnsgard 1987). 

Tufted Puffins are deep divers, probably exceeding 330 
feet in depth in their wing-propelled pursuit of prey (Ainley 
6k Boekelheide 1990, Chap. 11). They apparendy feed 
mostly early and late in the day, alone or occasionally in 
small flocks. Puffins hold many fish at a time crosswise in 
dieir bills, often in an alternating left- and right-handed 
manner, for delivery to young (Johnsgard 1987). At the 
Farallon Islands, the chick diet is mostly anchovies, rock- 



201 



Alcids 



MARIN COUNTY BREEDING BIRD ATLAS 



Alcids 



fish, and squid (Ainley & Boekelheide 1990, Chap. 3, n = 
728). Limited data suggest that rockfish predominate in 
cold-water years, when abundant in the diet of other 
Farallon breeding seabirds, and squid replace fish in warm- 
water years. The diet appears to diversify as the nesding 
period progresses, largely as a function of decreasing use of 
anchovies (which move inshore), increasing use of squid 
and unidentified fish (probably Pacific saury, which dwell 
in slope and pelagic waters), and stable use of rockfish. At 
the Farallon Islands, adult Puffins, like Murres, may also 
feed on euphausiids, but they do not feed them to chicks 
as they do to a limited degree at other locations (Hatch 
1984). Elsewhere, Tufted Puffins feed mosdy on a variety 
of fish— especially sandlance, rockfish, cod, prowfish, cape- 
lin, smelt, and herring— as well as euphausiids and poly- 
chaete worms (Vermeer 1979, Hatch 1984, Johnsgard 
1987). The diet, of course, varies annually, seasonally, and 
with location. 

Marin Breeding Distribution 

Since 1976, Tufted Puffins have been seen almost annually 
in the vicinity of the Point Reyes headlands from mid-April 
through July (ABN). These birds have been observed in 
passing flights, gathering algae, and carrying food up to 
cliffs, but solid confirmation of breeding is lacking because 
the inaccessibility of the cliffs has impeded efforts to find 
nests. Representative records for the adas period include 
up to seven birds on the water, diving to gather algae, and 
flying up to nearby cliffs at Chimney Rock from 1 2 June to 
16 July 1977; and up to six birds on the water and in 
passing flights at die Point Reyes Lighthouse from 29 May 
to 30 June 1980 (many observers -ABN). 



Historical Trends/ Population Threats 

Formerly, small numbers of Tufted Puffins came to the 
cliffs at Point Reyes during die nesting season (GckW 
1927), and two pairs were "apparendy nesting" on Bird 
Rock, Tomales Point, on 18 May 1930 (S6kP 1933). A 
paucity of records until the mid-1 970s (see above) may have 
indicated a decline and subsequent recovery, or perhaps 
just an upswing in sightings after a period of limited 
observer coverage drew to a close. 

Numbers of Tufted Puffins breeding at the Farallones 
declined from about 2000 birds in 1911 to 26 in 1959 
(Ainley 6k Lewis 1974). Subsequendy the population has 
grown slighdy and stabilized in the 1980s at about 80 to 
100 birds, except for the short-lived decline to less than 10 
birds during the 1982-83 El Nino event (Ainley 6k 
Boekelheide 1990, Chap. 11; PRBO unpubl. data). Ainley 
and Lewis (1974) speculated that the initial decline was 
caused by oil pollution and that Puffins had not made a 
full recovery because humans overexploited die Pacific 
sardine stock during the mid-1 940s (see Double-crested 
Cormorant account). Tufted Puffins have also contracted 
their range and numbers more widely and no longer breed 
south of the Farallones (Sowls et al. 1980, Garrett 6k Dunn 
1981) except at Prince Island in the Channel Islands 
(Carter et al. 1992). From surveys of the central and 
northern California coast, Sowls et al. (1980) estimated 
that 250 Puffins were breeding at 1 3 sites, and Carter et al. 
(1992) estimated 266 birds at 12 sites. The Tufted Puffin 
still faces threats from oil pollution and human distur- 
bance (Sowls et al. 1980), and it remains a Bird Species of 
Special Concern in California (Remsen 1978, CDFG 
1991b). 



202 



Pigeons and Doves 



SPECIES ACCOUNTS 



Pigeons and Doves 



Pigeons and Doves 

Family Columbidae 



ROCK DOVE Columbalivia 









A year-round resident. 








A fairly common, somewhat local 
breeder; overall breeding population 


^A° \^V V^\ ® %^\o \>\ o X^v Vf \*- 


\ °t-A _ 




small. 




/TO*. 




Recorded in 90 (40.7%) of 221 blocks. 

O Possible = 52 (58%) 
€ Probable = 13 (14%) 








--t- 


• Confirmed = 25 (28%) 
FSAR = 3 OPI = 270 CI = 1.70 






x^iV^A^Ar^^ 


^2o 








3v \<\ \^\° A 

^*-Ko\^ 







Ecological Requirements 

Flocks of pigeons feeding tamely at city dwellers' feet are 
sometimes the only "wildlife" they get to know. North 
American Rock Doves are the feral descendants of domes- 
tic pigeons brought to this continent by European settlers. 
Today, most of our pigeon populations still depend direct- 
ly or indirecdy on humans for their survival, but some 
birds do exist in a semiwild state. Breeding Rock Doves 
require elevated, enclosed sites for nesting and roosting, 
foraging grounds that are bare or covered only with short 
or scattered vegetation, and water for drinking (Goodwin 
1983, Cramp 1985). They avoid tall, dense vegetation of 
any kind, even grassland. Rock Doves are quite gregarious 
and commonly form flocks of varying size for virtually all 
activities away from their nesting territories. In the Old 
World, truly wild Rock Doves seek nest sites and shelter 
in cliffs (particularly on the coast) and feed in nearby 
treeless terrain. Today's feral birds are attached mosdy to 
human settlements, where they nest in artificial structures 
and feed in open urban settings or in agricultural land. 
Most Rock Doves in Marin County inhabit urban centers 
or agricultural ranchland, though small numbers frequent 
coastal cliffs. 



Although Rock Doves sometimes nest in solitary pairs, 
most breed in loose colonies. In large colonies, nests may 
be as close as 1.5 to 3.0 feet (Cramp 1985). They prefer to 
nest under cover in semidarkness (Goodwin 1983, Cramp 
1985). Wild (Old World) birds or semiwild feral birds 
usually nest in caves on sheltered ledges or in niches; or in 
holes, niches, or deep crevices in coastal or inland cliffs; 
less frequently they may nest in clefts in other rocks or in 
ruined buildings. Their nests are often far back in caves or 
deep in potholes or wells; birds will nest on a cave floor if 
suitable ledges are already occupied. Feral birds, however, 
have mosdy forsaken nesting in the wild for equivalent 
sites in or on farm buildings, church towers or steeples, 
large institutional buildings, large gabled houses, bridges, 
freeway underpasses, above-ground parking lots, and the 
like. Most nest sites are fairly high above ground. Although 
city-dwelling birds are tolerant of close human approach 
on the ground, they prefer abandoned buildings or in- 
accessible parts of structures for nesting. Rock Doves also 
nest in holes or hollows in trees, forks or depressions on 
large branches, or recesses in palm trees. Their nests are 
loosely constructed cups of roots, stems, and leaves, small 

203 



Pigeons and Doves 



MARIN COUNTY BREEDING BIRD ATLAS 



Pigeons and Doves 



pieces of driftwood, seaweed, and feathers; there is no tnie 
lining (Cramp 1985). Most pairs use two nest sites alter- 
nately, though normally not the same site for successive 
broods. The majority of pairs also use the same or adjacent 
nest sites over several years. 

Rock Doves form mainly communal, nocturnal roosts, 
though they sometimes roost singly or convene in die day 
during gloomy or stormy weather (Cramp 1985). Gregari- 
ousness at roosts is less pronounced among breeding 
birds, though off-duty males use communal roosts during 
incubation. The birds prefer roosts with cover all around, 
similar to nest sites. Although they prefer ledges that have 
broad overhangs and protection from the wind, they will 
use ledges more exposed to weather or artificial lighting. 
Roosts and breeding colony sites may be the same, or diey 
may be separate; there also may be several alternative roost 
sites. The roost site chosen may be die closest to the place 
where the birds last fed, or possibly the choice may be 
dictated by weather. Roosts are often used by birds from 
different feeding flocks and perhaps function in part as 
foraging information centers. However, flocks in some 
areas are discrete units attached to specific nesting, roost- 
ing, and feeding areas, while elsewhere they are chance 
aggregations. 

Rock Doves use exposed vantage points for long periods 
while surveying areas, waiting for feeding opportunities 
(Cramp 1985). Birds inhabiting agricultural or undevel- 
oped lands often feed near their nest sites. Birds from 
settled areas may feed exclusively in urban centers, or they 
may nest and roost there and fly to nearby fields to forage 
(Goodwin 1983, Cramp 1985). Rock Doves commonly 
forage up to one-third mile from nest sites, but sometimes 
as far as 4 to 5 miles (Cramp 1985). Upon arriving at fields 
to feed, feral birds normally circle, gain height, and circle 
again before landing in open areas with good visibility. 
Foraging birds walk or run about, pecking at the ground. 
Although most birds forage on bare or sparsely vegetated 
ground, feral (but not wild) birds will forage, rarely, for 
vegetable fare on the ground in woodlands or up in trees, 
bushes, or vines (Goodwin 1983, Cramp 1985). Flocks 
frequendy forage all at once, widi some birds flying from 
the rear to the front; there is a dominance hierarchy in feral 
flocks, with central birds obtaining more food (Cramp 
1985). In the country, Rock Doves feed around ranch- 
yards, in arable land, and on land grazed by domestic 
livestock. In urban areas, they feed on paved squares, 
sidewalks, roads, vacant lots, parking lots, parks, and 
gardens. This species is adapted chiefly to a seed diet of 
cereal grains, legumes, weeds, and grasses (Goodwin 1983, 
Cramp 1985). In agricultural areas, cereal grains predomi- 
nate in the diet over weed or grass seeds. Rock Doves 
occasionally eat acorns (usually broken or damaged), green 
leaves, buds, tender roots, flowers, berries, galls, and 
seaweed. In addition, they eat small amounts of inverte- 

204 



brate foods or small snails and mollusks, ticks and other 
arachnids, earthworms, slugs, moth larvae, and various 
insects. City-dwelling birds, on the other hand, subsist 
mosdy on a wide variety of artificial foods offered to them 
by humans or obtained by scavenging. Of these, bread is a 
staple. Other foods include grains, peanuts, popcorn, 
cheese, cooked meat, fat, bacon rind, fish, apple, banana, 
potato, chocolate, and ice cream! Overall there is consid- 
erable variation in diet with season and locality. Rock 
Doves also collect small stones and grit to aid in digestion, 
and eat mortar, presumably for the calcium content. 

Adults feed the young by regurgitation (Goodwin 1983, 
Cramp 1985). The diet of wild nesdings is similar to that 
of adults, but it includes a higher, though still minor, 
proportion of animal matter. "Crop (pigeon's) milk" is 
very important and is the sole food initially. At about four 
to five days of age, the young are fed morsels of soft food 
and small seeds. The importance of these and other solid 
foods in the diet increases until just before fledging, when 
the young consume only traces of crop milk. When their 
young are small, foraging adults take small seeds in prefer- 
ence to larger ones (Goodwin 1983). Although adults 
usually drink from the edge or in the shallows of water, 
they sometimes alight on the surface to drink or hover 
above it; young are brought water in die crop (Goodwin 
1983, Cramp 1985). 

Marin Breeding Distribution 

Marin County's breeding Rock Doves were concentrated 
in the lowlands of the eastern urban corridor along High- 
way 101 . They were scattered throughout the farm country 
of central and northern Marin but were relatively rare in 
ranchlands on the Point Reyes peninsula. Some of the 
birds in farming country were not truly feral, as some 
ranchers had built special lofts (dovecotes) for the birds 
and probably fed them. Representative nesting locations 
were ranch at E end of Clark Road, E of Tomales Bay (NE 
5/13/82 — DS); abandoned building near quarry on N 
side of Marshall-Petaluma Rd. near Soulajoule Reservoir 
(NE 6/23/82 — DS); and nest in depression in decrepit 
straw mattress in abandoned ranch house, Hicks Valley 
(NY 5/16/82 -DS, W&ST). Birds frequented the sea 
cliffs north of Slide Ranch (south of Stinson Beach) in the 
nesting season, but because access is difficult, observers 
did not confirm breeding there. 

Historical Trends/ Population Threats 

It seems likely that Rock Doves evolved in arid or semiarid 
and nearly treeless regions of Eurasia (Goodwin 1983), 
though the original distribution is obscure because of the 
long history of human domestication of the species for 
food, homing pigeons, and breeding of fancy varieties 
(Cramp 1985); these were apparendy the very first domes- 
tic birds. Many have become feral, especially in urban 



Pigeons and Doves 



SPECIES ACCOUNTS 



Pigeons and Doves 



areas, and these populations are still augmented by escaped 
birds. In the Old World, Rock Doves spread widely into 
many areas in response to the creation of suitable feeding 
grounds from agricultural and tree-cutting activities (Good- 
win 1983). Domestic Rock Doves were brought to the New 
World, to Nova Scotia, as early as 1 606, and later to other 
colonies along the eastern seaboard (Schorger 1952). 
Though they are now widespread (AOU 1983), the expan- 
sion of feral pigeons in North America appears to have 
been little documented. 

Mailliard (1900) and Stephens and Pringle (1933), for 
Marin County, and Grinnell and Wythe (1927), for the 
San Francisco Bay Area, did not list the Rock Dove at all 
in their avifaunal summaries. It seems likely that Rock 
Doves occurred in these areas at the time but went un- 
reported because of their domestic origins. Little additional 



attention has been paid to these feral homesteaders. Grin- 
nell and Miller (1944) gave only a skeletal account for this 
introduced species in California. They reported it "estab- 
lished in a free-living state about many cities." They did not 
mention its status in agricultural areas, its history of expan- 
sion, or the limits of its distribution at that time. A 
thorough search of historical archives would likely reveal 
the introduction of domestic pigeons to California at the 
time of Spanish missionaries or by enthusiasts during the 
Gold Rush. Rock Doves are now established throughout 
most settled and agricultural areas of California (McCaskie 
et al. 1979, Garrett <Sl Dunn 1981). Breeding Bird Surveys 
indicated that Rock Dove populations were still increasing 
in the Central Valley from 1 968 to 1 979 (Robbins et al. 
1986) and in California as a whole from 1968 to 1989 
(USFWS unpubl. analyses). 



BAND-TAILED PIGEON Columba fasciata 









A year-round resident; numbers can 




-W \ A^V, 




swell gready in winter (mosdy Sep-Mar) 


y3r^Op ; \0v 






but are quite variable then from year to 




K^^X^^^K^,' 




year. 




Y-- — \ ^V\ -jt<<\ J*r\ ^V^A y<^ \ 




A fairly common, fairly widespread 






breeder; overall breeding population of 


^^ < \3 >< v\v 


<\^K\\^k^\^\\A. 




moderate size. 


V\v5« 






Recorded in 117 (52.9%) of 221 


\\\°\ 






blocks. 










A o \>\ •■vca P'V-'A o V-^A-'P \^\J>-)*^\ '-QA 


„ 






V>\e V>t-Q.\>>t« \>)f o \>T opa.e l^<\ — 


— "T 






Ao\>a<\^Aov4« \>Ao v-"A « \>^\-vy 




O Possible = 83 (71%) 




'Oo 




v O ? r-\ 


© Probable = 29 (25%) 






*j?&5P?* ^~<_®>^oAj^p.\^^oAA<A •V%A 


\<:d> 


• Confirmed = 5 (4%) 






^5 \^\o\^^^^?^\ o V-Ao^ 






I U- 


^^^of^^k^W 








• 




FSAR = 3 OPI = 351 CI = 1.33 



Ecological Requirements 

This gregarious wild pigeon lives in Marin County s coni- 
fer, mixed conifer, and broadleaved evergreen forests d^at 
provide ample mast, berry, and small fruit crops. Appar- 
endy the best Band-tailed Pigeon habitat is forest land well 
interspersed with various age classes of trees and with 
openings; oaks are of particular importance in California 
(Jeffrey et al. 1977). The birds usually nest as isolated pairs, 
frequendy near permanent streams. Rarely (diough appar- 
endy not in California), they breed in small, loose colonies; 
as many as 1 7 nests have been found in a single tree (Bent 
1932, Neff 1947). Band-tails have nested in some part of 



their breeding range in every month of the year, but 
breeding may (Gutierrez et al. 1975) or may not (Michael 
1928) be a response to abundant mast or berry crops. 

Band-tailed Pigeons nest in a variety of trees or large 
bushes, and their nest heights range from 6 to 180 feet 
(most 15-40 ft.) above ground (Grinnell et al. 1918, Neff 
1947, Glover 1953b, MacGregor & Smith 1955, Peeters 
1962, Jeffrey et al. 1977). A sample of 33 nests in the 
Carmel area of Monterey County ranged from 1 2 to 95 feet 
(av. 36 ft.) above ground (MacGregor &. Smith 1955). 
Rarely, these pigeons place nests on the top of a stump, on 



205 



Pigeons and Doves 



MARIN COUNTY BREEDING BIRD ATLAS 



Pigeons and Doves 



the ground, or in blowholes and on ledges in sandstone 
rimrock (Neff 1947); apparendy no such records pertain to 
California. Most nests in coastal California are in conifers, 
oaks, alders, and occasionally in tall bushes, such as blue 
blossom (Ceanolhus thyrsi florus), or introduced trees near 
human habitation. Nests are usually placed in a fork, 
against a trunk, at variable distances out on a horizontal 
limb, or, infrequently, in dense branches. Tree nests are 
flimsy platforms constructed of dead coarse twigs of oaks, 
conifers, or other plant stems and forest litter (rarely, they 
are lined with pine needles); a ground nest was made of 
leaves and moss. Nest trees are frequendy on a slope or 
adjacent to a small precipice and near a clearing, leaving 
room for incubating birds to exit the nest with a momen- 
tum-gaining dive (Neff 1947, Peeters 1962). 

Band-tails feed singly or in flocks (even in the breeding 
season) and procure most of their food by plucking and 
gleaning from trees and shrubs and by gleaning from the 
ground. Birds searching in forest duff flip aside leaves and 
debris with horizontal movements of the bill (Smith 1968). 
Feeding flocks advance through the trees or across open 
ground by the progressive movement of individuals flying 
from the back of the flock to the front (Peeters 1962). 
Band-tails perform acrobatics while feeding in trees: they 
hang vertically by their feet from branches, half spreading 
their wings and tails to stabilize themselves as they reach 
for fruits and buds, then dropping to lower branches (Bent 
1932, Peeters 1962). 

Band-tailed Pigeons are almost exclusively vegetarians, 
specializing on mast, small wild and cultivated fruits and 
berries, grains, and other seeds; the few insects consumed 
appear to be taken incidentally. Band-tails tend to concen- 
trate on abundant food sources to the exclusion of other 
available items, and their diet varies considerably with 
season and locality (Neff 1947, Smith 1968, Jeffrey et al. 
1977, Grenfell et al. 1980). In California, acorns (espe- 
cially of live oaks), consumed whole, are the staple of their 
diet from fall through spring, when they are supplemented 
by wild fruits, such as madrone and toyon, and cultivated 
grains. Band-tails tend to select the smaller of available 
acorns, particularly with respect to width and weight (Fry 
1977). In spring, terminal buds, tender young leaves, and 
blossoms of oaks, madrone, and manzanita take on 
increasing importance. In summer and fall, fruits and 
berries become prominent dietary items. Important ones 
in California include elderberry, blackberry, raspberry, 
wild cherry and grape, dogwood, coffeeberry, salmonberry, 
thimbleberry, huckleberry, salal, chokecherry, and cascara. 
Cultivated grains, such as wheat, oats, and barley, are taken 
when sown in late winter and early spring, but particularly 
in summer and fall, when gleaned from stubble fields after 
harvest. Orchards supply buds, blossoms, and green fruit 
in spring and ripe fruits from spring through fall; plum, 
prune, apricot, peach, cherry, and almond orchards and 



vineyards are die favorites in California. Crop damage 
caused by the pigeons is most severe in spring and early 
summer, when wild staples are scarce; locally, cultivated 
grains then may be die predominant item in the diet 
(Smith 1968, Grenfell et al. 1980). Band-tails will also 
readily "clean out" grain from bird feeders. Infrequently, 
they eat seeds of alders, pines, grasses, forbs, and galls. 

For the first few days of life, adults feed the young 
exclusively "pigeon's milk," a fatty, yellow curdlike sub- 
stance produced in glands of the adult's crop. Regurgita- 
tions from the parents' crops contain progressively more 
berries and seeds until the food of the young is nearly 
identical to diat of adults (Neff 1947, MacGregor & Smith 
1955, Jeffrey et al. 1977). Band-tails collect grit to help 
grind food and perhaps for its mineral content, and they 
need fresh water daily (Smith 1 968). They frequendy ingest 
mineral salts from upland deposits, estuarine borders, and 
the water of mineral springs. It has been suggested that the 
minerals may aid in digestion of mast (Smith 1 968) or that 
they are a supply of calcium necessary for egg formation 
and crop gland function (March ck Sadlier 1972). 

Marin Breeding Distribution 

The only previously published nesting record from Marin 
County was of a nest found at Lagunitas on 30 July 1912 
(Mailliard 1912). During the adas period, the distribution 
of nesting Band-tails in Marin closely approximated the 
distribution of conifer and dense mixed evergreen forest. 
A representative nesting location was Upland Ave., Mill 
Valley (NE summer 1980 or 1981 -KY). 

Historical Trends/Population Threats 

Unrestricted sport and market hunting in the past, particu- 
larly early in this century, led to a decline in the species and 
an outcry for legal protection (Chambers 1912, Grinnell 
1913, Grinnell et al. 1918). In the winter of 1911-12, 
diere was intense shooting at Band-tailed Pigeon concen- 
trations in southern California. One hunter alone shipped 
2000 birds to San Francisco and Los Angeles hotels, and 
a single trainload of some 100 "enthusiasts" shot an 
estimated 3000+ birds per day (Chambers 191 2). A closed 
season was in effect in California from 1913 to 1932, and 
die fortunes of the pigeons rose accordingly (G&.M 1944, 
Neff 1947, Smith 1968, Jeffrey et al. 1977). Numbers of 
Band-tailed Pigeons detected on Breeding Bird Surveys in 
California were relatively stable from 1968 to 1989 but 
decreased from 1980 to 1989 (USFWS unpubl. analyses). 
Clear-cutting has destroyed vast expanses of suitable 
breeding habitat: this species nests only in forests at least 
20 years old (Glover 1953b). Although favored berry- and 
fruit-producing trees and shrubs are abundant in the early 
stages of forest regeneration after logging or fire, these are 
often eliminated by herbicide spraying that targets broad- 
leaved species and favors conifers (Grenfell et al. 1980). 



206 



Pigeons and Doves 



SPECIES ACCOUNTS 



Pigeons and Doves 



MOURNING DOVE Zenaida macroura 







A year-round resident; numbers 


nA^vpj^^T >^\ ® jv^to V A « \3A©x>-Y© \££v, y 




depressed somewhat (at least on Pt. 




Reyes) from Oct through mid-Mar. 
A common, nearly ubiquitous breeder; 


^V^V °i^\ • 3r^\ ° A^v D ; WVo Y>A© V>V© \5!M. 




overall breeding population very large. 


\ < Cv w N A. • J^X • .V'x • V-^V © Y-<^X V<TA -•€► V^CoX^---^ 
VS2k2tw ®A<r\ ®V^T\ '°J^\ ,» jp^^^v o \>r\ o J 




Recorded in 212 (95.9%) of 221 






blocks. 


V5©ve^*3c^®Jv^®A^^°V^\ Y>-\©YJ>n. 






XV^H 3A^VA ® Jv<\ ® V3A o Y^\ o \^\ © \J>vr© \ 






Yy° ^*<\^s \>\ © wv^v>\o y>-\ o \ /Voi\>h-s. 












O Possible = 71 (33%) 
C Probable = 101 (48%) 




^W\^^^ 




• Confirmed = 40 (19%) 




L^WoJkVf o \>\ © Y-^v ^^A^^S^A© WA • x - 








•H^ka**^ — ^-5 J^x • Jv<\ o Y'^© A^Ao V>^\ • \J<: 


v • 7 ^ 






J%*A >2^ ^" < --0^\ c Jr<\ ° Jf-^TX °3r<\ • A%L\ 

IT ^^^^^A^V^ 


V</& 


FSAR = 4 OPI = 848 CI = 1.85 




!>* xl/ ^- 4 <^ < g5 







Ecological Requirements 

These fast-flying, graceful doves are edge adapted. They 
forage in open country— grassland, pastureland, weed 
fields, croplands, roadside edges and ditches, and subur- 
ban yards and parks— and seek shelter and nesting sites in 
forest and woodland edges and woodlots. Although they 
may nest on the open edges of almost any of Marin's 
wooded habitats, they are most frequendy found in oak 
woodlands, suburban plantings, riparian woodlands, and 
planted woodlots or windbreaks (particularly eucalyptus). 

Mourning Doves build loose nest platforms of twigs, 
roodets, and grass stems. They usually place them on a 
horizontal branch or in crotches of limbs. Nests range up 
to 40 feet high in trees, but most are about 10 feet above 
the ground. Birds also construct nests on the ground, in 
low bushes, in piles of bark, on shelves on cut banks, in 
used nests of other birds, and on wooden ledges of human 
structures (Bent 1932, Cowan 1952). Pairs usually nest 
solitarily. Nests may be grouped in close proximity in 
limited favored nesting areas in open country— for example, 
in isolated woodlots or windbreaks in expansive grass- 
lands. 

Mourning Doves feed almost entirely by pecking from 
the ground and only very rarely feed in trees. During 
breeding, foraging birds occur singly or in small flocks. 
The diet is almost exclusively grains (Browning 1962). A 
few insects are taken only incidentally, and small amounts 
of snail shells and bone fragments perhaps satisfy a physio- 
logical need of nesting birds for calcium. These latter items 
might also be a source of grit or may be mistaken for seeds 



(Grenfell et al. 1980). Year round in California, seeds of 
162 species of plants make up 99.9% of the diet— 22 are 
principal food items that account for 10% of the total food 
in any month or region (Browning 1962, n=1016). 
Mourning Doves obtain about two-thirds of their seeds 
from annual weeds and the remainder from cultivated 
grains. The diet varies somewhat with season and locality 
(Browning 1959, 1962). In the inner Coast Range of San 
Luis Obispo County, Mourning Doves use 55 species of 
plants, of which 10 are principal food items (n = 183). 
Early maturing annuals, such as buckthorn weed, red 
maids, miners lettuce, and California poppy, constitute 
about 70% of the April and May diet and together with 
Napa thisde and prostrate pigweed provide over 75% of 
the June and July diet. Turkey mullein is the most import- 
ant item in late summer and early fall. Cultivated wheat 
and barley and, secondarily, milo are most important from 
late fall through winter, when they are taken as waste grain 
from fields after harvest. Other important annuals in the 
area are sunflower, hydra stickleaf, popcorn flowers, vine- 
gar weed, vetch, filaree, phacelia, valley spurge, bur clover, 
and lambs quarters. Mourning Doves also take advantage 
of birdseed spread on die ground or at feeders. 

As with other members of the pigeon and dove family, 
Mourning Doves initially feed their nestlings mostly 
"dove's (pigeon's) milk." At the age of 1 to 3 days, the 
young are fed 75%-90% dove's milk, but by 4 to 12 days, 
only 25%. Then regurgitated seeds form a progressively 
larger fraction of their diet (Browning 1959). 

207 



Pigeons and Doves 



MARIN COUNTY BREEDING BIRD ATLAS 



Pigeons and Doves 



Marin Breeding Distribution 

The Mourning Dove was one of Marin County s most 
widespread breeding birds. It reached its greatest abun- 
dance in lowland valleys of ranch and crop lands through- 
out the county and in suburban areas along die Highway 
101 corridor in eastern Marin. Representative nesting 
locales were the long eucalyptus grove SE of Abbott's 
Lagoon (NE 6/20/82 — DS); S side of Nicasio Reservoir 
(NE on shelf on road cut 7/6/82 -DS); Mt. Burdell, 
Novate (NB-NE-NY 4/9-5/1/81 -DS); and Dominican 
College, San Rafael (NB 3/28/79 -DS). At the Abbott's 
Lagoon eucalyptus grove, six nests were found in less than 
a half-hour's search of a stretch of 100 to 200 yards (four 
were 1.5-4 ft. above the ground in stick and bark litter). 



Historical Trends/ Population Threats 

In the early part of this century, there was limited evidence 
of local population declines in California (Grinnell et al. 
1918). Mourning Doves appear to have increased greatly 
since that time, aided by human activities, such as cultiva- 
tion, grazing, and ditch and roadside clearing, that have 
enhanced their food supply and created more of the dis- 
turbed areas they prefer for foraging (Grenfell et al. 1980). 
Although some of their habitat is currendy being lost to 
increased herbicide use, clean farming, and urbanization, 
Mourning Doves adapt by exploiting recendy cleared for- 
ests and new residential developments. Nevertheless, 
Mourning Dove numbers decreased on Breeding Bird 
Survey routes in California from 1968 to 1989 (USFWS 
unpubl. analyses). 




>*A^*toO I 



Band-tailed Pigeons perform acrobatics that enable them to gobble up 
berries from dangling clusters. Drawing fry Keith Hansen, 1 989. 



208 



Roadrunners 



SPECIES ACCOUNTS 



Roadr 



Roadrunners 

Family Cuculidae 



GREATER ROADRUNNER Geococcyx califomianus 



Formerly a year-round resident; extirpated by at least the 1960s. 



Ecological Requirements 

This legendary ground-dwelling cuckoo inhabits arid open 
land with scattered brush and thickets, which in coastal 
northern California consists primarily of the interface 
between broken chaparral and oak savannah woodlands or 
grasslands. Historically, fires have probably done much to 
maintain this habitat mix. Roadrunners build bulky nest 
platforms of loosely interlaced sticks and twigs, lined (or 
not) with finer miscellany such as manure flakes, bark 
strips, grass tufts, leaves, roots, feathers, or snakeskin 
(Bryant 1916, Dawson 1923, Bent 1940). They usually 
place their nests about 3 to 10 feet above the ground in 
clumps or thickets of thorny shrubs (or cactus in other 
regions). Rarely, they locate them up to 20 feet above the 
ground in a tree, direcdy on die ground, in a cranny of a 
cliff, or on an artificial structure (Bryant 1916, Dawson 
1923, Bent 1940). The birds situate their nests to receive 
full sun in the early morning hours, when adults are off 
hunting lizards, and partial shade in the heat of the day 
(Ohmart 1973). 

Roadrunners usually forage on the ground by slowly 
stalking their prey, then making a short dash to finalize the 
capture. They also chase down mobile prey, jump up and 
snatch insects from the air, or glean insects from bushes, 
from the ground, or by climbing into shrubs (Bryant 1916, 
Bent 1940). In southern California, the diet of adults is 
90% animal matter (by volume) and 10% fruit and seeds 
(Bryant 1916, n = 64). In the summer months, Roadrun- 
ners eat an even greater percentage of animal fare, consist- 
ing largely of insects, especially grasshoppers, crickets, and 
beedes, with lesser numbers of caterpillars, true bugs, flies, 
ants, bees, wasps, and scorpions. Vertebrates, including 
lizards, small birds, and small mammals, make up only 
about 10% (by volume) of the total diet (Bryant 1916); 
birds are probably taken to a greater degree in winter when 
cold-blooded prey are inactive (Zimmerman 1970). Snakes 
also are eaten (Bent 1940, Ohmart 1973). Even though the 
young are initially fed insects, by die time they are five to 
six days old they are fed mosdy lizards. Through most of 



the nesding period, young are also fed a clear viscous liquid 
by regurgitation (Ohmart 1973). Eggs hatch asynchro- 
nously, and adults and young are occasionally cannibalistic 
on the smallest young in their nests. Females also lay larger 
clutches when prey are more available, for example during 
the rainy season in Arizona. These observations suggest 
that Roadrunners are adapted to a limited or irregular food 
supply (Ohmart 1973). 

Marin Breeding Distribution/ 
Historical Trends/Population Threats 

Roadrunners formerly lived in Marin County in small 
numbers (Mailliard 1900, S&T 1933) and undoubtedly 
bred here, though there is no documentation. The last 
reported sightings were at Homestead, Locust Station, Mill 
Valley, on 22 April 1939 (Gull 21, No. 5); at San Rafael 
Hill on 24 February 1941 (Gull 23, No. 3); and on Mount 
Tamalpais sometime in the 1950s (JW). 

Grinnell and Miller (1944) noted declines or local 
extirpation from areas throughout California that had been 
thickly settled or heavily hunted. Roadrunners were widely 
persecuted at one time because (based on limited evidence) 
they were thought to prey heavily on the eggs and young of 
quail (Bryant 1916). Grinnell and Wythe (1927) noted a 
trend of increasing rarity of this species in the San Fran- 
cisco Bay Area that has continued to this day, with Road- 
runners now persisting in small numbers only in the 
hinterlands of the region. This decline has followed the 
intense development, habitat alteration, and disturbance 
attending a rapidly expanding human population. Verner 
et al. (1980) speculated that numbers in the Sierra foothills 
may have declined as chaparral-type habitats grew increas- 
ingly dense after decades of fire suppression activities. 
Numbers of Roadrunners were relatively stable on Breed- 
ing Bird Surveys in California from 1968 to 1989 but 
decreased from 1980 to 1989 (USFWS unpubl. analyses); 
this likely reflects trends largely in soudiern California, 
where the species is most numerous. 



209 



Barn Ou'ls 



MARIN COUNTY BREEDING BIRD ATLAS 



Barn Owls 



Barn Owls 

Family Tytonidae 



BARN OWL Tytoalba 













A year-round resident. 


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An uncommon, local breeder; overall 






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Recorded in 34 (15.4%) of 221 blocks. 


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€ Probable = 5 (15%) 


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Ecological Requirements 

The whitish, ghosdike silhouette and eerie screeching calls 
of this monkey-faced owl of subtropical and tropical origins 
enlivens the darkness in a number of Marin County s 
open habitats. Barn Owls hunt in open ranchlands, grass- 
lands, broken woodland and brushland, weedy fields, and 
marshes. They nest nearby in a wide variety of artificial and 
natural sites in wild, rural, or urban settings. Pairs usually 
nest solitarily, but nest sites are often limiting (Bloom 
1979) and birds sometimes nest in loose colonies at 
particularly attractive sites (Smith et al. 1974). Nest sites 
provide a roof and usually at least a modicum of, if not 
pitch, darkness. Natural nest sites include clefts, holes, 
grottoes, and caves of rock cliffs, quarries, or mines; 
natural cavities or ground squirrel holes in earthen banks, 
roadcuts, or gullies; deserted badger burrows; and hollows 
of trees (Bent 1938, Johnsgard 1988, Voous 1988). In 
some areas where the earthen substrate is soft, Barn Owls 
perform much excavation themselves, using their feet 
(Johnsgard 1988). Though tree nests are becoming less 
numerous, Barn Owls still nest in cavities of sycamores 
and live oaks in California (Bloom 1 979). Entrances to tree 
cavides in California range from 5 to 16 feet above the 
ground, and the height of actual nest sites varies from 

210 



ground level to about 40 feet. Unlike some large owls, 
Barns do not use abandoned nests of diurnal raptors, but 
on rare occasions they use deserted crow nests in Califor- 
nia (Bent 1938, Voous 1988). Holes in cliffs and banks 
may have been the original nest sites, as the pale Barn Owl 
plumage blends well with sandstone cliffs and caves 
(Voous 1988). Also, the Barn Owl's elliptical egg seems 
better adapted to the flat bottoms of cave niches (where the 
round eggs, typically laid by other owls, would be in 
constant danger of rolling off) than to enclosed tree hol- 
lows. 

In today's civilized surroundings, most Barn Owls nest 
in lofts, attics, and dark recesses of human structures, such 
as barns and farmhouses, church steeples, derelict build- 
ings and sheds, large industrial plants, old deep wells, 
mine shafts, water pipes, windmills, discarded agricultural 
machinery, and haystacks both inside and outside of barns 
(Bent 1938, Bloom 1979, Johnsgard 1988, Voous 1988). 
Rarely, they will nest in an exposed, unprotected situation, 
such as the flat roof of an occupied dwelling. Barn Owls 
also readily nest in large wooden nest boxes, preferably 
placed inside barns, cabins, and lofts. Nests inside build- 
ings are usually close to a ready exit. 



Barn Owls 



SPECIES ACCOUNTS 



Barn Owls 



Females lay their eggs in a crude shallow cup formed 
among dried pellets and heaps of broken bones or skull 
fragments, right on top of the hay in haylofts, or on wood 
chips or other natural debris in tree cavities (Bent 1938, 
Johnsgard 1988, Voous 1988). Favorable nest sites may be 
used for many successive years by the same or different 
pairs. Birds laying second clutches may deposit them in the 
same site as the first clutch, or they may choose a new site. 
Day roosts may be at nest sites, similar cavities or struc- 
tures, or in the thick foliage of trees (including palms). 

Barn Owls are highly nocturnal. They hide by day and 
typically become active and start hunting well after sunset 
and retire before dawn (Voous 1988). Observations of 
daytime hunting are rare and probably reflect unusual 
circumstances, such as a previous night of poor hunting 
because of inclement weather. Barn Owls are very versatile 
rodent hunters. With their acute hearing, they can strike 
and capture sound-producing prey in complete darkness 
(Johnsgard 1988, Voous 1988). Light, graceful, searching 
flights carry them over open country, where they bank, 
hover, and drop to the ground erratically (Bent 1938, 
Voous 1988). Barn Owls pursue prey on foot more than 
most owls do. To a limited degree, they also hunt from 
roadside poles (Voous 1988), drop from perches into 
bushes below to capture roosting sparrows, and catch bats 
flying out of caves (Smith et al. 1 974). Barn Owls that bred 
at Casde Rock, Del Norte County, and fed extensively on 
Leach's Storm-Petrels (Bonnot 1928) must also have 
caught them on the wing. Foraging owls range about 0.6 
to 1.9 miles from nest and roost sites (Johnsgard 1988, 
Voous 1988). 

Barn Owls are restricted, rather than generalist, feeders 
like Great Horned Owls and prey primarily on small 
terrestrial rodents of field and marsh (Voous 1988). When 
alternative food items are available, shrews rather than 
birds (at least in Europe) are usually the main prey. This 
differs from Long-eared and Short-eared owls and most 
other rodent-hunting species, whose secondary prey is 
birds. The North American diet varies by prey numbers 
from 1 00% down to 70% mammals— mosdy meadow voles 
and deer mice— with birds and large insects making up 
most of the balance (Voous 1988). Mammal prey range 
from a wide variety of small terrestrial rodents to pocket 
gophers, ground squirrels, moles, muskrats, jackrabbits, 
and skunks (Bent 1938). Bird prey in North America are 
mostly small songbirds but include rails, shorebirds, 
Coots, and Green-backed Herons, documenting that these 
owls do hunt over wet grasslands and marsh. Other 
vertebrate prey here include nocturnal lizards, turtles, 
frogs, toads, and fish. As might be expected, the diet varies 
geographically, seasonally, and year to year, depending on 
prey availability (Marti 1974, Voous 1988). 



In California, 61% of prey numbers are small voles, 
37% white-footed mice, 0.2% shrews, and 0.5% birds 
(Voous 1988), but another analysis showed 95% mam- 
mals, 3% birds, 2% insects, and less than 0.1% reptiles 
and amphibians (Jaksic et al. 1982, n = 8236). Mean 
weight of small mammal prey in California is 2.4 ounces 
(Jaksic et al. 1982, n = 7827). The diet of Barn Owls in the 
San Francisco Bay Area (including Marin County) is 99% 
small mammals (96% rodents). Meadow voles, pocket 
gophers, and deer mice account for 85% of the total, with 
the balance made up of various small mice, shrews, small 
rabbits, woodrats, moles, small birds, and a few Jerusalem 
crickets (Smith ck Hopkins 1937, n = 338). 

Barn Owls generally eat considerably smaller prey by 
weight than coexisting Great Horned Owls (Marti 1974, 
Knight & Jackman 1984) and somewhat larger prey than 
coexisting Long-eared Owls (Marti 1974). For example, in 
Colorado, mean prey weight is 1 .6 ounces for Barn Owls, 
6.2 ounces for Great Horned Owls, and 1.1 ounces for 
Long-eared Owls (Marti 1974). In some areas, Barns and 
Great Horns may capture prey of similar average weight, 
and they may overlap nearly completely in their food 
niches (Knight & Jackman 1984). Nonetheless, foraging 
separation occurs between the species (Rudolph 1978, 
Knight 6k Jackman 1984). Barn Owls are more nocturnal 
than Great Horns and can occupy more extensively open 
areas because they forage mosdy on the wing rather than 
from perches. On the other hand, because of their greater 
size and strength, Great Horns generally capture a wider 
range of prey. Litde is known of the dietary differences 
between male and female Barn Owls, but in Italy the food 
taken by females was more varied in species and size than 
that caught by males (Voous 1988). 

Barn Owls are prolific breeders, and providing food for 
the young can be a prodigious accomplishment. They vary 
their clutch size with food availability and sometimes lay 
two or more clutches in a year (Voous 1988). Females 
begin incubation with the first egg, and consequendy the 
difference in size between siblings is as large as or larger 
than for other owl species (Voous 1988). Females may 
begin laying the second clutch before the youngest owlets 
of the first brood have fledged (Johnsgard 1988). Males 
feed their mates, and in good years they stockpile food at 
the nest, sometimes even before the eggs are laid (Smith et 
al. 1974, Voous 1988). When the eldest nesdings are 
about three to four weeks old, the female resumes hunting, 
presumably because the dark, concealed nest site allows 
her to leave the young unprotected for periods of time 
(Voous 1988). During periods of food shortage, parents 
and larger young will eat smaller nesdings that behave 
abnormally or are already dead. 

211 



Barn Ou>!s 



MARIN COUNTY BREEDING BIRD ATLAS 



Barn Owls 



Marin Breeding Distribution 

During the atlas period, Barn Owls bred at scattered sites 
throughout the lowlands of Marin County in bodi rural 
and urban-suburban settings. Representative breeding 
locations were Lower Pierce Ranch, Tomales Point (NY 
spring 1981 fide JGE); ranch north of the NW corner of 
Nicasio Reservoir (NY 7/24/82 -DS); Willow Ridge Sta- 
bles, Point Reyes Station (NY 6/4/81 -JGE); Olema (NY 
4/28/77 -RMS); and Rancho Baulinas, N of Bolinas 
Lagoon (NE-NY spring/summer 1977 & 1978 -JKip). 

Historical Trends/ Population Threats 

Grinnell and Miller (1944) thought that Barn Owls had 
increased historically in California because of the increase 
in suitable nesting sites and the "reduction in numbers of 
owl-persecuting falconids." Numbers might also have been 
augmented by the clearing of land and the proliferation of 
rodents in agricultural areas. Times have changed, and, 
though still relatively abundant in parts of California, the 
species has declined steadily in recent years because of 



habitat loss from suburban and industrial developments 
(Bloom 1979). Barn Owls are now also virtually nonexist- 
ent in certain intensively cultivated agricultural areas of the 
Central Valley. Breeding Bird Surveys indicated that Barn 
Owl numbers were relatively stable in California from 
1968 to 1989 but decreased from 1980 to 1989 (USFWS 
unpubl. analyses). These surveys are geared toward diurnal 
species, limiting their usefulness for detecting trends in 
highly nocturnal species like the Barn Owl. The species 
was on the Audubon Society's Blue List from 1972 to 1981 
and on their list of Species of Special Concern since 1982 
(Tate 1981, 1986; Tate & Tate 1982). 

Traffic accidents are at present among the most serious 
human-induced mortality factors, as is readily evident from 
the carcass-littered roadsides of major highways, such as 
Interstate 5 in the Central Valley, that pass through prime 
Barn Owl country. Cases of poisoning by mercury, thal- 
lium, and organic biocides and thinning of eggshells have 
been recorded, but the extent of the threats posed to Barn 
Owls are unknown (Voous 1988, Marti &. Marks 1989). 




212 



Typical Owls 



SPECIES ACCOUNTS 



Typical Owls 



Typical Owls 

Family Strigidae 



WESTERN SCREECH-OWL Otus kennicottii 











A year-round resident. 


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Recorded in 42 (19.0%) of 221 blocks. 


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€ Probable = 22 (52%) 
• Confirmed = 3 (7%) 








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Ecological Requirements 

The bouncing-ball-like call of this tiny ear-tufted owl wafts 
softly through the night air in Marin County's oak wood- 
lands and open broadleaved evergreen hardwood forests 
where there is little understory. Screech-Owls are absent 
from Marin's coastal riparian thickets, but further study is 
needed to determine if in the county's interior they occupy 
the limited amounts of riparian woodland, an important 
breeding habitat elsewhere in California's lowlands 
(GckM 1944). Screech-Owls typically nest in natural cavi- 
ties of trees or stubs (elsewhere also saguaro cactus); old 
holes of large woodpeckers, such as Northern Flickers; and 
sometimes nest boxes (Dawson 1923, Bent 1938, Johns- 
gard 1988, Voous 1988), even those meant for Wood 
Ducks (S. Simmons pers. comm.). Birds have also used a 
bark-filled crotch of a eucalyptus tree, an old woodrat's 
nest, Black-billed Magpie nests, and even cliff cavities 
(Dawson 1923, Bent 1938). Screech-Owls sometimes nest 
in secluded recesses of buildings. Nests range from about 
4 to 60 feet above the ground, but most are at moderate 
heights (Dawson 1923, Bent 1938, Marti & Marks 1989). 
Nest heights probably reflect the availability of suitable 
cavities and woodpecker holes rather than any preference 



by the owls for particular heights. The owls do not make 
an actual nest but instead lay their eggs on the rotten wood 
chips, dead leaves, feathers, castings, or other debris accu- 
mulated at the bottom of the cavity (Dawson 1923, Bent 
1938). 

Screech-Owls are strictly nocturnal in their activities and 
retreat by day to roost in thick foliage, close against the 
camouflaging bark of trees, in tree hollows, in old build- 
ings, or other secluded spots (Bent 1938, Johnsgard 1988, 
Voous 1988). Little has been observed of foraging behav- 
ior. The most frequendy used hunting technique seems to 
be for an owl to swoop down from a perch on a twig 
projecting slighdy from the foliage or beneath the canopy 
of a tree to capture prey on bare or grassy ground (Voous 
1988). Many aspects of Screech-Owl biology are poorly 
known, but, like the Eastern Screech-Owl, males of the 
Western Screech-Owl probably bring food to incubating 
and brooding females and sometimes build up a store of 
food, such as voles, at die nest. The diet of Western 
Screech-Owls consists of varying amounts of insects and 
other arthropods, small birds, small mammals, crayfish, 
fish, and occasionally lizards, snakes, frogs, and salaman- 

213 



Typical Owls 



MARIN COUNTY BRFHDING BIRD ATLAS 



Typical Owls 



ders (Bent 1938, Johnsgard 1988, Voous 1988). Prey 
remains from nests in die San Joaquin Valley are mostly 
small mammals and birds, along wilh some fish and 
crayfish (S. Simmons pers. comm.). Reported invertebrate 
prey items include grasshoppers, crickets, locusts, Jerusa- 
lem crickets, mole crickets, walking sticks, praying man- 
tids, roaches, sowbugs, waterbugs, large moths, 
caterpillars, cutworms, beedes, ants, scorpions, spiders, 
harvestmen, and centipedes. Among the vertebrate prey 
not already mentioned are voles, pocket gophers, pocket 
mice, deer mice, harvest mice, grasshopper mice, woodrats, 
kangaroo rats, shrews, and many small and medium-sized 
birds up to the size of Northern Flickers, Steller's Jays, and 
American Robins. Screech-Owls have also attacked and 
sometimes partially eaten Golden Pheasants, Ring-necked 
Pheasants, Bantam hens, and domestic ducks. Litde is 
known of geographic or seasonal changes in diet, but birds 
to the south probably consume more insects and other 
cold-blooded prey, and birds in all regions probably eat 
less of these items in winter. 

Marin Breeding Distribution 

During the adas period, we found Screech-Owls breeding 
primarily in the eastern and north-central, oak-dominated 
portions of Marin County. They were lacking from die 



grassland-dominated areas on outer Point Reyes and 
around Tomales and from most of the moist dense coastal 
forests, particularly where there was a thick understory. 
Representative breeding records were San Anselmo (NE in 
attic 5/4/79); Mill Valley (NY/FL 6/2/79); and Fairfax 
(FL 7/22/79). All these confirmed records were obtained 
years after the fact from California Center for Wildlife 
records (S. Hershon in litt.), hence the locations plotted on 
the atlas map are only approximate and observer names are 
lost to posterity. 

Historical Trends/ Population Threats 

Grinnell and Miller (1944) felt the species may have 
increased in California in historical times from the "open- 
ing up" of heavy forests. On the other hand, many logged 
areas regrow with dense brushy understories that are 
unsuitable, while the clearing of areas of hardwoods for 
firewood, agriculture, or development must have had det- 
rimental effects on the species. Further urban-suburban 
development is likely to displace some of these owls, 
though providing nest boxes might mitigate the loss of 
some habitat. Pesticide residues have caused slight eggshell 
thinning in wild Screech-Owls, but these pollutants appar- 
endy have not impaired reproductive success (Marti 6k. 
Marks 1989). 




214 



Typical Owls 



SPECIES ACCOUNTS 



Typical Owls 



GREAT HORNED OWL Bubo virginianus 




A year-round resident. 

A fairly common, widespread breeder; 
overall breeding population fairly large. 

Recorded in 149 (67.4%) of 221 
blocks. 



O Possible 
© Probable 
W Confirmed 



68 (46%) 
54 (36%) 
27 (18%) 



FSAR = 3 OPI = 447 CI = 1.72 



Ecological Requirements 

This solemn, ferocious master of the night inhabits the 
edges of all Marin County's major woodland and forest 
habitats, as well as planted woodlots and windbreaks, 
where they border on open tracts of grassland, meadow, 
and field. Older stands of trees provide more potential nest 
sites and also offer more subcanopy flying room because 
they have few low branches (Johnsgard 1988). Elsewhere, 
Great Horned Owls also occupy extensive treeless areas 
with cliffs or rock outcrops for nesting and perches from 
which to hunt and regions ranging from boreal to tropical 
forests and deserts. Habitat preferences for a species that 
lives over such a broad latitudinal range are hard to define. 
The main requisites seem to be sheltered nesting and 
roosting sites, relatively open foraging grounds that supply 
a good mammal population, and suitable elevated hunting 
perches. 

Great Horned Owls usually lay their eggs in an aban- 
doned nest of a diurnal raptor (especially Red-tailed 
Hawks) in trees or sometimes cliffs; various species of 
raptor may alternate in their use of a given nest (Bent 1938, 
Johnsgard 1988, Voous 1988). Crow, raven, magpie, 
heron, or squirrel nests will also do as platforms for Great 
Horned Owl eggs. Other suitable nesting sites are ledges 
or caves in rocky cliffs, fissures of rocks, niches in cut- 
banks, rooms in Native American cliff dwellings, hollows 
in tree trunks and snags, depressions in the tops of old or 
dead trees, crotches in trees lacking debris or nest materi- 
als, a box in a tree filled with leaves, and occasionally lofts 
of barns. Rarely, these owls have nested on the ground 
next to a boulder or the base of a large tree, under a stump, 



in a hollow log, in long grass near a windmill, under 
bushes in the desert, and in an old Canada Goose nest on 
a tussock of grass in a pond. 

Great Horned Owls are mainly nocturnal but frequendy 
hunt at dawn and dusk and occasionally during the day. 
They usually swoop down silendy on their prey from an 
elevated perch in a direct, low, rapid flight (Marti 1974, 
Rudolph 1978). Typical perches include tall trees, tele- 
phone poles and wires, rock outcrops, and fence posts. 
Sometimes Great Horns hunt in harrierlike flights or catch 
bats in the air as they emerge from caves (Marti 1974). 
These owls also make night raids on the nests of other 
owls, hawks, and crows, and snatch crows from their night 
roosts (Voous 1988). 

Great Horned Owls are generalized and opportunistic 
predators and feed on a wider range of prey than is known 
for any odier owl or bird of prey in North or South 
America (Voous 1 988). A complete listing of all the prey 
eaten is not necessary, but about 77.6% of prey numbers 
in North America are mammals, 6.1% birds, 1.6% other 
vertebrates, and the balance insects, spiders, scorpions, 
crabs, and other invertebrates (Voous 1988). Great Horns 
will also feed on carrion or trapped fur-bearing mammals, 
particularly in winter. Their largest prey are mammals like 
muskrats, porcupines, skunks, and foxes, and birds the 
size of grouse, pheasants, domestic poultry, ducks, geese, 
swans, herons, and gulls. Great Horns prey on a variety of 
diurnal raptors at least as large as Red-tailed Hawks and on 
just about any species of owl they encounter. In semiarid 
regions of California, die percentages of important prey (by 

215 



Typical Owls 



MARIN COUNTY BREEDING BIRD ATLAS 



Typical Owls 



numbers) are 76.6% mammals, 15% insects (especially 
Jerusalem crickets), 42% birds, 1 .6% reptiles, 1 .8% arach- 
nids, and 0.8% amphibians (Jaksic & Marti 1984, n = 
2235 prey items). The most abundant prey in California 
are voles, woodrats, pocket gophers, and cottontails; their 
contribution to the diet by biomass is almost in the reverse 
order. In essence, they prey on almost all the available 
small mammals in a region, mostly those above 0.7 ounce 
in weight; diurnal, fossorial, and arboreal forms are mosdy 
absent in the diet. In areas of overlap, Great Horns eat 
considerably larger prey on average than do Barn and 
Long-eared owls (Marti 1974). See Barn Owl account for 
information on food niche separation. In the Sierra foot- 
hills, Great Horns subsist mainly on small to medium- 
sized mammals (Fitch 1947). Cottontail rabbits and 
woodrats represent 79% of the diet there by weight; other 
important items are kangaroo rats, pocket gophers, gopher 
snakes, and ground squirrels (Fitch 1947, n = 654). Vari- 
ous mice, opossums, skunks, bats, small landbirds, quail, 
Screech-Owls, American Kestrels, Coots, snakes, lizards, 
toads, Jerusalem crickets, and beedes were also taken. As 
might be expected, there is regional variation in diet 
reflecting prey availability and both seasonal and long-term 
variation in the diet reflecting fluctuations in prey popula- 
tions (e.g., Errington et al. 1940, Rush et al. 1972, Marti 
1974, Jaksic & Marti 1984; see also Voous 1988). Al- 
though females are considerably larger than males, there 
are no data on average or absolute prey sizes between the 
sexes (Voous 1988). The male hunts for die incubating 
and brooding female and for the nesdings (Voous 1988). 
He usually provides an abundance of prey, which he 
deposits, and which may accumulate, at the nest. The size 



of food items brought to young increases with nesding age 
(Johnsgard 1988). 

Marin Breeding Distribution 

During die adas period, the Great Horned Owl was the 
most widespread breeding owl in Marin County and 
perhaps the most widespread of our larger breeding birds. 
The adas map shows many small gaps in the distribution, 
particularly away from roads. This apparendy was mosdy 
an artifact of our limited coverage of nocturnally active 
species. Complete coverage of owls would probably have 
revealed Great Horned Owls in almost every adas block. 
Representative breeding locations were the Fish Docks, 
Point Reyes (FL 6/20/80 -DS); Morning Sun Ave., Mill 
Valley (NE-NY 3/5-5/15/81 -MC); and Tiburon (DD 
5/27/82 -BiL). 

Historical Trends/Population Threats 

Grinnell and Miller (1944) reported that Great Horned 
Owls had become scarce locally in California but were 
holding up remarkably well, even in areas closely setded by 
people and despite much hunting of "vermin." This assess- 
ment is still valid today for this remarkably adaptable 
species. Numbers of Great Horned Owls were relatively 
stable on Breeding Bird Surveys in California from 1968 
to 1989 (USFWS unpubl. analyses). Many owls are still 
shot, trapped, killed along highways by vehicles, and elec- 
trocuted by overhead powerlines (Voous 1988). Great 
Horned Owls have been poisoned by biocides, but this has 
not been as widespread or as well documented as for 
certain diurnal birds of prey. 



216 



Typical Owls 



SPECIES ACCOUNTS 



Typical Owls 



NORTHERN PYGMY-OWL Glaucidium gnoma 









A year-round resident. 




\ yCs 




A rare, very local breeder; overall breed- 


\^\? ; iA> i Tl/W 






ing population very small. 








Recorded in 2 (0.9%) of 221 blocks. 




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O Possible = 2 (100%) 


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ij^Kv^Z^i Jk\Jt\ 


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FSAR=1 OPI = 2 CI = 1.00 


J >j^=> 









Ecological Requirements 

Though in large part a creature of daylight, this midget of 
owldom still possesses the true spirit of that enigmatic clan, 
as one only rarely stumbles upon little gnoma, and then, 
likely as not, one beset by a mobbing, scolding throng of 
chickadees and nuthatches that advertises its presence far 
and wide. Because of the seeming rarity of Pygmy-Owls in 
Marin County, it is hard to determine habitat preferences 
here. Most encounters in Marin are of calling birds on the 
edges of Douglas fir forest or mixed evergreen forest 
dominated by Douglas firs or oaks. In California as a 
whole, Pgymy-Owls prefer open or broken forests or wood- 
lands of conifers and oaks at low to mid-elevations (Skin- 
ner 1938, G&M 1944, Gaines 1988). Although the 
surrounding forest type may sometimes be moist and 
dense, the owls are strongly attracted throughout their 
range to forest edges along meadows, clearings, or other 
openings (Skinner 1938, Johnsgard 1988, Voous 1988). 
Meadows harbor a choice supply of small mammal prey, 
and forest edges are renowned for attracting a diversity and 
abundance of avian prey as well. Pygmy-Owls tend also to 
nest in forests or woodland edges close to these important 
open foraging grounds (Skinner 1938). 

Pygmies lay their eggs in the bottom of abandoned 
woodpecker holes or natural cavities of trees or stumps in 
the scanty, naturally accumulated debris of wood chips, 
pine needles, twigs, feathers, and leaves, or the cast-off 
bones or beetle wings left from the previous year's dining 
(Skinner 1938). Pygmies sometimes nest in the same hole 
for several successive years. Known nest heights range 
from 5 to 75 feet above the ground (Skinner 1938), but 



also probably as high as suitable cavities exist. Johnsgard 
(1988) reported an average nest height of 19 nests at 20.7 
feet and a seeming preference for broadleaved trees over 
conifers for nest sites. It seems likely, however, that nest 
heights are largely determined by the availability of wood- 
pecker holes and natural cavities. The same goes for the 
types of trees in which these cavities exist, though broad- 
leaved trees may be more prevalent on forest edges where 
Pygmies tend to nest. 

Northern Pygmy-Owls are active and hunt during day- 
light hours, especially near dawn and dusk, though they are 
not averse to snoozing at times during the day (Skinner 
1938, Johnsgard 1988, Voous 1988). Foraging owls often 
perch on top of low to moderate-sized trees and snags in 
semiopen places. They travel from perch to perch shrike- 
like, dropping and buzzing along just above the ground, 
then rising sharply to the next perch. Pygmies are deter- 
mined, no-nonsense predators that rely on speed and 
surprise, though their flight is more "noisy" than many 
other owls (Skinner 1938). They pounce from perches 
onto prey on the ground, seize birds from perches in trees 
while flying, and pull adult and young woodpeckers out of 
their nest holes. They also sometimes hunt mice in open 
barns and cabins. These plucky owls often attack relatively 
large prey. Sometimes they pounce on a rather large mam- 
mal that drags its captor along with it for a considerable 
distance before succumbing to the onslaught. One owl 
carried off an Olive-sided Flycatcher shot by a collector! As 
a rule, they grasp avian victims by the neck. The male does 
all the hunting and brings in the food to a perch near the 

217 



Typical Owls 



MARIN COUNTY BREEDING BIRD ATLAS 



Typical Chuls 



nest, from which he calls his mate. The male typically 
decapitates and partly skins food before turning it over to 
die female. The female comes out to accept his offering and 
either remains to eat it or retires to the nest hole. Upon the 
hatching of the eggs, the male continues to bring food, first 
delivering small items to die young, and then larger items 
when the nesdings are more vigorous. There are sugges- 
tions in the literature of prey caching by this owl (Skinner 
1 938, Voous 1 988), though this needs further documenta- 
tion. 

The diet of Northern Pygmy-Owls consists primarily of 
small mammals, small birds, insects, and other inverte- 
brates, along with smaller amounts of cold-blooded verte- 
brates (Skinner 1938, Earhart 6k Johnson 1970, Snyder 6k 
Wiley 1976, Johnsgard 1988, Voous 1988). Insects and 
other invertebrates account for 61% of the prey items in 
the diet, mammals 23%, birds 1 3%, and reptiles and 
amphibians 3% (Snyder 6k Wiley 1976, n = 163). Mam- 
mals and birds undoubtedly are most important in terms 
of biomass. Earhart and Johnson (1970) reported that in a 
sample of 70 stomachs, 81% contained vertebrates and 
about one-third contained insects. Mammals are repre- 
sented by various species of voles, deer mice, house mice, 
pocket gophers, chipmunks, and shrews. A wide variety of 
small birds (mosdy passerines) are taken, including sizes 
up to American Robins, Steller's Jays, and sapsuckers. 
Pygmies occasionally take quail and chipmunks more dian 
twice the size/weight of the owls. Amphibians and reptiles 
in the diet include toads, frogs, various lizards, and small 
snakes. Insect prey are grasshoppers, cicadas, beedes, crick- 
ets, Jerusalem crickets, katydids, dragonflies, butterflies, 
and large hawk moths. Geographical differences in the diet 
likely occur, with mammals probably taken more in the 
north and reptiles and insects more to the south. Reptiles 
are taken only in the spring and summer when available 



(Earhart 6k Johnson 1970). On an annual bisis-, the larger 
females feed more on mammals (52% vs. 37%) and less 
on birds (21% vs. 34%) than do males. The smaller males 
presumably are more agile and better able to capture 
elusive avian prey than are females. 

Marin Breeding Distribution 

During the adas period, Pygmy-Owls were recorded in 
Marin County in the breeding season on only single dates 
at three localities: on 17 May 1980 at the east end of Big 
Rock Ridge (PckMSh); on 9 May 1981 on Mt. Burdell, 
Novato (ScC); and on 23 March 1982 on Mt. Vision, 
Inverness Ridge (RS). Although they undoubtedly would 
have been found in more areas if more nighttime work had 
been done, the species is quite scarce during the breeding 
season in the county. In contrast, Pygmy-Owls are relatively 
numerous in bordering Sonoma County, whereas North- 
ern Saw-whet Owls are uncommon there— the converse of 
the situation in Marin County. This difference defies easy 
explanation as broad areas of roughly similar habitat occur 
in both counties. 

Historical Trends/ Population Threats 

Grinnell and Miller (1944) did not report on any trends 
in populations of diis species in California. Johnsgard 
(1988) felt Pygmy-Owls generally were not seriously 
affected by human activities and, if anything, partial clear- 
ing of forests may improve hunting conditions for the 
species. This may be true, but conversely it seems that 
large-scale development, snag removal, or clear-cutting of 
forests may have detrimental effects on Pygmy-Owl popu- 
lations. Pygmy-Owl numbers were relatively stable on 
Breeding Bird Surveys in California from 1968 to 1989 
(USFWS unpubl. analyses). 



218 



Typical Owls 



SPECIES ACCOUNTS 



Typical Owls 



BURROWING OWL Speotyto cunicularia 









Occurs year round, though almost exclu- 




\ pCV_ 




sively as a winter resident from mid-Sep 


jOk^K^^^PXy^-^. 




through Mar. 


^f\%(\J\\J>^ 




~- - 


A very rare, very local breeder; overall 
breeding population very small. 




\C\p?<^C\j>^ 




Recorded in 1 (0.4%) of 221 blocks. 


\5*^>S^r\l^ 






O Possible = (0%) 


\Xk^\^^^^> 






d Probable = 1 (100%) 






^V '-^V^ \^A \^\ \^\ 


^V^Ky ,r- 


• Confirmed = (0%) 




1 \^^ 


"\^C^^>^C\^ 




FSAR =1 OPI = 1 CI = 2.00 



Ecological Requirements 

A fleeting glimpse of an owl silhouette at dusk is one thing, 
but leisurely views of the antics of a family of long-legged 
"Billy Owls" (Dawson 1923) preening, sunning, bowing, 
and bobbing in front of their burrow in broad daylight is 
another— one long to be remembered by die person new to 
the world of birds. These diminutive, ground-dwelling 
owls inhabit relatively dry, flat, very open grasslands and 
disturbed areas with very short vegetation. Habitat prefer- 
ences in Marin County are difficult to explain. Only one 
pair apparendy bred here during the adas period (see 
below). Elsewhere in northern California, Burrowing 
Owls occupy grasslands, pasturelands, edges of agricultural 
fields, abandoned fields and lots, and disturbed sites with 
sparse low-growing vegetation. Though receding before die 
tide of civilization, Burrowing Owls can tolerate a certain 
amount of noise and dismrbance if certain other require- 
ments are met, as evidenced by birds breeding at large 
airports, golf courses, and in small pockets of habitat in 
rapidly developing areas. The main requirements are ade- 
quate nest sites, productive open foraging grounds, and 
perching sites, such as raised rodent mounds, dikes or 
levees, fences, or utility poles and lines (Coulombe 1971, 
Voous 1988,Johnsgard 1988). 

As dieir name implies, diese owls usually nest inside die 
earthen burrows of mammals, or even tortoises and other 
animals (Voous 1988). Sometimes they select as nest sites 
burrows beneadi rock faces, natural rock cavities (Rich 
1986), drainpipes (Collins 1979), or, in Soudi America, 
Inca ruins or derelict, abandoned houses (Voous 1988). In 
lowland nordiern California, diey rely mostly on the exca- 



vations of Beechy ground squirrels (Citellus beecheyi), 
which they enlarge and improve (Thomsen 1971). The 
major factor controlling Burrowing Owl numbers appears 
to be the availability of suitable burrows (Coulombe 1971), 
diough sometimes birds dig their own burrows from 
scratch using their feet and, to a limited extent, dieir beaks 
(Thomsen 1971, Martin 1973, Voous 1988). The impor- 
tance of burrows to diis owl's ecology is further empha- 
sized by use of diem as social centers, sites for food storage, 
refuges from predators, and as a stable, tempered environ- 
ment during periods of extremely hot or cold weather 
(Coulombe 1971, Thomsen 1971). At die Salton Sea, 
temperatures at die entrances did not differ from those in 
die depdis of the burrows. Humidities, diough, were much 
higher inside burrows, which would reduce the water loss 
of owls diere (Coulombe 1971). Tunnels usually slope 
down about 15° from the entrance and always have a sharp 
turn widiin about diree feet of the surface (Coulombe 
1971). Burrowing Owls also readily accept artificial bur- 
rows of wood or pipe, as long as die tunnel has one turn 
diat maintains the nest chamber in complete darkness 
(Collins & Landry 1977, Collins 1979). Unlike most 
owls, Burrowing Owls transport nest materials and line 
die nest and die burrow entrance liberally with dried 
mammal dung, dried grasses, human litter, and divots 
from golf courses (Thomsen 1971, Martin 1973, Evans 
1982, Johnsgard 1988, Voous 1988). Nest materials may 
function to provide insulation or to camouflage the owl's 
scent or that of its prey from mammalian predators (Martin 
1973). The owls later remove nest materials from burrows, 

219 



Typical Owls 



MARIN COUNTY BREEDING BIRD ATLAS 



Typical Owls 



apparently during the early nestling phase (Thomsen 
1971). Young Burrowing Owls make hissing and rasping 
sounds, which, as apparent vocal mimicry of rattlesnakes, 
may discourage prowling carnivores (Voous 1988); it 
should be noted that tree cavity- nesting Saw-whet Owl and 
Screech-Owl young also make similar-sounding begging 
calls (Thomsen 1971). When the young owlets (about two 
weeks old) first come to the mouth of the burrow, they (and 
their families) use nearby auxiliary burrows and usually use 
two or three different burrows in succession before they 
fledge (Evans 1982). Burrows invariably swarm with fleas 
(Collins 1979). 

Burrowing Owls limit defense of their territories to the 
immediate vicinity of their burrows, and hence adjacent 
pairs often share mutual foraging areas (Coulombe 1971). 
In most parts of their range, Burrowing Owls are highly 
crepuscular, feeding mosdy in the dim light near dawn and 
dusk (Coulombe 1971, Thomsen 1971, Collins 1979, 
Johnsgard 1988, Voous 1988). They also feed actively 
during daylight hours and at night; in some areas noctur- 
nal foraging appears to increase in winter. Regional or 
seasonal differences in the timing of daily foraging activities 
may reflect those of their most frequent prey items or 
changes in environmental temperatures. Based on prey 
numbers, the Burrowing Owl diet is dominated by arthro- 
pods, mosdy insects, but apparendy small mammals are 
the most important prey in terms of biomass (Snyder &. 
Wiley 1976, Jaksic 6k Marti 1981 , Johnsgard 1988, Voous 
1988). The North American diet by prey numbers is 
90.9% invertebrates (mosdy insects), 6.9% mammals, 
2.0% reptiles and amphibians, and 0.3% birds (Snyder 6k 
Wiley 1976, n = 3564). The California diet by prey num- 
bers is 70.6% insects, 23.6% mammals, 3.5% birds, 2.2% 
amphibians, 0.1% isopods, with trace amounts of reptiles, 
scorpions, and centipedes (Jaksic 6k Marti 1981, n = 
3794). Beetles account for 49.2% and orthopterans 
(mosdy Jerusalem crickets) for 50.3% of the invertebrate 
prey in California. Vertebrate prey here are dominated by 
voles (69.5% of total), followed by birds (12.1%), amphib- 
ians (7-4%), jackrabbits and cottontails (6.4%), pocket 
gophers (2.4%), house mice (1.0%), Norway rats (1.0%), 
bats (0.2%), and reptiles (trace). The diet of birds at the 
Oakland Airport, Alameda County, consists mosdy of 
insects such as Jerusalem crickets and beedes and verte- 
brates such as meadow voles, young jackrabbits, pocket 
gophers, small to medium-sized birds, and toads; appar- 
endy a high percentage of vegetation in the pellets is 
consumed direcdy by the owls (Thomsen 1971). Near the 
Salton Sea in southern California, these owls feed primar- 
ily on arthropods and insects (earwigs, crayfish, crickets, 
and beedes) but also eat a few small mammals, birds, frogs, 
toads, snakes, and some carrion (Coulombe 1971). There 
is additional regional variation in the diet throughout the 
range and increasing use of mammals in winter and insects 



in summer (Johnsgard 1988, Voous 1988). The mean 
weight of small mammal prey in California is 1 .9 ounces 
(Jaksic 6k Marti 1981). Males provide food to incubating 
females, and they may temporarily cache food almost 
anywhere in their territory, usually within 100 feet of their 
burrows (Johnsgard 1988). A cache in southern California 
contained six freshly decapitated Mourning Doves, each of 
which represented 80%- 84% of the body weight of a 
Burrowing Owl. Males bring most of the food to the 
young, and females help distribute it (Thomsen 1971); 
females begin to forage when the young are three to four 
weeks old (Martin 1973). Unlike most North American 
owls, male Burrowing Owls are slighdy larger and heavier 
than females, but no difference has been found in average 
prey size between males and females (Voous 1988). 

Because of their crepuscular and sometimes diurnal 
habits, Burrowing Owls probably locate most prey by sight 
(Voous 1988). Hearing must also play a role, though they 
have a relatively poor ability to locate prey in the dark 
compared with many North American owls. Burrowing 
Owls capture prey in their talons after short flights or glides 
to the ground from elevated perches; by flycatching sorties 
from perches; by direct aerial chases; by hovering (heights 
of about 25-50 ft.) or flying low over fields, then pouncing 
on prey on the ground; and by walking or running down 
prey on the ground (Coulombe 1971, Thomsen 1971, 
Martin 1973, Johnsgard 1988, Voous 1988). At Oakland, 
Alameda County, males performed 98% of hovering 
attempts (Thomsen 1971), perhaps because this is a spring 
and summer foraging method when males do most of the 
hunting. Birds probably also take insects that live in their 
burrows (Coulombe 1971). 

Marin Breeding Distribution 

The only evidence that Burrowing Owls bred in Marin 
County during the adas period was the repeated observa- 
tion of a pair in the Terra Linda area of San Rafael all 
spring (through May) in 1976 and 1977 QSt). The near- 
absence of ground squirrels in West Marin may limit 
Burrowing Owl populations there, but where ground 
squirrels are more numerous in East Marin, other factors, 
perhaps extensive urbanization, must hinder them. 

Historical Trends/ Population Threats 

Both Mailliard (1900) and Stephens and Pringle (1933) 
considered the Burrowing Owl a year-round resident in 
restricted areas of Marin County. Given the current status 
documented by die adas work, Burrowing Owls appear to 
have declined historically in Marin County. 

Grinnell and Miller (1944) reported that Burrowing 
Owls were becoming scarce in setded parts of California. 
This owl is currendy on the state's list of Bird Species of 
Special Concern (Remsen 1978, CDFG 1991b). Data on 
recent trends in the population are conflicting. As of this 



220 



Typical Owls 



SPECIES ACCOUNTS 



Typical Owls 



writing, numbers of breeding Burrowing Owls continue to 
decline in the San Francisco Bay Area because of ongoing 
urban development (L. Feeney pers. comm.). Numbers 
also declined on Christmas Bird Counts throughout Cali- 
fornia from 1954 to 1986 (James 6k Ethier 1989) but were 
relatively stable on Breeding Bird Surveys in the state from 
1968 to 1989 (USFWS unpubl. analyses). Widespread 
grid-based surveys are currently being conducted to better 
determine the status of the breeding population in lowland 
California (D.F. DeSante pers. comm.). The U.S. Fish and 
Wildlife Service has variously categorized the Burrowing 
Owl as "rare" in 1966, dropped that classification in 1968, 
and assigned it a status of "undetermined" in 1973 (Johns- 



gard 1988). The species was on the Audubon Society's 
Blue List from 1972 to 1981 and has been on their list of 
Species of Special Concern since 1982, with the central 
and northern California population considered low or 
declining (Tate 1981, 1986; Tate ck Tate 1982). This 
decline has been blamed mainly on rodent control pro- 
grams that reduced nesting sites for the owls; on direct loss 
of nesting and foraging habitat to urban, industrial, and 
agricultural development; and perhaps to pesticides that 
have reduced food supplies and direcdy poisoned owls 
(Zarn 1974, Evans 1982, Johnsgard 1988, Marti ck Marks 
1989). Shooting has been an important source of mortality 
locally and in former times (G6kM 1944, Evans 1982). 




i^4.i+h l-ldnso-n 



With a woodrat clutched tightly in its talons, an adult Spotted Owl gazes down on a dark- 
eyed, fuzzy youngster absorbing its new world. Drawing by Keith Hansen, 1 989. 



221 



Typical Ou/ls 



MARIN COUNTY BREEDING BIRD ATLAS 



Typical Owls 



SPOTTED OWL Strix occidentalis 











A year-round resident. 


j*^* \Y^r^ 


r^5^-^ K 






An uncommon, very local breeder; 


yA^vOp 






overall breeding population very small. 




\^\^%^f\^\ \ La 


OrA0r\>^" 




Recorded in 16 (7.2%) of 221 blocks. 




cjtA^XAjPvp 


<^CV>V^ 




O Possible = 6 (38%) 


\~\~ 


\V^J^ V-Ajo\ 






C Probable = 4 (25%) 
• Confirmed = 6 (38%) 




^i^&\^\\.'L^^J>^ 


-A^I^VaVV •' 












— -r' 


FSAR = 2 OPI = 32 CI = 2.00 




i i 


v OVA -\J-*A -\>"\ V>fA 


Vo 





Ecological Requirements 

This recluse of the shadowy depths of ancient forests has 
in recent years been thrust into the national spodight. The 
Spotted Owl symbolizes both the plight of vanishing old- 
growth ecosystems— supporting a fragile web of species 
coadapted over thousands of years of evolution— and the 
difficulty of overcoming political obstacles even when a 
species' existence hangs in the balance. Mounting concern 
over the owl's predicament, and the ensuing controversy 
surrounding the difficulties of meeting both the owl's and 
perceived human needs, spawned extensive research aimed 
at developing management strategies to ensure long-term 
survival of viable owl populations in remaining habitat 
(e.g., Forsman et al. 1984; Gutierrez 6k Carey 1985; 
Dawson et al. 1987; Simberloff 1987; USFWS 1987a, 
1989c; USFS 1988; Thomas et al. 1990). 

Spotted Owls have long been known to breed in a 
variety of moist primeval conifer forests. In Marin County, 
at the southern limit of the range of the Nordiern Spotted 
Owl (S. o. caurina), they breed mosdy in forests dominated 
by coast redwoods, Douglas firs, and bishop pines and in 
forests where any of these conifers mix together or blend 
extensively with mixed evergreen hardwoods. These habi- 
tats are typical of those used in much of the species' range. 
Spotted Owls also breed to a limited extent in Marin's 
mixed evergreen hardwood forests, such as those at Toma- 
les Bay State Park dominated by coast live oaks. Elsewhere 
in the state, some populations of the California Spotted 
Owl (S. o. occidentalis) also breed in riparian habitats 
dominated by oaks and other hardwoods in canyons of the 

222 



western Sierra Nevada (Neals et al. in Thomas et al. 1990) 
and in southern California (Gould 1977). 

Recent research has demonstrated that superior Spotted 
Owl habitat is most commonly found in old-growth forests 
or mixed stands of old-growth and mature trees, usually 
1 50 to 200 years old (Thomas et al. 1 990). Throughout 
their range and in all seasons, Spotted Owls consistendy 
concentrate their foraging and roosting in old-growth or 
mixed-age stands of mature and old-growth trees. They 
select nest sites primarily in old-growth trees, whether in 
old-growth stands or in remnant old-growth patches. The 
appropriate structural characteristics that suit the owls can 
sometimes be found in younger forests, particularly when 
they include remnants of earlier stands affected by fire, 
windstorms, or inefficient or selective logging. Neverthe- 
less, with few exceptions, nest and major roost sites are 
located where elements of the earlier stands remain. That 
a particular suite of structural elements, radier than age of 
the forest, is important to the owls is demonstrated by 
habitat use in coastal redwood forests of northwestern 
California. There exceptional conditions lacking in most 
of the owls' range produce stands 50 to 80 years old that 
support relatively high numbers of owls. The combination 
of relatively high rainfall, a long growing season, the fast 
growth and stump sprouting abilities of redwoods, and the 
early intrusion of other conifers and hardwoods in the 
understory produce a structurally precocious forest with an 
abundant prey base. Aldiough Spotted Owls in California 
depend on old growth at higher elevation sites dominated 
by Douglas fir or Douglas fir-true fir forests, this does not 



Typical Owls 



SPECIES ACCOUNTS 



Typical Owls 



appear to be the case in the state's coastal redwood belt 
(G.I. Gould, Jr., in litt.). 

The structural characteristics of superior forest habitat 
for the owls are moderate to high canopy closure (60%- 
80%); a multilayered, multispecies canopy dominated by 
large (30 in. diameter at breast height) overstory conifer 
trees and an understory of shade-tolerant conifers or hard- 
woods; a high incidence of large tall trees with large 
cavities, broken tops, dwarf mistletoe infections, or plat- 
forms of branches capable of accumulating organic matter 
suitable for use as a nest; numerous large standing dead 
trees; a forest floor with heavy accumulations of logs, dead 
limbs, and other woody debris to support abundant prey 
populations; and a canopy open enough to allow the owls 
to fly within and beneath it (Thomas et al. 1990). In 
California, 90% of the Spotted Owls surveyed by Gould 
(1977) inhabited forests with at least 40% canopy closure, 
and most favored northern exposures; 89% of their terri- 
tories were on the lower slopes of canyons, and 90% were 
within 330 feet of a water course. 

One reason Spotted Owls may prefer older forests is 
because the layered structure of the canopy provides a 
range of roosting environments and hence the most pro- 
tection under a variety of weather conditions (Forsman et 
al. 1984). In Oregon, these owls roost in the day, primarily 
on limbs in trees or large woody shrubs, but also on limbs 
or logs on the forest floor. Owls diere use large trees in the 
forest overstory for roosting during cool or wet weather and 
smaller trees or shrubs in the forest understory during 
warm weather. Despite the use of roosts low in the under- 
story during warm weather, the majority of roost sites in 
spring and summer at one Oregon study area had south- 
ern exposures. During rainy or snowy weather, these owls 
tend to roost against the trunk or under the shelter of an 
overhead projection of the tree. In California, Spotted 
Owls tend to use the same roost trees repeatedly through- 
out the summer but shift roost sites frequendy during 
winter (Barrows 1981). Spotted Owls there tend to select 
roosts in cooler microclimates in the lower portion of the 
canopy in summer, apparendy to reduce heat stress (Bar- 
rows 6k Barrows 1978, Barrows 1981). Summer roosts in 
California tend to have dense canopies above and to be on 
north-facing slopes and in ravines; winter roosts are more 
variable and do not share these characteristics. 

Like most owls, Spotted Owls do not build nests. 
Instead they lay their eggs in natural cavities or on elevated 
natural platforms (Bent 1938, Forsman et al. 1984, Johns- 
gard 1988, Voous 1988, Thomas et al. 1990). Nests 
typically are inaccessible and have well shaded, cool micro- 
climates. Suitable cavities include those at the top of 
brokenoff trees, ones lower in the tnink of live or dead 
conifers or hardwoods, potholes or cavities of rocky cliffs, 
shelves of larger caves, and washouts in clay banks. Some 
nest sites in cliffs are on the remains of abandoned Com- 



mon Raven or Golden Eagle nests. Cliff nests are reported 
mosdy from southern California and the Southwest, where 
tree cavity nests also seem frequent (Bent 1938). Truly 
exceptional nests were one on bare ground at the base of 
a large rock (Bent 1938) and another on a pigeon coop 
(Johnsgard 1988). 

The platform nests the owls use in trees may simply be 
naturally accumulated debris. More often these are old 
nests of raptors (Red-tailed Hawk, Northern Goshawk, or 
Cooper's Hawk) or arboreal mammals (squirrels or wood- 
rats), often built among the dense clusters of deformed 
limbs ("witches-brooms") of old trees infected with dwarf 
misdetoe (Forsman et al. 1984). Platform nests tend to be 
next to or close to the trunk. Within the range of the 
Northern Spotted Owl in Washington, Oregon, and 
northern California, there is geographical variation in the 
predominance of platform versus tree cavity nests, presum- 
ably reflecting regional availability of these types of sites 
(Forsman et al. 1984, Thomas et al. 1990). Spotted Owls 
in Marin County select both tree cavity and platform nests, 
but it is not established if one type is used here more often 
than the other. In Oregon, Forsman et al. (1984) reported 
that the height of tree cavity nests averaged 99 feet (range 
38-181 ft., n = 30) and platform nests averaged 72 feet 
above the ground (range 33-123 ft., n = 17). Nests there 
tend to be located on the lower slopes of hillsides and 
widiin about 800 feet of water (Forsman et al. 1984), a 
pattern similar to diat noted above for territories in Cali- 
fornia (Gould 1977), which also seems to hold for Marin 
County. It is unclear if the owls select nest sites close to 
water for use in drinking or bathing or whether other 
factors they select for coincidentally tend to occur near 
water. Johnsgard (1988) reported an average nest height of 
31 feet for a small sample (n = 13) of sites, including cliff 
ledges or cavities, tree platforms, tree cavities, and one 
artificial structure. 

Regardless of site, females scrape out a depression for 
the eggs, which may sit on bare soil; on debris such as 
rotted wood, conifer needles, pine cones, and small twigs 
left naturally or by the previous occupants; on bones, 
pellets, or feathers accumulated by the owls themselves; or 
on a mixture of the above (Bent 1938, Forsman et al. 
1984). On rare occasions, females apparendy will add a 
few sprays of green conifer needles to the nest (Forsman et 
al. 1984). 

Spotted Owls dine on a wide variety of prey, but overall 
small arboreal or semiarboreal nocturnal mammals pre- 
dominate in the diet, whether measured by numbers or 
biomass consumed (Thomas et al. 1990, n= 15,100+). 
Throughout the range, diet studies often reveal that 70%- 
90% of prey biomass is contributed by just two or three 
dominant species, such as northern flying squirrels, dusky- 
footed or bushy-tailed woodrats, and various hares or 
rabbits. Pocket gophers, red tree voles, and deer mice may 



223 



Typical Ou/ls 



MARIN COUNTY BREEDING BIRD ATLAS 



Typical Owls 



be regionally important. Broad geographic differences in 
owl diets are manifest by a predominance of flying squir- 
rels in moist conifer forests at relatively high latitudes or 
elevations and woodrats in drier conifer forests, mixed 
evergreen forests, or oak woodlands at relatively low lati- 
tudes or elevations. In California, woodrats, flying squir- 
rels, deer mice, and red tree voles or other voles are die 
main prey items by numbers. After mammals, birds (in- 
cluding small owls) are the next most important group of 
prey. Birds contributed as much as 10%- 18% of die 
number of prey items at 5 of 1 4 study sites in California. 
Various reptiles, amphibians, insects, arthropods, and 
other invertebrates are of minor importance to the diet; 
insects may be consumed in numbers in some areas but 
always represent only a small fraction of total prey biomass 
(Forsman et al. 1984, Thomas et al. 1990). In Marin 
County, 88% of 16 pellets found near Palomarin con- 
tained dusky-footed woodrats, 69% white-footed mice, 
31% Band-tailed Pigeons, and 6% Steller's Jays; other 
observations indicate that brush rabbits and Sonoma chip- 
munks also serve as prey here (Beebe & Schonewald 
1977). 

Seasonal shifts in the diet in Oregon relate to changes 
in seasonal abundance or vulnerability of prey (Forsman et 
al. 1984). Various mammals are preyed on more during 
periods of juvenile dispersal or while juveniles of larger 
species are still of manageable size; insects are taken pri- 
marily in summer and early fall. Dietary composition at 
particular sites also appears to vary among years. Compar- 
ing both percent composition and mean prey weights, 
Forsman et al. (1984) found no difference in the diet of 
males and females at two sites in Oregon. Several studies 
(Barrows 1985, 1987; citations in Thomas et al. 1990) 
have shown a positive association between prey size and 
owl reproductive success (and breeding status), but it is 
unclear whether this reflects differential capture by the owls 
or merely differential transport of large prey to nests 
(Thomas et al. 1990). 

Spotted Owls are primarily nocturnal predators that 
leave their roosts to hunt soon after sunset and return 
shortly before dawn (Forsman et al. 1984). They forage 
primarily in old-growth or mature timber, from the ground 
to the upper canopy. Beyond roosting, diurnal activity is 
usually restricted to occasional short flights to capture prey 
below the roost tree, to retrieve cached prey, to change 
roost trees, or to drink or bathe in nearby streams. Laymon 
(1991) found some pairs of owls in the Sierra Nevada 
hunting regularly in the daytime when feeding large depen- 
dent young that had already left the nest. He speculated 
that diurnal foraging may be a compensatory response of 
adults that are unable to meet the demands of feeding 
young owls solely by nocturnal foraging when food is 
limited. Just prior to and during incubation and when the 



young are small, the male supplies all the food for the 
female, young, and himself. When young are two to three 
weeks old, females begin to forage for progressively longer 
periods and farther away from the nest each night (Fors- 
man et al. 1984). The male initially brings food to the 
female (on or near the nest) and transfers the decapitated 
prey to her, beak to beak; later he will leave the food at the 
nest if the female is away. The female always feeds the 
young. 

Whether capturing prey in trees or on the ground, 
Spotted Owls usually dive on their victims from an elevated 
perch (Forsman et al. 1984). After unsuccessful first 
attempts at catching squirrels in trees, the owls often hop 
or fly from limb to limb in pursuit of the fleeing animals. 
Insects are captured either on the ground or on limbs of 
trees (rather than in the air), usually by pouncing on them 
with the feet or by landing and picking them up with the 
beak. It seems likely that most birds or bats are taken when 
active, leaving or entering roost sites, rather than when 
concealed and asleep (S.A. Laymon pers. comm.). Spotted 
Owls hold vertebrate prey in the feet and kill them by 
crushing, tearing, or breaking the base of the cranium or 
neck (Forsman et al. 1 984). They sometimes eat small prey 
whole, but usually they at least partially dismember ani- 
mals larger than deer mice and consume the heads first. 
Year round, Spotted Owls regularly cache decapitated and 
partially eaten remains of excess prey and later retrieve 
them. They securely wedge the remains for storage on top 
of limbs in trees or on the ground beside logs, trees, or 
large rocks. 

Marin Breeding Distribution 

During the adas years, Spotted Owls were found breeding 
primarily on Inverness Ridge, Bolinas Ridge, canyons of 
the Mount Tamalpais watershed, and nearby ridges north 
to the vicinity of San Geronimo and Tocaloma. This 
distribution mirrors that of Marin's old-growth and mature 
conifer and associated mixed evergreen forests. Represen- 
tative breeding locations were Palomarin, PRNS (NE-NY 
4/12-6/5/77 -GBe, SJ, BSo); near Phoenix Lake (ON- 
NY 3/14-4/18/76 -RMS); and Bolinas Ridge along Boli- 
nas Lagoon (NY 6/2/81 -ARo, DS). 

As part of a study of Spotted Owl vocalizations, Seth 
Bunnell in 1989 surveyed all of Inverness Ridge and 
limited parts of the remainder of West Marin for these 
owls. He located 19 pairs and knew of five other sites 
where they had been reported previously. He estimates that 
more complete surveys of all potential habitat in Marin 
would reveal at least 30 pairs of Spotted Owls (S. Bunnell 
pers. comm.). This compares with another recent estimate 
of 25 pairs in the county (USFWS 1987a). 



224 



Typical Owls 



SPECIES ACCOUNTS 



Typical Owls 



Historical Trends/ Population Threats 

Early in this century, the true status of this owl was cloaked 
in mystery. Mailliard (1900) did not include the Spotted 
Owl among the owls listed in his avifaunal summary of 
Marin County's landbirds. Reporting on the San Fran- 
cisco Bay region, Grinnell and Wythe (1927) considered 
the Spotted Owl "very rare," with only three records for the 
area, all from Marin County. Stephens and Pringle (1933) 
added four additional Marin County records, all from near 
Phoenix Lake, 1931 to 1933. With the accumulation of 
additional knowledge, Grinnell and Miller (1944) con- 
cluded that numbers of this owl in California were "no- 
where large ... at best to be rated as only fairly common." 
They further surmised that "no change in range or num- 
bers [were] apparent from data in hand." Surely many of 
these owls had already been displaced at that time by the 
lumbering activities that now threaten the species' exis- 
tence. Population decline has undoubtedly been greater in 
recent decades because most timber cutting in California 
has occurred since the end of World War II in 1945 
(Thomas et al. 1990). From 1973 to 1977, Gould (1977) 
conducted the first statewide survey of Spotted Owls in 
California. He found evidence of 122 pairs of Northern 
Spotted Owls and 195 pairs of California Spotted Owls, 
largely within the range previously described by Grinnell 
and Miller (1944). Gould was unable to estimate the total 
size of the California population, but, applying conserva- 
tive assumptions to data from resurveys of historical sites, 
he concluded that the state's population had declined a 
minimum of 28% over prior levels. 

Based solely on inventories during the period 1985 to 
1989, Thomas et al. (1990) documented a minimum of 
2022 pairs of Northern Spotted Owls in Washington 
(360), Oregon (1129), and northern California (533). 
They suspected the true number was somewhere between 
3000 to 4000 pairs. From 1973 to January 1991, 1392 
Northern Spotted Owl territories and 1439 California 
Spotted Owl territories (1142 Sierra Nevada, 297 s. Calif.) 
have been located in California (G.I. Gould, Jr., in litt.). 
These figures do not represent breeding pairs, as they have 
not yet been adjusted for habitat loss at some sites, lack of 
surveys in about 20% of the range, intermittent occupation 
of some sites, sites maintaining only single owls, and 
territories with pairs that rarely breed. Because data are 
lacking on the size of the historic population, the true 
extent of the decline of the Northern Spotted Owl is 
unknown. Nevertheless, population reduction has been 
severe. Outspoken concern for the viability of the popula- 
tion ultimately led to protection in 1990 when the subspe- 
cies was finally listed as federally Threatened. The 
California Spotted Owl remains a Candidate (Category 2) 



for federal listing as Threatened or Endangered (USFWS 
1991) and a Bird Species of Special Concern in California 
(Remsen 1978, CDFG 1991b). Neither subspecies is in 
immediate danger of extinction in California, but there is 
major concern at the lack of a regulatory mechanism to 
ensure that continued habitat loss will not fragment the 
population into small isolated groups (G.I. Gould, Jr., in 
litt.). The population decline is attributed to habitat reduc- 
tion from clearing for agriculture, urban development, 
natural events such as fire and windstorms, and most 
importandy from logging (Thomas et al. 1990). By some 
estimates (perhaps conservative), forest habitat of this owl 
has been reduced by 60% since 1800 (mosdy since 1900) 
and continues to be lost at a rate of l%-2% per year. 
Beyond habitat loss, forest fragmentation can lead to edge 
effects, such as increased blowdowns of large trees during 
storms; higher predation rates on Spotted Owls by Great 
Horned Owls; competition with Barred Owls now rapidly 
expanding into the Spotted Owl range; and potential loss 
of habitat or microhabitats that lessen the effects of 
weather, provide habitat for prey species, or serve as refugia 
during catastrophic events. Also, fragmentation can 
increase the risks of local extirpation because of the greater 
likelihood that small populations will exhibit or be affected 
by loss of genetic variability, deleterious demographic pat- 
terns (such as skewed sex or age ratios or poor reproductive 
success), environmental variation, or the inability of dis- 
persing individuals to find and recolonize suitable habitat. 
Although most Spotted Owls here reside in protected 
parklands, isolation of the Marin County population, 
among others, is of concern for the above reasons (USFWS 
1989c). 

The Interagency Scientific Committee to Address the 
Conservation of the Northern Spotted Owl has proposed 
a management strategy based on a network of Habitat 
Conservation Areas throughout the subspecies' range 
(Thomas et al. 1990). Where possible, these areas should 
support 20 pairs of owls and should be separated from 
other areas by a maximum distance of 12 miles, features 
designed to minimize the effects of habitat fragmentation. 
Although the Forest Service has endorsed this strategy, it 
has yet to complete its own recovery plan. In the meantime, 
political forces are proposing plans more favorable to the 
timber industry and also attempting to weaken the Endan- 
gered Species Act (Liverman 1 990). The outcome of this 
struggle is a true test of our society's ability to adequately 
cope with the stark and obvious realities that natural 
resources are limited and that we are not alone on this 
planet. 



225 



Typical Owls 



MARIN COUNTY BREEDING BIRD ATLAS 



Typical Owls 



LONG-EARED OWL Asio otus 



U^\\'-'P^\^ 


>>s^^ \ ^Oa_ 


Occurs irregularly year round, though pri- 
marily as an irregular winter resident 
from late Sep through March. 
A very rare, very local breeder; overall 


\*\\^ \XjO 


f^V>^r^c3^^Vv^cA 


breeding population very small. 

Recorded in 2 (0.9%) of 221 blocks. 


\\ l 


^-^\V A^ • \**^ \ ^^\^^V^^V^'\ ^---V^^V ----dr^^v^kV^^X^^^^-v 


O Possible = (0%) 




^S^S^ 


© Probable = 1 (50%) 




^\\^c^)^ •-=" 


• Confirmed = 1 (50%) 






FSAR = 1 OPI = 2 CI = 2.50 



Ecological Requirements 

Regardless of its uncanny ability to conceal itself during the 
day, this sleek medium-sized owl is one of the most elusive 
and puzzling of this mysterious clan. In some regions, it is 
numerous and breeds with regularity (Marks 1986); else- 
where, birds are nomadic, breeding when and where prey 
populations permit. Hence owl populations fluctuate 
markedly from year to year at some sites, and at others 
birds breed only irregularly (Voous 1988). Marin County 
appears to fall into the latter category. Long-eared Owls 
often hunt in open country of grasslands, meadows, and 
fields, where they seek small mammal prey, particularly 
microtine rodents. Apparendy they also forage in open 
conifer stands or along their edges (Bull et al. 1989). 
Studies in Europe have linked fluctuations of Long-eared 
Owl population densities and clutch sizes to annual varia- 
tions in microtine prey populations (Johnsgard 1988, 
Voous 1988). Such a link has not yet been documented in 
North America (Marks 1986), but, given dietary prefer- 
ences here (see below), it undoubtedly will be widi further 
work. Although no studies were conducted on their mam- 
mal prey, confirmed and apparent breeding of Long-eared 
Owls (and Short-eared Owls) in Marin County during the 
adas period coincided with a population peak of voles 
(Microtus). These mice were so abundant that year that 
observers often found them scurrying underfoot in the 
daytime Q.G. Evens pers. comm.). At the lone site where 
Long-eareds were confirmed breeding in Marin County 
during the adas period, the owls nested in a nonnative pine 
plantation surrounded by grassland and scattered stands 
of open coastal scrub. They were also seen displaying at the 

226 



edge of a coast live oak and California bay-dominated 
mixed evergreen forest bordering on open coastal scrub 
mixed with grassland. Hence, the main nesting require- 
ments of Long-eared Owls are open or semiopen short- 
grass or sparsely vegetated foraging areas, an abundance of 
small mammal prey, and suitable nesting and day-roost 
sites on the edges of adjacent dense forests, thickets, or 
planted woodlots (Getz 1961, Marks 1986, Johnsgard 
1988, Voous 1988). These owls do not select specific plant 
communities for breeding in California (GckM 1944). 
Nesting birds range from coastal lowlands to interior 
deserts and seem to favor riparian groves, planted wood- 
lots, and belts of live oaks paralleling stream courses, 
perhaps because these habitats tend to occur in the fertile 
bottomlands most often used for foraging. Long-eareds 
probable also nest in conifer forests in California, as they 
do in Oregon (Bull et al. 1 989). Dense cover is important 
to inhibit nest predation (Marks 1986, Bull et al. 1989) 
and predation of adults by Great Horned Owls (Voous 
1988). 

Long-eared Owls usually lay their eggs in vacant stick 
nests of crows, magpies, ravens, Buteo or Accipiter hawks, 
other birds, or squirrels in trees or large bushes (Bent 
1938, Stophlet 1959, Craig &. Trost 1979, Marks 6k 
Yensen 1980, Marks 1986, Johnsgard 1988, Voous 1988). 
In Oregon's conifer forests, they prefer to nest in dwarf 
misdetoe brooms (Bull et al. 1989). Rarely, they nest in 
shallow cavities in trees or stumps, in natural cavities in 
cliffs, in semiopen nest boxes or baskets constructed for 
other birds, or on the ground in sheltering vegetation 



Typical Owls 



SPECIES ACCOUNTS 



Typical Owls 



(references above). There is limited evidence that Long' 
eareds occasionally modify nests by adding sticks or small 
amounts of nest lining (Bent 1938, Craig <&. Trost 1979, 
Voous 1988, Bull et al. 1989). Maximum nest height is 
about 61 feet, and average nest heights in various studies 
range from 7 to 32 feet (Stophlet 1959, Craig <St Trost 
1979, Marks & Yensen 1980, Peck &. James 1983, Marks 
1986, Johnsgard 1988, Bull et al. 1989) and probably vary 
locally with the stature of the vegetation and available stick 
nests or other nest sites. In Idaho, Long-eareds prefer 
partially canopied Black-billed Magpie nests over open 
magpie or crow nests, presumably because the former 
provide additional concealment (Marks 1986). Corvid 
nests used by the owls there tended to be wider and slighdy 
higher above ground than unused corvid nests; wider nests 
provide ample room for eggs and young and probably 
better concealment from ground-based predators. Nests 
there are usually near midheight in trees, and in clumps of 
trees rather than in isolated trees or rows of trees. Nesting 
on the edge of clumps may reflect the availability of corvid 
nests (Marks 1986) and/or selection for proximity to open 
foraging areas. In Oregon's conifer forests, Long-eareds 
appear to nest in the smallest, most inconspicuous plat- 
forms that can accommodate them (Bull et al. 1 989). There 
the average distance of nests from openings (2.5 acres in 
size) is 344 feet. Probably because birds do not defend 
foraging territories (Voous 1988), Long-eared Owls some- 
times nest in loose "colonies" of three to ten pairs; nests 
are as close as 52 feet in Idaho (Marks 1985). Some owls 
return to reuse the previous year's nest site, particularly if 
they were successful the year before (Marks 1 986). 

Long-eared Owls are primarily nocturnal, but occasion- 
ally they hunt in daylight at far-northern latitudes, or when 
they are forced to by exceptionally short food supplies at 
the end of winter (Voous 1988). The owls typically start 
hunting 25 to 30 minutes after sunset and retire 30 to 45 
minutes before sunrise, with peak activity in the few hours 
after activity starts and before it diminishes. These owls 
have very acute hearing and a highly perfected ability to 
locate prey by sound. Widi their light bodies and long 
wings, Long-eared Owls hunt from buoyant, moddike 
flight resembling a nocturnal harrier. This enables them to 
hunt and hover efficiendy low or high, and to capture prey 
in the short vegetation of open fields, farmlands, grass- 
lands, marshes, and deserts (Marti 1976, Voous 1988). In 
winter, they sometimes hover over bushes to try to force 
out communally roosting landbirds (Voous 1988). 

Long-eared Owls have a restricted diet. They specialize 
in capturing voles and mice, and, unlike some species, 
most Long-eareds leave the area to breed or winter else- 
where in times of food shortage (Voous 1988). Aldiough 
the diet varies between locations and habitats, it typically 
consists of relatively few species of small mammals and 
varies little over time. Mammals (of 45 species) account for 



98.2% of 23,888 prey items in North America and voles 
and deer mice make up 82.2% of the total; mammals as 
large as young rabbits are taken (Marti 1976). Voles 
account for 53.7% of total prey here and, as the dominant 
species, range from 29.8% to 94-4% of total prey items in 
local studies. Deer mice, pocket mice, pocket gophers, and 
kangaroo rats have also been the dominant prey in some 
studies here (Marti 1976, Craig & Trost 1979, Marks 
1984, Barrows 1989, Bull et al. 1989). The preponderance 
of voles in the North American diet may be in part an 
artifact of limited data on the diet in deserts or conifer 
forests (Marks 1984, Bull et al. 1989). In the Great Basin 
Desert of Idaho, Long-eared Owls feed on a greater diver- 
sity of small mammals than in other North American 
studies and predominandy on three genera of mammals 
rather than on one or two, as is the case in studies 
elsewhere (Marks 1984). In the deserts of soudiern Cali- 
fornia, the diet of Long-eared Owls in winter is about 54% 
pocket mice and 32% kangaroo rats (Barrows 1989, n = 
956). In Oregon's conifer forests, the diet of breeding 
Long-eareds is predominandy pocket gophers, with voles a 
distant second (Bull et al. 1989, n = 1123). Small to 
medium-sized birds represent 1.7% of North American 
prey items (35 species, Marti 1976), including birds as 
large as quail, Ruffed Grouse, and Mourning Doves (Bent 
1938). Bats, reptiles, amphibians, fish, crayfish, and 
insects are very rarely taken (Marti 1976, Marks 1984). In 
Europe, birds may be important prey for Long-eared Owls 
in winter, with a higher proportion of birds taken in poor 
vole years (Voous 1988). Prey size of mammals rather than 
prey type may be the most important factor in prey selec- 
tion (Marks 1984). In North America, most prey weigh 
less than 3.5 ounces (Marti 1976, Marks 1984), and 
average prey weight is 1 .3 ounces (range 0.04-28 oz., Marti 
1976, n = 23,888). In Colorado, average prey weight is 1.1 
ounces for Long-eared Owls, compared with 1 .6 ounces for 
Barn Owls, and 6.2 ounces for Great Horned Owls (Marti 
1974). An increase in pocket mice in the postfledging diet 
in Idaho may reflect a corresponding increase in numbers 
of diese mice in the environment from spring to summer, 
or, alternatively, young owls may have begun to capture 
prey at that time and either have a preference for smaller 
prey or have more difficulty catching other species of mice 
(Marks 1984). Insects, though rare in the diet, are more 
frequendy taken in the postfledging period, suggesting they 
are captured by young owls. Although females are slighdy 
larger and heavier dian males and have much stronger and 
heavier talons, there is as yet no evidence of differential use 
of prey or hunting niches between the sexes (Earhart & 
Johnson 1970, Voous 1988). 

Although die female may occasionally hunt early in the 
incubation period (Johnsgard 1988), the male does most 
of the hunting for the female and the young (Voous 1988). 
The eggs hatch asynchronously over an extended period— 

227 



Typical Owls 



MARIN COUNTY BREEDING BIRD ATLAS 



Typical Owls 



up to 1 1 or 1 2 days in a nest with six owlets (Johnsgard 
1988). As an adaptation to predation, the young leave the 
nest after about three weeks of age to climb about the 
branches, roosting solitarily in dense foliage until attaining 
flight and leaving the nest area at about five weeks (Marks 
1986). Hence, where pairs nest close together, broods can 
mix and perhaps are sometimes fed by adults other than 
their parents (Marks 1985). On rare occasions, second 
clutches are reported (Voous 1988). 

Marin Breeding Distribution 

During the adas period, Long-eared Owls were confirmed 
breeding by the observation on 12 May 1979 of three 
adults and a full-sized juvenile (with some down still in its 
plumage) at a pine plantation along the Estero Trail at the 
head of Home Bay, PRNS (JLo et al.). In addition, a 
territorial bird was at Palomarin, PRNS, from 9 May to 1 3 
July 1979 (PRBO). Because of their almost stricdy nocturnal 
habits, their propensity for irregular breeding, and the fact 
that winter visitant Long-eareds still gather at communal 
roosts at a time when local breeding birds are already 
incubating in March or early April (Voous 1988), it can be 
difficult to confirm breeding of this species. We likely 
would have confirmed breeding for this species in more 
blocks if we had concentrated our efforts at the first sign of 
the population explosion of voles on the Point Reyes 
peninsula in 1979, or soon after stumbling upon the one 



nest that year. Subsequendy, a nest with five eggs was 
found at the pine plantation at Home Bay on 3 May 1983 
(CCu fide JE). The only known prior breeding record for 
Marin County was of a nest with eggs observed in an oak 
near Novato on 6 and 20 April 1904 (Ray 1904). 

Historical Trends/ Population Threats 

Although they considered the species "common" or even 
"abundant" locally, Grinnell and Miller (1944) noted a 
reduction in Long-eared Owl numbers in California, 
apparendy mosdy because of the clearing of bottomlands 
for farming. Numbers have continued to decline, and the 
species is currendy on the state's list of Bird Species of 
Special Concern (Remsen 1978, CDFG 1991b). This owl 
was also on the Audubon Society's Blue List in 1980 (Tate 
1981). In addition to the destruction of riparian habitat, 
causes of the decline in California may be collisions with 
traffic, shooting and harassment at nest sites (Remsen 
1978), and land use changes that have caused reductions 
of the small mammal prey base. Long-eared Owls have 
been lethaHy contaminated by heavy metals, insecticides, 
fungicides, rodenticides, and PCBs, but the overall effect on 
their populations has been less severe than in other species 
of owls and diurnal birds of prey, probabiy because, unlike 
buteo hawks, Long-eared Owls do not eat carrion (Voous 
1988). 




228 



Typical Owls 



SPECIES ACCOUNTS 



Typical Owls 



SHORT-EARED OWL Asioflammeus 







Almost exclusively an irregular winter res- 




iP^r^-^ ^ ^ 


ident from mid-Sep through Mar; excep- 


-4. ^^\' \ jgS*^? 




tional in summer. 


Vfl 


yc\^r\\\ 


A very rare, very local breeder; overall 
breeding population very small. 




^^^r^r\^^^^^<^C^\ 


Recorded in 1 (0.4%) of 221 blocks. 




-^vs^vV^^V \^\ \^<\ \^*\ \ ^--V'x ^c^\ 


O Possible = (0%) 




}k'T>^^ 


© Probable = (0%) 




^ar-'V'v '' 


• Confirmed = 1 (100%) 




j^Wh$^^^PC\^^ 


FSAR =1 OPI = 1 CI = 3.00 









Ecological Requirements 

This arch nomad wanders and migrates only to settle 
where it finds high vole populations, gathering sometimes 
by the hundreds and staying to nest only as long as the 
food abundance lasts (Clark 1975, Voous 1988). Although 
Short-eared Owls are resident in some suitable marsh- 
lands, these habitats are increasingly rare; thus the species 
is an unpredictable migrant in most of its range. These 
owls occur irregularly in Marin County, and since they are 
known to have bred here only once (see below), it is 
difficult to describe local breeding habitat preferences. 
Though the exact site of the Marin breeding record is 
unknown, it was generally in an area where dunes, coastal 
swales, and grasslands intermingle on Point Reyes. In 
California, these owls breed in fresh, brackish, and salt 
marshes; in lowland meadows and moist grasslands; in 
irrigated fields; and in fallow or stubble fields (G&M 
1944, D. Shuford pers. obs.). The main requirements for 
breeding are low nesting and resting cover and open 
foraging grounds supporting an abundance of small mam- 
mals, particularly voles (Clark 1975, Johnsgard 1988, 
Voous 1988). 

Short-eared Owls defend breeding territories that vary 
inversely in size with prey abundance and sometimes nest 
close enough together to suggest the existence of nesting 
"colonies" (Clark 1975, Voous 1988). They typically lay 
their eggs in depressions on the ground, and, unlike most 
owls, they construct their own nests (Townsend 1938, 
Clark 1975, Johnsgard 1988, Voous 1938). Birds usually 
make a scrape in the substrate in open country and line it 
sparsely with grass stems, weed or stubble stalks, thin 



twigs, or other vegetation, sometimes gathered at some 
distance from the nest. On occasion, nest sites may be 
mere scrapes in the ground or only the flattened or dead 
vegetation of the spot chosen. Nests may be entirely 
exposed to the light but more often are shielded by clumps 
of grasses, weeds, grains, or low-growing marsh vegetation. 
Short-eareds tend to pick drier sites for their nests than 
Northern Harriers do (Clark 1975). Very rarely, nests have 
been found in a patch of low bushes, on the top of a 
broken tree stump in a clearing, in old crows' nests, on a 
ledge, and apparendy in burrows or cavities in the ground 
(Townsend 1938, Clark 1975, Voous 1988). Exception- 
ally, birds will nest in the exact site in successive years 
(Townsend 1938). As an adaptation of ground nesting to 
avoid predation, the young, after hatching asynchronously, 
leave the nest at the early age of 12 to 16 days, scattering 
around in the safety of "runs" they make in the vegetation 
(Clark 1975, Johnsgard 1988, Voous 1988). Chicks may 
disperse up to nearly 200 yards in four days. 

Short-eared Owls are primarily crepuscular and second- 
arily nocturnal hunters, diough they also forage more in 
the day than do most medium-sized owls (Clark 1975, 
Johnsgard 1988, Voous 1988). These owls apparendy 
forage in the daytime mostly when they are unable to 
obtain enough preferred foods in the night, for example 
when feeding growing young; birds breeding at extreme 
northern latitudes must of necessity forage during daylight 
hours. Short-eared Owls seem somewhat less dependent 
on hearing for hunting than are Long-eared Owls, though 
dieir hearing is more acute than that of their diurnal 



229 



Typical Owls 



MARIN COUNTY BREEDING BIRD ATIAS 



Typical Ou/ls 



ecological counterparts, Northern I larriers (Voous 1 988). 
The flight of Short-eared Owls seems even lighter, more 
huoyant and mothlike than that of Long-eared Owls. Short- 
eareds take most prey by surprise as they quarter low 
(mostly 1 -6.5 ft., rarely over 10 ft.) over fields and marshes 
in slow tilting flight, alternately flapping and gliding on stiff 
wings (Townsend 1938, Johnston 1956b, Clark 1975, 
Johnsgard 1988, Voous 1988). In such coursing flights, 
the owls usually head into the wind and often hover 
momentarily before pouncing (Clark 1975). If they over- 
shoot their prey, they frequently turn 180° and pounce, 
seemingly headfirst, heading downwind. With no wind, 
the owls seem to prefer to hunt into or away from the light 
rather dian at right angles to the light. Short-eared Owls 
also hunt from extended ternlike hovering flight (about 
6-100 ft. in height), maintaining themselves in one place 
by varying amounts of wing flapping. When hovering, the 
birds descend rapidly by raising the wings in a deep 
dihedral. This may be followed quickly by pouncing, or by 
checking the flight at a lower height before pouncing or 
flying elsewhere to hunt. Clark (1975) felt hovering was 
not a response to wind, but radier is used when prey is 
scarce. Occasionally diese owls land on fence posts, tree 
stubs, grass tufts, or the ground and wait for their prey to 
appear before launching an attack. This technique is fre- 
quendy used when weadier conditions are unfavorable. In 
interspecific encounters widi harriers, buteo hawks, and 
falcons, Short-eared Owls may rob them of dieir prey, or 
vice versa (Voous 1 988). 

Short-eared Owls specialize in catching voles, lemmings, 
and mice, though diey adapt to a variety of odier prey when 
local conditions dictate. The year-round diet (data mosdy 
from winter) in North America is 94-8% mammals, 5.1% 
birds, and 0.1% insects, frogs, and lizards (Clark 1975, n = 
9640). Voles (Microtus) account for 60.6% of die mammal 
prey, but pocket gophers, deer mice, pocket mice, brown 
lemmings, Norway rats, and numerous small terrestrial 
species are also taken. Clark (1975) thought that Short- 
eared Owls did not really prefer voles but instead opportu- 
nistically took whatever prey species were most available in 
open habitat. Colvin and Spaulding (1983), on the other 
hand, felt that the owls preferred the larger voles over 
smaller deer mice for energetic reasons and concentrated 
their hunting times during major periods of vole activity. 
In North America, at least 60 species of bird prey have 
been recorded, especially open-country and marsh species, 
such as Red-winged Blackbirds, Western Meadowlarks, 
Horned Larks, various sparrows, shorebirds, and rails. 
Many birds are probably captured at night (Clark 1975) or 
at least when there is little light. Birds are taken much more 
extensively during migration or in winter (e.g., Page 6k 
Whitacre 1975) and in coastal or island situations (Clark 
1975). The owls sometimes prey on birds at seabird 
colonies (Townsend 1938, Voous 1988). Insect remains 



found in pellets of young in poor vole years are probably 
captured by the young while walking around. It is doubtful 
diat parents would bodier carrying insects to them (Clark 
1975). Average prey weight of Short-eared Owls is about 
1.2 ounces versus 1.05-1.1 ounces for Long-eared Owls 
(Voous 1988). 

The male hunts for the female while she is involved in 
egg laying, brooding, and caring for the young (Clark 
1975, Voous 1988). The female flies out to retrieve food 
from die male. Consumption of die prey is begun with the 
head. The female initially tears off pieces to feed to the 
begging owlets. The female usually begins foraging again 
when die chicks first scatter from the nest, well before 
fledging; in good vole years, when the male is apparendy 
able to provide enough food for both the female and 
young, she remains close to the nest throughout nesting. 
When food is plentiful, surplus prey is often cached at the 
rim of the nest. Clutch size varies with die food supply, but 
birds only rarely lay second clutches and raise second 
broods, even in years of vole abundance. 

Marin Breeding Distribution 

During the adas period, Short-eared Owls were confirmed 
breeding by the observation of up to seven owls, including 
diree recendy fledged young (with traces of white down in 
dieir plumage), near the north entrance to Point Reyes 
Beach, PRNS, from 24 to 26 June 1979 (S6kMK et al.). 
This is the only known breeding record for Marin County 
and it coincided widi the 1978-79 vole (Microtus californi- 
cus) outbreak on Point Reyes which also appeared to 
induce breeding efforts of Long-eared Owls nearby. 

Historical Trends/ Population Threats 

Grinnell and Miller (1944) commented on a decline in 
winter numbers from shooting by duck hunters but made 
no mention of a change in status of breeding birds. 
Nevertheless, the elimination of 60%-95% of the former 
marshlands around San Francisco Bay (Nichols 6k Wright 
1971, Josselyn 1983) and habitat loss elsewhere undoubt- 
edly have gready reduced both summer and winter num- 
bers. The Short-eared Owl has been on the Audubon 
Society's Blue List from 1976 to 1986 (Tate 1981, 1986; 
Tate 6k Tate 1982). It is also currendy on California's list 
of Bird Species of Special Concern (Remsen 1978, CDFG 
1991b). Numbers of Short-eared Owls were relatively sta- 
ble on Breeding Bird Surveys in California from 1968 to 
1989 (USFWS unpubl. analyses). Grazing may be another 
factor responsible for the decline of the species in the state. 
Where Short-eared Owls nest in extensive farmlands, nests 
and young often are destroyed by mechanized farm equip- 
ment (Voous 1988). Pesticide residues have been found in 
tissues and eggs of Short-eared Owls, but the effects of 
diese contaminants on owl populations are unknown 
(Marti 6k Marks 1989). 



230 



Typical Owls 



SPECIES ACCOUNTS 



Typical Owls 



NORTHERN SAW- WHET OWL Aegolius acadicus 



-'vgt^. 






A year-round resident; numbers appar- 
endy swell somewhat from Sep through 


J\\^\^k > 


J^>s^^ \ JC\, 




Mar. 
An uncommon, local breeder; overall 


r\vh 


'\L^V\13c^\°^ArOr^\JV-Avir^^\ 




breeding population very small. 




X^s^^^i^^c^kA 




Recorded in 32 (14.5%) of 221 blocks. 


r-^' 


\±^(^^\^\^^\ ^Jr^V \^\ \^\ \-^\ 




O Possible = 21 (66%) 








€ Probable = 9 (28%) 






A^Va 


• Confirmed = 2 (6%) 
FSAR = 2 OPI = 64 CI = 1.41 




3r \^w^&-Va^ 


^s5^^ 






■ 1 ^-^#\ J^<$^>^r\ ° 








.>fe=> ~X^_/ ^^~<j 


Pv^V^AO 





Ecological Requirements 

An observer imitating the repetitive, penetrating whistled 
calls of a Saw-whet Owl in the heart of its territory is likely 
as not to be aggressively dive-bombed as the occupant 
challenges the intruder. Saw-whet Owls inhabit Marin 
County's Douglas fir, bishop pine, coast redwood, and 
mixed evergreen forests and woodlands, as well as her 
alder riparian thickets. These habitats are all relatively 
moist and have diverse ground cover with variably open or 
fairly dense understories of shrubs and ferns. Mixed ever- 
green forests used by these owls vary from a mixture of 
dominant trees to pure bay laurel stands. Throughout their 
range, these owls are usually found in conifer-dominated 
zones, but often where there is a broadleaved component 
to the forest (Johnsgard 1988, Voous 1988). 

Saw-whet Owls most frequently nest in deserted wood- 
pecker cavities and also in namral tree cavities and nest 
boxes, including the spacious ones designed for Wood 
Ducks (Bent 1938, Johnsgard 1988, Voous 1988). Nest 
trees may be either broadleaved or coniferous species; 
cavities may be in snags or dead stubs of live trees. The 
woodpecker cavities used are those of the larger species- 
Northern Flickers, Pileated Woodpeckers, and Hairy 
Woodpeckers— with entrances at least 2.8 inches in diam- 
eter (Johnsgard 1988). Nest heights probably reflect what 
is available, principally from the dominant large woodpeck- 
ers in a given area; an extreme nest height was 63 feet 
above the ground (Bent 1 938). In Ontario, the height of 
13 nests ranged from 8 to 44 feet above the ground, with 
most from 12 to 20 feet (Peck 6k James 1983). Like most 
owls, Saw-whets do not build nests; instead they lay their 



eggs on the bottoms of the cavities on wood chips, other 
naturally accumulated debris, or the nest materials of 
previous rodent occupants, mixed with feathers of the owls 
(Bent 1938, Johnsgard 1988, Voous 1988). 

Saw-whet Owls are considered stricdy nocturnal in their 
activity patterns. By day they roost in thick patches of 
forests in dense layers of foliage (or stranded debris) in tree 
canopies or under cover of vines in bushes (Bent 1938, 
Johnsgard 1988, Voous 1988). They generally prefer roost 
sites between 6 and 12 feet, though rarely as low as 6 
inches, off the ground. Saw-whets forage at irregular inter- 
vals through the night, apparently largely within forests or 
woodlands or at their edges (Randle 6k Austing 1952, 
Forbes 6k Warner 1974, Johnsgard 1988, Voous 1988). 
Their highly developed hearing suggests diat they can 
capture prey in total or near-total darkness (Johnsgard 
1988). Saw-whets have bodies radier light in weight relative 
to the surface area of their wings, which provides for good 
maneuverability and allows them to hunt in somewhat 
shrub-dominated cover. There are few observations of 
actual foraging behavior, but apparendy birds hunt mosdy 
from relatively low perches and swoop down to the ground 
or, less frequendy, into the foliage of trees to catch their 
prey. 

The Saw-whet Owl diet is about 96.8% mammals, 1 .6% 
birds, 1.4% insects, and 0.2% frogs (Snyder 6k Wiley 
1976, n = 435). The mammal prey consists mosdy of small 
terrestrial species, particularly woodland-inhabiting deer 
mice, voles, and shrews, but also other mice, small rats, 
young squirrels, flying squirrels, chipmunks, and bats 

231 



Typical Owls 



MARIN COUNTY BREEDING BIRD ATLAS 



Typical Owls 



(Bent 1 938, Johnsgard 1 988, Voous 1 988). The owls seem 
to take birds to a greater degree when their numbers 
increase during migration (Graber 1962, Catling 1971)- 
Nesting Saw-whets at Silver Valley Lakes, San Mateo 
County, relied mostly on deer mice and stored extra mice 
in the nest (Santee ck Granfield 1939). The diet varies 
between habitats and regions (Johnsgard 1988, Voous 
1988). Mean prey weight is about 0.7 to 0.8 ounce (Voous 
1988). Female Saw-whets are heavier than males, but 
differences between the sexes in average prey size or feeding 
habits have yet to be demonstrated. Since only the female 
incubates the eggs (Johnsgard 1988), the male must pro- 
vide her with food at least during this period. 

Marin Breeding Distribution 

During the adas period, Saw-whet Owls were found in the 
breeding season (Feb-Jul) mosdy in various habitats on the 
moist conifer- and mixed evergreen-dominated coastal 
ridges, particularly Inverness Ridge, where it was the most 
common owl (large or small) in moderately dense and 
dense forests. Saw-whets were also found calling locally in 
dense mixed evergreen forests in the interior of the county. 
If more nocturnal field work had been undertaken, we 
undoubtedly would have recorded Saw-whets in more 
blocks, particularly on the conifer-dominated ridges of the 



Mount Tamalpais and Lagunitas Creek watersheds. Saw- 
whets were most vocal in February and March at a time 
when local numbers are swelled by wintering birds; but 
territorial calling is apparendy restricted to breeding birds, 
there being no evidence that these owls call in regions 
where they occur only as winter residents (Johnsgard 1988, 
Voous 1988). Saw-whets can be heard calling in almost 
every month of the year; a bird at a breeding site at 
Palomarin frequendy called during the middle of the day 
(D.F. DeSante pers. comm.), though this is apparendy 
unusual (J. Winter pers. comm.). 

The only confirmed breeding records for Marin County 
were established during the adas period. A pair success- 
fully fledged seven young from a nest box on the edge of 
an evergreen hardwood forest at Palomarin, PRNS (ON-FL 
spring/summer 1 979 — PRBO), and another pair occupied 
a nest hole on the edge of an alder riparian thicket at 
Laguna Ranch, PRNS (ON 5/2-24/79 -JGE et al.). 

Historical Trends/ Population Threats 

No good information is available on any changes in status 
(G&M 1944, Johnsgard 1988, Voous 1988), but logging 
in California has likely reduced numbers of this forest- and 
woodland-inhabiting owl. 




232 



Poorwills 



SPECIES ACCOUNTS 



Poorwills 



Poorwills 

Family Caprimulgidae 



COMMON POORWILL Phalaenoptilus nuttallii 









A year-round resident; numbers appar- 




\ vfv 




endy swell in summer. 


{\M^%^X^^ 




A fairly common, very local breeder; 


JV**\ ^A^\ ~^<i\ 


^-V'X 'Jf^A .3c"\ -^""A 3^^\ 




overall breeding population very small. 
Recorded in 2 (0.9%) of 221 blocks. 










W\\yflfCA 


j^\\ ^\% y<^\ y<r\ y^\ j\ 




O Possible = 2 (100%) 


\\ L^vv^Y 


V^XA'^&o^V^V^cv 




€ Probable = (0%) 


\ \ Ar^^\ V~^^ 


^^V/a^Vv 




• Confirmed = (0%) 






^y -r- 




iwil 


vTX_ 


FSAR=3 OPI = 6 CI = 1.00 




^fTA^' 


Nf^\ \- > Sl t -0^^\ "'"fOf'X V 


^^^ 






__pz x > 









Ecological Requirements 

The burning ember-like eyeshine and mellow self-descrip- 
tive whistled calls of breeding Poorwills are trademarks of 
relatively open chaparral in the Coast Range, including 
Marin County. Poorwills also nest on gravel streambeds, 
in proximity to Lesser Nighthawks (Unglish 1929), and 
likely use clearings in open dry pine forests, recently 
burned forests, and clearcuts. They lay their eggs on the 
ground on bare soil, gravel, sparse leaf and bark litter, 
moss, or flat rocks. They often locate their nests at the base 
of a bush, grass clump, or rock overhang, diough they 
sometimes place them in the open (Aldrich 1935, Bent 
1940, Swisher 1978). Undisturbed nesdings make short- 
distance movements (<10 ft.) from the nest site, presum- 
ably for thermoregulation (avoiding wet areas or direct 
sunlight) or concealment from aerial predators (Swenson 
ck Hendricks 1983). Long-distance movement by young 
birds may be an antipredator adaptation triggered by dis- 
turbance. 

Poorwills are primarily crepuscular and nocturnal feed- 
ers. They forage from die ground or from low perches- 
rocks or fence posts— in openings such as clearings or roads 
and capture prey by short, fluttery, mothlike flycatching 
flights, returning to the ground with their victims. Appar- 



endy Poorwills capture some food on the ground (Bent 
1940). Their diet is exclusively insects, primarily small 
night-flying types such as moths, beedes, chinch bugs, 
grasshoppers, and locusts. 

Marin Breeding Distribution 

During the adas period, Poorwills were found in the 
breeding season in only two areas— on Mount Burdell, 
Novato, and on Carson Ridge. On Mount Burdell, a 
single bird was calling from a small patch of broken 
chaparral on 9 May 1981 (ScC). On Carson Ridge, up to 
six birds were calling in an extensive area of open serpen- 
tine chaparral on 21 May 1977 QGE et al.). Though 
certainly not widespread in Marin, Poorwills would un- 
doubtedly have been recorded in more locations if we had 
conducted more nighttime work in their rugged, relatively 
inaccessible breeding habitat. The only confirmed breed- 
ing record for Marin is of a nest found on 22 July 1908 
"among some fragments of serpentine rock from which we 
had burnt die low, scrubby manzanita brush the previous 
winter" (Mailliard 1909a); die site was above San Geron- 
imo, likely on or close to Carson Ridge. Recent controlled 

233 



Poonuills 



MARIN COUNTY BREEDING BIRD ATIAS 



Swifts 



burns in chaparral on Marin Municipal Water District 
lands in the Mount Tamalpais area should favor this 
species. 

Historical Trends/ Population Threats 

Historical trends are unclear, though it is likely that fire 
suppression has reduced Poorwill habitat; conversely, log- 



ging, especially clear-cutting, has increased it. Horn and 
Marshall (1975) felt that clear-cutting had increased the 
species' range in Oregon. In California, Poorwill numbers 
were relatively stable on Breeding Bird Surveys from 1968 
to 1989 but decreased from 1 980 to 1 989 (USFWS unpubl. 
analyses). 



Swifts 

Family Apodidae 



VAUX'S SWIFT Chaetura vauxi 









A summer resident from mid-Apr 




^^-^ \ JC\^ 




through mid-Oct; numbers swell notice- 


j0v> Vjr 




ably during fall migration in Sep and 


Ajx JL. 




(- 


Oct. 

A rare, very local breeder; overall breed- 


V^c^ 


Or^PvA^r^SH^Vv^PC 




ing population very small. 
Recorded in 12 (5.4%) of 221 blocks. 








O Possible = 12 (100%) 






\(<*r C *° 


€ Probable = (0%) 




i \jkvx \\r\ JrV \^\\^\o Xdv 




• Confirmed = (0%) 




Vs\i' XX\ \^\ \^\o\^f^)\Z^%^\^ 








^^p7z^"^ ^-<^_^\^--\ X<^\ 0.\>^\ \ 


^^5^^ 


FSAR=1 OPI = 12 CI = 1.00 






■<^LV>K 





Ecological Requirements 

On the coast, these airborne apparitions prefer redwood 
and Douglas fir forests, especially old-growth or fire-swept 
stands that provide decaying trees for nesting. Evidence 
from Oregon suggests that Vaux's Swifts may need old- 
growth forests to satisfy their nest site requirements (Bull 
6k Cooper 1991). 

Unlike closely related Chimney Swifts, Vauxes usually 
nest in natural settings and build their nests inside hol- 
lowed-out trees or stumps that are either heavily decayed or 
dead. In northeastern Oregon, 21 nests were found in 20 
large-diameter old-growth grand fir fAbies grandis) trees (1 7 

234 



live, 3 dead); two nests were in one tree in separate 
chambers (Bull <St Cooper 1991). All nest trees were 
hollowed-out by a fungus that decays the heartwood; 
entrance was made via holes excavated by Pileated Wood- 
peckers, as broken-topped trees, that sometimes provide 
swifts access to potential nest chambers, are scarce in most 
forests. Vaux's Swifts also nest in chimneys, water tanks, 
and even metal boilers on occasion (Taylor 1905, Bent 
1940, Baldwin 6k Hunter 1963, Baldwin 6k Zaczkowski 
1963). Tree nests are usually well below the top of the stub 
and sometimes are very close to the bottom of the cavity or 



Swifts 



SPECIES ACCOUNTS 



Su/ifts 



even below ground level if a stump has been undermined 
by fire (Dawson 1923, Bent 1940, Bull ck Cooper 1991). 
In Oregon, 19 nests averaged 6.7 feet below the entrance 
hole used for access (some had two holes) and 5.2 feet from 
the bottom of the chamber; 2 nests were 1.3 and 8.2 feet 
above the entrance hole (Bull 6k Cooper 1991). Entrance 
holes averaged 39.4 feet above ground (range 26.2-60.0 
ft.). 

Vaux's Swifts construct compact and shallow nests that 
are narrow and saucer shaped. They use small pieces of 
twigs or conifer needles that they stick together and attach 
to the wall of the nest chamber with their gluey saliva. They 
sometimes line their nests with conifer needles. Birds 
apparendy situate their nests inside chimneys or trees in 
locations providing protection from rain (Bent 1940). 

Vaux's Swifts forage on the wing in open airways at 
variable heights. Their aerial domain appears to be the 
insect-productive air space over moist forested habitat, 
canyon bottoms, stream courses, and lakes. Birds propel 
themselves with a number of very rapid strokes, then sail 
or circle with their long, narrow wings curved backward 
and downward. The diet is poorly known as the only 
evidence appears to be from examining the gullets of 
young, in one nest, which contained mosdy leafhoppers 
(Bent 1940). Presumably the diet is exclusively aerial 
insects of the general types taken by the Chimney Swift. 

Marin Breeding Distribution 

During die adas years, Vaux's Swifts appeared to nest in 
Marin County only in small numbers. Most of the breed- 
ing season sightings then were from the vicinity of red- 
wood or Douglas fir forest on the southern part of Bolinas 



Ridge. This species was seen most frequendy near Kent 
Lake, where it probably nested upslope on Bolinas Ridge 
in an area replete with dead snags and stubs of redwood 
and fir that were charred by a 1945 fire. To the best of my 
knowledge, nesting of this species has never been con- 
firmed in Marin, but breeding has been surmised from its 
nesting season occurrence here (Allen 1880, Mailliard 
1900, SckP 1933, GckM 1944, this study). Representative 
locations of breeding season sightings during the adas were 
ridge on S side of Lucas Valley Rd. (5/30/82 -BiL); 
Carson Ridge and vicinity of Kent Lake (4 on 6/5/82 
— DS, ITi); and Garden Club Canyon along Bolinas 
Lagoon (2 on 7/4/82 -DS). 

Historical Trends/ Population Threats 

Given that Vaux's Swifts nest inside large-diameter, 
decayed trees which tend to occur in old-growth stands, it 
is likely that numbers of breeding swifts have declined 
historically in Marin County (as well as in much of their 
range) because of extensive logging in the last century. The 
effect of fire suppression on availability of swift nest sites is 
unclear. Fire may damage trees that soon decay to leave 
hollow interiors. On the other hand, fire may kill many 
smaller trees that otherwise would have made good nest 
sites at maturity. Whether swifts will nest in forests opened 
by logging or fire, as long as suitable nests trees are left 
standing, or whether they require other characteristics of 
old-growth forests for nesting needs further study. Num- 
bers of Vaux's Swifts were relatively stable on Breeding 
Bird Surveys in California from 1968 to 1989 (USFWS 
unpubl. analyses). 



235 



Swifts 



MARIN COUNTY BREEDING BIRD ATLAS 



Swifts 



WHITE-THROATED SWIFT Aeronautes saxatalis 









A year-round resident. 


t^\ '■ \ ^^"\^ \ -**^7 :>< ~~^ 


\ jr^v 




An uncommon, local breeder; overall 






breeding population very small. 




\yPk\^V^\^^\^t^' 




Recorded in 42 (19.0%) of 221 blocks. 




(j^^^0c\^Pk^A 




O Possible = 34 (81%) 


\^-^>^\\z>^^ 


-V\ AAV \^\ V^\o V-iCo \ 




€ Probable - 4 (10%) 
• Confirmed = 4 (10%) 


\ \ auf^x''--"^*^ 












\' Jr\ j>^\j \^2\ o v-"\ y>^a !•? 

^ < ^^\°\^\o )^K -kAo vfcr^ 

\^i\ • V^rr^ Y^vo \^^a\-^ Y3*t>- 
^J^^^^X^A Hot 


^O 


FSAR = 2 OPI = 84 CI = 1.29 



Ecological Requirements 

These masters of the air course over almost all terrains and 
habitats within foraging range of their breeding haunts. In 
California, they breed solitarily or in small colonies from 
near sea level probably up to 8000 or 9000 feet (Gaines 
1988). Some traditional colony sites have a history of use 
of over 50 years (Dobkin et al. 1986), but many of diese 
have probably been occupied by breeding swifts for much 
longer. White-throated Swifts usually nest in vertical or 
horizontal cracks or crevices in steep cliffs (Bent 1940). 
The main nest site requirements are narrow recessed 
nooks and crannies inaccessible to terrestrial predators. 
Proximity to good foraging areas is probably also important 
(Dobkin et al. 1986). Throughout much of their range they 
nest in granite cliffs, but also in a variety of other rock 
types. Granite occurs in Marin County only on the Point 
Reyes peninsula and is perhaps serviceable to White- 
throated Swifts only at the Point Reyes headlands; most 
White-throateds in Marin nest in other substrates. Else- 
where, these swifts also nest in caves, in dug-out Rough- 
winged Swallow holes (Bent 1940), and in recesses of 
buildings and wharves (Bailey 1907, Collins 6k Johnson 
1982). Nests take the shape of the crack in which they are 
built and are made of feathers, plant down, weed stems 1 , 
grasses, and bark, all glued together by the birds' saliva 
(Bent 1940). 

White-throateds are considered die fastest of North 
American swifts (Bent 1940). They forage over a wide 
variety of habitats at variable heights. Their flight is more 
or less direct, but birds feeding hundreds of feet from the 
ground may dart and swoop erratically or alternately flap 

236 



and soar. Grinnell and Miller (1944) suggested that "pos- 
sibly the daily cruising radius of these birds is greater than 
any other species, even the California Condor," though no 
data are available on how far diat might be. Excluding the 
isolated Point Reyes sightings, all others made during the 
Marin adas project were within five to seven miles of 
known breeding sites, in the southern and eastern part of 
the county. The White-throated Swift's diet is exclusively 
aerial insects, especially flies, beedes, bees, wasps, ants, and 
true bugs (Bent 1940). 

Marin Breeding Distribution 

Except for a nest site at a bayshore quarry, all confirmed 
and probable Marin County breeding records pertained to 
sea cliffs. Representative breeding locations were cliffs at 
Double Point (ON Apr-Jun 1977 — SGA) and a quarry at 
Larkspur Landing (ON/DD 5/30/82 -SSm). Three 
observations of aerial copulations near Palomarin (6/5/80 
PRBO) were not recorded as confirmations because of the 
uncertainty of the actual blocks in which the birds were 
nesting; these may have been Double Point rather than 
Palomarin breeders. 

Historical Trends/Population Threats 

The White-throated Swift: was unrecorded in Marin 
County by Mailliard (1900), whose ornithological explora- 
tion here focused mostly on the vicinity of San Geronimo. 
Stephens and Pringle (1933) considered it "very rare" in 
Marin County, and Grinnell and Wythe (1927) thought it 
was established in the San Francisco Bay Area only in the 



Swifts 



SPECIES ACCOUNTS 



Hummingbirds 



vicinity of Mount Diablo, Contra Costa County. Although 
our atlas data for Marin and casual coastal observations for 
the rest of the Bay Area (ABN) suggest White-throated 
Swifts are now more numerous, this most likely reflects 
increased observer coverage, resulting in the identification 
of more nesting sites of this local breeder. On the other 



hand, the nesting substrate of White'throated Swifts may 
inadvertently have been augmented by excavation of quar- 
ries and construction of buildings— sites that these birds 
increasingly use for nesting elsewhere. From 1968 to 1989, 
numbers of these swifts were relatively stable on Breeding 
Bird Surveys in California (USFWS unpubl. analyses). 



Hummingbirds 

Family Trochilidae 



ANNA'S HUMMINGBIRD Calypte anna 



-1 itjl 






A year-round resident. 


Att&i 






A fairly common, very widespread 






breeder; overall breeding population 
fairly large. 








Recorded in 163 (73.8%) of 221 




Mm«gsM 




blocks. 




\ Q J\r^^9o^Z\t> X^\iS> V-"\ «\/ao V^a © J 






\^~ 


nJ^-oVv o x>\ \>"Y © \^\ © \^\ • \Z^\ 






x\- 


5k? , v^<C>^^ 




O Possible = 55 (34%) 




SA JHW\ ^C<^\rJ^k °3r^®J^A® J^\* \><Zx " 




€ Probable = 76 (47%) 
• Confirmed = 32 (20%) 




/^\ iV-^V' Jv-rC Jv^>«A^\0 \><© i^CPJ 7 ^ 








<^\\^V^\ • 3r<\ ®Jr^\ o J^dro V^Y» WTi A^£v 








•7 VMCV # J\P\ < iV\ '^\ *^>T© L^sY© V>^\ fe Y~s 








t S>*d3e»^ — $o>-'\ • v<a © \^\o &^\*> w-Y» \Z< 




FSAR = 3 OPI = 489 CI = 1.86 






^®. 






>T V^°J\^^3<^^3r^^ cr 3P , ^l 








"M ^®J^?i\ ®3c^S£Ar^\ ® >-^\ • J? 








[ji. ^^^*^^^ 







Ecological Requirements 

In December, daredevil display dives punctuated by 
ground squirrel-like squeaks are signs that our only resi- 
dent hummingbird, unique perhaps among our breeding 
landbirds, is initiating nesting activities while day length is 
yet decreasing. For breeding, Anna's hummers frequent 
oak woodlands, chaparral, broadleaved evergreen forests, 
riparian woodlands, coastal scrub, eucalyptus groves, and 
suburban plantings and gardens of Marin's hills and 
lowlands. Where they overlap widi Allen's Hummingbirds 
near the coast, Annas generally avoid dense, moist habi- 
tats. Instead they prefer more open, sunny habitats with 
less understory, and they range into more upland situa- 
tions. In the drier interior of the Coast Range (away from 
Marin), Annas are somewhat intermediate in habitat pref- 



erence between Costas, which occupy very xeric and open 
habitats, and Black-chinned Hummingbirds, which fre- 
quent more mesic situations, particularly in or adjacent to 
riparian woodlands. The habitat preferences of Anna's 
Hummingbirds overlap those of Black-chins to a much 
greater degree than diose of Costas (Stiles 1973). 

Male Annas defend territories in broken vegetation of 
scattered trees or brush or of forest edges that provide a 
commanding view of dieir surroundings (Pitelka 1951a). 
The core area of the territory often consists of a patch of 
low shrubs of relatively uniform height, with taller bushes 
or trees all around, from each of which die male can scan 
most or all of the area (Stiles 1973). To maintain a vigorous 
and consistent territorial defense, breeding males also 

237 



Hummingbirds 



MARIN COUNTY BREEDING BIRD ATLAS 



Hummingbirds 



require rich, dependable, and easily exploitable nectar 
sources. Preferably, floral sources are on the territory, but 
they may also be a considerable distance away; males may 
go about 55 to 765 yards or, rarely, as much as 0.6 miles 
for nectar sources. In coastal sage scnib and chaparral of 
southern California, the timing of establishment of breed- 
ing territories has evolved to coincide with the seasonal 
blooming of the chaparral currant (Ribes malvaceum), 
which begins after the first heavy winter rains. Males' 
territories are often centered around patches of chaparral 
currant, particularly the later blooming R. speciosum (gen- 
erally occurring on sheltered exposures in canyon bottoms 
and on northern and eastern exposures), though manza- 
nita, eucalyptus, or profusely blooming ornamentals may 
be important, as local conditions dictate. Anywhere in 
their range, certain dominant flowering plants exert a 
profound influence on the local distribution of Anna's 
Hummingbird breeding territories. 

Females nest mosdy in shaded woodlands, especially of 
live oak and eucalyptus in this region, or in gardens. They 
apparendy first locate a reliable nectar source, then situate 
the nest site nearby. Since incubating and brooding 
females do not conserve energy by entering nocturnal 
torpor, they must have a handy nectar source available at 
dawn and dusk, when insects are least available (Stiles 
1973). Females defend the nest site and flower clumps, but 
once incubation begins, little time or energy is available for 
defending anything but the nest itself. Females also have 
one or more prominent perches in their regular rounds 
from which to launch insect-catching sallies. 

Anna's Hummingbirds usually build their nests in trees 
or bushes on a large solid surface (Aldrich 1945, Legg 6k 
Pitelka 1 956). Nest heights range from 1 .5 to 30 feet above 
the ground in southern California (Chambers 1903), from 
5 to 30 feet (most 11-15 ft.) in Santa Barbara (Pitelka 
1951b), and from 10 to 50 feet (av. 28 ft.) in Santa Cruz 
(Legg 6k Pitelka 1956). Birds also select nest sites in 
orchards, in hanging vines on cliffs, and in artificial situa- 
tions, such as insulated electrical wires hanging from 
service poles, light fixtures, hanging baskets, and coils of 
wire in outbuildings (Dawson 1923, Woods 1940). The 
nests of Anna's Hummingbirds are smaller and shallower 
than those of Aliens, and they build them of plant down, 
shredded plant fibers, mosses, and plant stems bound with 
cobwebs. They line them with plant down or feathers and 
adorn them on the outside with lichens and plant seeds 
(Dawson 1923, Woods 1940, Legg ck Pitelka 1956). 
Females may move early-season nests several times before 
they lay eggs (Legg ck Pitelka 1956), and they may lay eggs 
when only a minimum of the cup is built, construction 
continuing during incubation (Dawson 1923, Woods 
1940, Stiles 1973). 



From hovering flight or perches, Annas secure carbohy- 
drate food by probing tubular flowers for nectar or (less 
often during breeding) feeders for sugar solutions. Infre- 
quently, they sip at sapsucker drillings and the juices of 
fruits pecked at by other birds (Woods 1940, Foster 6k 
Tate 1966). Although they use a wide variety of native 
flowering plants, important ones in the breeding season in 
California are currant (Ribes), monkey flower (Diplacus), 
sage (Salvia), Penstemon, manzanita, and paint brush 
(Castilleja). Annas use numerous cultivated varieties, and 
probably the most important naturalized species are tree 
tobacco (Nicotiana) and eucalyptus (Woods 1940, Stiles 
1973). Insects are also a staple that birds procure by 
gleaning and probing trunks, limbs, and leaves in hovering 
flight; by flycatching from exposed perches; and by extract- 
ing entangled individuals from spider webs. From hover- 
ing flight, Annas also startle stationary insects into flight, 
then snap them up (Mobbs 1979). Gnats, small flies, ants, 
wasps, bees, true bugs, and spiders are the main prey 
(Woods 1940). Females spend much more time foraging 
for insects in the breeding season than do males, perhaps 
partly because males defend the best nectar sources and 
also because females have greater protein demands for egg 
laying and feeding the young (Stiles 1973). The female 
feeds the young by regurgitation. Females also apparendy 
need additional minerals in the breeding season, which 
they procure from mortar, plaster, or sand (Woods 1940). 

Marin Breeding Distribution 

Although they bred widely in Marin County during the 
adas period, Anna's Hummingbirds were more evenly 
distributed and more numerous in the eastern part of the 
county, where open woodlands are more prevalent. They 
were widespread and numerous in residential areas along 
the Highway 101 corridor. Representative nesting localities 
were Muddy Hollow, PRNS (NE 4/22/78 -JGE, DS et al.); 
Palomarin, PRNS (NY 3/1/78 -GBe); the ridge NE of 
Santa Margarita Valley-Los Gallinas area (NE 4/23/82 
-BiL); E end Big Rock Ridge (NE 4/30/83 -BiL); and 
Cascade Canyon, Fairfax (NE-NY 4/1 1 -5/1 2/77 -DS). 

Historical Trends/ Population Threats 

California is one of the few areas on Earth that have 
undergone such an extensive and successful series of plant 
introduction and naturalizations. These changes have dras- 
tically altered the spatial and temporal array of flowers, 
causing far-reaching effects on the distribution, numbers, 
and movements, both local and long-distance, of hum- 
mingbirds (Stiles 1973). The establishment of exotic flow- 
ering species has enhanced the suitability of many breeding 
territories. It has also augmented the food supply and, 
hence, survival of migrants and particularly the survival of 
birds from late summer to early winter, when native 
flowers reach a low ebb in dry lowland areas. For these 



238 



Hummingbirds 



SPECIES ACCOUNTS 



Hummingbirds 



reasons, Anna's Hummingbirds have increased gready in 
California in historical times (Woods 1940, G&.M 1944, 
Stiles 1973). In the 1960s to early 1970s, Annas continued 
to expand considerably in range and abundance north- 
ward on the Pacific Coast (records to Alaska), particularly 
in California and Oregon, as diey did throughout much of 
the Southwest and adjacent Mexico (Zimmerman 1973). 
Most expansion has been noted in suburban areas, again 
where ornamental and garden plants and a proliferation of 



hummingbird feeders provide extensive nectar sources. 
The widespread expansion of the species may be the result 
of the birds' success in human-altered habitats in the core 
of the California range, ensuring a large population for 
continuing emigration and recruitment elsewhere. From 
1968 to 1989, numbers of Anna's Hummingbirds were 
relatively stable on Breeding Bird Surveys in California 
(USFWS unpubl. analyses). 







"'arj.se.n W 



Allen's Hummingbirds bring enviable intensity and brilliance to coastal climes. 
Drawing fry Keith Hansen, 1 989. 



239 



Hummingbirds 



MARIN COUNTY BREEDING BIRD ATLAS 



Hummingbirds 



ALLEN'S HUMMINGBIRD Selasphorus sasin 







A summer resident from late Jan 


X~rit%Vw?$>v^ \ w~V- 




through Jul, sparingly through Aug. 






A common, nearly ubiquitous breeder; 




overall population very large. 


<^\ $ Vox ® >--^r\ ° Jv^\ o \^r\- ■'V-^\ © V-'A o v V-"A °^A 




Recorded in 198 (89.6%) of 221 


XXwOt 3 ^ © >^\ © V"\ o \>\ » v>Afl§ v>\ © ixT © J 
\ ^-r^ ^><\ o Y>A © J^\ © Xf-'A ©T^A © v-'V o -v^a 




blocks. 






VA»V\0 V-^\ ©A^A © V^v c V^^\ © V-^\ o V-^\ o j 






\^^JWC®JV 5 i\ c 3^\ V-'A© V^AQ V>A • V^A 
n^Hvt^A © V-"A "WaoV/\o \>ao\>a«j 
Y-V* 3k\v® V-'IX © .A^chS-v-'A o v^A o X^\ ©A^vcr-^^ 




O Possible = 62 (31%) 


\ YV\©nY AAA • V^A « i&?3u« v^-QA^^^Jt-^A • V^aT ) 






YU-A.© v^A • V-"A * A-'-Aa© v-^V* V---A© YA-A © W??3*> 




C Probable = 101 (51%) 


^,- 


• Confirmed = 35 (18%) 


1 0;3K\ ° \^\~<i AAT\ V^V® V-^A O \A-A © V-*'A-f > ^--' 






L>Ak rA>A •■ A>A • Wao \3d^ \J>A"©\AkA © AArA 








i©\J* > 7Y c L\r\ ©A-'.x •A > \ P>si\ © Av<5\ • V'x *>• ATIV 








>wopn • v<\ • v^a© \^y^K^\6 V"A © v^a • 
"IcrijU-Ttk^ 1° v-^\ •Y-<\ © V^Ao?a2-Aa« aaA; © v<: 


o 7 ~> 


FSAR = 4 OPI = 792 CI = 1.86 






^V@> 






"•Jr A-<A • v>^®^V^A:^>^'v© v -N^- >< ^i 


T 






-T7 v© V^iC«A>^? \x\o Wa © ^ 


Pi ^ 






i ^^^V-^S-'^^v* V-^A © y>A 







Ecological Requirements 

With buzzing wings and feverish activity, Allen's Hum- 
mingbirds are harbingers of spring, arriving on the breed- 
ing grounds in late January and early February while winter 
rains still replenish the land. In Marin, females nest in 
riparian, mixed evergreen, Douglas fir, redwood, and 
bishop pine forests, as well as in moist north-facing coastal 
scrub, eucalyptus, and cypress groves, and to a limited 
extent oak woodland. In most habitats, they place their 
nests in dense understory vegetation, but this need not be 
the case in eucalyptus and cypress groves or in the rare 
instance when birds nest in oak woodland or in human 
structures. Nest sites are frequendy near running water, 
where thickets and dense tangles abound. Males defend 
territories, separate from the females nest sites, that are 
usually on the edge of the above habitats and adjoining 
scrub (especially coastal scrub). In narrow linear habitats 
such as riparian thickets or eucalyptus groves, males may 
establish territories above the females' nesting sites. Males 
rarely frequent the interior of dense shaded habitat, as they 
need a commanding view of their surroundings (Aldrich 
1945, Legg ck Pitelka 1956). 

Nest heights usually range from 0.5 to 50 feet (rarely, to 
90 ft.) above the ground (Bent 1940, Aldrich 1945) and 
generally are lower dian those of Annas in areas of overlap. 
In Santa Cruz, Allen's Hummingbird nests ranged from 
1.5 to 40 feet (av. 18 ft.) above ground, whereas those of 
Annas were from 10 to 50 feet (av. 28 ft.) (Legg &. Pitelka 
1956). Female Aliens may nest in close proximity to each 
other, suggesting that where habitat is limiting, such as in 
cypress and eucalyptus groves, loose colonies may exist 

240 



(Bryant 1925, Aldrich 1945). Unlike Annas, which build 
on top of a solid support, Aliens usually build with part of 
the supporting stnjcture incorporated in the sides of the 
nest. These structures include limbs and twigs less than 
one inch in diameter as well as vine runners, fern stems 
and fronds, and the like (Aldrich 1945; see for further 
details on nest site selection and attachment). In Marin, 
Allen's Hummingbirds use many species of trees, espe- 
cially eucalyptus and cypress, for nest support. In under- 
story thickets, they prefer blackberry brambles and sword 
ferns. Females here also select artificial sites inside build- 
ings—in rope or wire slings, on iron hooks, and under 
rafters (Mailliard 1913, PRBO nest records). They build 
tiny deep nest cups, using mosdy moss on the outside and 
willow down and pappus from composite seeds in the 
lining. Other important nest materials are lichens (on the 
outside), feathers, shredded leaves, grass fibers, and hair. 
The females bind them all together and attach them to the 
supporting structure with spider webs (Aldrich 1945). As 
in the case of Annas, the female Aliens also sometimes lays 
eggs on a mere platform that is completed later and 
typically adds nest material throughout nesting (Aldrich 
1945). 

Little has been written on the foraging and food prefer- 
ence of this species, but diey appear to be quite similar to 
those of the Anna's Hummingbird (see account). When 
numbers of Aliens first appear in Marin in February, they 
use Ribes flowers extensively. The timing of migratory 
movements may have evolved to coincide with the bloom- 
ing of these shrubs. 



Hummingbirds 



SPECIES ACCOUNTS 



Kingfishers 



Marin Breeding Distribution 

During the atlas period, the Allen's Hummingbird was 
one of the most common and widespread of all of Marin 
County's breeding species, but it was in the zone of 
persistent summer fogs near the immediate coast that this 
species was most numerous and evenly distributed. 
Although some birds bred on moist ridges, most were 
concentrated in thickets of the lowlands and canyon bot- 
toms. In the interior of Marin, most breeding activity 
centered around riparian thickets and eucalyptus groves; 
most forests in this region generally lack dense understory 
cover. Representative breeding locations were cypress 
grove at Mendoza Ranch, Point Reyes (NE 5/30/80 — DS); 
near stream at Bear Valley, PRNS (NE 3/22/81 -DS); 
cypress by streamside at Tennessee Valley (NE-NY 3/28- 



4/25/77 — GMcM); and a live oak in open oak woodland 
on Big Rock Ridge near Marinwood (NE 3/24/79 — DS). 

Historical Trends/ Population Threats 

It seems likely that the planting of early-blooming exotics 
and an increase in hummingbird feeders has benefited the 
species, especially early in the nesting season, when contin- 
uing winter rains curtail available food and foraging time. 
Allen's Hummingbirds have undoubtedly extended their 
range locally into areas where suitable nesting habitat 
formerly was limited but where eucalyptus and cypress 
have since been planted in grassland. Numbers of Allen's 
Hummingbirds were relatively stable on Breeding Bird 
Surveys in California from 1968 to 1989 (USFWS unpubl. 
analyses). 



Kingfishers 

Family Alcedinidae 



BELTED KINGFISHER Ce^le alcyon 











A year-round resident. 


iSr 


t\^>-\ 






An uncommon, somewhat local 


joV^V 


iJ\\ j^\\ <?^-^^ ^r*J~^-Ar-~ J \. Jt >< \^ 






breeder; overall breeding population very 




V o b<^\ \^\ i^\ i Y^nf V-^A \^\ 

s AJ^rV °3r\* Jer^v Y^C Wv X^K 


><VoJ 




small. 
Recorded in 73 (33.0%) of 221 blocks. 










O Possible = 46 (63%) 
€ Probable = 5 (7%) 






/jJr"\ v^^v*C Xs-^y \^*^\~0 \^\ Y, 




x?o» 


• Confirmed = 22 (30%) 
FSAR = 2 OPI = 146 CI = 1.67 






"iT ^-o^v*5<^ 


^S^\°3r\3 










j£z> ~xJv 


^^^^r\ 







Ecological Requirements 

These dashing crested piscivores raise their rattling hue 
and cry along streams, freshwater ponds, and shallow 
estuarine margins that provide favorable food supplies. 
Kingfishers usually excavate nest burrows, using their bills 
and feet, in well-drained eartdien banks widi vertical or 
slightly overhanging faces that are bare or sparsely vege- 
tated (Bent 1940, White 1953, Cornwell 1963). Often 



these are naturally created by water erosion, but road cuts, 
gravel excavations, ditch banks, and other human modifi- 
cations afford nest sites as well. Kingfishers may excavate 
burrows at any height but usually do so at least five feet 
from the bottom and not more than two feet from the top 
of die bank, to limit access by mammalian predators. They 
dig burrows that vary in length from about three to seven 



241 



Kingfishers 



MARIN COUNTY BREEDING BIRD ATLAS 



Kingfishers 



feet, shorter ones generally in harder substrates, and dig 
them on a horizontal plane unless they encounter obstacles 
(White 1953). The nest chamber is usually spherical or 
ovoid and somewhat flattened on the bottom, with the nest 
below the level of the tunnel. Females lay eggs either on 
bare earth or on the debris of regurgitated food pellets. 
When there is a limited supply of nest sites, Kingfishers 
occasionally nest in the open tops or decayed sides of 
stumps or trees and in earth adhering to roots of large 
upturned trees; they even nest in sawdust piles, though 
generally unsuccessfully (White 1953). A dead or dying 
tree or other suitable perch generally overlooks each nest 
site. Although most nests are over or near water, they may 
be up to a mile from a water source (Cornwell 1963). 

Kingfishers feed either from perches or from hovering 
flight 20 to 50 feet in the air. They dive headlong, nearly 
vertically or at an angle, into relatively shallow and calm 
water, securing prey with their large bills. Kingfishers dive 
mosdy in water less than two feet deep, and even in deeper 
water they probably do not catch prey below this depth 
(White 1953). They often subdue or stun prey before 
swallowing by whacking them against a perch. Foraging 
flights take the birds 0.5 to 5 miles from the nest site; most 



trips are within a 1-mile distance, though daily flights of 2 
miles are not uncommon (Cornwell 1963). Although the 
diet is often up to 90%- 100% fish (Salyer 6k Laglar 1946, 
White 1953), at some sites crayfish occasionally predomi- 
nate (White 1953, Eipper 1956). Other minor food items 
are large insects, other invertebrates, salamanders, frogs, 
lizards, young birds, mice, shrews, and even berries (Bent 
1940, Salyer 6k Laglar 1946, White 1953). Small adult and 
larval insects found in Kingfisher pellets undoubtedly are 
from the ruptured stomachs of their fish prey (White 
1953). Adults feed the young relatively large fish, such that 
one to three will fill the stomach of a single nesding. 

Marin Breeding Distribution 

Breeding Kingfishers were patchily distributed in Marin 
County during the adas period because of limitations of 
suitable feeding and nesting sites. Breeders concentrated 
along the west side of Tomales Bay, where steep banks are 
almost continuous from Inverness north to Tomales Point. 
Representative nesting sites were Abbott's Lagoon (FL 
6/14/82 — DS); near Sacramento Landing, Tomales Bay 
(FL 6/18/82 -DS); Salmon Creek along the Marshall- 




J*4~ 




y 



' 




; v 




BE 

Fish obscures face as this Belted Kingfisher brakes to enter its burrow and feed its growing young. Photograph b} Ian Tail. 



242 



Kingfishers 



SPECIES ACCOUNTS 



Woodpeckers 



Petaluma Rd. (FY 6/21/82 -DS); and Stafford Lake, 
Novate (ON 5/1/82 -ScC). 

Historical Trends/Population Threats 

Numbers were formerly reduced (G&M 1944) by shoot- 
ing resulting from fishermen's disfavor. After it was real- 
ized that Kingfishers help control some fish species 
destructive to trout's eggs and young, human persecution 
of Kingfishers decreased and the birds' populations 
increased. Fishermen's needs have also been met by cover- 
ing rearing ponds with wire mesh. Although the creation 



of reservoirs may have increased habitat in areas such as 
Marin County, where natural lakes and permanent 
streams are few, elsewhere damming of year-round streams 
and riprapping for bank protection have probably more 
than counterbalanced these positive effects. Human deple- 
tion of fish resources upon which Kingfishers depend 
likely has also been to the species' detriment. Numbers of 
Kingfishers were relatively stable on Breeding Bird Surveys 
in California from 1968 to 1989 but decreased from 1980 
to 1989 (USFWS unpubl. analyses). 



Woodpeckers 

Family Picidae 



ACORN WOODPECKER Melanerpes formicivorus 









A year-round resident. 


if%>- 






A fairly common, somewhat local 


A-^^O^A 




breeder; overall breeding population 


A%t 


A JV"A -A^A'OA' •\QA^^®3^\°3r\L^ 
"\ Ar\ Ar\ °A^A A-""A • A-"A o V-r \ 




small. 

Recorded in 86 (38.9%) of 221 blocks. 




r\ J<\ J<^\ 3r^-3r^°3PA^A^A 

\ A-^\ -^yr\ A^V V^xoA^A A^\ • J 
vKA -J<\ Ar\ A-^A A^\ A-"\ • V-^A 




O Possible = 40 (47%) 




«JiAk \ ^A^\ ^Vv\ ^V\ ^V\ j^\«Jl 

A^A A-^A • jtr^A *Ai>A • v<2\-'0 \^AA» \A--A ^L\ 
<^Ak tfV^*iV^*At^*^Sr»>^®A-^ 

H \ ^->7V I V>"^ V^\ *A^\ <T^--"A • «^c1\» \J-^\ Q \" 


— --r" 


€ Probable = 14 (16%) 
• Confirmed = 32 (37%) 

FSAR=3 OPI = 258 CI = 1.91 




J><V vV\ VA ° \^o\> 5 nA^«' VA \>\oS^ 
><X~>^ ^-_V"A G J^\ ■ ■JpzX*>%<\ °Ar«^\ </ /A 






i^^ \~^j ^~^*^*iSQ<^**\ 







Ecological Requirements 

As their name suggests, Acorn Woodpeckers are intimately 
dependent on the fruits of oak trees, mostly of die genus 
Quercus. They are permanent territorial residents in almost 
all California forests and woodlands with a reasonable 
density and diversity of oaks. In Marin County, their 
habitat includes oak woodlands and savannah; relatively 
open Douglas fir, redwood, mixed evergreen, and riparian 
forests; and residential areas where oaks have been pre- 
served or planted. 



Acorn Woodpeckers nest exclusively in cavities diey 
excavate in living trees or dead snags, including pines, 
oaks, sycamores, and palm trees; they may also use utility 
poles. Holes are often reused for nesting and are also used 
for nocturnal roosting outside the breeding season. 

Nesting, as well as all other phases of Acorn Wood- 
pecker home life, is performed in groups, and up to 12 
individuals may participate in incubation and feeding of 
nestlings (Ritter 1938, MacRoberts & MacRoberts 1976, 

243 



Woodpeckers 



MARIN COUNTY BREEDING BIRD ATLAS 



Woodpeckers 



Koenig ck Mumme 1987). Only one nest at a time is 
attended by a group. Within a group, up to three females 
may jointly lay eggs, and up to at least four males may 
compete for matings with the breeding females. These 
cobreeding sets of birds are almost always close relatives, 
usually siblings or else a parent and its offspring of the 
same sex. There is considerable competition among joint- 
nesting females within a group, and a female that lays her 
eggs first frequendy has them removed from the nest and 
subsequendy eaten by her cobreeding female relatives. 
Additional group members consist of offspring of both 
sexes that may remain in their natal group as nonbreeding 
"helpers" for one to five years prior to dispersal (Koenig 6k 
Mumme 1987). Breeding takes place in die spring and 
early summer (Apr to mid-Jul, with die peak of egg laying 
near the end of Apr), although in bumper acorn years it 
may also occur in the fall (Aug-Nov). The latest docu- 
mented first-egg date is 23 September, at Hastings Reserva- 
tion in Monterey County (Koenig 6k Mumme 1987). 

Acorn Woodpeckers are critically dependent on mast. 
The birds store acorns or other mast, harvested direcdy 
from the trees in fall, one by one in individual holes drilled 
in communally defended storage trees or "granaries." 
Though granaries typically contain about 1000 to 4000 
storage holes, they may host up to 20,000 or more! These 
storage facilities are usually in snags, dead limbs, thick 
bark, or structures such as the wooden eaves or roofs of 
houses. They may be in pines, oaks, sycamores, redwoods, 
Douglas firs, incense cedars, cottonwoods, fence posts, 
utility poles, palm trees (Ritter 1938), or, exceptionally, in 
large pine cones (J en l 1979). The birds usually store 
acorns, but also, to a lesser degree, cultivated nuts (Ritter 
1929), pine seeds (MacRoberts ck MacRoberts 1976, 
Stacey 6k Jansma 1977), and such bizarre items as stones 
(Ritter 1938) and Douglas fir cones (W.D. Koenig pers. 
obs.). These latter items are probably secured by young 
birds that, in their zeal to store food, fail to discriminate 
between suitably shaped edible and nonedible items. 

Stored acorns are eaten diroughout the winter mondis 
and are critical to successful spring reproduction in at least 
some areas. Insect predators of acorns, especially weevil 
larvae, may also be eaten if encountered, but the myth that 
acorns are stored as "nurseries" for these insects is untrue 
(see MacRoberts 1974). Acorn Woodpeckers also eat 
diverse other foods that they capture with a variety of 
techniques (MacRoberts 1970). Flycatching is common at 
all seasons during good weather. Sapsucking occurs regu- 
larly in late summer (Jul and Aug) and late winter (Jan- 
Mar). Oak catkins are eaten in late winter and early spring, 
and bark gleaning occurs regularly. Finally, birds have even 
been recorded hovering and picking at wild oat seeds 
(MacRoberts ck MacRoberts 1976). Major food items 
include acorns, sap, catkins, ants, beedes, bees, wasps, true 
bugs, and earwigs. Observers at Hastings Reservation occa- 



sionally have even seen Acorn Woodpeckers eating lizards 
(genus Sceloporus). Exceptionally, diese birds prey on the 
eggs or young of other species of birds (Shuford 1985). 
They particularly prize crane flies as food for their nesdings 
but also feed die young a wide variety of insects, as well as 
acorn fragments (Beal 1911, MacRoberts ck MacRoberts 
1976, Koenig 6k Mumme 1987). 

Marin Breeding Distribution 

During die adas period, Acorn Woodpeckers were wide- 
spread breeders in Marin County's forested regions and, 
as elsewhere in their range, were limited to extensive stands 
of oaks. Most coastal birds were dependent on coast live 
oaks, but valley oaks became increasingly important in the 
northeastern corner of the county near Novate Acorn 
Woodpeckers were absent from the largely treeless tracts of 
land on outer Point Reyes and east of Tomales Bay, and 
they occurred only to a limited extent along the shore of 
San Francisco and San Pablo bays. Representative breed- 
ing localities were Mt. Burdell, Novato (ON 5/?/82 — 
ScC); Pioneer Park, Novato (ON 5/?/82 -ScC); and 
Olompali, Novato (ON 4/V79 —ScC). Of interest is a 
small population of birds with golden, rather than red, 
crowns that was resident in die San Geronimo area up 
until at least 1980 (W.D. Koenig pers. obs.). 

Historical Trends/ Population Threats 

Locally, birds are threatened by the loss of oaks to develop- 
ment, but often they are able to adapt well to exotic 
conditions as long as mast trees remain. Perhaps a more 
important long-term threat is the possible future loss of 
large stands of oaks because of a lack of recruitment of 
young oaks; the causes of this include grazing pressures of 
cattle and deer on seedlings and damage to acorns from 
insects and rodents (Griffin 1977). Although logging prac- 
tices tend to open up dense forests and favor hardwood 
trees such as oaks, these species are generally slow growing 
and are often removed by forest managers to favor econom- 
ically important conifers. 

The only major hole competitor of the Acorn Wood- 
pecker is the introduced European Starling which can, at 
least occasionally, render breeding difficult for groups of 
woodpeckers by usurping their favored holes. However, 
the only study of competition between these species per- 
formed to date reported that reproductive success and 
group size of Acorn Woodpeckers were not affected by 
Starlings (Troetschler 1976). It remains to be seen whether 
this conclusion will hold at other study sites, or if Starlings 
continue to expand in California. Numbers of Acorn 
Woodpeckers on Breeding Bird Surveys in California were 
relatively stable from 1968 to 1989 but increased from 
1980 to 1989 (USFWS unpubl. analyses). 

WALTER D. KOENIG 



244 



Woodpeckers 



SPECIES ACCOUNTS 



Woodpeckers 



RED-BREASTED SAPSUCKER Sphyrapicus ruber 











Occurs year round, though primarily as a 






\ J«Oa_ 




winter resident from late Sep through 


y\^\js^ 


C^t\^ 




mid-Mar. 


S^CX^k 




\\ %<^\\^\\^\ \ 


V^- - 


A rare, very local breeder; overall breed- 
ing population very small. 






-^^AfAp^V^C^ 




Recorded in 5 (2.3%) of 221 blocks. 




P^kv 






O Possible = 3 (60%) 






,\^Vo JSe*?\' \^<Jjf^ \^\ 


^VA^r^y -r- 


€ Probable = (0%) 
• Confirmed = 2 (40%) 






\ — -^3''\VV*'A A^V J$r' 


^^5^°- 


FSAR=1 OPI = 5 CI = 1.80 


i 


^>^> 









Ecological Requirements 

In California, the nasal breeding calls of Red-breasted 
Sapsuckers emanate primarily from moist conifer forests or 
woodlands, as well as bordering riparian zones dominated 
by aspens or alders (Shuford 1986). In Marin County, 
Red-breasted Sapsuckers breed in mixed evergreen wood- 
lands or riparian groves of alders and willows adjacent to 
Douglas fir forests. They drill their nest holes from 4 to 70 
feet above the ground; in the Sierra Nevada, average nest 
height of 49 nests was 42 feet (Raphael 6k White 1984). 
Birds use dead trees, limbs, stubs, or live trees for nesting; 
occasionally they occupy previously used holes (Lawrence 
1967). Although they appear to prefer deciduous trees over 
conifers (Bent 1939, Howell 1952, Crockett ck Hadow 
1975), in the Sierra Nevada, Red-breasted Sapsuckers may 
select predominandy conifer snags (Raphael ck White 
1984). Sapsuckers find live aspens with tough outer shells 
of sapwood and centers decaying from "infection" by shelf 
fungi (Fomes) especially attractive; such sites provide better 
protection than dead trees do from the ravages of predators 
like raccoons (Bent 1939, Kilham 1971). Nest hole orien- 
tation appears to favor light and warmth, with a tendency 
for southern or eastern exposures, lower height in open 
surroundings, and location on the edges of clearings (Law- 
rence 1967, Inouye 1976). 

Unlike most woodpeckers, these birds rarely, if ever, dig 
deep into decaying trees for wood-boring insects and lar- 
vae. Instead they primarily glean active exposed insects 
from trunks and limbs and, secondarily, flycatch against 
clusters of leaves or needles. They also make flycatching 
sorties into the air from perches, fly to the ground after 



visible insects, and search for ants on the ground in the 
manner of flickers (Bent 1939, Howell 1952, Kilham 
1977, Raphael ck White 1984). They use conifers exten- 
sively for insect foraging. Although they pick many wild 
fruits from trees and bushes (and hang chickadeelike on 
branch tips to pluck off aspen buds), most vegetable matter 
that sapsuckers consume comes from the characteristic 
small square holes that they drill in trees, most frequendy 
in series of horizontal and vertical rows (Tate 1973). 
Sapsuckers show a strong preference for drilling these 
holes in young trees in dense stands, in trees marked by 
previous sapsucker working, and in those already weak- 
ened or wounded in some manner (Kilham 1964, Law- 
rence 1967). In California, they drill them preferentially in 
native members of the willow and birch families, though 
they also use conifers to a lesser extent (Bent 1 939, GckM 
1944). They also use orchard trees extensively. The birds 
drink sap from the holes and also eat the soft inner bark, 
or cambium, of the trees. Although some authors feel that 
sapsuckers take many insects that are attracted to the 
oozing sap (Tate 1973), others feel this practice is infre- 
quent (Kilham 1977). 

Adults feed the young primarily insects (Howell 1952) 
and also some sap (Kilham 1977), though perhaps inciden- 
tally (Lawrence 1 967). The size of insects they feed to young 
increases with the nesdings' age (Lawrence 1967). The 



Some information on aspects of natural history were obtained 
from studies of the Yellow-bellied Sapsucker (S. varius), formerly 
considered conspecific with the Red-breasted Sapsucker, whose 
habits do not appear to differ materially from its congener. 

245 



Woodpeckers 



MARIN COUNTY BREEDING BIRD ATIAS 



Woodpeckers 



September-to-March diet of the Red-breasted Sapsucker 
(exclusive of sap) is 69% animal and 31% vegetable (Beal 
1911, n = 34); in montane California, the birds' summer 
diet is 96% animal and 4% vegetable (Otvos &. Stark 
1985, n = 19); in the Sierra Nevada, its June-to-July diet is 
88% animal and 1 2% vegetable (Dahlsten et al. 1985, n = 
8). Animal fare is mosdy made up of ants and scale insects 
along with smaller amounts of beedes, bees, wasps, aphids, 
termites, and miscellaneous insects. Vegetable fare consists 
of fruits and berries, cambium, and seeds; bark use peaks 
in spring (Tate 1973). 

Marin Breeding Distribution 

Red-breasted Sapsuckers were unknown as breeders in 
Marin County before the adas project began (Mailliard 
1900, S6kP 1933, GckM 1944). We found them to be very 
local breeders in the Olema Valley and vicinity. Represen- 
tative breeding locales were Bear Valley Headquarters, 
PRNS (NY 6/22/80 -DS) and Five Brooks, PRNS (FL 
10/18/82 —EH, MAS). The young at the former site were 



apparently destroyed by Acorn Woodpeckers (Shuford 
1985). See Shuford (1986) for additional possible and 
probable breeding records for Marin County. 

Historical Trends/Population Threats 

Grinnell and Miller (1944) mapped the breeding distribu- 
tion in coastal California as extending south as far as 
central Mendocino County. Shuford (1986) reviewed 
recent records and concluded that Red-breasted Sapsuck- 
ers breed continuously south to northern Sonoma County, 
with a small disjunct population in Marin County. A 
search of historical sources revealed that there had been 
very little ornithological investigation of any kind in the 
area of apparent range extension until recent years. This 
strongly suggested that sapsuckers previously had been 
overlooked in this region, rather dian that they had 
expanded their breeding range. Current adas work in 
Sonoma County is likely to add further details to the 
known breeding range. 



NUTTALL'S WOODPECKER Picoides nuttalli 









A year-round resident. 


vv^>5rv\ ^V\ aVa\ Avaap A^tf* K\o \>-\c> A 
\ A-cSX w^\ A-^\ °\^\ Q\^\ o^c^\ o\^*C< &\^\ 

V5 X>=fA AA^A oVA * \7\o l/TowcM 




A fairly common, somewhat local 




breeder; overall breeding population 
small. 

Recorded in 72 (32.6%) of 221 blocks. 

O Possible = 51 (71%) 


\ A 


vXA Ar \ Ar"f\ A-^A A^"\ ° V-^\ ®A 

^fCv-T V\ Jk\^<\ ®V<\ a^ca ftA-"\o-^ 

\ Ar^J3?<\ A^V*>A<i.jO VATo V^A ' A-^A * r 
#r\ -A-^A_jA<^A' ••'jA'A o Xs^\ * A^"A o \^\ * \^j 
Q\ J<\+^c\ 'A\^A \A\ j^OA^Vo^^T • 
^A"\ At \ Ar'lAA > "'i\ A<T\ A--'\,Q-A > A '--_\ 
'A.i — \ AV^\ *AV^\'--O^A & A-^\ A-^A * A^-A 


' 


€) Probable = 14 (19%) 
• Confirmed = 7 (10%) 




Air \ Ay^\ A-^\ A"^\J \-^\ A-^\ o \*<\~\ A' 




FSAR = 3 OPI = 216 CI = 1.39 






^p& 





Ecological Requirements 

The dry rattling calls of Nuttall's Woodpeckers resound 
from open, primarily deciduous oak woodlands, and bor- 
dering riparian groves, which may be especially attractive 
for nesting. In Monterey's Carmel Valley, Nuttalls usually 
center their home ranges around drainages (Miller 6k Bock 
1972). In the interior of Marin County, they apparendy 
also sometimes nest in open eucalyptus and cypress groves, 

246 



far from oak woodlands, but then usually near streamside 
vegetation. Elsewhere in California in ecological zones 
supporting oaks, Nuttall's Woodpeckers may also breed in 
open riparian forests devoid of oaks or where gray pines 
(Pinus sabiniana) mix with oaks. Nevertheless, throughout 
its range as a whole, perhaps the most consistent element 
of Nuttall's Woodpecker foraging habitat is a dominance 



Woodpeckers 



SPECIES ACCOUNTS 



Woodpeckers 



of oak trees (Block 1991). Openness also seems to be a key 
character of Nuttall's Woodpecker habitat. In the breeding 
season these woodpeckers generally avoid Marin's coastal 
riparian habitat which tends to be dense and is never 
bordered by true oak woodland. 

Although Nuttalls drill many of their nest cavities in 
oaks (Block 1991), they appear to prefer soft-wooded spe- 
cies of trees when available, perhaps because their foraging 
style leaves them ill-equipped for deep digging in hard 
wood (Miller 6k Bock 1972). Nest holes range from 3 to 
45 feet (av. 1 7 ft., n = 54) above the ground and are usually 
in dead limbs or trunks. 

Nuttall's Woodpeckers are much more versatile and 
acrobatic foragers than either Downy or Hairy woodpeck- 
ers (Miller 6k Bock 1972). Nuttalls forage for insects 
principally on the surface and shallow subsurface of trees 
mosdy by light pecking and tapping, bark scaling, probing, 
gleaning, and foliage and twig scanning (Miller 6k Bock 
1972, Jenkins 1979, Block 1991). To a limited degree, they 
drill and excavate extensively in classic woodpecker fash- 
ion, sapsuck, flycatch, and pick seeds from pinecones. At 
times, birds climb through foliage clusters, fluttering and 
balancing with their wings and hanging upside down to 
procure fruits (Miller 6k Bock 1972). Nuttalls generally 
glean more but peck and probe less during breeding than 
in the nonbreeding season (Block 1991). The proportional 
reliance on various foraging techniques also varies between 
habitats, as does use of foraging substrates. Across various 
habitats, Nuttall's Woodpeckers perform about 90% (n = 
907) of their foraging maneuvers on branches or trunks; 
about 53% involve twigs and small branches (Block 1991). 
About 75% of foraging attempts are directed at live stems 
of trees and only about 2% at fruits, cones, and leaves. On 
average, trees used for foraging are larger indiameter and 
taller than those generally available; die size characteristics 
(height, trunk diameter, and canopy size) of trees used for 
foraging differs among study sites and between seasons 
and years. Nuttall's Woodpeckers tend to forage mosdy 
about two-thirds up the height of the tree and 40%- 70% 
of the distance from the center to the edge of the canopy, 
which corresponds to the location of small and medium- 
sized branches, the two most frequendy used foraging 
substrates (Block 1991). 

Although closely tied to oaks at all seasons, Nuttall's 
Woodpeckers vary their preference for foraging trees both 
among sites and between seasons, as indicated by selection 
of tree species out of proportion to their availability (Miller 
6k Bock 1972, Block 1991). In the Carmel Valley during 
the breeding season— when deciduous oaks are newly 
leafed out— birds forage almost exclusively in oaks, primar- 
ily on and around branchlets and leaves (Miller 6k Bock 
1972). At other times, birds there shift much of their 
foraging to live oaks, particularly during winter and early 
spring when other trees are bare. Block (1991) confirmed 



the tendency of Nuttalls to concentrate foraging in the 
breeding season on white (deciduous) oaks, but at some 
sites found that birds increased foraging in trees such as 
gray pines, rather than live oaks, during the nonbreeding 
season. 

In one study in the Carmel Valley, Jenkins (1979) 
detected sexual differences in foraging behavior during the 
postbreeding season (Jun-Oct). There females fed primar- 
ily by gleaning. They foraged more frequendy on smaller 
branches, twigs, and foliage, and used more obliquely 
oriented positions than did males. Males fed primarily by 
surface tapping and also fed lower in trees than did 
females. In contrast, Block (1991), working at two sites in 
the Sierra Nevada and one in the Tehachapi Mountains, 
found little variation between the sexes in either foraging 
behavior or foraging habitat during breeding (Apr-Jun) 
and nonbreeding (Nov-Feb) seasons. Males and females 
at his study sites used similar foraging maneuvers, sub- 
strates, and macrohabitats. The size of trees used by males 
and females differed slighdy, and males tended to forage 
relatively higher in trees than did females (contra Jenkins 
1979). These slight differences may have allowed the sexes 
to partition limited resources or may reflect dominance by 
one sex over the odier (Block 1991). 

The diet in California is 79% animal matter, primarily 
beedes, true bugs, caterpillars, and ants and other hyme- 
nopterans (Beal 1911, n = 53). Consumption of animal 
matter varies from about 87% of the diet in winter, to 80% 
in spring and summer, to 65% in fall (Martin et al. 1951, 
n = 52). Vegetable fare includes wild fruits and seeds, such 
as elderberry, blackberry, raspberry, poison oak, and 
acorns, along with cambrium and flower buds (Beal 1910, 
1911). Sap is taken as the opportunity arises (Miller 6k 
Bock 1972). 

Marin Breeding Distribution 

During the adas period, breeding Nuttall's Woodpeckers 
were concentrated in the northern interior of Marin 
County, especially around Novate They were extremely 
rare on the immediate coast during the nesting season but 
occurred there with greater regularity during postbreeding 
dispersal, starting in mid- to late June (see adas map). At 
that time, some birds shift to coastal eucalyptus and cypress 
groves and riparian woodlands. Representative nesting 
locations were Mt. Burdell, Novato (ON 5/7/82 — ScC); 
Stafford Lake, Novato (ON 4/V82 -ScC); and Novato 
area (NY 5/6/77 —RMS). An old nesting record for Ross 
(see below) and a pair entering a nest hole at Bear Valley 
Headquarters, PRNS, on 1 1 May 1985 (DS) indicate occa- 
sional nesting in southern and coastal Marin County. 

Historical Trends/ Population Threats 

In his annotated list of Marin County's landbirds, 
Mailliard (1900) considered die Nuttall's Woodpecker an 

247 



Woodpeckers 



MARIN COUNTY BREEDING BIRD ATLAS 



Woodpeckers 



"exceedingly rare visitant in major portion of county, but 
more numerous near northern boundary." Stephens and 
Pringle (1933) listed it as "uncommon" here, and a record 
of a pair feeding young at Ross on 16 June 1929 (Gull 1 1 , 
No. 7) was considered noteworthy. These reports occurred 



before much exploration of the Novato area, where we 
found the species numerous during the adas period. From 
1968 to 1989, numbers of Nuttall's Woodpeckers were 
relatively stable on Breeding Bird Surveys in California 
(USFWS unpubl. analyses). 



DOWNY WOODPECKER Picoides pubescens 









A year-round resident. 


A — C*^ 






An uncommon, widespread breeder; 


vVJ^ 




overall breeding population small. 
Recorded in 145 (65.6%) of 221 


YssrS XxV\ °3<<a ° A^v • j^v. \<c\o V^C \^\ 
\c5oNa-^ --jJ*r\ jxr\°b^\ V^\ \^\ \^\ 




blocks. 

O Possible = 81 (56%) 
© Probable = 25 (17%) 




U-^v Jen® v^ ^^^^^^^^^^ jA\ ° v 


-•r- 


• Confirmed = 39 (27%) 
FSAR=2 OPI = 290 CI = 1.71 




^ 





Ecological Requirements 

This petite lively woodpecker inhabits Marin County's 
riparian tracts and moist mixed evergreen forests, espe- 
cially those dominated by California bay trees. Downies 
also occasionally forage in adjacent shrubby habitats and 
tall weed fields. They overlap to a limited degree widi die 
larger Hairy Woodpecker in broadleaved forests, but for 
the most part they avoid the conifer forests preferred by 
that species. Downies also overlap to a small degree with 
Nuttall's Woodpeckers in riparian groves or at the ecotone 
between broadleaved evergreen forests and oak woodlands, 
but the latter species prefers more open woodlands. Down- 
ies usually drill their nest holes in the soft wood of dead 
trees, dead branches of live trees, and stumps, and occa- 
sionally in live trees; most nest cavities are close to the tops 
of brokenoff stubs of dead trees (Bent 1939, Lawrence 
1967, Conner et al. 1975). Nest heights range from 3 to 
50 feet or more above the ground (Bent 1939). Downies 
excavate their nest cavities in shorter, smaller-diameter 
trees and at lower heights than do Hairies (Conner et al. 
1975; see Hairy Woodpecker account). Eleven Downy 
Woodpecker nests in Ontario ranged from 12 to 45 feet 
and averaged 29 feet above the ground (Lawrence 1967), 
whereas 19 nests in Virginia ranged from 4 to 38 feet and 

248 



averaged 16 feet above the ground (Conner et al. 1975). In 
some areas, nest holes appear to be oriented with respect 
to light and warmth and tend to have southern or eastern 
exposures and occur at lower heights in open surround- 
ings (Lawrence 1967); elsewhere, a northeasterly orienta- 
tion may provide shelter from wind and rain (Conner 
1975, 1977). The slope of die trunk appears to be the most 
important factor in nest orientation (Conner 1975, 1977); 
nest holes facing slightly downward prevent rain from 
entering the cavity and aid in defense of the hole from 
predators. 

Downies forage primarily by drilling and scaling but 
also by gleaning, probing, and occasionally by hovering 
and flycatching. They direct most foraging attempts at bark 
but do some gleaning from leaves and flower clusters. 
Though Downies forage more frequendy in the lower 
zones of the canopy, they can feed from the tops of trees to 
the ground (Willson 1970, Williams 1975). Downies also 
exploit insects in galls on tall weed stalks (Confer 6k Paicos 
1985); birds involved seem always to be males (Grubb 6k 
Woodrey 1990). Compared with the closely related but 
larger Hairy Woodpecker, Downies forage more on the 
smaller branches and twigs of trees; they probe and glean 



Woodpeckers 



SPECIES ACCOUNTS 



Woodpeckers 




Downy Woodpeckers often select nest holes on 
where gravity can hinder tree<limbing 

to a greater extent; they do not drill as often or penetrate 
to as great a depth; and they feed on different tree species 
(Koplin 1969, Kisiel 1972, Conner 1981). See Nuttall's 
Woodpecker for comparison to that species. 

It is well documented that the sexes of Downy Wood- 
peckers exploit different foraging niches. Males drill more 
on small branches and twigs than do females, which tend 
to forage more on larger branches and trunks (Jackson 
1970; Kilham 1970; Willson 1970; Kisiel 1972; Williams 
1975, 1980). Experimental studies suggest that males may 
choose the more productive portions of the forest and 
exclude females from these sites (Grubb ck Woodrey 
1990). On the whole, there is no consistent relationship 
between the vertical foraging distribution on trees of males 
versus females, as has been found in individual studies 
(Grubb &. Woodrey 1990). Because of the above substrate 
preferences, males generally tend to forage more by dril- 
ling, females more on the surface by probing and gleaning 
(Jackson 1970, Kisiel 1972, Williams 1980). In winter, 
males feed more on dead substrates and on a wider array 
of tree species than do females (Williams 1980). In general, 
Downies forage more on live than on dead trees (Jackson 
1970, Kisiel 1972, Williams 1975), though their use of 



the downward-oriented side of leaning trees, 
predators. Photograph b)i Ian Tait. 

dead trees increases in winter when it can be roughly 
equivalent to their use of live trees (Jackson 1970). Since 
surface foraging techniques are more important on live 
trees and subsurface techniques on dead trees, Downies 
increase their drilling and decrease surface probing and 
gleaning in winter (Jackson 1970, Conner 1981). Travis 
(1977) also noted that in winter, Downies forage more on 
furrowed wood and larger trees and less on branches; see 
Conner (1981) for additional differences in seasonal forag- 
ing methods. Between-habitat foraging differences have 
also been noted (Williams 1975). 

The annual diet of the Downy Woodpecker in Califor- 
nia is 77% animal and 23% vegetable (Beal 1910, n = 80), 
which is remarkably similar to the species' continentwide 
diet of 76% animal and 24% vegetable (Beal 1911, n = 
723). In the mountains of California (season unspecified), 
the diet of males (n = 12) is 95% animal and that of 
females (n = 1 7) 99% (Otvos ck Stark 1985; see for further 
slight sexual differences in diet). In the East, reliance on 
animal matter varies from 86% in summer to 71% in fall 
and winter (Martin et al. 1951, n = 828). In Illinois in 
winter, females consume more homopterans and spiders 
than do males, presumably because they probe more in 

249 



Woodpeckers 



MARIN COUNTY BREEDING BIRD ATLAS 



Woodpeckers 



bark crevices and select rough-barked trees to forage in 
more than males do (Williams 1980). Males take more 
ants than do females, apparently because they peck into 
smaller limbs and are more adept at removing them witJi 
their larger tongues. The bulk of the species' animal food 
is insects, especially beedes (particularly wood-boring lar- 
vae), ants, caterpillars, and true bugs (scale and plant lice); 
other insects, spiders, millipedes, pseudoscorpions, 
sowbugs, and snails are taken infrequendy. Although 
wood-boring beede larvae are important to both Downy 
and Hairy woodpeckers, Downies eat only about half the 
amount that Hairics do, substantiating the fact that Down- 
ies peck into wood much less frequently (Beal 1911). The 
size of insects fed to young increases with nesding age 
(Lawrence 1967). The vegetable component of die diet is 
largely various fruits, mast, and seeds, though grain, galls, 
flower petals and buds, and cambium are eaten to a limited 
extent. Sap is also eaten occasionally (Foster 6k Tate 1966). 



Marin Breeding Distribution 

Although Downy Woodpeckers were distributed widely 
diroughout the forested regions of Marin County during 
die atlas period, they occurred primarily in the lowlands 
and along moist drainages. Representative nesting loca- 
tions were Chileno Valley (FY 5/30/82 -DS); along 
Miller Creek, E end Big Rock Ridge (FY 6/6/82 -BiL); 
near Alpine Lake (NY 6/5/82 — DS); and Panoramic 
Hwy., W of Pantoll (FY 6/2/81 -DS). 

Historical Trends/Population Threats 

Few prior data exist, but numbers of Downy Woodpeckers 
decreased on Breeding Bird Surveys in California from 
1968 to 1989 (USFWS unpubl. analyses). 



HAIRY WOODPECKER Picoides villosus 














A year-round resident. 








An uncommon, somewhat local 








breeder; overall breeding population 


^~~<\ vx^C-'x ^\\ >-VaV Jv \ \/YoV/a VX, j 






small. 








Recorded in 82 (37.1%) of 221 blocks. 


x^^N^^vA^vA^J^^V^rjXv 














O Possible = 54 (66%) 


V^^VCvn- V^\ A^b3^^^\^A^\^V^C 






€ Probable = 10 (12%) 








T- 


• Confirmed = 18 (22%) 
FSAR = 2 OPI = 164 CI = 1.56 




^3^^--^>iv^Y\^c^V 


AO\>A 










5^€ 

^S^ 


VD 




. l^> -^<—%^ 


<sWr- 







Ecological Requirements 

Although near look-alikes except for subtle differences in 
size and plumage, Hairy and Downy woodpeckers are 
nonetheless quite distinct ecologically. The Hairy Wood- 
pecker occupies Marin County's conifer, mixed conifer, 
and moist evergreen hardwood forests and coastal riparian 
thickets. In the latter two forest types, it overlaps to a 
limited degree with the Downy Woodpecker. Hairies exca- 
vate their nest cavities in live trees, dead trees, or dead parts 
of live trees. Some authors have reported that Hairies 

250 



prefer live trees for nesting (Kilham 1965, Lawrence 1967), 
but they use dead trees or dead portions of live trees more 
commonly in the Sierra Nevada (Raphael ck White 1984). 
Nest holes range from 3 to 1 00 feet above the ground (Bent 
1939) and on the average are in firmer wood, in larger 
trees, and higher above the ground than those of Downies 
(Conner et al. 1975). The heights of 1 1 Hairy Woodpecker 
nests in Ontario ranged from 1 5 to 45 feet and averaged 
35 feet (Lawrence 1967); those of 10 nests in Virginia 



Woodpeckers 



SPECIES ACCOUNTS 



Woodpeckers 



ranged from 8 to 65 feet and averaged 29 feet (Conner et 
al. 1975); and those of 19 nests in the Sierra Nevada 
averaged 16 feet (Raphael 6k White 1984). In some areas, 
nest holes appear to be oriented with respect to light and 
warmth, with most having a southern or eastern exposure 
and those in open surroundings occurring at lower heights 
(Lawrence 1967). Conner (1975, 1977) reported that the 
slope of the trunk appears to be the most important factor 
in nest orientation. Nest holes facing slighdy downward 
prevent rain from entering the nest cavity, though locally a 
northeasterly orientation may also provide shelter from 
prevailing winds and rain. The downward orientation also 
aids in defense of the nest hole from predators. Lawrence 
(1967) reported that Hairy Woodpecker nest holes are 
often excavated near some sort of protrusion that provides 
camouflage and protection from weather. 

Birds forage mosdy by rapid drilling, pecking, probing, 
scaling, gleaning, and excavating on live and dead trees, 
stumps, downed logs, and occasionally on the ground; they 
sometimes hover and flycatch (Kilham 1965, Stallcup 

1968, Kisiel 1972, Conner 1981, Raphael & White 1984, 
Lundquist 6k Manuwal 1990). Hairies sometimes take 
advantage of insects uncovered by Pileated Woodpeckers' 
prying off thick bark or digging deep holes (Maxson 6k 
Maxson 1981). In Virginia in winter, Hairies rely more on 
scaling and excavating and less on pecking (Conner 1981 ; 
see for additional seasonal differences). In die Washington 
Cascades, Hairies increase pecking, decrease probing, and 
eliminate gleaning from spring to winter (Lundquist 6k 
Manuwal 1990). Compared with Downies, they tend to 
drill more often and deeper and to forage more on larger 
branches and trunks and on different tree species (Koplin 

1969, Kisiel 1972, Conner 1981). 

Hairy Woodpeckers expand their foraging niche by the 
males and females working different species of trees, or by 
each sex using different proportions of various foraging 
techniques, substrates, or heights. Females tend to special- 
ize in surface foraging methods on larger branches and 
limbs, whereas the males excavate deeper in dead trees and 
spend more time on smaller branches (Kilham 1965, 
Kisiel 1972). Morrison and With (1990) described the 
seasonal changes in the foraging niche of male and female 
Hairy Woodpeckers (relative to White-headeds) in the 
mixed conifer zone of the Sierra Nevada. In summer, 
males and females forage both in trees of similar height 
and at similar heights in these trees. Although both sexes 
forage at greater heights and in taller trees in winter, males 
exceed females in both categories then but still forage at die 
same relative height in trees as do females. Both sexes 
choose similar-diameter trees at both seasons. Males and 
females both concentrate their foraging on trunks at both 
seasons, but both sexes increase use of limbs during 
winter; they seldom use twigs in either season. Bodi sexes 
divide foraging activities relatively evenly between dead and 



live substrates in summer and winter. On the other hand, 
males make relatively even use of available tree species for 
foraging during both seasons, whereas females concentrate 
foraging on white fir and ponderosa pine in summer and 
incense cedar and black oak in winter. Males and females 
concentrate their foraging during the first five hours after 
sunrise in summer and the five to nine hours after sunrise 
in winter. Lundquist and Manuwal (1990) reported sea- 
sonal changes in Hairy Woodpecker foraging in the Wash- 
ington Cascades but did not distinguish between the 
patterns of males and females. 

The Hairy Woodpecker annual diet is about 78% ani- 
mal and 22% vegetable (Beal 1 91 1 , n = 382). It varies litde 
seasonally, with animal matter comprising 74%-76% in 
winter and fall and 80%-82% in spring and summer 
(Martin et al. 1951, n = 405). In the mountains of Califor- 
nia (season unspecified), the diet is about 92%-93% 
animal matter (Otvos 6k Stark 1985, n = 69). The main 
animal foods are wood-boring beede larvae, ants, caterpil- 
lars, weevils, true bugs, and scale insects, along with other 
insects, spiders, and millipedes. Although their diets are 
generally quite similar to Downies', Hairies consume 
about twice as many wood-boring beede larvae, attesting to 
dieir more frequent and deeper drilling (Beal 191 1). Com- 
pared with female Hairies, the males consume more wood- 
boring beetle larvae, bark beetles that inhabit thicker bark, 
and carpenter ants, again because die larger-billed males 
drill deeper; females consume more scale insects found 
under the loose, scaly bark of incense cedar (Otvos 6k Stark 
1985, Morrison 6k Widi 1990). The size of insects fed to 
young increases with the age of nesdings (Lawrence 1967). 
Vegetable foods include fruits, seeds, grain, mast, cam- 
bium, and sap. In the nonbreeding season, mast and pine 
seeds may be important foods (Beal 191 1 , Stallcup 1968). 

Marin Breeding Distribution 

During the adas period, the breeding distribution of the 
Hairy Woodpecker in Marin County was much more 
restricted than that of the Downy Woodpecker. Hairies 
were confined largely to Inverness Ridge, Bolinas Ridge, 
and the Mount Tamalpais watersheds. Breeding birds 
away from these areas were usually found in patchily 
distributed conifer or moist evergreen hardwood forests in 
canyons or on north-facing slopes. Representative nesting 
locations were Inverness (ON 5/2/80 — DS); Bear Valley, 
PRNS (NY 5/26/76 -RMS); Alpine Lake (FY 6/5/82 
-DS); and Bon Tempe Lake (FY 5/1 2/76 -RMS). 

Historical Trends/ Population Threats 

Few prior data exist, but numbers of Hairy Woodpeckers 
were relatively stable on Breeding Bird Surveys in Califor- 
nia from 1968 to 1989, despite a decrease from 1980 to 
1989 (USFWS unpubl. analyses). 



251 



Woodpeckers 



MARIN COUNTY BREEDING BIRD ATLAS 



Woodpeckers 



NORTHERN FLICKER Colaptes auratus 



-i.-rtft 






A year-round resident; numbers swell 


yCK^A?? 






substantially from Sep through Mar. 






An uncommon, widespread breeder; 


x°2^ 






overall breeding population small. 


^X\°Jk 


C3^\°V\^MPcv\o^^ 




Recorded in 143 (64-7%) of 221 








blocks. 

O Possible = 85 (59%) 








© Probable = 44 (31%) 






^& 


• Confirmed = 14 (10%) 
FSAR = 2 OPI = 286 CI = 1.50 




•>* w ^- <*^g 







Ecological Requirements 

Once highly prized for ornamentation by Native Ameri- 
cans, the intermittendy flashing, fiery orange-red flight 
feathers of Red-shafted Flickers still draw our admiration 
as birds course overhead in undulating flight. The Red- 
shafted subspecies of Northern Flicker, like its eastern 
counterpart, the Yellow-shafted Flicker, is wedded to grass- 
land edges of forests or to woodlands or openings within 
them. Flickers nest in all of Marin's forests, woodlands, or 
planted groves as long as open ground for foraging is 
available either within the habitat, in nearby meadows, or 
in grasslands. Flickers excavate most of their nest holes in 
dead trees, dead limbs of live trees, or stubs, but they 
sometimes select sides of houses, posts, and earthen banks. 
They rarely use live trunks or limbs of trees, as Flickers 
prefer very soft wood for excavating (Raphael ck White 
1984). Nests range from ground level to 100 feet, though 
most are from 8 to 30 feet above the ground (Bent 1939). 
The height of 68 nests in the Sierra Nevada averaged 25 
feet; some were enlarged nest cavities of other species of 
woodpeckers (Raphael & White 1 984). 

Of our woodpeckers, Flickers are least dependent on 
foraging on or beneath die bark of trees. They obtain much 
of their food by pecking and probing in the ground widi 
their beaks, scratching the surface of the ground, and 
picking fruits from trees and bushes; they sometimes hang 
almost upside down from swaying branches to procure 
elderberries. Birds also pick items from the surface of the 
ground, dig (peck and tear) into rotten stumps or logs, 
glean insects from trees and bushes, and, rarely, flycatch 
(Bent 1939). In ponderosa pine forest in Arizona, North- 

252 



ern Flickers show annual variation in their use of foraging 
techniques and other resource-use behaviors (Szaro et al. 
1990). The diet of the Red-shafted Flicker in California is 
about 54% animal food and 46% vegetable matter (Beal 
1910, n = 118), compared with 61% and 39%, respec- 
tively, for the Yellow-shafted form of the East (Beal 1911, 
n = 684). The animal food is overwhelmingly dominated 
by ants, particularly ground-dwelling forms, though bee- 
tles, crickets, grasshoppers, caterpillars, miscellaneous 
insects, spiders, sowbugs, snails, and myriopods contrib- 
ute minor amounts. Animal food may vary from 99% of 
the summer diet (n = 35) to only 33% in the fall (n = 48), 
when fruits predominate (Martin et al. 1951). Vegetable 
food consists primarily of small wild fruits, such as elder- 
berries and gooseberries, cultivated fruits, acorns and other 
mast, poison oak seeds, grains, and, infrequendy, cam- 
bium and weed seeds. Flicker young are fed by regurgita- 
tion (Lawrence 1967). 

Marin Breeding Distribution 

Although Flickers bred widely in Marin County during the 
adas period, diey were concentrated more toward the coast 
dian the interior. The coastal area is more heavily forested 
and has generally thicker and moister soil in openings, 
which presumably aids ground foraging. Representative 
nesting locations were Laguna Ranch, PRNS (ON 7/20/79 
— JGE), and beside a small pond near Soulajoule Reservoir 
(FY 6/17/82 -DS). 



Woodpeckers 



SPECIES ACCOUNTS 



Woodpeckers 



Historical Trends/Population Threats 

Although data are lacking, it seems likely that Flickers have 
increased as breeding birds in historic times as a result of 
the opening up of dense coastal forests by clearing for 
human needs. Although placed on the Audubon Society's 



Blue List for 1971 (Tate 1981), based on recommenda- 
tions from southern California and the central Rockies, 
numbers of Flickers were relatively stable on Breeding Bird 
Surveys in California from 1968 to 1989 and increased 
from 1980 to 1989 (USFWS unpubl. analyses). 



PILEATED WOODPECKER Dryocopus pileatus 









A year-round resident. 




^^-^ \ , r X^_ 




An uncommon, very local breeder; 


fj<^s3^ 




overall breeding population very small. 




\\%r^t%\ \^K^<\\r\t^C 




Recorded in 28 (12.7%) of 221 blocks. 




^S^O^^c^c^c^^C^ 




O Possible = 18 (64%) 


\dj~5 


•>&<A^^^ 




€ Probable = 7 (25%) 


Vu 


V Jr^^O^vX \^*\l£^\& \^\ jf^\ 'Jr-i 




• Confirmed = 3 (11%) 




^^^^^^Jk^^^0<\^ 


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-T^St— — - " r " 


FSAR = 2 OPI = 56 CI = 1.46 






\ jr~"s 








W^?°* 





Ecological Requirements 

This primeval-looking woodpecker inhabits Marin 
County's Douglas fir and redwood forests, moist mixed 
evergreen forests dominated by Douglas fir, and, to a 
limited degree, mixed bishop pine-hardwood forests. Pile- 
ateds are adapted to mesic forests with large-diameter trees, 
characteristic of ancient stands, but may not be dependent 
on old-growth forest since they use isolated patches of 
decay in highly productive conifer forests less than 75 years 
old (Harris 1982). Pileateds select structural features of nest 
trees and forest patches around their nests that are inde- 
pendent of the productivity or age of the stand. In Califor- 
nia, stands of trees around nest sites are characterized by a 
high density of large trees and dead material, particularly 
clusters of dead trees (Harris 1982). Pileateds in California 
need about four 1-acre patches of dense, naturally occur- 
ring tree mortality per 247 acres of forest. Although they 
use stumps and logs for foraging, the densities and volume 
of these are variable around nesting sites and are therefore 
not good indicators of the suitability of a stand for nest 
location. Although Pileateds do not use the venerable 
remnant trees from older forests for nesting, such trees 
may be a crucial habitat feature since carpenter ant galler- 



ies, a prime food source, are usually found in large diame- 
ter logs, stumps, standing dead trees, and live trees with 
basal wounds (Harris 1982). Although Pileateds often 
select nest trees close to water (Harris 1982), or sometimes 
standing in it (Carriger & Wells 1919), this appears to be 
an artifact of the moist forests they inhabit rather than a 
factor the birds consider when choosing a nest site (R.D. 
Harris pers. comm.). 

In California, Pileateds excavate nest cavities most fre- 
quendy in conifers such as white fir, ponderosa pine, 
Douglas fir, red fir, coast redwood, and giant sequoia, as 
well as in broadleaved trees such as quaking aspen, black 
oak, madrone, black cottonwood, big-leaf maple, elm, and 
white alder (Harris 1982). In this state, diey choose trees 
with an average diameter (at breast height) of 26 inches 
(range 20-36 in., n = 24) for nesting, even when larger 
snags are available; this compares widi an average of 30 
inches (range 15-47 in., n = 58) in Montana (McClelland 
1979), 30 inches (range 23-39 in., n = 13) in Oregon (Bull 
6k Meslow 1977), and 22 inches (range 13-36 in., n = 18) 
in Virginia (Conner et al. 1975). In the respective studies, 
nest hole heights averaged 59 feet above the ground in 

253 



Woodpeckers 



MARIN COUNTY BREEDING BIRD ATIAS 



Woodpeckers 



California, 50 feet (range 18-98 ft.) in Montana, 43 feet 
(range 23-62 ft.) in Oregon, and 67 feet (range 24-120 ft.) 
in Virginia. Nest holes may rarely be as low as about 2 feet 
off the ground (Bent 1939). Of the California nest trees, 
those of the pine family were all snags, usually with broken 
tops (Harris 1982). Nests in trees with intact tops were 
located just above midbole, whereas those in trees with 
broken tops were near the top, where decay would be most 
likely. Of the remaining nest trees, five of ten hardwoods 
were alive, two giant sequoias were alive, and one of two 
redwoods was alive. Of the eight nest cavities in live trees, 
one was in a live portion of a redwood, while the others 
were in dead portions of the live trees. All of the California 
nests were in trunks (radier than limbs), had clear flight 
paths to the entrance, and were not consistendy oriented 
with any specific compass directions. They tended to be 
located on the underside of the incline of a nest tree, as 
noted also in Virginia (Conner 1975), presumably because 
this affords protection from rain and predators. The 
entrance holes of Pileateds are larger relative to the bird's 
body size than holes of odier California woodpeckers and 
are distincdy oblong from top to bottom, instead of round. 
Since the nest cavity is usually excavated through advanced 
decayed heartwood, the chamber shape follows that of the 
decayed portion of the wood; the sound sapwood around 
the cavity apparendy provides protection from predators. 
Like other woodpeckers, in most cases Pileateds keep the 
bottom of the nest cavity practically bare, with only a few 
chips of wood left from excavation. One wonders if these 
woodpeckers bring in the sand or the few pebbles that on 
rare occasions are found on die floor of the nest chamber 
(Hoyt 1957). Pileateds often use nest trees in successive 
years in die East (Hoyt 1957), but not in California (R.D. 
Harris pers. comm.); only very rarely do they use a previous 
year's nest cavity again (McClelland 1979). Pileateds have 
been known to re-lay in the same cavity after the first set of 
eggs is taken (Carriger 6k Wells 1919, Bent 1939). In 
addition to nesting, snags are also important for roosting 
cavities (Bull 6k Meslow 1977). 

Pileateds put their massive bills to good use by drilling 
gaping holes deep into the rotting timber of large snags, 
logs, and stumps or by scaling off large pieces of bark with 
glancing blows in search of sequestered prey. The home- 
spun name "stump-breaker" apdy fits birds working on 
downed logs, as they strike alternate blows from side to 
side in the manner of lumberjacks, sending wood chips 
several inches long flying in every direction. Birds also peck 
and probe on the wood's surface, tear up anthills on the 
ground, or swing by their feet, head down, like giant 
pendulums as they grasp branches and gobble berries. The 
annual diet is about 73% animal, primarily carpenter ants 
and wood-boring beede larvae, and 27% vegetable, princi- 
pally wild fruit, along with a few seeds, mast, and a little 
cambium (Beal 1 91 1 , n = 80). The diet varies with season, 

254 



as do the methods of procurement. Animal matter in the 
diet varies from 94% in spring (n = 1 5) to 51% in fall (n = 
30) (Martin et al. 1951). During spring and summer, 
surface foraging methods predominate (Hoyt 1957, Con- 
ner 1981), and a wider variety of insects is eaten (Hoyt 
1957). At least in areas with harsh winters, breeding birds 
forage more on fallen logs, low stumps, and the lower 
portions of snags and live trees, presumably because of 
insect sources newly available on these substrates after 
snowmelt (Hoyt 1957, McClelland 1979). Later in the fall, 
wild fruits and mast take on increased importance. In the 
winter, birds eat chiefly insects, especially carpenter ants 
(the main prey throughout their range), secured predomi- 
nandy by excavating deep into the heart of trees, stumps, 
logs, power poles, and even decaying buildings (Bent 
1939, Hoyt 1957, Conner 1981). Although decayed wood 
is a preferred foraging substrate, Pileateds also feed on live 
trees, particularly when infested with insects (Bull 6k 
Meslow 1977). In Oregon, prime feeding areas are dense 
mixed-species forests widi high snag densities and more 
than 10% of the ground covered with logs (Bull 6k Meslow 
1977); logs without limbs and bark are preferred, as are 
natural stumps over cut stumps. Pileateds forage in selec- 
tively logged areas, as long as substantial numbers of snags 
and logs are available (McClelland 1979), but rarely on 
logs and stumps in cut-over areas less than 40 years 
old— probably because dense shrub and sapling cover limit 
access to them (Mannan 1984). 

Marin Breeding Distribution 

In Marin County, Pileated Woodpeckers were restricted 
during die adas period to the conifer forests of Inverness 
Ridge, Bolinas Ridge, and the Mount Tamalpais water- 
sheds. A representative nesting record was Lake Lagunitas 
(NE-FL 4/10-5/28/79 -RDH). 

Historical Trends/Population Threats 

Unrecorded in Marin County by Mailliard (1900), the 
Pileated Woodpecker's range on the California coast was 
thought to extend "casually" south to Marin County at the 
time of Grinnell and Miller's (1944) comprehensive avifau- 
nal summary. Knowledge of its status in Marin County 
then was apparendy based solely on sightings by C. Hart 
Merriam at Larkspur and along San Geronimo Creek in 
die vicinity of Lagunitas "on various occasions between 
1911 and 1918" (G6kW 1927). Grinnell and Miller 
(1944) noted declines of the species in California following 
the expansion of lumbering operations. 

Although the Pileated Woodpecker's range in Marin 
County today is probably similar to what it was early in this 
century, numbers may have increased since the era of 
intensive logging here. Harris (1982) noted that mainte- 
nance of Pileated populations was not incompatible with 
some logging. He felt that young-growth high-productivity 



Woodpeckers 



SPECIES ACCOUNTS 



Tyrant Flycatchers 



sites now probably supported these woodpeckers because 
the harvest methods and management strategies in vogue 
60 to 80 years ago left standing many large trees of low 
commercial value. He cautioned that today's intensive 
management practices may threaten to extirpate Pileateds 
from major portions of their California range, especially in 



low-productivity sites where old-growth forest is the only 
suitable habitat. Despite these concerns, Pileated Wood- 
pecker numbers on Breeding Bird Surveys in California 
increased in the period 1 968 to 1 979 (Robbins et al. 1 986), 
increased slightly from 1968 to 1989, and were relatively 
stable from 1980 to 1989 (USFWS unpubl. analyses). 



Tyrant Flycatchers 

Family Tyrannidae 



OLIVE-SIDED FLYCATCHER Contopus borealis 









A summer resident from mid-Apr 


A^\^P\ 


^V^-_ \ JCba~ 




through late Sep. 


f \^\®2<r 






A fairly common, fairly widespread 




J>c\ y<r\ \**x\^\ \^\\^\ J\ 




breeder; overall breeding population 
small. 




<r\ J>c\ -A"XA ji^\ * \^\ *> \^\ \^\ 




Recorded in 96 (43.4%) of 221 blocks. 


V-c"^- 


\Jr\>A^\ ( lr\ 3r"\ Jv^v c Jv^\ \ 






Vu 






O Possible = 16 (17%) 




v ° 3r<7>%r9?<\ \^\ x^\ -© v-^a v-^\ v^\ ^ 




€ Probable = 72 (75%) 






-r- 


• Confirmed = 8 (8%) 




<V^>?\ ®>V<\ ®3r<\ « ><^» >^T© \-^\VPr< 








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