BLM LIBRARY

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8802675"

SNAKE RIVER
BIRDS OF PREY

RESEARCH PROJECT

', .

ANNUAL REPORT
1976

U.S. DEPARTMENT OF THE INTERIOR
BUREAU OF LAND MANAGEMENT
JOISE DISTRICT
IDAHO

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Bureau Of Land Management
Denver Service Center

As the Nation's principal conservation agency, the
Department of the Interior has responsibility for most of
our nationally owned public lands and natural resources.
This includes fostering the wisest use of our land and water
resources, protecting our fish and wildlife, preserving the
environmental and cultural values of our national parks and
historical places, and providing for the enjoyment of life
through outdoor recreation. The Department assesses our
energy and mineral resources and works to assure that their
development is in the best interests of all our people. The
Department also has a major responsibility for American
Indian reservation communities and for people who live in
Island Territories under U.S. administration.

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SNAKE RIVER BIRDS OF PREY RESEARCH PROJECT

ANNUAL REPORT

1976

NOT FOR PUBLICATION

The data presented herein are preliminary and may be
inconclusive. Permission to publish or cite any of these
materials is therefore withheld pending specific authoriza-
tion of the Boise District, BLM, and the specific Principal
Investigator.

lichael N. Kochert
Project Leader

D. Dean Bibles

Boise District Manager

A. /*^iKX.f~

William F. Mathews
Idaho State Director

Bureau Of Und Management

Library

Denver Service Cent*

ACKNOWLEDGMENTS

The Bureau of Land Management wishes to thank those agencies
and individuals who assisted with the project. Appreciation is
extended to the Idaho Fish and Game Department, the U.S. Fish and
Wildlife Service; especially those personnel of Region I who
helped with the fall and winter aerial eagle survey. Thanks is
extended to the Idaho Cooperative Wildlife Research Unit;
University of Idaho for furnishing the original background data
and nest site locations; Mr. Morlan Nelson; and to those
individuals who volunteered their services.

Special appreciation is extended to Mr. E.T. Evans for
providing the area for the Melba field camp and other assistance.

SNAKE RIVER BIRDS OF PREY NATURAL AREA STAFF

William F. Mathews
D. Dean Bibles
Alan B. Tripp

Michael N. Kochert

Albert R. Bammann
Robert D. Rheiner
Douglas A. Smithey

Idaho State Director

Boise District Manager

Chief Resource Program Management,
Boise District

Project Leader, Snake River Birds
of Prey Research

Assistant Raptor Research Biologist

Bruneau Resource Area Manager

Management Biologist, Snake River
Birds of Prey Natural Area

COOPERATING AGENCIES

Idaho Fish and Game Department
U.S. Fish and Wildlife Service

PREFACE

The BLM became actively involved in raptor habitat preservation
and management on October 12, 1971 when Secretary of the Interior,
Rogers C.B. Morton, signed Public Land Order Number 5133 forming
the Snake River Birds of Prey Natural Area (Natural Area) . This
withdrawal preserved a 53 km (33 mile) portion of the Snake River
Canyon for the protection of nesting habitat for this raptor
population.

Work in the Natural Area commenced when the BLM hired a
research biologist in June 1972 to determine management and research
needs for the area. A management biologist was hired in October
1973 to develop and implement the management alternatives outlined
by research.

The Official Snake River Birds of Prey Study Area (BPSA) was
established in April 1974 while funding for the project came in the
summer and field work commenced in the winter of that year. A
moratorium on most land development on public lands within the
Study Area boundaries was declared in 1975 pending the outcome of
the Research Project.

The Snake River Birds of Prey Research Project was designed
when the basic research revealed that information on habitat
requirements, land tenure, predator-prey relationships and life
requirements of the raptors was needed to insure long-term manage-
ment and stability of this population. The objectives of the
Project are as follows:

1. Ascertain the necessary requirements in terms of habitat,
successional stage, food, and space needed to sustain a
breeding pair of the major raptors and rear their young.

2. Predict the effects of habitat alteration to the composi-
tion and demography of the prey base and ultimately on
reproductive performance and population dynamics of the
major raptors.

3. Develop a long-term management model for the Natural Area
and BPSA.

The Project now consists of eight integrated studies (seven
university contracts and a BLM in-house study) designed to investi-
gate all components of the ecosystem. Research efforts are supervised
and directed by the BLM's Project Leader.

This report presents results of the second year of this four
year integrated research project. The information gathered from
each of the component studies will be incorporated into a descriptive
ecosystems model to describe the interrelationship of the raptors
(and mammalian predators) to prey and prey to habitat. This
information will provide the basis of establishing a new permanent
boundary for the Snake River Birds of Prey Natural Area and will
generate the predictive capacity to estimate the effects of land
use changes. A final report will be submitted by January 1, 1980.

TABLE OF CONTENTS

Study

Title

REPRODUCTIVE PERFORM-
ANCE, FOOD HABITS, AND
POPULATION DYNAMICS
OF RAPTORS.

II RESPONSES OF NESTING
RAPTORS TO HUMAN
DISTURBANCE.

Ill ACTIVITY, HUNTING

PATTERNS, TERRITORI-
ALITY, AND SOCIAL
INTERACTIONS OF BIRDS
OF PREY.

Principal
Investigator (s)

Page

Michael N. Kochert 1

(Project Leader)

and
Albert R. Bammann

BLM

Dr. Steven R. Peterson 58
Dept. of Wildlife Resources
University of Idaho

Dr. Thomas C. Dunstan 63

Western Illinois

University

IVA DENSITY AND SPECIES
COMPOSITION OF SMALL
MAMMALS AND BIRDS.

IVAS ECOLOGY OF REPTILES
IN THE SNAKE RIVER
BIRDS OF PREY NATURAL
AREA.

IVB DYNAMICS OF NON-AVIAN
PREDATION UPON THE
RAPTOR PREY BASE.

V POPULATION ECOLOGY

AND HABITAT REQUIRE-
MENTS OF TOWNSEND
GROUND SQUIRRELS.

VII FEEDING ACTIVITY AND
BEHAVIOR OF PRAIRIE
FALCONS .

Drs. Michael L. Wolfe 133

and John Neuhold
Utah State University

Dr. Donald R. Johnson 165

and Mr. Lowell Diller
Dept. of Biology
University of Idaho

Dr. Maurice Hornocker 181
Idaho Cooperative Wildlife
Research Unit
University of Idaho

Dr. Donald R. Johnson 203
Dept. of Biological Science
University of Idaho

Dr. Steven R. Peterson
Dept. of Wildlife Resources
University of Idaho

227

Mike Kochert, Study I, bands a juvenile turkey vulture at the
first known vulture eyrie in the Study Area (BLM photo by
Bammann) .

STUDY I: Reproductive performance, food habits and

population dynamics of raptors in the Snake
River Birds of Prey Natural Area.

INVESTIGATORS: Michael N. Kochert, Principal Investigator.

Albert R. Bammann, Assistant Research Biologist.
John H. Doremus, Research Technician.
Michael DeLate, Research Technician.
Jill Wyatt, Research Aid.

PROJECT SUPPORT: BLM "in house" study.

OBJECTIVES :

1. To annually determine absolute nesting density and
reproductive performance of the major nesting raptors.

2. To annually ascertain quantitative food habits of the
major nesting raptors and to attempt to estimate
absolute food intake for select species.

3. To ascertain seasonal species composition, population
densities, age compositions and mortality rates of
raptors within the study area.

ANNUAL SUMMARY

Nesting surveys indicated that at least 474 pairs of raptors
and ravens occupied sites within the Birds of Prey Study Area in
1976. Prairie falcons were the most numerous raptors. Within the
Intensive Study Area, raptors averaged 2.1 pairs per km . Nest
density appears to be related to amount of available cliff.
Nesting phenology and reproductive performance were similar to
previous years. We banded 326 ravens and raptors and color
marked 195. Food habits information for five species was collected
from 36 nests and stratified according to land use category.
Preliminary results show that nest visits by researchers do not
reduce productivity. 1976 research efforts included fostering
experiments, road counts of raptors, and aerial transect censuses.
Numbers of eagles counted from transects in October 1976 were the
lowest ever recorded since the census began in 1972.

METHODS

Definition of Terms

The "breeding season" is the time elapsed between first
courtship and observation of the last young in the vicinity of its
natal nest. "Courtship," as used in this study refers to activity
associated with pair bonding, territorial defense, or reproduction
by adult birds in the vicinity of a potential nest site.

A "traditional site" (site) refers to an area of cliff or a
group of nests used in this or in previous years by a mated pair
of raptors. A site is "vacant" if no adults were seen during the
breeding season, or if an adult with no site tenacity was observed.
A !'one bird site" is a site where we repeatedly observed a lone
adult bird but no breeding attempt. An "occupied site" is any
site occupied or defended by two or more adult raptors (a "pair"')
at any time during the breeding season. Pairs that occupied sites
but showed no evidence of laying an egg were categorized as "non-
breeding. "

A "nesting attempt" refers to an occupied site that contained
an incubating adult, eggs, young, or any field sign that indicated
eggs were laid regardless of success. A "successful nesting
attempt" is a nesting attempt that produced one or more young that
reached fledging age (i.e. the age at which most young are capable
of flight). Active golden eagle (Aquila chrysaetos) and Buteo
spp. nests discovered after young had fledged were considered
successful if the nest platform was well worn with sufficient prey
remains and fecal matter, and if no dead young birds were found
within a 50 m radius of the nest. We considered renesting attempts
as separate new attempts in calculating productivity and chronology
data.

"Maximum brood" is the maximum observed number of hatching
chicks in a nesting attempt (Olendorff 1973).

Nesting Population, Density, Chronology, and Reproductive Performance

Systematic searches for breeding raptors began in January and
followed procedures outlined by Kochert et al. (1975) . We continued'
intensive surveys of the Birds of Prey Study Area (BPSA) during
the nesting season with special emphasis on the Intensive (Ogden)
Study Area (Fig. 1). This year, we surveyed several areas within
the BPSA omitted in previous years. We also surveyed portions of
the Snake River and its tributaries outside the BPSA from Civens
Hot Springs to Bliss, Idaho (Kochert 's (1972) golden eagle study
area) (Fig. 2) once early in the nesting season and once during
the banding season. Most site locations from Swan Falls to
Halverson Lake were provided by Study VII personnel.

We calculated average minimum distance (Ratcliffe 1962)
between nests as the direct line distance between closest active
nests. For non-breeding pairs, we computed distances between the
most recent active nest site or most active roost.

The Snake River Canyon from river mile 431.5 (near Givens Hot
Springs) to Bliss, Idaho (Fig. 2) was divided into 10 km (6.2 mi)
sections measured along the river. The western end of the Snake
River Birds of Prey Natural Area (BPNA) was used as the starting
point, and sections upstream and downstream were measured from

Fig. 1. Snake River Birds of Prey Study Area (BPSA) including the Intensive (Ogden)
Study Area.

BOISE

MARSING

GIVEN!

HOT
SPRINGS

10

—I

Kl LOMETERS

BLISS

?ig. 2. Location of the Snake River Birds of Prey Study Area and other areas surveyed
in 1976.

this point. The number of kilometers of available cliff (any-
outcropping higher than 5 m) and the number of occupied sites of
each species in each section were recorded.

To determine occupancy of sites and nesting phenology,
traditional sites, selected prior to the nesting season, were
repeatedly visited until these parameters were determined. Sites
studied for phenology were visited at least weekly. Most potentially
active nest sites were entered and examined for eggs, eggshell
fragments, young, or other sign of reproductive activity.

Reproductive performance was assessed from visits to the
preselected traditional sites, nests surveyed every four days for
food habits, and other sites where clutch, maximum brood, and
fledge were recorded. Other nests were visited once to determine
occupancy and once (at banding time) to determine success.

Reproductive data collected consisted of clutch size, maximum
brood, fledging success, and chick mortality factors. Since
"maximum brood" does not distinguish early nesting mortality from
embryonic mortality, we minimized this bias by assessing maximum
brood size only when broods were less than 10 days old. Nests
were not visited during inclement weather or immediately after
hatching to prevent nest desertion or temperature stress to eggs
or young.

Average laying, hatching, and fledging dates were calculated
using Julian dates. Some nests were inadvertently entered at
hatching, and therefore, we obtained some exact hatch dates.
Otherwise, hatch dates were assigned to young under 10 days old.
Weight and external morphology (eye, beak, toenail, and/or skin
color and feather development) of known age birds were used to
establish approximate aging criteria for each species. We back-
dated 45 days from observed or estimated hatch dates for golden
eagles (Beecham 1970) and 30 days for all other species (Bent
1938, Smith and Murphy 1973) to estimate laying dates.

Pre-fledging mortality rates of the, major species were
calculated from direct counts of dead young in the nest and
tallies of young that disappeared from nests prior to fledging
age. To ascertain post-fledging mortality rates, we made intensive
searches for dead birds above and below the cliff for several
hundred meters to the limit of all known perches of the post
fledge young. Banded young found dead away from their natal area
but within the BPSA were included in the post fledge mortality
totals. Dead young were categorized "pre-f ledge" if 25 to 50
percent of the primary vane was still in sheath. Dead young
were categorized "post-fledge" if primary vanes were less than 25
percent in sheath or if field sign indicated the young died more
than 50 m from the nest.

We collected dead nestlings and unhatched eggs to determine
cause of death. Decomposed birds found in the field were only
examined for obvious causes of mortality and destroyed. Fresh
carcasses and unhatched eggs, as in past years, were frozen for
eventual examination. With missing nestlings, the cause of
mortality often could not be determined unless there was evidence
of predation.

Investigator Disturbance

The effects of researcher activity on raptors and ravens were
determined by comparing the number of young fledged from nests
visited or disturbed with the number fledged from unvisited
(control) nests. A "disturbance" is any nest visit where an adult
or young visibly or vocally responded to the presence of researchers.
Control nests were observed only from a distance to verify occupancy
prior to counting fledgings. The control group included nesting
attempts that failed before the nest was located. Attempts that
failed after the first visit, even when the failure was caused by
natural events, were included in the appropriate visit category.

We stratified disturbed nests in relation to number of
visits. We categorized food habits sites separately since visits
there always involved entering the nest. Chicks at food habits
nests were sometimes treated for disease or protected from the
elements by construction of shade devices. Many of these nests
are used for food habits investigations each year and the adults
may be habituated to human disturbance.

Banding and Color Marking

Nestling raptors and ravens (Corvus corax) were banded with
U.S. Fish and Wildlife Service leg bands. All birds banded by
personnel in Study I, III, and VII were under the BLM Master
Station Permit, and data are pooled. We measured the foot pad on
all banded golden eagles, red-tailed hawks (Buteo jamaicensis) ,
ravens, and prairie falcons (Falco mexicanus) and placed wing
markers and/or painted most of them following the procedure
outlined by Kochert et al. (1975). Over 700 requests for information
on sightings of marked raptors were prepared and distributed to
organizations and agencies in the western United States and Canada.

Food Habits

Food habits for golden eagles, prairie falcons, red-tailed
hawks, ferruginous hawks (Buteo regalis) and ravens were determined
by analysis of castings (regurgitated pellets) and prey remains
collected from nests visited every four days. Collections began
within 10 days after hatching or when the nest was first located,
and lasted until fledging. Edible remains were identified,
counted, marked and left in the nest (Kochert et al . 1975).

[unMwirnwiwiB^iM

Prey remains and castings were analyzed by Mr. Jackson Whitman
at the University of Idaho and by BLM research staff. We calculated
prey numbers in castings from a maximum count of femurs and/or
toenails and/or mandibles (Kochert et al. 1975). We stratified
individual prey items into age and sex classes, assigned appropriate
weights to each class, and calculated prey biomass (Appendix 1).
Unknown size and age classes of each species were assigned an
average weight.

We selected previously studied nests to assess yearly changes
in food habits (Tables 1-5). Some traditionally studied pairs did
not breed or failed during incubation which necessitated collections
from nests not previously studied.

We stratified golden eagle, prairie falcon, red-tailed hawk,
ferruginous hawk and raven sample nests according to the native
vegetation type, amount of agricultural land, type of farming, and
density of Townsend ground squirrels within a 5 km (3 mi) radius
of the nest (Tables 1-5). Nests were classed as "agricultural"
where at least 50 percent of the land was farmed, "intermediate"
where 10 to 50 percent was farmed, and "range" where less than
10 percent was cultivated. "Clean" farming is the practice of
eliminating rank vegetation along fence lines, irrigation ditches,
and roadsides. "Old" farms are those with uncultivated rank
vegetation surrounding most plowed fields.

In addition to the five major species, we obtained burrowing
owl (Speotyto cunicularia) , great horned owl (Bubo virginianus) ,
and barn owl (Tyto alba) food habits by periodic collections of
fresh castings at the mouths of burrows and under roosts and
active nests.

Raptor Populations

Raptor seasonal trends were monitored monthly by a 112 km
(70 mi) raptor road transect (Kochert 1973, Kochert et al. 1975).
Species, age, sex, and activity of raptors within 0.4 km (0.25 mi)
of the vehicle as well as vegetation type were recorded. An
effort was made to conduct transects at the same time of day and
under the same weather conditions.

Golden eagle aerial transects were flown in January and
October over 17,920 km2 (7,000 mi2) of the Snake River floodplain
(Fig. 3) following the procedures of Boeker and Bolen (1972) and
Kochert et al. (1975).

Table 1. Stratification of golden eagle nests visited for food habit collections, Snake River
Birds of Prey Study Area, 1976.

Nest
Name

No. of Land Use Desert
Collections (Farm Type) Vegetation

Breeding Years
Chronology Studied

Otis

Walter's Butte

Chalk Gulch

Bobcat

Pump Station

Feedlot

Strike II

Cabin

Lower Reynolds

14

12

L3

10

8**

14

15*

12

11

Agricultural Big Sage (Artemisia tridentata) Early

(old) Grass (Bromus tectorum)

Agricultural Big sage (Artemisia tridentata) Late

(old) Grass (Bromus tectorum)

Agricultural Big sage (Artemisia tridentata) Average

(clean) Grass (Bromus tectorum)

Intermediate Big sage (Artemisia tridentata) Average
(ranch)

1971, 73,

74, 75

1970, 71, 73,

74, 75

1971

1971, 73, 74,

75

Intermediate Greasewood (Sarcobatus vermiculatus) Average 1971, 73, 75
(clean) Big sage (Artemisia tridentata)

Intermediate Shadscale (Atriplex conf ertif olia) Average
(pasture)

1971

Range
Range
Range
Range

Shadscale (Atriplex conf ertif olia) Average
Budsage (Artemisia spinescens)

Shadscale (Atriplex conf ertif olia) Late
Winterf at (Eurotia lanata)

Big sage (Artemisia tridentata) Average
Shadscale (Atriplex conf ertif olia)

1974, 75
1970, 71, 73

San Sebastian

* Young died of trichomoniasis, 23 May.
** Collected by Study III personnel.
*** Includes seven collections by Study III personnel.

3ig sage (Artemisia txideatata) Average 1973

Table 2. Stratification of prairie falcon nests visited for food habit collections, Snake River
Birds of Prey Study Area, 1976."

Nest
Name

No. of Land Use Desert
Collections (Farm Type) Vegetation

TGS** Years
Density Studied

Castle

Pump Station

Chalk Gulch

Sandclif f

Tick II
Tom

Upper Bigfoot
Cabin

Strike II

3
6

7
9

Intermediate Greasewood (Sarcobatus vermiculatus) Moderate 1974

(clean) Shadscale (Atriplex conf ertif olia)

Intermediate Big sage (Artemisia tridentata) Moderate

(clean) Grass (Bromus tectorum)

Intermediate Grass (Bromus tectorum)

(clean) Big sage (Artemisia tridentata)

Moderate

Intermediate Shadscale (Atriplex conf ertif olia) Low
(old)

Range
Range

Range
Range

Range

1975

1974, 75
1974, 75

Winterfat (Eurotia lanata) High

Winterfat (Eurotia lanata) High

Shadscale (Atriplex conf ertif olia)

Shadscale (Atriplex conf ertif olia) Moderate

Shadscale (Atriplex conf ertif olia) Low 1974, 75
Winterfat (Eurotia lanata)

Shadscale (Atriplex conf ertif olia) Low
Budsage (Artemisia spinescens)

* All nests, average breeding chronology except Chaulk Gulch which was earlier than normal
...... rpQg _ Towns end Ground Squirrel

Table 3. Stratification of red-tailed hawk nests visited for food habit collections, Snake
River Birds of Prey Study Area, 1976.

Nest No. of Land Use Desert TGS Breeding Years
Name Collections (Farm Type) Vegetation Density Chronology Studied

Halverson I 6 Agricultural Big Sage (Artemisia tridentata) High Early 1975

(old & clean) Grass (Bromus tectorum)

Castle Butte 9 Agricultural Greasewood (Sarcobatus vermiculatus) None Average

(clean) Shadscale (Atriplex conf ertif olia)

Black Butte 5* Intermediate Greasewood (Sarcobatus vermiculatus) Low Late

(clean & old) Shadscale (Atriplex conf ertif olia)

Pump Station 7 Intermediate Big sage (Artemisia tridentata) Moderate Late

(clear)

Tick 8 Range Winterfat (Eurotia lanata)

Tom Draw 8 Range Winterfat (Eurotia lanata)

Shadscale (Atriplex conf ertif olia)

Spoon 9 Range Shadscale (Atriplex conf ertif olia)

High Average

1973,

74,

75

High Average

1973,

74,

75

Moderate Average

1973,

75

Montini 11 Range Shadscale (Atriplex conf ertif olia) None Average

(old) Bi°: sage (Artemisia tridentata)

* Includes one collection by Study III personnel.

Table 4. Stratification of ferruginous hawk nests visited for food habit collections, Snake
River Birds of Prey Study Area, 1976."

Nest
Name

No. of Land Use Desert
Collections (Farm Type) Vegetation

TGS**

Density

Years
Studied

Pump Station 7

Rattlesnake 7

Powerline 6

Upper Bigfoot 6

Intermediate Big sage (Artemisia tridentata) Moderate
(clean) Greasewood (Sarcobatus vermiculatus)

Range

Range
Range

Big sage (Artemisia tridentata) Moderate
Grass (Bromus tectorum)

Shadscale (Atriplex conf ertif olia) Low

Shadscale (Atriplex conf ertif olia) Low

* All nests, average breeding chronology
;•:< TGS = Townsend Ground Squirrel

1975

Table 5. Stratification of common raven nests visited for food habit collections, Snake River
Birds of Prey Study Area, 1976."

Nest

Name

No. of Land Use Desert
Collections (Farm Type) Vegetation

TGS**

Density

Years
Studied

Halverson Road 3**

Fawn Feedlot 7

Balls Basin 5

Tom Road 9

Murphy Rim 5

Agricultural Big sage (Artemisia tridentata) High

(clean) Grass (Bromus tectorum)

Intermediate Shadscale (Atriplex conf ertif olia) Low

(clean) Greasewood (Sarcobatus vermiculatus)

Range
Range

Range

Winterfat (Eurotia lanata) High

Winterfat (Eurotia lanata) High
Shadscale (Atriplex conf ertif olia)

Greasewood (Sarcobatus vermiculatus) None
Shadscale (Atriplex conf ertif olia)

1975
1974, 75

* All nests, average breeding chronology
"" TGS = Townsend Ground Squirrel
'** Adult shot; young died of starvation between collections.

u

Fig. 3. Location of the golden eagle aerial survey with transect lines numbered.

14

RESULTS

Breeding Chronology

The breeding chronology for all species was similar to that
observed in 1973-75 (Table 6) . The first sighting of courtship by
golden eagles, prairie falcons and ravens occurred on 22 January
1976. Golden eagles were the first of the five major species
raptors in the area to lay eggs (average laying date, 2 March),
and ferruginous hawks were the latest layers (average laying date,
23 April). Most ravens, red-tailed hawks, and prairie falcons
laid eggs in the first two weeks of April (Table 6) .

Average hatching dates were 17 April for golden eagles, 4 May
for ravens, 6 May for red-tailed hawks, 13 May for prairie falcons,
and 23 May for ferruginous hawks. Average fledging occurred in
the first two weeks of June for the five major species except
ferruginous hawks (Table 6) .

The duration of time young are in the nest cannot be directly
calculated from the average hatch to average fledge dates. A high
proportion of the hatch dates were from late breeding pairs
because of our reluctance to flush brooding birds during cool
spring temperatures. Our sample of fledge dates is more representa-
tive of the true population.

Golden eagles had the longest breeding season while ferruginous
hawks had the shortest. Ferruginous hawks were the last major
species to return to the nesting area, last to initiate courtship,
and last to lay and hatch eggs. Although the onset of ferruginous
hawk dispersal was later than for the other major raptor species,
the ferruginous hawk dispersal period was shorter. Young ferruginous
hawks disappeared from their nest sites by the same time as young
of other species.

Some late nesting or renesting prairie falcons and red-tailed
hawks were observed this year. Four red-tailed hawk pairs renested
and two were successful. The latest red-tailed hawk attempt
hatched three young on 20 June but the young were found dead two
weeks later. A prairie falcon pair hatched two young on 30 June.
One young was still alive but emaciated (weight 355 gm) on 30 July
when banded. It was missing from the nest on 3 August. The band
from this individual was found within a great horned owl cast near
the nest in spring 1977. Data from this exceptionally late brood
is excluded from Table 6.

In all species of raptors, except ferruginous hawks, young
did not fledge from any nest later than the end of June. Broods
that would have fledged later than June all died prior to fledging.
High temperatures, decreasing prey population (Townsend ground
squirrel estivation) , possible reduced prey vulnerability (mammals

Table 6. Breeding chronology of ravens and raptors in Che Snake river Birds of Prey Study Area, 1976.

Hatch Fledge
Laying x x First** Last Last Length of
First First x range range Young Young Adult Breeding
Species Arrival Courtship range N N Dispersed Seen'1'** Seen Season (Days)

Golden Eagle * 22 Jan 2 Mar 17 Apr 6 Jun

8 Feb-22 Mar 24 Mar-6 May 23 May-27 Jun 2 Jun 11 Aug * 202
18 8

Red- tailed hawk 4 Feb 13 Feb 6 Apr 6 May 11 Jun

23 Mar-21 May 22 Apr-20 Jun 29 May-21 Jun 8 Jun 8 Aug 24 Aug 177

8 12

Prairie falcon 18 Jan 22 Jan 13 Apr 13 May 9 Jun

1 Apr-6 May 1 May-5 Jun 21 May-26 Jun 12 Jun 21 Jul 5 Aug 181
13 15

Ferruginous hawk 4 Mar 26 Mar 23 Apr 23 May 22 Jun

14 Apr-7 May 14 May- 6 Jun 14 Jun-12 Jul 29 Jun 27 Jul 30 Jul 123
6 5

Raven * 22 Jan 4 Apr 4 May 1 Jun

17 Mar-28 Apr 16 Apr-28 May 15 May-10 Jun 5 Jun 6 Aug * 197

8 14

* Year round residents
** First date that a fledgling cannot be located near its natal site if it is not observed on subsequent visits.
*** Date that last young of species was seen in vicinity of natal site.

16

becoming more wary with age) , and behavioral changes in adult
raptors may explain higher mortality of late broods.

Nesting Density

9 9

Within the arbitrary limits of the 135 km (60 mi ) Intensive

(Ogden) Study Area, 278 pairs of ravens and raptors (Table 7)

averaged 2.1 pair per km , similar to the 2.0 pair per km^ observed

in 1975.

The average minimum distance between golden eagle nests in
the BPSA was 4.93 km, and the average minimum distance between
red-tailed hawk nests was 2.47 km. Raven and prairie falcon nests
averaged 1.56 and 0.66 km apart respectively. Results were similar
to those in 1975 which were derived from the Intensive (Ogden)
Study Area.

The number of occupied raptor and raven sites and the linear
amount of potential nesting cliff per 10 km section of the Snake
River are presented in Figure 4. The highest total raptor and
raven density occurs between Halverson Lake and Swan Falls Dam.
This area also possesses the longest stretch of high cliffs (70 m
or higher) in the BPSA. The second largest density occurs in the
C.J. Strike Reservoir area which also contains a large amount of
cliff.

More than 50 percent of the variation in red-tailed hawk,
prairie falcon, and raven pair numbers can be attributed to the
amount of cliff available (r^ = .77, .57, and .55 respectively).
However, only 38 percent of the variation in golden eagle density
could be attributed to cliff availability (r^ = .38). Although
raptor density may be partially determined by availability of
cliff sites, other factors including prey density and territorial
requirements probably influence number of pairs present. Golden
eagles are known to have specific territorial requirements (Brown
and Watson 1964) , and this may explain the low correlation between
cliff availability and eagle density. Ravens, red-tailed hawks,
and prairie falcons have less rigorous territorial requirements
and are more responsive to availability .of cliff nesting sites.
Numbers are higher in areas with more nesting habitat.

Nesting Population

We located 474 pairs of raptors and ravens within the BPSA
(Table 7). Prairie falcons were most numerous, comprising 195
(41 percent) of the total pairs. Ravens were the second most
numerous large breeding birds on the area (98 pairs or 21 percent
of the population). Red-tailed hawks (52 pairs) and golden eagles
(32 pairs) were the next most numerous raptors.

17

Table 7. Number of raven and raptor pairs located in the
Natural Area, Ogden Study Area and the BPSA.

Ogden

Natural

Study

Species

Area

Area

BPSA

Golden Eagle

11

14

32

Prairie Falcon

114

127

195

Red-tailed Hawk

23

31

52

Kestrel

11

17

20

Ferruginous Hawk

4

6

13

Swainson's Hawk

0

0

1

Marsh Hawk

6

7

19

Great Horned Owl

5

9

15

Barn Owl

4

7

14

Screech Owl

2

2

2

Long-eared Owl

2

2

2

Burrowing Owl

0

0

10

Turkey Vulture

1

1

1

Common Raven

40

55

98

223 2 78 474

18

40

30

§

0

TO

60

50

BE

0

BC

03

z?

30

20

"■■■ ■ ™" All Raptors

' — "-■ — Prairie Falcons

1,1 "" Golden Eagles

w

-* •,*-./,,0»"'''*''''.,...---*','-*fr

-(

r

%w£_wl

1 f "C"F*

t

WEST

SWAN

C.J- BRUNEAU

EAST

GLENNS

END

FALLS

STRIKE

END

FERRY

BPSA

DAM

DAM

BPSA

BLISS

Fig. 4. Number of raptor pairs in relat

ion to cliff availability

in the Snake River Canyon from Givens Hot Springs t

Bliss, Idaho, 1976.

19

Of the 474 pairs, 223 were within the Natural Area and 278
were within the Intensive (Ogden) Study Area (which includes the
Natural Area) (Table 7). We believe' the 1976 totals for large
cliff nesting raptors are complete, but the tally of riparian and
farmland nesting species, as well as cliff nesting kestrels (Falco
sparverius) , is still inadequate. A tripling of the 1976 manpower
would be necessary to complete a census in all habitats.

The BPNA had more nesting raptors than any other areas along
the Snake River Canyon. For example, we located only five prairie
falcon, eight red-tailed hawk, six raven, and six golden eagle
sites along the Snake River and adjacent side draws from Hammett,
Idaho to the mouth of Malad Canyon (58 km or 36 mi) . Although the
53 km (33 mi) BPNA was more intensively surveyed, the population
differences between these two stretches of similar length are
striking.

Known prairie falcon pairs in the Intensive (Ogden) Study
Area increased from 117 in 1975 to 127 in 1976 (eight percent
increase). Compared with 1975, red-tailed hawk and raven pair
numbers in the Intensive (Ogden) Study Area increased from 28 to
31, and 49 to 55 respectively. Eagle pair numbers remained the
same for both 1975 and 1976.

Ferruginous hawk pairs in the Intensive (Ogden) Study Area
decreased from seven to six, though there was an increase in
occupied sites (8 to 13) found in the total BPSA. Searches in
sections of BPSA not previously surveyed facilitated the discovery
of more ferruginous hawk and raven nests.

Known barn owl pairs in the Intensive (Ogden) Study Area
increased from three in 1975 to seven in 1976. Sites occupied by
great horned owls decreased from 14 in 1975 to 9 in 1976. Although
some barn owls and great horned owls may have been missed, the
area was repeatedly and intensively surveyed both years.

We observed vacant traditional sites of all the major species
(Table 8) . Ferruginous hawks had the greatest proportion vacant
(5 of 10 preselected sites). We also observed unmated prairie
falcons and ravens at traditional sites. Despite the vacancies,
we identified 3 new red-tailed hawk, 11 new prairie falcon, and
1 new ferruginous hawk sites occupied by pairs in the Intensive
(Ogden) Study Area. These results indicate that the presence of
vacant sites does not imply a population decline. In contrast,
there is an apparent shifting of sites with new sites being occupied
and old sites being abandoned. An influx of new pairs has apparently
caused a net increase of the five major raptors in the BPSA.

20

Table 3. Occupancy at preselected traditional sites of ravens and the
major raptors in the study areas, 1976.

No .

No.

No.

Total No.

Sites

One bird

Occupi

ed

Preselected

Species

Vacant

Sites

Sites

-:■■

Sites

Golden eagle

3

0

58

61

Red- tailed hawk

5

0

23

33

Common raven

3

2

39

44

Prairie falcon

3

1

27

31

Ferruginous hawk

5

0

5

10

TOTAL 19 3 157 179

* Sites with decorated nests were assumed to be occupied. Sites without
either decorated nests or white wash in the vicinity were assumed to be
vacant.

2]

Reproductive Performance

Golden eagle. Average clutch size of golden eagles was 1.76,
and maximum brood averaged 1.04. We observed the first three egg
clutch since 1971. All three eggs hatched, but one young died
soon after hatching.

Eagles fledged 0.69 young per pair, 1.11 young per attempt,
and 1.62 young per successful attempt (Table 9). Loss of eggs and
young occurred mostly in nests where a single egg had been laid.
Average clutch and maximum brood size was slightly lower this year
than in any year since 1973, and the number of breeding pairs was
seven percent less than last year. The numbers fledging per
attempt and per pair were slightly higher than observed in those
years (Kochert 1973, Kochert et al. 1974, 1975). Increased juvenile
survival apparently caused this contradiction. Golden eagle pairs
did not breed at 19 of 56 (34 percent) occupied sites (Table 10).
The eagle population appears to be reproducing normally and is
beginning to respond to an increase in rabbit numbers in some
habitats.

Prairie falcon. Clutch size of prairie falcons averaged 4.21
and maximum brood averaged 3.16. Falcons fledged 2.91 young per
pair, 3.16 young per attempt, and 4.0 young per successful attempt
(Table 9). Average clutch, maximum brood, and fledglings per
breeding attempt were below the 1975 levels (Kochert et al. 1975).
However, average fledglings per pair was higher than all previous
years because of a decrease in non-breeding. In 1976, only one of
25 preselected pairs (four percent) did not breed (Table 10) .
Non-breeding in 1975 was 14 percent (Kochert et al. 1975) and
Ogden and Hornocker (1977) estimated between 10 to 14 percent non-
breeding for this population in 1972. Although yearly fluctuations
in productivity have occurred in this population since 1971, they
have not been appreciable (Kochert et al. 1974, 1975; Ogden and
Hornocker 1977) .

Red-tailed hawk. Clutch and maximum brood size of red-tailed
hawks averaged 3.05 and 2.18 respectively. Red-tailed hawks
fledged 1.54 young per pair, 1.77 young per attempt, and 2.30
young per successful attempt (Table 9) . One pair laid five eggs
and fledged all young. This was the only pair to lay five eggs in
1975. Average clutch size was down from 1973 and 1975 but above
the 1974 level (Kochert 1973, Kochert et al. 1974, 1975). Maximum
brood and numbers fledging per attempt were similar to 1975, but
fledgings per pair were slightly lower. This was a result of an
increase in non-breeding from 7 percent in 1975 (Kochert et al.
1975) to 22 percent in 1976 (Table 10). The 1976 level of non-
breeding was similar to that observed in 1973 and 1974 (Kochert
1973, Kochert et al. 1974).

Table 9. Raptor and raven productivity in the Snake River Birds of
Prey Study Area, 1976.

Golden

Prairie

Red-tailed

Ferruginous

Common

Eagle

Falcon

Hawk

Hawk

Raven

Clutch Size

No.

13

23

19

3

10

X

1.76

4.21

3.05

4.00

5.20

range

1-3

1-6

2-5

4

4-7

Maximum Brood

No.

21

25

22

8

21

X

1.04

3.16

2.18

3.12

3.66

range

0-3

0-6

0-5

0-5

0-7

Fledged/Pair*

No.

56

25

26

6

40

X

6.9

2.91

1.54

2.54

2.55

range

0-2

0-6

0-5

0-4

0-6

Fledged /At tempt

No.

35

43

26

12

34

X

1.1

3.16

1.77

2.75

2.88

range

0-2

0-6

0-5

0-4

0-6

Fledged /Successful

Attempt

24

34

20

10

No.

26

X

1.62

4.0

2.30

3.30

3.88

range

1-2

2-6

1-5

2-4

1-6

NO

Preselected sites.

Table 10. Breeding performance of the major raptor species and ravens in the Study Areas, 1976.

■ ■-

Species

No

. pairs

Nesting
attempts

Attempts
successful

% pairs
breeding

Nest success
attempts

(%)
pairs

Golden eagle

56

37

26

66.1

70.3

46.4

Prairie falcon

25

24

20

96.0

83.3

80.0

Red-tailed hawk

29

26*

19

79.3

73.1

65.5

Ferruginous hawk

13

12

10

92.3

83.3

76.9

Common raven

38

34

26

89.5

76.5

68.4

* 3 renesting attempts

24

Ferruginous hawk. Ferruginous hawk production was the best
ever observed in the BPSA with 83 percent of the nesting attempts
being successful. Clutch size averaged 4.0, and maximum brood was
3.12. Ferruginous hawks fledged 2.54 young per pair, 2.75 young
per attempt, and 3.3 young per successful attempt (Table 9). More
young fledged in 1976 than in all previous years combined. This
is due to an increased number of pairs located and better nestling
survival. Only eight percent of the • ferruginous hawk pairs failed
to breed (Table 10) . This is considerably lower than 1975 when
57 percent of the pairs did not breed.

Raven. Clutch size of ravens averaged 5.2, and maximum brood
averaged 3.66. Ravens produced 2.55 fledging young per pair,
2.88 per attempt, and 3.88 per successful attempt (Table 9).
Overall raven reproduction was slightly higher than in 1975 and
has been steadily increasing since 1974 (Kochert et al. 1974,
1975) . Non-breeding was 11 percent (Table 10) which was similar
to 1975.

Owl. We made no systematic attempt to observe great horned
owls in 1976 because most owls nested close to red-tailed hawks or
prairie falcons, and visits to owl nests disturbed the other
species. An average of 2.28 fledging great horned owls were seen
at seven sites. This fledging rate is similar to that observed in
1973-75 (Kochert 1973, Kochert et al. 1974, 1975).

Two long-eared owl (Asio otus) broods were located. One
attempt fledged two young, one of which was later found dead. The
other attempt fledged four. Two known screech owl (Otus asio)
pairs fledged four and five young. Screech owl and long-eared owl
pairs were widely spaced in riparian habitat. Pairs appear to be
restricted to groves of large trees along the Snake River.

Barn owl scrapes were located throughout the study area; most
were in close proximity to agriculture. Only cliff nests were
located, and we made no effort to locate nests in buildings. One
pair laid nine eggs; six hatched; none fledged. Two succcessful
attempts fledged five young each.

Harrier. A single marsh hawk (Circus cyaneus) with three
fledging age young was located by Jon Montan and Jan Wassink
(Study IVA) . At other sites, the general location of marsh hawk
pairs was noted but not searched due to time limitations.

Vulture. The first recorded turkey vulture (Cathartes aura)
nest in the BPSA was located by Study VII personnel. We entered
the nest and banded and weighed one young with fully developed
primaries (weight 1.512 kg or 3.3 lbs). The other young escaped
to the rear of the cave. Other nesting sites are suspected but
have not been located.

25

Mortality

Egg losses. Hatching failures, missing eggs, predation and
wind caused most egg losses for the major species (Table 11). Of
all eggs laid, golden eagles lost 35 percent, red-tailed hawks
lost 30 percent, and prairie falcons lost 26 percent. Hatching
failure was the greatest single cause of egg losses.

Nestling mortalities. Human activity, predation, falling
from the nest, and disease accounted for most nestling losses
(Table 11) . Predation and human activity not associated with
Study I were the most common known causes of nestling mortality.
Missing young were classed under mortality from unknown causes.
Nestling losses were 20 percent for golden eagles, 18 percent for
red-tailed hawks, and 14 percent for prairie falcons. Ogden and
Hornocker (1977) reported 17 percent nestling mortality for this
prairie falcon population in 1971 and 1972.

Post fledging mortality. We found 12 dead post fledged
prairie falcon young at 30 sites that fledged 98 young (minimum
post fledge mortality of 12.2 percent). Estimated post fledged
mortality for this population in 1975 was 22 percent. Great
horned owl predation was the major cause of mortality among post
fledging prairie falcons.

We located two dead post fledged red-tailed hawk young at
10 sites that fledged 26 young. We feel the post fledging mortality
rate (eight percent) may be low because of the mobility of the
post fledged young and the difficulty in finding dead individuals.

We attempted to search some golden eagle sites for post
fledging mortalities but the activity and mobility of the young
and their long post fledging period made it impossible to adequately
locate dead young.

Effects of Disturbance

There was no apparent relationship between nest visits by
researchers and number of young fledged from nests (Table 12).
The average number of young fledged from golden eagle and ferruginous
hawk control nests was the same as the average number that fledged
from all visited nests. The number fledged from red-tailed hawk
control nests was lower than that for visited nests, but the
average number of ravens and prairie falcons fledged was lower in
visited nests (Table 12) .

Nests sampled for food habits at four day intervals had
higher fledging rates in all species except ferruginous hawks.
Increases in numbers of researcher visits were associated with
higher golden eagle, prairie falcon, red-tailed hawk, and raven
fledging rates. This is a partial artifact of our observation

Table 11. Causes for raptor and raven egg and nestling losses in the Birds of Prey Study Area, 1976.

Species

Hatch
Failure

EGGS

Missing

Predation

Wind

Human
Activity

Abandoned

Golden Eagle

Red-tailed Hawk

Ferruginous Hawk

Prairie Falcon

Raven

TOTALS

11
4

17

10

42

2

2

4

_]_
15

N5

Species

Human
Activitv

YOUNG

Predation

Fell

Out Disease

Starvation Heat

Unknown

Golden Eagle

Red-tailed Hawk

Ferruginous Hawk

Prairie Falcon

Raven

TOTALS

10
13

4

4

3

2

9
22

''Trichomoniasis

Table 12. Number of young fledged per attempt from control and disturbed nest;
of ravens and the major raptors In the BPSA, 1976.

Species

(Number of Nests)

00

mean number of fledged ± SD ( x ± SD

Control 2-3 visits 6-15 visits

Visited for
Food habits

Golden eagle

Prairie falcon

Red-tailed hawk

Ferruginous hawk

common raven

(12)
1.0810.90

(14)
3.50±1.70

(9)
1.2211.30

(4)
3.00±2.00

(13)
3.1511.63

(14)
0.93±0.92

(27)
2.70±1.94

(11)
1.36+1.63

(4)
3.5010.53

(13)
2.2211.93

(11)
1.2710.79

(10)
3.6012.17

(9)

3

0011
(4)

12

2

5011.

(5)

00

/,

2012.

39

(12)
1.2710.79

(9)
4.4411.11

(8)
3.0011.20

(4)
2.5011.00

(5)
4.2012.39

All visited
nests

(25)

(37)
2.9512.01

(20)
,1011.62

(8)
3.0010.92

(23)
2.6512.14

JX

method. Nesting attempts that failed were not visited again so
that early failures were more likely to have been visited few
times. Nests that did not fail early were available for additional
visits. Preliminary analysis suggested that none of the differences
in fledging rates were statistically significant.

These data do not prove that egg and nestling survival are
independent of human activity. It only indicates that careful
research techniques do not necessarily cause a major decrease in
fledging rates.

Four ferruginous hawk pairs successfully nested less than
400 m (0.25 mi) from agricultural fields; one was only 90 m (100
yds) from farmland. The three closest nests were below the canyon
rim or around a rock corner and were not visible from cultivated
land. Two successful attempts were near existing roads. One was
180 m (200 yds) from Interstate 80. The other was 70 m (80 yds)
from an improved county road and was one of the nests close to
cultivated fields.

Banding and Marking

We banded 326 ravens and raptors in 1976. Of these, 195 were
wing-marked (Table 13). Prairie falcons, ravens, red-tailed
hawks, and golden eagles comprised 94 percent of the birds banded.
This year was the first year bald eagles (Haliaeetus leucocephalus) ,
marsh hawks, burrowing owls, and turkey vultures were banded
during the study.

At least 37 sightings of our wing-marked birds were reported
in 1976. Band returns and marked bird sightings continue to be
reported, and information is presently being analyzed. The
increased use of wing markers in the United States and Canada has
resulted in confusion over the identity of many observed birds.
Most sightings now require follow-up questioning, and some must be
discarded due to possible misidentif ication. The most common
problems result from use of the same or similar colored marker on
the same or similar species, misidentif ication of species, and
misuse of markers.

Researchers on this project using spotting scopes and binoculars
frequently observe wing markers on golden eagles at distances of
0.8 km (0.5 mi) and on prairie falcons at distances of 0.4 km
(0.25 mi). At these distances it is easy for persons unfamiliar
with raptors to misidentif y species, especially those in immature
plumages. Color identification of markers by any observer at
these distances is very difficult except under ideal light conditions.

29

Table 13. Numbers of ravens and raptors banded and marked

in the Snake River Birds of Prey Study Area, 1976.

Species Banded Wing-marked Other
Markings''

Prairie Falcon

Golden Eagle

Ferruginous Hawk

Red-tailed Hawk

Marsh Hawk

Great Horned Owl

Barn Owl

Burrowing Owl

Bald Eagle--

Turkey Vulture

Common Raven
Totals

* Includes radio transmitters, paint, and wingmarkers singly
or in combination.

"" Trapped and banded by Study III personnel.

+ Includes 21 birds banded by Study VII and 20 birds banded
by Study III.

++ Includes 5 birds banded by Study VII and 17 birds banded
by Study III.

.62

100

24

28

21

5

28

37

33

3

3

2

8

4

1

1

1

•

52

41

4

26+

195

34++

30

Food Habits

Golden eagle. As in all previous years, black-tailed jack-
rabbits (Lepus californicus) were the single most important prey
species. Cottontails (Sylvilagus nuttali) and pheasants (Phasianus
colchicus) were the next most frequent of the 31 species identified
in food habit collections (Table 14) .

The frequency of occurrence and percent biomass of jackrabbits
in the diet of eagles (23 and 47 percent respectively) were the
lowest ever reported since the jackrabbit population crash in 1972
(Kochert 1973, Kochert et al. 1974, 1975). The occurrence of
cottontails (the second most important prey species) has changed
little from pre-jackrabbit crash levels while the occurrence of
pheasants has increased slightly. The occurrence of Townsend
ground squirrels has steadily declined since 1974, but the 1976
Townsend ground squirrel frequency of occurrence (13 percent) is
four times greater than frequencies recorded before the jackrabbit
crash (Kochert 1972, 1973; Kochert et al. 1974, 1975). Ground
squirrels still contribute little (three percent) to the total
prey biomass. Medium birds, small mammals, and reptiles increased
in percent biomass compared to previous years.

Pheasants were the most important golden eagle prey species
in agricultural habitat, and black-tailed jackrabbits were the
most important in range and intermediate habitats (Fig. 5) . The
pattern of different diets in relation to land use type has remained
similar for all sample years (Kochert 1972, 1973; Kochert et al.
1975).

Prairie falcons. Townsend ground squirrels were the most
important prey species in both numbers and biomass (Table 15) .
The proportion of Townsend ground squirrel biomass in the food
habits of these birds has changed little during four years of
study (Kochert 1973, Kochert et al. 1974, 1975).

Lagomorphs were the only other species comprising a large
portion of the prey biomass (15 percent). Lagomorph remains were
found at five of nine study scrapes. Most of the deer mice
(Peromyscus maniculatus) and meadow mice (Microtus spp.) were
found at a single scrape on the south side of the river surrounded
by agriculture. However, various species of mice were identified
in five of eight nests studied.

Red-tailed hawks. As in previous years, Townsend ground
squirrels were the most numerous prey in red-tailed hawk diets,
but lagomorphs comprised a larger portion of the biomass (Table
16) . Several changes in prey numbers and species occurred in 1976
compared to previous years. Townsend ground squirrels and reptiles
were lower in the 1976 red-tailed hawk diet, but lagomorphs
increased in both occurrence and percent biomass. Pocket gophers

■~™—™°~~— ™°"

31

Table 14. Golden eagle food habits in the Snake River Birds of Prey Study
Area, 1976.*

—

Number

Estimated

Percent

Percent

Size

of

Total

of

of Total

Species

Class

Individuals

Biomass (g)

Total Prey

Biomass

MAMMALS

Black-tailed

Jackrabbit

A (Male

6

11,268

1.2

2.7

A (Female) 3

7,026

0.6

1.7

A

45

94,950

9.2

22.7

M

26

40,482

5.3

9.7

J

2

942

0.4

0.2

U

30

41,370

6.1

9.9

total

112

196,038

22.9

47.0

Cottontail

A

45

37,215

9.2

8.9

M

13

6,500

2.7

1.6

J

10

2,150

2.0

0.5

U

19

9,766

3.9

2.3

total

87

55,631

17.8

13.3

Townsend Ground

Squirrel

A (Me

le)

5

1,300

1.0

0.3

A(Femal

e)3

561

0.6

0.1

A

4

844

0.8

0.2

M

5

835

1.0

0.2

U

45

7,245

9.2

1.7

total

62

10,785

12.7

2.6

Townsend Pocket

Gopher

A

32

7,680

6.5

1.8

U

12

2,400

2.4

0.6

total

44

10,080

9.0

2.4"

Yellow-bellied

Marmot

M

6

9,000

1.2

2.2

U

8

13,336

1.6

3.2

total

14

22,336

2.9

5.4

Kangaroo Rat,Unid.

A (Ma

le)

4

222

0.8

1.0

Coyote

J

2

4,086

0.4

1.0

Bushy-tailed Woodrat U

2

574

0.4

0.1

Woodrat, Unid .

U

2

406

0.4

0.1

White-tailed

Antelope Ground

Squirrel

U

2

212

0.4

0.1

Mule Deer

J

1

6,300

0.2

1 .5

Desert Woodrat

U

1

212

0.2

0.1

Mouse, Unid.

U

1

20

0.2

+

SUBTOTAL

336

310,461

68.7

74.4

BIRDS

Ring-necked

Pheasant

A (Ma

le)

12

16,560

2.5

4.0

A(Femal

e)35

35,595

7.2

8.5

A

10

11,990

2.0

2.9

total

57

64,145

11.7

15.4

3 2

Table 14. Continued

Species

Number Estimated Percent Percent
Size of Total of of Total

Class Individuals Biomass(g) Total Prey Biomass

Chukar

5,139

1.1

1.2

Great Horned Owl

A

1

1,505

0.2

0.4

M

2

2,460

0.4

0.6

U

2

2,736

0.4

0.7

total

5

6,701

1.0

1.6

Mallard

A

3

3,444

0.6

0.8

Western Meadowlark

A

3

288

0.6

0.1

Common Raven

M

2

1,742

0.4

0.4

Gull, Unid.

I

2

923

0.4

0.2

Gray Partridge

A

2

752

0.4

0.2

Duck, Unid.

A

1

660

0.2

0.2

J
total

2

150
810

0.2
0.4

+
0.2

Passerine, Unid.

U

2

34

0.4

+

Great Blue Heron

U

1

2,807

0.2

0.7

Ferruginous Hawk

M

1

1,254

0.2

0.3

Red-tailed Hawk

A(Fema

le)l

1,224

0.2

0.3

Pintail

A(Fema

le)l

822

0.2

0.2

Prairie Falcon

M

1

715

0.2

0.2

Ring-billed Gull

M

1

419

0.2

0.1

Black-billed Magpie M

1

173

0.2

+

Starling

U

1

84

0.2

+

SUBTOTAL

122

100,326

24.9

24.0

REPTILES

Snake, Unid.

A

10

3,320

2.0

0.8

J

1

33

0.2

+

U

10

1,840

2.0

0.4

total

21

5,193

4.3

1.2

Gopher Snake

A

4

1,092

0.8

0.3

U

1

146

0.2

+

total

5

1,238

1.0

0.3

Long-nosed Snake

A

1

51

0.2

+

Leopard Lizard

A

1

26

0.2

+

Reptile, Unid.

J

1
29

9
6,517

0.2
5.9

+

SUBTOTAL

1.6

INVERTEBRATES

Beetle

0.4

+

TOTAL PREY ITEMS

hi

417,305

*See appendix for assigned weights and scientific names.
A = Adult J = Juvenile + = Trace

M = Medium U = Unknown (Average)

40

>s 30

0

o

E
'1",

0

a.

20

10

I,

Jaclcrabbit Cottontail

J3 Agricultui

= Intermediate

Range

^

Townsend Pheasant

Ground Squirrel

I I

Othe

U)

^ig. 5. Food habits of golden eagles in relation to land use, 197f

34

Table 15. Prairie falcon food habits in the Snake River Birds of Prey Study
Area, 1976.*

Species

Class

Number
of
Individuals

Estimated
Total
Biomass (g)

Percent of
Total Prey

Percent of
Total Biomass

MAMMALS
Townsend ground

squirrel

A (Male)

2

520

0.5

0.9

A

2

422

0.5

0.7

J

1

106

0.2

0.2

II

258

41

,538

61.8

72.1

t

.otal

263

42

,586

62.9

73.9

Deer Mouse

U

51

867

12.2

1.5

Meadow Mouse

U

18

1

,080

4.3

1.9

Kangaroo Rat, Unid

U

8

444

1.9

0.8

Townsend Pocket

CTopher

U

7

1

,400

1.7

2.4

Cottontail

u

6

3

,084

1.4

9.4

Black-tailed

Jackrabbit

D

4

5

,516

1.0

5.4

Harvest Mouse

U

5

75

1.2

0.1

Antelope Ground

Squirrel

U

3

318

0.7

0.6

Pocket Gopher Unid

u

2

410

0.5

0.7

Woodrat, Unid.

u

2

406

0.5

0.7

Desert Woodrat

u

1

118

0.2

0.2

Shrew, Unid.

u

1

18

0.2

+

Mouse, Unid.

0

1

20

0.2

+

SUBTOTAL

372

56

,342

89.0

97.8

BIRDS

Western Meadowlark

u

4

384

1.0

0.7

Sparrow, Unid.

u

4

68

1.0

0.1

California Quail

u

1

198

0.2

0.3

Mourning Dove

u

1

113

0.2

0.2

Brewer's Blackbird

u

1

68

0.2

0.1

Red-winged

Blackbird

u

1

54

0.2

0.1

Sage Sparrow

u

1

30

0.2

+

White-crowned

Sparrow

u

1

30

0.2

+

Horned Lark

u

1

28

0.2

+

Yellow Warbler

u

1

20

0.2

+

SUBTOTAL

16

993

3.8

1.7

REPTILES

Whiptail Lizard

u

9

135

2.2

0.2

Horned Lizard

u

2

36

0.5

0.1

Leopard Lizard

u

1

23

0.2

+

Lizard, Unid.
SUBTOTAL

u

1
13

17
211

0.2
3.1

+

0T4

INVERTEBRATES

Beetle

u

10

30

2.4

+

Scorpion

SUBTOTAL

u

7
17

10
40

1.7
4.1

+
0.1

TOTAL PREY ITEMS

418

57,586

* See appendix for assigned weights and scientific names

+ = Trace

A = Adult
M = Medium

Juvenile
Unknown

35

Table 16. Red-tailed hawk food habits in the Snake River Birds of Prey-
Study Area, 1976.*

Species

Size
Class

Number

of

Individuals

MAMMALS

Townsend Ground

Squirrel

A(Male

) 6

A(Fema

le)4

A

7

M

10

J

5

U
total

157
189

Townsend Pocket

Gopher

A

3

U

41

total

44

Cottontail

A

2

M

3

J

6

U

23

total

34

Black-tailed

jackrabbit

U

11

Kangaroo Rat, Uni

d

u

7

Harvest Mouse

u

5

Yellow-bellied

Marmot

J

1

u

3

total

4

Woodrat, Unid.

U

3

Deer Mouse

u

3

Muskrat

M

J

U

1

total

2

Domestic. Cat

U

1

Pocket Gopher, Una

d

u

1

Long-tailed Wease

1

u

1

Meadow Mouse

u

1

Estimated

Total

Biomass(g)

SUBTOTAL

306

1,560

748

1,477

1,670

530

25,277

31,262

720
8,200
8,920

1,654

1,500

1,290

11,822

16,266

15,169

388

75

50n
5,001
5,501

609
51

1,052
1,157
2,209

1,800

205

178

60

82,693

Percent
of Total
Prey

1.4
1.0
1.7
2.4
1 .2
37.9
45.7

0.7

9.9

10.6

0.5
0.7

8.2

2.7

1 .7
1.2

0.2
0.7
1.0

0.7
0.7

0.2
0.2
0.2
0.?

Percent
of Total
Biomass

1.6
0.8
1.5
1 .7
0.6
26.3
32.5

0.7
8.5
9.3

1.7

1.
1.

12.

16.9

15.9
0.4
0.1

73.9

1.1
1.2
2.3

1.9
0.2
0.2
0.1
36.0

36

Table 16. Continued

Number

Estimated

Percent

Percent

Size

of

Total

of Total

of Total

Species

Class

Individual s

Biomass

(g)

Prey

Biomass

BIRDS

Passerine, Unid.

U

8

136

1.9

0.1

Rock D ove

U

3

900

0.7

0.9

Chukar

A

2

1,142

0.5

1.2

Cliff Swallow

A

2

30

0.5

+

Ring-necked Pheasant

U

1

1,199

0.2

1.2

Mallard

A

1

1,148

0.2

1.2

Gray Partridge

A

1

376

0.2

0.4

Califorr i.a Quail

A

1

198

0.2

0.2

Black-billed Magpie

M

1

173

0.2

0.2

Mourning Dove

M

1

113

0.2

0.1

SUBTOTAL

21

5,415

5.1

5.6

REPTILES

Gopher Snake

A

U

total

Ground Snake

U

Striped Whipsnake

U

Whiptail Lizard

U

Lizard, Unid.

U

Rattlesnake

U

Snake, Unid.

U

Reptile, Unid.

U

Racer

V

Western Fence Lizard U

Night Snake

u

SUBTOTAL

INVERTEBRATES

Beetle

u

Scorpion

u

Grasshopper

u

Spider

u

SUBTOTAL

TOTAL PREY ITEMS

7
33

40

6
5
4
2

1
1
1
1
1
1

63

18
3
2
1

24

414

1.911

4,818
6,729

48

485

60

34

329

184

101

68

17

14_

8,069

5
4
1

1
11

96,189

1.7
8.0
9.7

1.4
1.2
1.0
0.5
0.2
0.2
0.2
0.2
0.2
0.2
15.2

4.3
0.7
0.5
0.2

2.0
5.0
7.0

+

0.5

0.1

+

0.3

0.2

0.1

0.1

+

+

8.4

+
+
+
+

*See appendix for assigned weights and scientific names,
A = Adult J = Juvenile + = Trace

M = Medium U = Unknown (average)

37

(Thomomys spp.), not previously identified in food habit collections,
comprised about 10 percent by numbers and biomass in 1976. Pocket
gopher remains were identified in five of seven sites, and lagomorph
remains occurred at all sites studied. Hawks, nesting near farmland,
captured more pocket gophers than hawks nesting near range lands.

Ferruginous hawks. Townsend pocket gophers (Thomomys townsendii)
and Townsend ground squirrels comprised 54 percent of prey numbers
and 90 percent of prey biomass at the four ferruginous hawk nests
studied (Table 17) . Adult ferruginous hawks remove castings and
prey remains from their nests (L. Powers pers. comm. ) . This
decreases the accuracy of the four day interval collections.
Ferruginous hawk food habits were last studied in 1973 when a
small number of collections at two nests identified mostly lagomorphs,
woodrats (Neotoma spp.), and Townsend ground squirrels. In 1976,
more pocket gophers were found in nests located near farmlands
than those in range land areas.

Ravens . As in previous years, mammals comprised almost all
the prey biomass identified in castings collected at raven nests
in 1976 (Table 18). Townsend ground squirrels were still most
prevalent. However, their biomass contribution decreased from 93
to 70 percent between 1975 and 1976 due to a decrease in the
frequency of occurrence of ground squirrels in the diet, an
increase in prey diversity and an increase in a small number of
lagomorphs that contribute a large biomass. This may have been an
artifact of our sampling procedure. Young ravens at the Murphy
rim nest consumed large numbers of beetles and scorpions. Some
ground squirrel remains were found here late in the fledgling
period although there are no known ground squirrel demes nearby.
Townsend ground squirrels were the most commonly found prey species
at all other nests regardless of habitat or land use practice.

Raptor Census

We ran monthly road transects in 1976 as in 1975, 1974, and
1973. Monthly totals varied widely for some species between
years. As in past years, ravens were the most numerous. The
number of rough-legged .hawks observed from January through March
was one fourth of the number counted during the same period in
1975. We also counted fewer red-tailed hawks than in 1975.
Golden eagle, prairie falcon and kestrel numbers remained relatively
constant (Table 19) .

Average yearly results (Table 20) for all species by month
show general trends of density. Golden eagles, prairie falcons,
red-tailed hawks, kestrels, marsh hawks, ravens and shrikes
(Lanius spp.) occupy the BPSA year round. Rough-legged hawks
(Buteo lagopus) are fall and winter residents. Merlins (Falco
columbarius) , Cooper's hawks (Accipter cooperii) , and goshawks
(Accipter gentilis) winter or migrate through in very low numbers

3b

Table 17. Ferruginous hawk food habits in the Snake River Birds of Prey
Study Area, 1976.*

Species

Number
Size of
Class Individuals

Estimated Percent Percent of
Total of Total

Biomass (g) Total Prey Biomass

MAMMALS
Townsend Ground

Squirrel

A

2

422

1.7

M

1

167

0.9

U

32

5,152

27.4

total

35

5,741

29.9

Townsend Pocket ■

Gopher

A

16

3,840

13.7

LI

12

2,400

10.3

total

28

6,240

23.9

Kangaroo Rat,Unid.

U

2

11.1

1.7

Mouse, Unid.

U

2

40

1.7

Cottontail U

Woodrat, Unid. U
SUBTOTAL

BIRDS

Rock Wren U

Say's Phoebe U

Passerine, Unid. U
SUBTOTAL

REPTILES

Western Fence Lizard I!

Whiptail Lizard U

Woodhouse's Toad U

Lizard, Unid. U

Snake, Unid. U

Reptile, Unid. U
SUBTOTAL

INVERTEBRATES

69

1

1
1
5
2
_1
11

Beetle

U

28

Grasshopper

U

2

Jerusalem Cricket

U

1

Cicada

u

1

Wasp

u

1

SUBTOTAL

33

TOTAL PREY ITEMS

117

514

40

12,849

13,279

0.9

0.9
59.0

15

1.7

21

0.9

17
53

0.9

3.4

17

0.9

15

0.9

20

0.9

17

4.3

184

1.7

101

0.9

354

9.4

18

23.9

1

1.7

2

0.9

1

0.9

1

0.9

23

28.2

3.2

1.3

38.8

43.2

28.9
18.1
47.0

0.8

0.3

3.9

0.3
96.8

0.1
0.2
0.1
0.4

0.1
0.1
0.2
0.1
1.4
0.8
2.7

0.1
+
+
+

0.1

" See appendix for assigned weights and scientific names.
A = Adult J = Juvenile + = Trace

M = Medium U = Unknown (Average)

39

Table 18. Raven food habits in the Snake River Birds of Prey Study Area, 1976. ;

Species*-

Number

of
Individual s

Estimated

Total
Biomass (g)

MAMMALS

Townsend Ground Squirrel

Deer Mouse

Kangaroo Rat, Unid.

Townsend Pocket Gopher

Cottontail

Black-tailed Jackrabbit

Desert Woodrat

Woodrat, Unid.

Harvest Mouse

Great Basin Kangaroo Rat

Mouse, Unid.

House Mouse

SUBTOTAL

BIRDS

Passerine Egg
Sparrow, Unid.
SUBTOTAL

REPTILES and AMPHIBIANS
Side-blotched Lizard
Gopher Snake
Striped Wbipsnake
Collared Lizard
Toad, Unid.
Horned Lizard
Whiptail Lizard
SUBTOTAL

INVERTEBRATES
Beetle
Scorpion
Grasshopper

SUBTOTAL

163
12
10
7
6
3
3
2
2

1
1

1_

211

26,234

204

555

1,400

084

137

354

406

30

65

20

15

36,513

Percent

of Total
Prey

21.1
1.7
1.4
1.0
0.8
0.4
0.4
0.3
0.3
0.1
0.1
0.1

29.3

Percent of

Total
Biomass

70.3
0.5
1.5
3.8
8.3

11.1

0.1
+
97.8

2

4

0.3

l

1

17

0.1

+

3

21

0.4

0.1

3

12

0.4

1

2

292

0.3

0.8

1

97

0.1

0.3

1

26

0.1

0.1

1

20

0.1

0.1

1

18

0.1

+

1
11

15
497

0.1
1.5

+
173

360

108

50.1

0.3

132

191

18.4

0.5

2

2

0.3

+

494

301

68.7

0.8

TOTAL PREY ITEMS

719

37,332

* See appendix for assigned weights and scientific names.
"'"'All prey items unknown (average) size class.
+ = trace

40

Table 19 . Birds of prey observed on the monthly raptor road transect
in the Birds of Prey Natural Area, 1976.

Number

of Bi

rds

Species

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Golden Eagle

4

3

3

1

9

3

2

3

3

Prairie Falcon

3

1

5

2

2

4

1

1

2

7

4

Kestrel

4

8

2

2

1

3

4

5

13

6

6

8

Red-tailed Hawk

1

1

1

1

1

3

3

2

1

Rough-legged
Hawk

4

1

3

2

5

7

Swainson's Hawk

2

1

Cooper's Hawk

1

1

1

Marsh Hawk

3

1

2

4

1

1

3

1

Common Raven

19

13

5

6

8

7

7

17

6

4

4

Loggerhead

Shrike 1 2 23 92412

Turkey Vulture 1

Burrowing Owl 2 11

Unknown Buteo 1 2

Unknown Falcon 1

Unknown Raptor 2

TOTAL 40 28 16 22 15 25 33 27 41 26 33 28

Table 20. Monthly averages of birds of prey counted on the road transect
in the Natural Area, October, 1973 - December, 1976.

Species

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Golden Eagle

6.0

6.0

4.0

3.0

6.7

10.7

7.0

3.5

1.3

1.7

1.7

3.5

Prairie Falcon

2.7

4.0

8.3

7.4

2.0

2.3

5.5

1.0

0.7

1.0

4.3

2.3

Kestrel

5.7

6.0

4.3

2.7

3.7

4.0

10.5

5.0

11.7

3.8

3.0

7.8

Red-tailed Hawk

3.3

2.0

1.7

2.3

1.7

1.0

2.5

1.0

1.7

2.0

0.67

2.5

Marsh Hawk

3.0

2.0

1.3

2.3

1.7

1.3

+

+

0.8

2.0

3.8

Common Raven

14.0

15.0

16.0

11.3

15.7

9.3

+

13.7

6.2

4.7

12.2

Loggerhead Shrike

1.7

1.0

0.7

1.3

12.0

6.0

2.0

2.8

1.7

1.8

Rough-legged Hawk

11.0

6.0

8.3

5.3

+

2.7

7.2

Ferruginous Hawk

+

Swainson's Hawk

1.3

0.7

+

+

Goshawk

+

Cooper's Hawk

+

+

+

0.5

+

Bald Eagle

+

Merlin

+

+

Turkey Vulture

+

+

Short-eared Owl

fAsio flammeus)

+

+

Burrowing Owl

+

5.3

3.0

+

+

Barn Owl

+

Unknown Buteo

+

+

+

+

+

+

Unknown Falcon
Unknown Raptor

+

1.3

+

+

0.8

+ Observed only in one transect.

42

and are rarely observed. Ferruginous hawks, Swainson's hawks
(Buteo swainsoni) and turkey vultures are summer breeders but are
rare or occupy habitats not sampled by the transects.

Numbers of each species vary with arrival of winter or spring
immigrants, courtship and brood rearing requirements, fledging of
young, departure of summer emigrants and perhaps weather. Golden
eagle numbers peak in June and decrease in July with dispersal.
Dispersal of adults and young raptors during the hot periods of
July results in low numbers observed from August until December.
This period is typified by resident breeders and transient individ-
uals. Wintering species begin to arrive in November and leave in
March. The period when most birds are counted extends from January
through March when breeding birds, winter residents and transients
are present.

These data do not completely show the relative numbers of
each species in the BPSA because of the biases of the route. It
did not sample all habitat types in proportion to their availability.
In addition, it followed a powerline for more than 60 percent of
its length and passed nests of five burrowing owls and the only
Swainson's hawk in the BPSA. These transects will not be continued
in 1977. Instead, seasonal abundance of each species and habitat
utilized will be determined from observations made each month by
Study I personnel.

Golden Eagle Population

Forty-one golden eagles were counted from aerial transects in
January 1976, but only seven eagles were counted in October 1976.
The January density (3.2 eagles/100 km2 or 8.2/100 mi2) and adult
to immature ratio (73:23) were similar to previous years and have
not changed appreciably since 1974. The October density (0.5
eagles/100 km2 or 1.4/100 mi ) was the lowest recorded since
counts began in 1972 (Table 21). Also, as in October 1975, no
immature eagles were observed on the survey. However, the sample
size was too small to compute any valid age ratios.

The 'Statistically significant decline in golden eagle numbers
between 1972-73 and 1973-74 transects appeared to be related to a
decline in the jackrabbit population and decreased golden eagle
reproductive performance in the BPSA (Kochert et al, 1975). This
does not appear to be the case with the October 1976 decline in
numbers. The jackrabbit population appears to be increasing
(Study IVA 1976 Annual Report) and golden eagle reproductive
performance has not shown any significant decline since 1973. A
1976 range fire that destroyed 64,750 ha (250 mi ) of jackrabbit
habitat in the transect area, conversion of large areas to cultiva-
tion, weather and error may account for the decline. The matter
will be further analysed.

Table 21- Results of aerial transects on 17,920 km2 (7000 mi) of the Snake River floodplain, 1972-76.

Golden Eagles Other

No.

No.

No .

Date

Adults

Immature

Unknown

Oct 72

10

11

8

Feb 73

33

33

IS

Oct 73

7

3

7

Jan 74

20

9

12

Oct 74

4

2

10

Feb 75

17

S

7

Oct 75

10

0

5

Jan 76

24

9

6

Oct 76

4

0

3

Adult : Immature Eagle/100 m±2
Total ratio (256 km2)

29 43:52

34 50:50

17 70:30

41 69:31

16 67:33

32 63:32

15 100:00

41 73:27

7 ** 1.4

5

.3

6

.3

3

.4

8

2

3

2

6

4

0

o

O

9

Other

No.

raptors*

Coyotes

56

16

72

19

28

0

82

14

32

8

43

16

19

9

69

6

54

2

* includes bald eagles, red-tailed hawks, rough-legged hawks, ferruginous hawks, prairie falcons,
American kestrels, marsh hawks, and unknown raptors.

** Adult: Immature ratio not calculated because of small sample size.

44

Management Experiments

Shade construction. Heat prostration is a common cause of
eaglet mortality in the study area (Kochert 1972, 1973; Kochert et
al. 1975). In 1976, two golden eagle pairs nested on traditional
sites with southwest exposures where young had previously died
from heat prostration. At the control nest, a six week old eaglet
was found dead below the nest on 2 June following several days of
hot weather. At the other site, a single rock was placed on one
side of the nest when the two occupant young were about two weeks
old. The nest was visited every four days and the structure did
not seem tall enough to provide mid-afternoon protection as the
young exhibited signs of heat stress. When the young were about
one month old, rocks were piled about 40 cm (16 inches) high in
front and back of the nest. Plywood was laid across the top,
weighted down, and partially covered with nest material. About
half the nest surface was protected by the shade device. The
eaglets could choose where to sit. On all nine subsequent visits,
one or both young were under the shade. Both young fledged on
approximately 15 June. The shade device did not cause any apparent
change in food delivery by parents.

An enclosed shade was constructed of rocks at an extremely
late prairie falcon nest on 14 July where two young exhibited
signs of heat stress. A single remaining young was inside the
structure on both subsequent visits. By 3 August, prior to its
expected fledging date, the young had disappeared from the scrape.

At a ferruginous hawk nest that lacked shade, rocks were
placed on one side to give some afternoon protection. The young
were not observed using the shade. Two of the three original
chicks and one of the two foster chicks (see below) fledged.
There was no observed change in parental behavior.

Fostering and moving of young. Five emaciated young ferruginous
hawks were found at the Loveridge Site with a dead adult hawk
nearby and one adult defending the area. The two youngest and
most emaciated birds were taken to the ferruginous hawk nest where
the shade device had been built (see above) . The remaining three
young left in the nest fledged four and eight days later. The
smallest of the fostered young was found dead below the nest where
it had been placed four days earlier. The older of the fostered
young fledged with the original two nestlings when the site was
visited eight days after the introduction. Adult behavior did not
change, and the foster nestlings were apparently accepted.

At one of the few golden eagle nests located in farmland, the
single ten day old nestling was missing on 13 May. The chick was
alive on 9 May. Since the nest was vital to our eagle food habits
study, a 3.5 week old eagle (the oldest from a nest with two
young) was introduced on 15 May. The chick was hand fed on

45

introduction and again on 17 May when the foster adults had not
returned. On 19 May, a foster adult eagle was with the young. We
resumed four day interval food habits visits on 21 May. Seven
subsequent visits were made until 21 June. The eaglet fledged on
27 June and was repeatedly seen in the vicinity of the nest until
11 August. The female was more attentive to this nest than most
other female eagles in the BPSA, and attentiveness continued
longer than typically observed.

Four 3-week old prairie falcons were found below their nest.
The scrape was highly infested with hemiptera parasites. We moved
the young to the 1975 scrape, 100 m from the parasitized nest.
Parasites did not become numerous in the new site. The adults
continued to feed the young and defended the new site instead of
the original 1976 site. All young fledged and used the 1975 site
for resting. We observed no changes in the behavior of the
adults.

Disease treatment. In 1976, eaglets in food habits study
nests were treated with Dimetridazole (Emtryl (R) , Salsbury
Laboratories, Charles City, Iowa 50616). Emtryl was administered
in gelatin capsules with 250 mg given to chicks less than one
month and 500 mg to older chicks.

In two of three cases where lesions were less than 10 mm^
when discovered, lesions disappeared after a single treatment.
The third case required four treatments of 250 to 500 mg Emtryl
administered every four days before symptoms disappeared.

In two cases, a three week old chick had small lesions that
increased in size after treatment began. One young died despite
three treatments; lack of food may have been a contributing
factor. The other young recovered after six treatments at four
day intervals and showed no sign of new lesions in the following
16 days it was observed before fledging. A third serious case
suddenly developed in a 7.5 week old eaglet between a four day
visit interval. The emaciated bird was first treated with 500 mg.
It was stronger four days later but still had large lesions. We
again administered 500 mg Emtryl. Four days later, the bird's
keel was within the breast muscle, but lesions had not decreased
in size. We hand fed the chick until the crop was full and then
gave it 750 mg Emtryl. The eaglet was alive and active 13 days
later. On two subsequent visits, the eaglet was not observed in
the area of the nest and is assumed to have fledged.

In past years chicks with trichomoniasis were not treated.
Birds with advanced cases were taken to a veterinarian, and these
birds subsequently died. This year all but one treated bird
lived to fledging age while most chicks with frounce in control
nests died.

46

ACKNOWLEDGMENTS

We wish to thank W. Mullins for field assistance. M. Collopy
and J.R. Oakley analyzed some of the data presented in the report,
and K. Steenhof helped the authors to prepare the final draft. We
also acknowledge the cooperation of other study personnel in
providing sightings of marked birds, locating nests, and assisting
with field work. We are grateful to R. Dodge and P. Lewis for
typing the final draft.

LITERATURE CITED

Baldwin, S.P. and S.C. Kendeigh. 1938. Variations in the
weight of birds. Auk 55:416-467.

Balph, D. 1973. Curlew Valley Validation Site Report:
1972 progress report. Research Memorandum. 73-1.
Vol. 2:218-263.

Beecham, J.J. 1970. Nesting ecology of the golden eagle in
southwestern Idaho. M.S. Thesis. Univ. of Idaho, Moscow.
48 pp.

Bellrose, F.C. 1976. Ducks, geese, and swans of North
America. Stackpole Books. Harrisburg, PA. 543 pp.

Bent, A.C. 1938. Life histories of North American Birds of
Prey. Vol. I and II. U.S. Natl. Mus. Bull. 170,
Washington, D.C. 409 and 482 pp.

Boeker, E.L. and E.G. Bolen. 1972. Winter golden eagle
populations in the southwest. J. Wildl. Manage.
36(2) :477-484.

Brown, L.H. and A. Watson. 1964. The golden eagle in relation
to its food supply. Ibis 106:78-100.

Christensen, G.C. 1970. The chukar partridge: its introduction,
life history and management. Nevada Dept. Fish and Game.
Biol. Bull. No. 4. 82 pp.

Craighead, J.J. and F.C. Craighead, Jr. 1956. Hawks, owls,
and wildlife. Stackpole Co., Harrisburg, PA. and Wildl.
Mgmt . Inst., Washington, D.C. 443 pp.

Donohoe, R.W. 1966. Muskrat reproduction in areas of controlled
and uncontrolled water-level units. J. Wildl. Manage.
30(2) :320-326.

Esten, S.R. 1931. Bird weights of 52 species of birds (taken
from notes of Wm. Van Gorder) . Auk 48:572-74.

47

Evans, F.C. and J.T. Emlen, Jr. 1947. Ecological notes on
the prey selected by a barn owl. Condor 49(1) :3-9.

Hall, E.R. 1946. Mammals of Nevada. Univ. Calif. Press,
Berkeley.

Hornocker, M.G., J. P. Messick, J.S. Whitman and G.W. Smith.
1976. Dynamics of predation upon a raptor prey base in
the Snake Ri^er Birds of Prey Natural Area, Idaho in Snake
River Birds of Prey Research Proj . Ann. Rep. 1976.
U.S. Dept. of Interior, Bur. of Land Manage., Boise, ID.

Howard, R.P. 1975. Breeding ecology of the ferruginous hawk
in northern Utah and southern Idaho. M.S. Thesis. Utah
State University, Logan. 60 pp.

Johnson, D.R. and L. Diller. 1976. Ecology of reptiles in
the Snake River Birds of Prey Natural Area in Snake River
Birds of Prey Research Proj. Ann. Rep. 1976. U.S. Dept.
of Interior, Bur. of Land Manage., Boise, ID.

Johnson, D.R. and W.E. Melquist. 1975. Population ecology
and habitat requirements of Townsend ground squirrels.
Pages 156-178 in Snake River Birds of Prey Research Proj .
Ann. Rep. 1975. U.S. Dept. of Interior, Bur. of Land
Manage., Boise, ID. 193 pp.

Kochert, M.N. 1972. Population status and chemical contamination
in golden eagles in southwestern Idaho. M.S. Thesis.
Univ. of Idaho, Moscow. 102 pp.

. 1973. Density, food habits, and reproductive

performance of raptors in the Snake River Birds of Prey
Natural Area. Ann. Prog. Rep. Bur. of Land Manage. ,
Boise, ID. 54 pp.

Kochert, M.N., R.P. Howard, and J. Silva. 1974. Reproductive
performance, food habits and population dynamics of raptors
in the Snake River Birds of Prey Natural Area. Ann. Prog.
Rep., U.S. Dept. of Interior, Bur. of Land Manage.,
Boise, ID. 20 pp.

Kochert, M.N., A.R. Bammann, R.P. Howard, J.H. Dor emus,

M. Delate, and D. Donahue. 1975. Reproductive performance,
food habits, and population dynamics of raptors in the
Snake River Birds of Prey Natural Area. Pages 1-50 in
Snake River Birds of Prey Research Proj. Ann. Rep. 1975.
U.S. Dept. of the Interior, Bur. of Land Manage.,
Boise, ID. 193 pp.

Kortright, F.H. 1942. Ducks, geese, and swans of North
America. 3rd Ed. The Stackpole Co. Harrisburg, PA.
and Wildl. Manage. Inst., Washington, D.C. 476 pp.

McGahan, J. 1966. Biology of the golden eagle. M.S. Thesis.
Univ. of Montana, Missoula. 78 pp.

Messick, J. P., M.G. Hornocker, I.T. Mc T. Cowan and J. Whitman.
1975. Dynamics of predation upon a raptor prey base in
the Snake River Birds of Prey Natural Area, Idaho.
Pages 137-154 in Snake River Birds of Prey Research Proj .
Ann. Rep. 1975. U.S. Dept. of Interior, Bur. of Land
Manage., Boise, ID. 193 pp.

Ogden, V.T. 1973. Nesting density and reproductive success
of the prairie falcon (Falco mexicanus) in southwestern
Idaho. M.S. Thesis. Univ. of Idaho, Moscow. 43 pp.

Ogden, V.T. and M.G. Hornocker. 1977. Nesting density and
success of prairie falcons in southwestern Idaho.
J. Wildl. Manage. 41(1):1-11.

Olendorff, R.R. 1973. The ecology of nesting birds of prey
of northeastern Colorado. U.S.I.B.P. Tech. Rept . No. 211.
233 pp.

Porter, R.D., and CM. White. 1973. The peregrine falcon

in Utah, emphasizing ecology and competition with the prairie
falcon. Brigham Young Univ. Sci. Bull. 18(1) :74 pp.

Ratcliffe, D.A. 1962. Breeding density in the peregrine
falcon (Falco peregrinus) and raven (Corvus corax) •
Ibis 104:13-39.

Smith, D.G., and J.R. Murphy. 1973. Breeding ecology of

raptors in the eastern Great Basin of Utah. Brigham Young
Univ. Sci. Bull. Biol. Ser. 18(3):l-76.

Wolfe, M. and J.R. Montan. 1975. Density and species

composition of the major raptor prey species in relation
to land use and habitat type. Pages 98-126 in Snake River
Birds of Prey Research Proj. Ann. Rep. 1975, U.S. Dept.
of Interior, Bur. of Land Manage., Boise, ID. 193 pp.

49

Appendix 1. Weight of prey items found in raptor nests, Snake River Birds
of Prey Study Area, 1976.

Approximate weight, grams

Species

(Larg

MAMMALS

Black-tailed

jackrabbit

(Lepus calif-

ornicus)

Male

1878

Female

2342

Unknown

2110

Mountain cotton-

tail

(Sylvilagus

nuttalli)

827

Townsend ground

squirrel

(Spermophilus

townsendi)

male

260

female

187

unknown

211

Whitetailed

antelope squirrel

(Ammo spermophilus

leucurus)

106

Adult Medium Juvenile Unknown
(Large) (Intermediate) (Small) (Average) Source

1557

500

471

215

1379

514

167

106

161

Stoddard
(Pers. Comm.)

This Study,
Wolfe &
Montan
(1975)

Johnson &

Melquist

(1975)

Hall
(1946)

Least chipmunk
(Eutamius minimus)

78

Smith &

Murphy

(1973)

Yellowbellied marmot
(Marmot a flavi-
ventris)

3000

1500

500 1667

Hall
(1946)

Townsend pocket
gopher
(Thomomys town-
sendii)

240

200

Hall (1946)
This Study

50

Species

Valley pocket
gopher
(Thomomys
bottae) 170

Pocket gopher
unidentified

(Thomomys spp.)

Harvest Mouse
(Reithrodontotnys
megalotis)

Great Basin pocket
mouse
(Perognathus

parvus) 15

Great Basin
Kangaroo rat
(Dipodomys microps)

Kangaroo rat,
Unidentified
(Dipodomys spp.) 55.5

Deer mouse
(Peromyscus mani-

culatus) 17

Meadow mouse
(Microtus spp.) 60

Adult Medium Juvenile Unknown
(Large) (Intermediate) (Small) (Average)

Source

House mouse
(Mus musculus)

Bushy-tailed wood
rat
(Neotoma cinerea)

Desert woodrat
(Neotoma lepida)

Woodrat, Unidentified
(Neotoma spp.)

205

15

65

lr.

287

118

203

Smith &

Murphy

(1973)

This Study

Evans &
Emlen (1947)

Smith &

Murphy

(1973)

Smith &
Murphy (1973)

Hall
(1946)

Smith &

Murphy

(1973)

Porter &

White
(1973)

Evans &
Emlen (1947)

Wolfe &

Montan

(1975)

Wolfe &

Montan

(1975)

This
Study

51

Species

muskrat
(Ondatra zibeth-
Ica)

Mouse, Unidentified
(Unknown)

Shrew, Unidentified
(Sorex spp. )

Adult Medium Juvenile Unknown
(Large) (Intermediate) (Small) (Average)

1261

1052

1157

20

18

Source

Donohoe
(1966)

This
Study

This
Study

Domestic cat
(Fells domesticus)

Long-tailed weasel
(Mustela f ren-
ata)

Coyote
(Canis latrans)

Badger
(Taxidea taxus)

178

2043

2392

This
1800 Study

Smith &

Murphy
(1973)

This
Study

Hornocker et al .
(1976)

Mule deer
(Odocoileus
hemionus)

BIRDS

6300

McGahan
(1966)

Gray partridge
(Perdix perdix)

376

Ring-necked

pheasant

(Phasianus

colchicus)

male

1380

female

1017

unknown

1199

California quail

(Lophortyx

californicus)

Chukar

(Alectoris

graeca)

1093

198

571

McGahan
(1966)

Wolfe &

Montan

(1975)

Porter &

White

(1973)

Christ-

ensen

(1970)

52

Species

Adult Medium Juvenile Unknown
(Large) (Intermediate) (Small) (Average) Source

Common Raven
(Corvus corax)

Black-billed
magpie
(Pica pica) 173

Rock dove
(Columba livia) 330

Mourning dove
(Zenaida
macr' ira)

Yellow warbler
(Dendroica
petechia)

Starling
(Sturnus vulgaris)

Says phoebe
(Sayornis saya)

Brewer's black-
bird
(Euphagus

cyanocephalus)

Red-winged black-
bird
(Agelaius phoeni-

ceus) 54

Western meadowlark
(Sturnella neglecta) 96

Horned lark
(Eremophila

alpestris) 28

Loggerhead shrike
(Lanius ludovic-
ianus)

871

300

113

20

84

21

68

52

This
Study

Smith &

Murphy

(1973)

This
Study

Balph
(1973)

This
Study

Smith &
Murphy (1973)

Porter
White (1973)

Porter &

White

(1973)

Porter &

White
(1973)

Wolfe &
Montan (1975)

Wolfe &

Montan

(1975)

Esten
(1931)

5 3

Species

Pintail

(Anas acuta)

Mallard
(Anus platyrhynchos)

male 1219

female 1077

unknown 1148

Adult Medium Juvenile Unknown
jLarge) (Intermediate) (Small) (Average)

Source

Unknown duck

Killdeer

(Charadrius
vocif erus)

Great blue heron
(Ardea herodias)

Ring-billed gull
(Larus delawarensis)

Gull, Unidentified
(Larus spp . )

Rock Wren
(Salpinctes obsoletus)

White-crowned
sparrow
(Zonotrichia
leucophrys)

Sage sparrow
(Amphispiza belli)

Sparrow, Unidentified
(Unknown)

697

150

982

106

2807

419

504

13

30

30

17

Bellrose
(1976)

Kortright
(1942)

Kortright
(1942)
Porter &
White (1973)

Porter &

White
(1973)

Esten
(1931)

Harris
(Pers. Comm.)

Harris
(Pers. Comm.)

Harris
(Pers. Comm.)

Baldwin &

Kendeigh

(1938)

This
Study

Balph
,1973)

Prairie falcon
(Falco mexicanus)

715

This
Study

54

Species

Adult Medium Juvenile Unknown
(Large) (Intermediate) (Small) (Average)

Source

Red-tailed hawk

(Buteo jamaicen-

sis)
male

1028

female

1224

unknown

1126

Great horned owl

(Bubo

virginianus)

1505

Ferruginous hawk
(Buteo regalis)

1056

1230

1254

Craighead &
Craighead
(1956)
This
1091 Study

Craighead &
Craighead (1956)
1368 This Study

Howard
(1975)

REPTILES AND AMPHIBIANS

Side-blotched
lizard
(Uta stansburiana)

Johnson &

Diller

(1976)

Western fence lizard
(Sceloporus occi-
dentalis

18

17

Johnson &

Diller

(1976)

Desert horned lizard
(Phrynosoma platy-
rhinos)

24

11

18

Johnson &

Diller

(1976)

Western whiptail
lizard
(Cnemidophorus
tigris

17

15

Johnson &

Diller

(1976)

Collared lizard

(Crotaphytus

collaris)

35

26

Johnson &

Diller

(1976)

Leopard lizard
(Crotaphytus
wislizenii)

26

11

23

Johnson &

Diller

(1976)

Lizard, Unidentified
(Unknown)

24

17

Johnson &

Diller

(1976)

5S

Species

Woodhouse's toad
(Bufo woodhousei)

Toad, Unidentified

(Unknown)

Gopher snake
(Pituophis melano-

leucus) 273

Western rattlesnake
(Crotalus

viridis) 622

Ground snake
(Sonora

semiannulata)

Long-nosed snake
(Rhinocheilus
lecontei)

Striped whipsnake
(Masticophis taenia-

tus) 101

Night snake
(Hypsiglena
torquata)

Racer
(Coluber

constrictor)

Unknown snake
(Unknown ) 332

FISH

Adult Medium Juvenile Unknown
(Large) (Intermediate) (Small) (Average)

Source

147

305

19

60

20

:•■()

20

146

329

184

33

51

97

14

68

184

Diller
(Pers. Comm.)

Diller
(Pers. Comm.)

Johnson &

Diller

(1976)

Johnson &

Diller

(1976)

Johnson &

Diller

(1976)

Johnson Si

Diller

(1976)

Johnson &

Diller

(1976)

Johnson &

Diller

(1976)

Johnson &

Diller

(1976)

Johnson &
Diller (1976)

Carp
'Cyprinus carpio)

500

This

Study

lb

Adult Medium Juvenile Unknown
Species (Large) (Intermediate) (Small) (Average) Source

INVERTEBRATES

Grasshopper
(Locustidae) 0.63

Scarab beetle
(Scarabidae) 0 . 30

Scorpion
(Scorpionidae) 1.45

Cicada
(Cicadidae) 1 . 0

Jerusalem Cricket
(Gryllacrididae) 2 . 0

Wasp
(Hymenoptera) 0 . 5

Smith &

Murphy(1973)

Smith &

Murphy(1973)

Smith &

Murphy (1973)

This

Study

Evans &

Emlen (1947)

This

Study

57

Notes

58

STUDY II:

CONTRACTOR:
INVESTIGATORS :

PROJECT SUPPORT:

OBJECTIVES:

Responses of nesting raptors to human
disturbance in the Snake River Birds of Prey
Natural Area, Idaho.

University of Idaho, Moscow, Idaho 83843.

Steven R. Peterson, Principal Investigator.
Norman Stewart, Research Assistant.

U.S. Department of the Interior, Bureau of
Land Management, Contract No. YA-512-CT6-69.

1. Identify human activities that affect the reproductive
success of raptors in the Birds of Prey Natural Area
(BPNA) .

a. Test specific hypotheses concerning these disturbances
to prairie falcons in relation to proximity of nest,
time of day, and point in the reproductive cycle.

b. Note the responses of other raptors to human
disturbances and possibly correlate these with
reproductive success.

2. Measure prairie falcon habituation to initially
disturbing events.

3. If prairie falcons can be habituated to specific
disturbances, suggest ways of exploiting the
phenomena for use in research and visitor development.

ANNUAL SUMMARY

This study was terminated during the fall due to lack of
funds. Reinitiation will depend upon adequate funding after
baseline data on undisturbed raptor behavior has been collected.
Because of initial difficulties in establishing a study area where
our activities did not conflict with other research on raptors in
the BPNA, data collection was minimal this first summer. However,
we have summarized our general impressions and have suggested some
additional avenues of research, if the project should be refunded.

METHODS

Most of our time was spent in observation, often in conjunction
with other researchers working in the study area, particularly
Study I. Most of our data pertaining to the establishment of
breeding territories were gathered in the Swan Falls Dam area.

59

When possible, research activities were coordinated so that
observations on the behavior of nesting raptors could be made
before, during and after visits to a nest site by an investigator.

Observations were also made on the response of raptors to
boating activity, both public boating and trips by BPNA research
personnel .

At the end of the breeding season, the research assistant
aided John Doremus in measuring the physical attributes of the
nesting sites.

RESULTS

Our observations do not indicate whether breeding abandonment
is the result of a single event or an accumulation of pressures.
Observations on prairie falcons (Falco mexicanus) suggest that
human activity well below a nest site resulted in little or no
response from the falcons. This was observed in river narrows as
well as locations inland. If the prairie falcon does fly off the
cliff, it lands soon after. A sudden loud noise such as a dropped
rock or gun report, will often cause a bird to fly off the cliff,
but once again it lands almost immediately.

Human presence above the scrape, while not always resulting
in flight of the birds due to less visual or auditory contact,
will keep prairie falcons in the air until the humans depart, if
the falcons were disturbed. Prairie falcons are more difficult to
force (bump) on the nest site when eggs are in a late stage of
incubation or when young chicks are present. Human activity in
relation to parent prairie falcons appears less important late in
the nesting season because the adults are often not at the nest
site during much of the day, whereas considerable human activity
early in the season, especially when territories are being established,
increases the probability of site abandonment.

Among larger raptors, particularly golden eagles (Aquila
chrysaetos) , that frequently fly out of view after being forced
from a nest site, the time away from the nest can vary greatly and
seems to have little correlation with site abandonment. Golden
eagles have not returned for several hours only to be seen active
near the nest several days later, while others have returned in
less than one-half hour and the nest was later found abandoned.
Time was not available to observe these nests intensively, and
human contacts that we are unaware of may have occurrred after the
initial observation. Our observations suggest that considerable
variability occurs within the golden eagle population utilizing
the BPNA in regards to their tolerance of humans.

A nesting site readily observed and easily accessible to
humans will usually fail. Generally renesting attempts in the

60

same vicinity will also fail. The latter failures may be due to a
number of causes, such as the presence of humans, high cliff face
temperatures and resultant over-heating of eggs when adults are
absent from the nest, or decreases in available prey.

SUGGESTIONS FOR FUTURE RESEARCH

Before any additional research is undertaken concerning the
effects of humans on nesting raptor populations, the term "disturbance"
in reference to humans, should be more precisely defined in the
context it will be investigated. It is unrealistic to assume that
when a human comes within sight of a raptor and no visual response
is elicited, that the raptor is not necessarily "disturbed." The
visual response may actually be the end-point triggered by a
series of events up to that time. A controlled study utilizing
pen-reared birds equipped with cardiac transmitters (this method
has been used successfully on ground squirrels) would suggest when
raptors are stimulated by humans under specific conditions.

On the other hand, what we perceive as a disturbance may not
actually disturb the specific raptor at all. For example, prairie
falcons emit little or no visible response to human activities
below a nest site, especially late in the breeding season. Just
because humans are within close visual contact with a raptor does
not necessarily mean the events are disturbing to that specific
raptor.

It is the opinion of this investigator that a study designed
to investigate human disturbances to a raptor population should
really be examining the entire entity, i.e. behavior and breeding
success, with human activity as just one important variable. This
approach would allow human activity to be examined in proper
perspective. Different human-raptor contacts would have to be
identified including previous exposure (reflected in land-use
patterns of the area) and present exposure as reflected by public
visits and research activity. The disturbance threshold, as well
as the intensity of disturbances, would have to be quantified and
ultimately related back to breeding success in specific areas. An
inherent problem in this type of study would be individual variation
within the raptor population. Data from the BPNA suggests, at
least in golden eagles, that some individuals are more tolerant of
human activity than others. Consequently, for the data to have
much meaning, a fairly large sample size would be necessary, such
as could be obtained with prairie falcons in the BPNA. It would
be unwise to conduct an in-depth study of this type on species
such as the ferruginous hawk (Buteo ragalis) , simply because an
insufficient breeding population exists in the study area.

In this type of study, it will be necessary to cause some
nest site abandonment by repeated human-raptor contacts, both
prior to and during egg laying. If this is not accomplished,

61

then the disturbance threshold for specific periods of the breeding
cycle will remain unknown. This aspect of the research will be
unpopular with various "raptor-loving" groups, but I believe it
will be necessary for the data to have validity.

A study site(s) will have to be secured in which all human
activity, investigative as well as recreational, will be controlled.

Lastly, 1 believe the time for this type of study is long
overdue. We are in an era of resource management in which humans
have a potentially dominant influence on that resource. In areas
such as the BPNA, humans are going to demand more opportunities
for recreation. Unless managers know what the effects human
activities have on that raptor population, the reproductive effort
of the raptors may decline, or people may be unjustly deprived of
enjoying a public resource to its fullest.

62

Thomas Dunstan, Study III, affixes a radio transmitter to one
of six golden eagles tracked by research personnel in 1976
(BLM photo by Dunstan) .

63

STUDY III:

CONTRACTOR:

INVESTIGATORS :

PROJECT SUPPORT:

Activity, hunting patterns, territoriality,
and social interactions of birds of prey
in the Snake River Birds of Prey Natural
Area, Idaho.

Department of Biological Sciences
Western Illinois University

Thomas C. Dunstan, Principal Investigator.
James F. Harper, Nat. Sci. Tech. Assistant.
Kenneth B. Phipps, Research Assistant.

U.S. Department of the Interior, Bureau of
Land Management Contract No. 52500-CT5-1013.

OBJECTIVES:

2.

To determine the space needed by breeding golden eagles,
prairie falcons, and red-tailed hawks for food gathering
within the Snake River Birds of Prey Natural Area.

To determine the daily activity rhythms of each of the
species and to ascertain the inter-relationships between
the three species.

3. To determine the fledging and dispersal activities of
the young of the three species.

4. To coordinate the above mentioned objectives with the
objectives of investigators studying the ecology of
the prey species of the three species of raptors
previously mentioned.

ANNUAL SUMMARY

The habitat use and behavior of golden eagles (Aquila chrysaetos)
at six nest sites, of red-tailed hawks (Buteo jamaicensis) at five
nest sites, and prairie falcons (Falco mexicanus) at eight nest
sites were studied in 1976 as part of a three year study. Radio
transmitters were placed on 2 adult and 3 young golden eagles,
1 adult and 2 young red-tailed hawks, and 9 adult and 10 young
prairie falcons. Horizontal space use, hunting behavior, daily
activities and rhythms, territoriality, behavior of the young,
dispersal, and interactions were studied.

PLANS FOR NEXT SEASON

To continue to monitor behavior and activities of all
three species in five study areas, including an eastern
extension outside the BPNA.

64

2. To capture and place radios on selected individuals with
primary emphasis on gathering quantitative data about
hunting behavior as related to habitat use.

3. To coordinate efforts of Study III personnel with the
personnel from other studies.

4. To analyze, compile, and evaluate data in the preparation
of the Final Report.

5 . To compare and discuss the relationship of the data
from Study III with other studies in the pertinent
literature.

STUDY AREA

Raptors were studied at four sites within the Snake River
Birds of Prey Natural Area (BPNA) , Idaho. The designated sites
were named: Western, Intensive, Mid-desert, and Eastern sites
(Fig. 1).

METHODS

Definition of Terms

The following are definitions of terms used in this study.
"Territory" is a defended area. "Total range" includes all of the
horizontal space used by study subjects when we monitored. For
this report, "home range" includes all area used except areas used
on only one flight, and areas used after the young dispersed.
"Sunrise" and "sunset" refer to official sunrise and sunset
respectively. "Hours monitored" is the length of time that the
location of a marked bird was known. A "spot check" was a random
monitoring of a bird's location, usually for 15 to 30 minutes as
we passed in the vicinity. During a "sample day" we monitored for
more than several hours, usually for the entire day. Two or three
consecutive sample days constituted a "sample period." A "perch
site" was a rocky ledge, power pole, cairn, or other object a bird
repeatedly used as a perch when hunting, loafing, or roosting. A
"trap day" was a day when trapping was attempted. "Man-hours" was
the time spent setting up a trapping apparatus and waiting nearby
for a capture. "Set-hours" included only the time that the trap
was set to capture a subject.

Frequently we monitored locations of several birds simultaneously.
If a second bird flew in a different direction while we were
tracking a subject, we only noted a flight was made, since we were
not able to plot the second bird's flight path. This distinction
is referred to as "flights followed/flights noted."

Home
Nest Ran%e Species

D
O

A

Fig. 1. Location of 20 nest sites and 14
home ranges of birds during 1976. Adjacent
prairie falcon ranges have been combined.
The 4 Study Sites are indicated in
parentheses .

66

Trapping

We used the same trapping techniques as in 1975 (Dunstan and
Harper 1975). In 1976, an additional technique, a drop net, was
used. A 1.8 x 1.8 m piece of gill net (6.3 cm mesh), reinforced
peripherally with 12-gauge plastic covered wire, was designed to
be dropped at night and entangle a bird perched below.

Instrumentation

Captured raptors were weighed, measured, and sexed. Numbered
U.S. Fish and Wildlife Service metal leg bands (pop rivet for
eagles, lock-tite for hawks) were applied, and an identification
code was assigned to each bird (Table 1) . Radio packages as
described by Dunstan and Harper (1975) were used to mark study
subjects. Some wing markers were also placed on subjects by
personnel from Study I (see this report, Study I). In 1976, two
radio packages used on the adult eagles contained two transmitters
within a green back sack. Each package weighed 180 g.

Monitoring Instrumented Subjects

Radio-tagged birds were tracked from ground vehicles, aircraft,
and on foot, using receivers and antennas previously described
(Dunstan and Harper 1975). In 1976, a second tracking truck, a
Dodge pickup, was used in addition to the International Travelall
used in 1975. On sample days, we continuously monitored the
position of marked birds from 0.5 hours before sunrise to 0.5 hour
after sunset. Due to vehicle breakdowns and the difficulty of
simultaneously following two to four birds in different directions,
there were periods of up to several hours in some sample days when
no data was gathered from certain individuals.

Prairie falcons in the Intensive Study Site were monitored
three consecutive days in every 11-day period. Falcons in the
Eastern Study Site were sampled on two consecutive days per week.
The remaining study subjects were tracked for one, two, or three
consecutive days at irregular intervals. Information gathered
during spot checks and trapping days added to the total hours
monitored.

Monitoring Non-Instrumented Birds

Unmarked members of families were also studied. Individuals
were identified by plumage characteristics (color variations,
various stages of molt) , and association with a nesting site or
other family members. Size dimorphism in golden eagles was
distinguishable, and eagles regularly used favored perches and
hunting areas. Because of their large size, eagles were seen and
followed to and from eyries. Red-tailed hawks were also large
enough to be followed visually.

67

Table 1. Data for 31 marked study subjects, 1976

I.D.

Band

Instru-

Location

Code

Sex

Age

Number

mented

Dispersed

GOLDEN EAGLE

Cabin Draw

CD

U

Yg

629-02605

19 July

26 Oct.

Cabin Draw

CD

u

Yg

629-02606

—

—

Rapids (RD)

GE-1

u

Yg

599-11928a

15 June '75

6-11 Feb.

Feedlot

FL

F

Ad

629-02602

12 May

(remained)

Feedlot

FL

u

Yg

629-026303

—

1-10 Aug.

Feedlot

FL

u

Yg

629-02631

19 June

17-20 July

Black Butte

BB

F

Ad

629-02603

3 July

8-9 July

Black Butte

BB

U

Yg

629-026353

3 July

8-9 Julyc

RED-TAILED HAWK

Priest Ranch

RT-1

M

Ad

987-16303

29 May '75

13-17 Aug.

Black Butte

B

M

Ad

987-16304

13 June

1-7 Sept.

Black Butte

B

U

Yg

987-16305

11 July

17-19 Aug.

Spoon

S

U

Yg

987-21 968a

7 July

5-19 Aug.

PRAIRIE FALCON

Spring

s2

F

Ad

877-37802

14 May

13 July

New

No*

M

Ad

836-77002,

987-16360°

987-21951°

27 March

21 July

Priest

P-l

U

Yg

17 June

23 July

Priest

P-2

u

Yg

17 June

19-20 July

PF-2

PF-2?

F

Ad

987-16302

27 March

28 June

Camera

§f

M

Ad

836-77006 .
967-57231°
987-16336°
987-16354°
987-21955°
987-21954°

30 April

10 July

Camera

F

Ad

3 April

12 July

Camera

C-l

U

Yg

12 June

1-2 July

Camera

C-2

U

Yg

16 June

7-8 July

Finger

F

u

Yg

20 June

15-16 Julyc

Powerline

W

u

Yg

17 June

14 July

Upper Bigfoot

D-4

U

Yg

987-219583

17 June

16 July

Upper Bigfoot

D-7

U

Yg

786-94379a

17 June

15-16 July

Loaf

If

M

Ad

836-77004,
967-57288°

23 April

4-6 July

Loaf

F

Ad

23 April

21-23 Junec

Humpback

5?

H

Ad

836-77003

12 April

mid-October

Humpback

F

Ad

877-37801

21 April

2 July

Humpback

H-ll

U

Yg

987-2191 la

10 June

3-6 July

Humpback

H-12

u

Yg

987-219123

12 June

29 June-
1 July

a. Banded by Study I personnel.

b. Banded and instrumented by Study VII personnel.

c. Died

d. Banded by V. Ogden, Idaho Coop. Wildlife Res. Unit, 1972.

68

Data Analysis

Golden eagle. To derive range maps of horizontal space use,
all sightings of an adult pair during the breeding season were
combined. Mated eagles regularly used the same perches and often
perched and flew together over the same areas. Although one of
the sexes used a specific location more than the other, both used
the same areas and therefore had identical ranges. At the feedlot
site, we monitored for full days and noted activity (perched
versus flying, and location). The data were obtained by noting
the activity and location of the subject every 10 minutes.

Prairie falcon. To clarify the increases in falcon range
size as the breeding season progressed, four ''periods" were
designated: incubation, nestling, fledgling, and dispersal.
These divisions were based on changes in the adult behavior,
mainly a noticeable increase in the distance of the furthest
flight monitored during a two or three day sample. Due to the
nature of our sampling schedule (three days "on" with eight days
"off" at the Intensive Study Site, and two days "on" with five
days "off" at the Eastern Site) , cutoff dates between the four
periods were not abrupt.

The four divisions also were closely related to the development
of the young. During the "incubation" period, adult females
rarely left the canyon. Approximately a week after the young
hatched, adult females began making several flights per day out of
the canyon, and this change in behavior marks the beginning of the
"nestling" (brood-rearing) period. The "fledgling7' (post-fledgling)
period began within a week after the young fledged, when the
adults began making flights distinctly further from the canyon
than previously monitored. The "dispersal" period included
increases in space used after fledglings dispersed from the BPNA,
and just prior to departure of the adults. Not all adults exhibited
definite increases in cumulative range with each change in period.
The four divisions demonstrate the changing nature of the horizontal
space used as the breeding season progressed.

In analysing daily rhythms, we measured the portion of the
diurnal day spent out of the canyon (minimum of 100 m from the
rim). The day was divided into half hour increments, and the
amount of time the adult bird spent out of the canyon was divided
by the amount of time we monitored the bird. For example, between
0900 and 0930 hours, we were monitoring on eight days (a total of
four hours), and the bird spent a total of 1.5 hours out of the
canyon on those eight days. The portion of time spent out of the
canyon is a decimal of 0.38 (1.5 hrs/4 hrs) . The probability of
finding the falcon out of the canyon during that particular half
hour on any given day would be 38 percent. By totaling the decimal
value for each half hour increment, and then dividing by the
number of half hour divisions in the diurnal day, the average time

69

spent out of the canyon is derived. By grouping the monitoring
days into the four developmental periods, we determined values for
the average time spent out of the canyon for each bird for each
period.

Similarly, we derived the average number of round trips north
of the canyon per day per bird. For every half hour increment in a
group of sample days, the numbers of round trips was divided by the
number of days monitored during that half hour. The total of all
the half hourly decimal values in a diurnal day is the average
number of round trips per day for that developmental period. For
example, during a developmental period, we may have been in a
position to monitor on 10 days between 0800-0830 hours, but could
only monitor on seven days between 2000-2030 hours. By deriving a
decimal value for each half hour increment, we minimized the effects
of data gaps in our full day sampling.

Flight speeds were determined by dividing the distance between
two known points by the time interval it took the bird to travel
that distance.

Red-tailed hawks. The methods of monitoring and data analysis
for red-tailed hawks were the same as for eagles and falcons.

RESULTS

Golden Eagle

Chronology. Eagles were the first of the three species to
initiate breeding activities (Fig. 2) . Nest repair began in
January and continued into February at the Rapids, Ogden, and
Beecham nests in the Intensive and Western Study Sites. We planned
to work with the Rapids (RD) eagles a second year, but after the
adults repaired the nest and courted, they laid no eggs. The Ogden
(OG) pair nested on the steep north canyon wall, which was inaccessible
for study. The Beecham pair repaired nests, but did not breed in
1976.

The eagles at Black Butte (BB) and Feedlot (FL) eyries had a
similar chronology, but the Cabin Draw (CD) pair nested about a
month later. During incubation, field personnel remained away from
the nest sites to minimize disturbance.

Nestlings remained in the eyries into June and July, and
fledging occurred at an age of 8-11 weeks. One radio-tagged
fledgling at the Feedlot site dispersed in mid-July, GE-1 from the
Rapids eyrie in 1975 did not disperse until February 1976. Adult
golden eagles remained within the breeding range for the entire
year.

Nest
Site 1

R CD

3 5 BB
O <!

u w

FL

March
10 20

1

Ap r i 1
10 20

1

May

10 20

J une

10 20

July
10 20

Augus t
10 20

I L_

Sept ember

1 10 20

i i )

LLL: :::::::::::::: :HHH==
LLLL: :::::::::::::: : HHHH================

LLLL: ::::::::::::: : HHH = = = = = = = = = = = = " = = = = =

= F-xD

= FF-

a

.J
H «

< is
o

W
Pi

LLL: :
LLL:

:::::::: : HH============

::::::::: :HH==x

LLLL: :::::::::: :HHHH = = =
LLL: :::::::::: :HHH=

=FFFFF-

= FFF-

L

D

!s

CJ

W

<

ta

F

w

l-l

C

n

«e

BJ

I

Oh

P

N

S

LLL: :::::::::: HHH======

LLL: ::::::::: :HHH = = =
LLL: :::::::::: HH-

LLLL: ::::::::: :HHH = -
LL ::::::::: : :HHH =

======FFF

-dDd

.DX

-dd

d

>

= = FFF d--dDD

= = = FFFF-- D

■d-d

■-D

LLLL:

:HHH =

= FFF ■. . . . .D

eggs in nest young in nest young flying no young remain

Fig. 2. Chronology of the 1976 breeding season.

Key

L egg laying

H hatching

F fled Ring

D adult dispersed

d young dispersed

X adult found dead

x young found dead

O

71

Trapping effort. A total of 207 man-hours and 147 set-hours
on 20 trapping days were expended to capture two adult golden
eagles. Approximately half this effort was spent in the Intensive
Study Site in February and March. The other half was spent in the
Mid-Desert and Eastern areas in June and July.

Horizontal space use. Movement and perch site data were
gathered from the Rapids, Ogden, Beecham, and Swan Falls Dam
eagles by direct observation from 2 January to 31 March, 1976.
During observations on 26 days (including census days for pole
use), 59 golden eagle sightings were noted perched on crossbars of
the tall power line (138 KV) . Five sections between poles numbered
195 through 278 were preferred (Fig. 3). The flight paths of the
Ogden and Rapids eagles, as well as 46 additional golden eagle
sightings away from the power line were noted. The clumping of
locations of certain sightings, when combined with flight path
data, indicate a relationship to specific nest sites.

Within the Intensive Study Site the tall pole line and the
buttes were the favored loafing and hunting perches. Eagles of
various age classes perched on top of or soared over Coyote Butte,
which protrudes 60 m above the desert flats. Raptors seemingly
were attracted to the butte because of its shape, which caused
windward updrafts, lee waves, and slope drafts suitable for soaring.
The same situation, but to a lesser degree, existed at Trio Butte,
which protrudes 30 m above the desert floor. During the winter of
1975-76, we also made eight bald eagle sightings.

The young eagle (GE-1) from the 1975 Rapids nest remained
within its home range until 6 to 12 February when it dispersed.

Breeding season horizontal space use information was collected
from the Cabin Draw, Black Butte, and Feedlot pairs. Neither
Cabin Draw (CD) adult was radio-tagged, and on the basis of 89
pairs of visual observations, the minimum home range of the pair
was 12 km^ . The birds used both the north and south sides of the
Snake River (Fig. 4). Favored perches near the eyrie were north
across the ravine along the rock cliff, and on the point 60 m west
of the nest. These perches were used at night for roosting, and
occasionally during the day when the young were still in the nest.
Other favored perches were located along the rim south of the
eyrie. Perches on the shaded north and west sides of Jackass
Butte were used often during the hot summer days. Mid-day thermal
and slope updrafts were used for soaring at Jackass Butte, and
adult eagles hunted along the rims and slopes. Additional favored
perch sites were on vertical uprights and horizontal crossbars of
the tall power line 1.5 km north of the eyrie. Both the male and
female favored pole 241 (short power line) , located close to a
draw with relatively tall vegetation. They also hunted from pole
238 on the flats. Poles 105 to 112 of the tall power line received
use as perch sites, as did several cairns and rock piles.

72

A 1976 nest
A 1975 nest
:;;; Farm
• Sighting

■ Flight path

Favored section

o f power 1 ine
/ c \ No . sightings /

section of poles

Total range of GE- 1 (21 km2 from 6/75-2/76) |.

1 Mi

-—•-Minimum total range of Rapids adults (29 km2) 1 Km

Fig. 3. Golden eagle sightings away from the nesting
areas from 2 January to 31 March, 1976. There were 59
sightings along 5 favored sections of the tall (138 KV)
power line, and 45 other sightings in random sampling.

73

Site

Area

Hours

Days

Cabin Draw

12 km2

89

20

Black Butte

21 km2

28

15

Samp le period

9 June-
1 3 June

19 No
-27 0

v ,
ct

1 Km

Jk Nest
• : • Farm
• Perch

Fig. 4. Area used by adult golden eagles at the Cabin
Draw (CD) and Black Butte (BB) sites, 1976.

74

The Black Butte (BB) adults frequented both sides of the
Snake River, covering at least 21 km2, determined by 28 hours of
monitoring (Fig. 4). The adult female was instrumented on 3 July,
but disappeared on 8 or 9 July after the death of her fledgling.
Favored perches near the eyrie were 500 m southwest along and just
below the rim. Two favored night roosts were used during the
breeding and post-breeding seasons. They were 15 and 12 m below
the rim and 150 and 375 m northeast of the eyrie respectively.
Other perches in sight of, but further from, the eyrie were at the
apex and west slope of Black Butte, and on the east end of Jackass
Butte. Birds perched on the east end of Jackass Butte could not
see Cabin Draw eagles perched on the west end of Jackass Butte.
Perches out of sight of the eyrie were along the short pole line
between poles 265-268, along the tall pole line between poles 88-
90, and on several cairns further north.

From the top, or on the slopes of Black Butte (which rises
115 m above the desert flats), the adult eagles could see almost
all of their home range, as well as a majority of the home range
of the Cabin Draw eagles to the west, and the Beer Case pair of
eagles to the east. The large surface area of Black Butte caused
slope drafts suitable for soaring. Updrafts were also common
along the ravines and draws in Section 11 west and northwest of
the butte, and this area was often used for hunting.

During the breeding season, the Feedlot (FL) adults used a
home range of 48 km2 (Fig. 5). No flights south of the Snake
River 4.5 km from the nest were noted. Between the nest and the
river, an intensively cultivated flood plain provided limited
habitat for Townsend ground squirrels (Spermophilus townsendi) and
black-tailed jackrabbits (Lepus californicus) , the prey most often
brought to the nest. At this site, the adult female and one of
two nestlings were instrumented. The adult female was identified
to be an eagle released as a one year old in 1964 by Morlan Nelson
(Morlan Nelson, pers. comra. ) .

Favored perches near the eyrie were 15 m to the south along
and just below the rim. The adults also used a rock point 150 m
directly across from the eyrie, and perched along the draws in
Section 22 west of the eyrie. Shaded perches below the rim were
used on hot summer afternoons. Preferred crossbars along the
short power line were on pole 347 (on top of a hill over looking
the ravines close to the eyrie) , pole 357 (near a creek bed) , and
pole 361. Preferred perches along the tall power line were at
poles 54, 48, and 33.

Dorsey Butte and adjacent ridge were often used for perching.
Dorsey Butte rises 60 m above the desert flats and provides a
vantage point with line of sight to all areas of the home range
except for below the rim. Additional favored perch sites were
near dry creek beds or rocky areas, which provided the tallest

75

Home range Area Hours Days Sample period
Breeding season 48 km2 249 62 10 May- 29 Jan ( 7 7 )

Post-breeding 18 ki

10 2

9 16 Nov-29 Jan(77)

m. Nest
• • ■ Farm
o Perch

Fig. 5. Area used by adult golden eagles at the Feed lot
(FL) site, 1976.

76

vegetation in the area, and habitat for favored prey, especially
black-tailed jackrabbits and desert cottontails (Sylvilagus
nuttalli) .

Between 24 June and 25 July, the Feedlot adult female made
four trips 60 to 70 km northeast to the foothills north of Glenns
Ferry and King Hill. On each occasion, the eagle remained in the
foothills overnight, and in one case, was absent from the home
range for at least 4.5 days (6-10 July). When monitored on three
departure and two return flights, the initiation of flight occurred
between 1230 and 1705 hours. In all cases, trips were made without
landing along the way, and the eagle remained airborne a maximum of
3.5 hours.

The radio-tagged fledgling dispersed between 17-20 July (when
the adult female was within the home range) .

The Feedlot adults used a 18 km^ post-breeding range as
determined by sampling between 16 November 1976 to 29 January 1977
(Fig. 4). The wintering eagles were monitored on full day samples
from 45 minutes before sunrise to 45 minutes after sunset. The
bird's locations were grouped into three geographic areas: 1) the
eyrie, including the draw just west, and the nearby small pole
line, 2) the rim and slopes southeast of the nest, and 3) north of
the eyrie including the tall power lines and Dorsey Butte. These
areas were the three general areas that the wintering birds seemed
to prefer for hunting. Other large areas within and adjacent to
these were not used. Our purpose in analyzing the data in this way
was to determine relative use of each of the three general areas as
they relate to hunting use. The three areas were identified as
Eyrie, South, and North respectively.

The marked adult female was monitored for 100 hours on nine
sample days, and spent 21 percent, 12 percent, and 67 percent of
the time in the Eyrie, South, and North areas respectively. The
adult male (not instrumented) was monitored for 45 hours on eight
sample days and spent 1 percent, 2 percent, and 97 percent of the
time in the Eyrie, South, and North areas, respectively. The male
and female were observed simultaneously, but the male occasionally
was lost from sight when it flew as we watched the female. The
male was sometimes not seen for several hours. This accounts for
the difference in hours monitored. The female repaired the 1976
eyrie, and also started a new nest in January 1977. Sticks and
grasses were gathered from nearby, and this accounts for the greater
percentage of time spent near the eyrie.

Boundaries for the post-breeding range were related to the
location of favored perches, which were related to hunting methods
used in taking favored prey (probably jackrabbits) during this
season.

77

Hunting behavior. Data on hunting were gathered by direct
observation and radio-tracking. All of the eagles studied used
several methods of hunting. The four methods of hunting seemed to
be related to the physical conditions for various types of flight,
the preferred prey, and the prey habitat.

Eagles hunted while watching from perches on cairns, rims,
and power line crossbars. Flights from perches were either direct
to the prey, or were out over habitat in search of prey and then
back to the perch. In the latter case eagles attempted to flush
prey or surprise it during the flights.

Eagles also hunted by watching for prey during long (time and
distance) flights. Flapping or soaring flights were of several
types. Flapping flights were common in morning, late afternoon,
and evening until after sunset. During flapping flights, eagles
flew rapidly just above the vegetation and crossed back and forth
over several square kilometers. Soaring flights occurred during
late morning, mid-day, and early afternoon at elevations of
approximately 20 to 500 m. To travel distances more than a
kilometer, eagles used downward glides that terminated by spiraling
upward. Hunting from soaring flights was not common.

A third method of hunting, commonly used from mid-day until
early evening, was a combination of flapping and soaring along
rocky areas with slope updrafts. Eagles slowly flew over and
along the rims and talus slopes with a hunting style typical of a
harrier (Circus cyaneus). These flights were made where there
were boulders and vegetation that appeared to be good habitat for
cottontails .

A fourth method of hunting flight was a combination of
flapping flight and short glides. This method was commonly used
when hunting ravines and draws. After soaring along the rim,
eagles entered ravines at fast speeds and glided through with
wings tucked in fairly close to the body. Although no prey was
seen taken in this manner, rock doves (Columba livia) were seen
flushing out of the ravines as the eagles flew through. We
speculate that any prey, either mammalian or avian, that is
exposed and vulnerable may be taken during such quick flights.

The adult eagles in Cabin Draw brought from 0 to 4 prey items
per day to feed two nestlings. This statement is based on data
gathered during observations on four trap days. Avian prey included
an adult red-tailed hawk, a fledgling great horned owl (Bubo
virginianus) , a nestling ferruginous hawk (Buteo regalis) , a
fledgling prairie falcon, a black billed magpie (Pica pica) , a
rock dove, and a ring-necked pheasant (Phasianus colchicus) .
Mammalian prey brought to the eyrie included one or more Townsend
ground squirrels, black-tailed jackrabbits, desert cottontails,
and pocket gophers (Thomomys townsendii) .

78

The Black Butte adults hunted from perches on Black Butte,
cairns, rock piles, and crossbars of pole lines. Rock piles north
of the power lines provided habitat for mammalian prey. A favored
area for the flapping-soaring method of hunting was the ravine and
bowl west of Black Butte. No data were gathered on food habits at
this eyrie.

The Feedlot adults hunted from specific power poles located
near likely rabbit habitat; the tallest and most dense vegetation
in the area. Other hunting perches were on rock piles, Dorsey
Butte, and along the canyon rim. During July, the adult female
hunted along the slopes and rim southeast of the eyrie using the
flapping-soaring method. The male also hunted this area but was
not marked with a radio and was more difficult to follow. Both
adults hunted just west of the eyrie in the grassy bowl where
there were updrafts for soaring. Prey found in the eyrie included
remains of black-tailed jackrabbit, Townsend ground squirrel,
desert cottontail, white-tailed antelope squirrel (Ammospermophilus
leucurus, and an unidentified snake.

The relative time the Feedlot adults spent perched versus
flying was studied on full day samples (sampling every 10 minutes)
between 16 November 1976 and 29 January 1977. During 100 hours on
nine sample days, the adult female (with radio) was perched 97
percent, and flying 3 percent of the day-light hours. In 45 hours
on eight sample days, the adult male perched 97 percent, and was
flying 3 percent of the time. Ground squirrels were not available
during this season and rabbits were probably the primary prey. On
four days, the male killed one jackrabbit each, and on three
occasions, shared it with the female. The small post-breeding
range was probably related to two factors: 1) less demand for
food, and 2) the location of hunting perches used during the
winter months.

Daily activities and rhythms. During January and February,
eagles were not often seen flying except between perch sites.
Most of the flights were flapping flights. The breeding eagles
usually roosted at night in the canyon, and were seen flying out
to the power lines from dawn to mid-day. On foggy days or windy
days with snow, eagles were less likely to leave the canyon.

During incubation (March and April) , the females remained at
the nest, and males seemed to spend much time near the nest. To
minimize human disturbance, we only spot checked at nest sites
during these months.

After the eggs hatched, the adults at the Ogden, Cabin Draw,
and Feedlot eyries tended the young closely until the nestlings
were two to three weeks of age. In subsequent weeks, the adults
often remained away from the eyrie for intervals of three to six
hours from mid-morning until late afternoon. When food was

79

brought to the nest, the adult female spent time at the eyrie,
tearing the prey apart and feeding the young. Until the young were
about eight weeks old, the female, and sometimes the male, perched
within line-of-sight of the eyrie.

Night roosts were often in the eyrie ravines where favored
perches were easily identified by "whitewash" on the rocks and
ground below. The power pole crossbars were commonly used for
night roosting, even through the winter. Automobiles passing in
the dark beneath the powerline did not flush roosting eagles.

Behavior and dispersal of young. The nestlings remained in
the eyries into June and July. Young in nest, with no shade and
exposure to the afternoon sun, tucked their heads into small crevices
or crawled into large crevices in the rocks, apparently to escape
the heat. The two young, at the Feedlot eyrie, crawled into a
large crevice during the afternoons. As they grew older, the
larger bird dominated the shade areas. The younger Feedlot sibling
was found on the ground below the eyrie on 24 May after a wind and
rain storm, and was also found on the ground on 1 June. On 23 June,
it was again found on the ground below the nest, but by this age
(10-11 weeks) it was able to glide into the ravine. The older
sibling had also recently fledged. The younger bird was placed
back in the nest, but it jumped out again the next day.

After fledging, the young perched in shaded areas near the
eyries during warm afternoons and often used favored perches of the
adults. The young remained close to the eyrie and often returned
to it to receive food from the adults. Young were very vocal and
called throughout the day for food.

It appears that the young are provided with food until a
certain date. The young then disperse and sometimes wander through
the range of other breeders. Carrion, which is easily located by
watching the activity of common ravens (Corvus corax) or black-
billed magpies, may be an important food source at this stage of
development. The young do not use the entire adult home range, but
tend to stay closer to the eyrie. The lack of food brought and the
short term absence (up to a day) of the adults from near the eyrie,
may prompt the young to disperse.

The date of dispersal varied between young from different
nests and between siblings (Fig. 2). The two young at the Ogden
nest were not marked and dispersal date was undetermined. The
Black Butte fledgling was found dead on 9 July in a draw by the
river. The radio transmitter had been cut off the bird and the
bird's head was laterally flattened. The cause of death was not
determined but the day before we found the carcass, a local resident
reported seeing an eagle with a yellow marker flying along the road
in the same vicinity.

80

The two Feedlot young dispersed independently of each other.
The younger sibling (band 629-02631), with the radio, was last
seen and received on 20 July. A young eagle (unmarked), presumed
to be the older sibling (band 629-02630) , was last seen near the
eyrie ravine on 30 July.

The two young at Cabin Draw were tethered to the nest plat-
form from 27 June to 6 July during trapping attempts. The larger
young (band 629-02606) fledged on 20 July and was seen in the
eyrie ravine during the following week. This eagle was not marked
with a radio, and was difficult to find and identify after it left
the nest cliff. The smaller sibling (band 629-02605) fledged on
24 July. By 11 August, it was seen perched in the company of the
adults on the west end of Jackass Butte (Fig. 4). On subsequent
days, the young eagle moved back and forth across the river. On
14 September, this eagle was caught by hand in an emaciated condition
(weight 1,400 grams), unable to fly. The bird was fed and cared
for until release on 19 October (weight 3,540 g) in apparently
healthy condition. This instrumented eagle stayed within the
range of the parents through 25 October when it moved west several
kilometers. It was chased by two adults at Castle Butte (6 km
west of the eyrie on the south side of the river) on 6 November.
The bird's radio signal was last detected south in the foothills
of the Owyhee Mountains on 17 November, 29 days after its release.

Red-tailed Hawk

Chronology. Very few red-tailed hawks reside in the Study
Area from late September until late February. Nest building and
repair began in mid-March and continued in late March. Five pairs
of hawks were studied for various intervals and intensities (Fig.
2). Hatching date at the Black Butte nest lagged about two weeks
behind hatching at the Halverson (H) and Priest Ranch (P) nests.
Our detailed study of the Black Butte (B) nest, as well as less
intensive observations of the Spoon (S) and Upper Bigfoot (U)
nests, began in June.

The adult male RT-1 (color marked and instrumented in May
1975) from the Priest Ranch nest returned to the 1975 home range
by 28 February 1976 (J. Messick, pers. comm. ) . RT-1 and mate were
seen copulating nine times from 22-24 March and building a new
nest 75 m east of the 1975 nest with incubation commencing by
2 April. The adults at the Priest Ranch nest were studied in
detail until the nestlings died between 7-12 May. Later information
was gathered during spot checks or on prairie falcon sample days.

At the Priest nest, two young hatched between 4 and 7 May,
but by 13 May, they were dead, possibly from overheating. In an
attempt to encourage the Priest Ranch hawks to continue using the
site, two young of a brood of four from the Halverson Lake nest
were placed in the Priest nest on 15 May. The Priest adults,

although remaining in the canyon, did not return to the nest, so
the foster young were removed after two days. Even though the
nesting attempt failed, the Priest adults remained within their
old home range for the duration of a typical breeding season.
RT-1 dispersed between 13 and 17 August. RT-1 dispersed on
14 August in 1975.

Trapping effort. A total of 101 man-hours and 68 set-hours
were expended in May, June, and July to capture one adult red-
tailed hawk (Black Butte) . One nestling was instrumented at the
Black Butte nest and a second nestling was instrumented at the
Upper Bigfoot nest. The 1975 transmitter on RT-1 (Priest Ranch
nest) continued to function through the 1976 breeding season.

Horizontal space use. Neither adult was marked at the
Halverson Lake (H) nest, but in 10 hours of observations during
spot checks and trapping visits, a minimum of 4 km^ was observed
used (Fig. 6). Adults brought prey from both sides of the Snake
River. The eyrie was on a ledge along a vertical cliff and the
adults perched on the nest ledge, the rim, and the cliff. The
male was darker than the female and we could visually differen-
tiate between the sexes without using radios.

o
The total area used by RT-1 (Priest Ranch nest) was 24 km .

Excluding areas used on only one day, the home range was 13 km^

(Fig. 7). There was no change in the home range after the nestlings

died. There was a change in time spent out of the canyon. The

1975 home range, while young were present, was 23 km^ and included

much of the same area used in 1976. In both years, we monitored

from the north side of the canyon, so the exact location of the

bird's south side range was not determined, but the bird was

seldom noted south of the bluffs above the nest.

Favored perch sites for both Priest adults included ledges
and rocks near the nest, trees at the Priest Ranch, and rocks and
big sage (Artemisia tridentata) along the south slope above the
nest. North of the canyon, RT-1 frequently used tall power line
poles 226-234 and occasionally perched out to 223-238. In 1975,
the extremes were similar, from 225-235. Poles of the short power
line were not used for perching during either year. Adults also
perched on big sage north of the canyon, as well as on the ground.
The canyon cliffs provided updrafts for soaring over the north and
south rims.

The home range boundaries for the unmarked Spoon and Upper
Bigfoot red-tailed hawks were based on a relatively small number
of sample hours (Fig. 8) . These home range measurements should be
considered as minimum values.

82

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Fig. 6. Area used (4 km2) by adult red-tailed hawks at
the Halverson (H) site during 10 hours of monitoring on
19 April through 22 May, 1976.

83

Monitored 463 hours
18 March-11 August

Fig. 7. Area used by RT- 1 , adult male red-tailed hawk,
at the Priest Ranch (P) site, 1976.

84

Site

Spoon

■— Upper Bigf oo t

Area Hours Days Sample period
5 km2 ' 29 5 4-24 June

km'

5

5 4-24 June

1 Mi A Nest
::: Farm
Perch

Fig. 8. Area used by adult red-tailed hawks at the Spoon
(S) and Upper Bigfoot (U) sites, 1976.

85

The Spoon (S) adults frequented a home range of 5 km2 during
brood rearing. Favored perches near the eyrie were on rocks along
and below the rim, within 60 m to the east and west. The adults
usually roosted at night on the canyon wall near the eyrie. They
avoided flying to the south where an aggressive pair of prairie
falcons defended a territory. Crossbars of the small power line
between poles 182-198 were used, as were poles 133-137 of the tall
power line. These adults soared from the eyrie and hunted south
over the desert and farm fields of the flood plain.

The Upper Bigfoot (U) adults frequented a home range of 8 km2
during brood rearing (Fig. 8) . Favored perches were along the
canyon wall within 25 m of the eyrie and east across the ravine.
These adults avoided the area of the rim west of the nest, where a
pair of ferruginous hawks nested. The red-tailed hawks also
perched on a power line extension that branched south from the
short power line at pole 213. They also used poles along the
short and tall power lines. The Upper Bigfoot adults were seen
soaring over the rims and slopes south and east of the eyrie as
well as over the desert and farmed fields toward the river.

The radio-tagged male at the Black Butte (B) nest used a home
range of 20 km2 during brood rearing, post fledging, and dispersal
(Fig. 9). By visual observations, we noted that the unmarked
adult female remained within the home range of the male and was
often found with the marked male when away from the nest site.
The total range of the male was 34 km2 including area for one long
soaring flight to the southwest (Fig. 9). The home range of the
female (20 km2) was relatively easy to determine because she often
flew with the male or with the radio-marked fledgling. The male
was reddish in color and the female was regular color. The female
had a secondary feather growing on the right wing that was easily
recognized until about 9 July.

The nest was on a ledge of a 10 m high cliff that was 800 m
south of the main canyon wall. Favored perches were on the rim
above the eyrie and 100 m west. A row of trees by the river,
south of the eyrie, provided shaded day perches for both the
adults and the fledgling. Two poles between two farms on the
south side of the river were used for early morning hunting,
loafing, and night roosting. These poles were in sight of the
nest. Both adults perched on hilly ground in areas with sparse
vegetation and along a rocky bluff in Sections 28 and 29. Small
power line poles and fence posts were also frequently used for
perching.

From 16 June to 26 August, the location of the Black Butte
adult male was noted every 15 minutes on 15 sample days (ranging
from 0.75 hour to 10 hour samples on 12 days, to three full day
samples). Based on 384 data entries (96 hours of monitoring), the
adult male was located on the north side of the river 28 percent,
on the south side 69 percent, and close to (within 125 m of)

Monitored 138 hours on 27 days
from 5 June to 1 September.

m y:^-

Home range 20 km
Total range 34 km
Used on only
one flight
Nest

'■'■'•'■ Farm
• Perch

Area used by adult red-tailed hawks at the Black
) site, 1976 .

00

the eyrie three percent of the time. The location of the adult
female sampled on 12 days (when observed) and based on 180 entries
(45 hours monitored) was 42 percent on the north side,- 48 percent
on the south side of the river and 10 percent near the nest. The
adult female was found near the eyrie and/or the young more often
than the male and almost always seemed to be within visual distance
of the nest during our visits to the nest for trapping or prey
collection.

From 5 to 16 August, the radio-marked young hawk was on the
north side of the river 68 percent, on the south side 32 percent,
and near the eyrie 0 percent of the time, based on 100 entries (25
hours monitored) on four days.

Hunting behavior. All the red-tailed hawks studied used
several methods of hunting and these methods were similar to those
used by golden eagles.

Red-tailed hawks hunted while watching from perches on cairns,
canyon rims, trees and lower vegetation, the ground, and power
line crossbars. As with eagles, flights from perches were either
direct to the prey or were out over habitat in search of prey,
usually to distances less than 100 m.

Red-tailed hawks also hunted while flying. Flapping or
soaring flights were over greater distances and of longer duration
than flights from perches. Soaring flights were common when
thermals and winds were favorable (from late morning to late
afternoon). Soaring flights lasted for up to an hour and appeared
to be used to locate prey.

A third method of hunting was a combination of flapping and
soaring low along slopes and rocky areas. This method was used by
all of the study birds to take lizards.

Hunting while hovering was a fourth variation used, usually
when hunting in flat terrain over fields of grass or dense vegetation.

A fifth method of hunting was a "fly and scramble" technique.
Typically, a short flight of less than 15 m followed by a running
scramble on the ground was used to capture lizards and snakes.

Before and after the nest failed, the adult male at the
Priest Ranch site often hunted within the canyon. The trees and
fields of the abandoned ranch provided varied habitat for prey.
Relatively little time was spent over the desert flats south of
the canyon.

The adults breeding at the Spoon and Upper Bigfoot sites used
similar hunting methods. High soaring flights (20 to 200 m) were
made out over the floodplain and the farmland close to the river.
Flapping-soaring flights were used along the long slopes

88

of the draws and ravines close to the eyries. Specific sections of
the tall and short power lines were used as hunting perches.

At the Black Butte site, hunting from the favored perches was
common. Hunting while hovering occurred over the desert, grass
fields, and along the edge of farm fields. The amount of farmed
area (stippled on Fig. 9) within the 20 km home range was 5 km^ .
Probably due to the amount of ground cover the hawks did not hunt
over the agricultural land until after the crops were harvested
(hay fields were mowed periodically) and the height of ground cover
was reduced. The "fly and scramble" technique was often used by
the Black Butte male during the latter portion of the breeding
season (August) when hunting over bluffs in Sections 28 and 29
(Fig. 9).

The diversity of prey brought to red-tailed hawk nests was
great. For example, prey remains found in the Black Butte nest
included: desert cottontail, black-tailed jackrabbit, Townsend
ground squirrel, bushy-tailed woodrat (Neotoma cinerea) , deer
mouse (Peromyscus maniculatus) , pocket gopher, common flicker
(Colaptes spp.), quail (Lophortyx spp.), gopher snake (Pituophis
melanoleucus) , western ground snake (Sonora semiannulata) , whip-
snake (Masticophis taeniatus) , garter snake (Thamnophis elegans) ,
bullfrog (Rana catesbeiana) , side-blotched lizard (Uta stansburiana)
and western fence lizard (Sceloporus occidentalis) . We often found
remains of snakes, small mammals, and rabbits in the other study
nests but did not sample and record prey.

Daily activities and rhythms. During the incubation period,
both the male and female at Priest Ranch were seen separately in
incubation posture on the nest. On several occasions, both adults
soared together over the north and south canyon rims during the
incubation period. The average number of round trips north of the
north rim per day by RT-1, as well as the average amount of time
spent to the north during the daylight hours is shown in Table 2.
After the nestlings died, RT-1 spent less than half as much time to
the north and made less than half as many round trips north per
day. Throughout the nesting season, RT-1 hunted within the canyon
and occasionally flew to the south but we were unable to quantitatively
sample those flights. When north of the canyon during the incubation
period, RT-1 spent 50 percent of the time flying and 50 percent of
the time perched on poles, sage or the ground.

The daily rhythm of the portion of the daylight hours spent
north of the canyon by RT-1 was also noted (Fig. 10) . During the
incubation period, the first flights out of the canyon by RT-1 were
usually after 0800. RT-1 was often found out of the canyon from
1700-1800 hours. After the young died, the adult male seldom

Table 2. Activities of RT-1, adult male red-tailed hawk,
at the Priest Ranch (P) site, 1976.

Stage of
breeding
season

Samp le
period

Hours

moni t .

Flights
NE of
canyon
moni t .

Courtship

and nest 18-24 March 55

building

Incubation 2-26 April 121

Hatching 5-7 May 40

After
death of
young

Total

15 May-3
August

216

247

14

23
14

51

21

Avg . time
spent to
NE/day

27%

24%
17%

Avg . no
trips to

NE/day

3.5

3 .9

4.6

1 . 5

Comparab le

1975 data 30 May-17

with young July

present

140

47

31%

5 .3

90

0.70

0.60H

0.50

0.40-

0.30

0.20

c

o

&0.10

a
u

0)

1976 Incubation
2-26 April (9 days)

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06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22

s0.401
o

£0.30

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a

xO.ioH

c
p.

<i)0.80 i
e

0.70

CO. 60
o

k0.50
o

cu

0.40
0.30
0.20 i

0.10

0

1976 After young died
15 May-3 August (23 days)

1975 While young present
30 May-17 July (16 days)

J]

FL

-T— ' — | 1 — ""| I 1 1 r— 1 1 T— — 1 I I

06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22
Time of day (00 hours) M.D.T.

Fig. 10. Daily rhythm (portion of the diurnal day spent
out of the canyon to NE) for adult male red-tailed hawk
RT-1 (Priest nest) .

91

ventured north of the canyon during the morning hours and spent
only 8 percent of the diurnal period to the north, as compared to
31 percent of the time to the north during the same weeks in 1975
when two young were successfully being raised. There were no
Townsend ground squirrel concentrations within the canyon or south
of the canyon within the home range so it appears that the influencing
factor for flights north of the canyon was the abundance of ground
squirrel prey.

In general, adults brought food to the nests and the young
were fed until they were at least four to six weeks old. One adult
of a pair, usually the female, was perched within line-of-sight of
the nest most of the time. When we gathered food samples or placed
traps, any adults present always called, circled, and sometimes
swooped in defense of the nestlings until we left the area. Most
often, the adult present was the female and after she called and
began aggressive displays, the less aggressive male flew in and
entered into the defense.

At the Black Butte site, data on time flying versus time
perched was gathered between 28 June and 26 August using 10 minute
sampling. On 11 sample days (489 entries, or 81 hours monitored),
the adult male (radio-tagged) was noted perched 67 percent and
flying 33 percent of the time. On seven sample days (65 entries,
or 11 sample hours), the adult female (not-instrumented) was noted
perched 68 percent and flying 32 percent of the time it was visible
to us. In four days of sampling between 5 and 16 August, the
radio-marked young was noted perched 87 percent and flying 13
percent of the time (90 entries or 15 hours monitored). In comparison
with the adults, the young hawk spent more time perched, was not
often seen hunting, and used a smaller area within the parental
home range.

Activities of the young. Nestlings fledged at six to eight
weeks of age and remained near the eyries. Food was brought by the
adults to the nest or to favored feeding perches. The radio-marked
young hawk at the Black Butte prematurely fledged from the nest on
10 June. The nest contained many parasite insects. This young was
placed in a man-made nest, tethered from 3 to 11 July and left the
nest on 12 or 13 July. The bird remained on the rocks close to the
eyrie and flew southwest to a group of deciduous trees along the
banks of the Snake River (Fig. 9). This young hawk seldom followed
the adults around the home range but began hunting about 5 August
(nine to ten weeks of age).

Young hawks dispersed from the parental range before the
adults (Table 1) . The fledgling from the Spoon nest was seen on 5
August but was absent on 11 August. The Black Butte young dispersed
between 17 and 19 August.

92

As with the eagles, "foreign" immature red-tailed hawks were
seen soaring with and perching near breeding adults during the
dispersal period. For example, on 30 June, an unknown immature
was seen soaring for about an hour with the adults from the Black
Butte nest. On 16 August, three other immature hawks were seen
hunting close to the Black Butte male in a favored hunting area.
On 19 August, another immature hawk was seen close to the nest
area, and no aggression was noted. Dispersal of the young appears
to be a combination of: reduced food supply from the adults, the
development of the hunting abilities of the young, and the relative
decrease in the amount of vulnerability of food available within
the home range primarily due to the estivation of ground squirrels
and growth of ground cover.

Prairie Falcon

Chronology. Adult falcons returned to the breeding areas in
February and early March. Adults were seen at the Dedication
Site, PF-1, PF-2, and Camera territories during a falcon census in
the Intensive Study Site on 25 February. There was about a three
week spread between the earliest hatching and latest hatching
falcon clutches and between other stages of the breeding season
(Fig. 2).

Trapping effort. A total of 79 man-hours and 32 set-hours

were expended to capture nine adult prairie falcons. Two of the

adult females had been banded by Verland Ogden of the Idaho
Cooperative Wildlife Research Unit in 1972 (Table 1).

Horizontal space use. Data for 1975 indicated that prairie
falcons had larger home ranges than golden eagles or red-tailed
hawks. In 1976, we placed our greatest effort on studying falcons
to determine home ranges for falcons nesting throughout the BPNA.
Adults at six nest sites within three general study sites were
instrumented (Fig. 1).

The home range sizes of adult prairie falcons studied to date
are shown in Table 3. The column "flights followed versus noted"
during the "hours monitored" represents the sampling effort that
was used for home range measurements. For example, the Spring (S)
female was monitored less than one third as many hours as the
others; thus, the boundaries of the home range were based on less
data.

The 1975 data on the PF-1 female (Fig. 11), PF-2 female
(Fig. 12), and the PF-3 male (Fig. 13), mate of PF-2, were re-
examined in terms of our current home range criteria (based on the
1976 monitoring). Falcons instrumented in 1975 were monitored
from 6 to 14 hours a day for 4 to 6 days a week on a random
schedule, as opposed to the full day sampling technique used in
1976. Without the four to eight day gaps between sampling periods,

93

Table 3. Adult prairie falcon home range summary

Study No . days
sub j ec t tagged

Flights
f o llowed
vs . noted

Hours
monitored

Total
range
(km2)

Home

range

(km*)

1976 Eastern Study Site

L$

59

14/21

227

140

77

Lo"

72

43/63

249

120

78

HS-

72

19/30

246

83

62

Ho71

182

70/77

249

40

35

1976 Intensive Study Siti

c?

100

34/38

377

149

65

Co"!

72

62/64

246

45

30

Nd"

117

96/102

413

45

37

PF-2°_

91

IP 1 3 3

333

90

68

s?

61

19/19

77

72

15

1975 Intensive Study Site
PF-1£ 84 55/55

PF-20.
PF-367'

44

71

44/44
56/56

363

228
365

59

142

78

5 6

107
66

Total females (7) 212/240

males (5) 327/362

Average female* 32/37
male 65/72

1 , 851
1 ,522

296
304

110
66

73
49

* Avg. female omitting SijL

9 4

3 Mi

Period Sample period

incubation (not tagged)

nestling 21 April-24 June

fledgling-, . * -» T i
,. i h/5 June-17 July
dispersal-1 J

home range

Flights Hours Area

40/40

15/15
55/55

250
113
363

59

N. I

56

Fig. 11
1975.

Area used by FF-1£, adult female prairie falcon,

9 5

j,|, j. Grass
.iliji field

Period

n-:f wJiv-4-.

Only 1
flight

WW

home range

Sample period

incubation (not tagged)

nestling -, 29 Aprii_24 June 44/44
fledglmg-i

home range 44/44

Flights Hours Area

228
228

142

107

Fig. 12
1975.

Area used by PF-2°_, adult female prairie falcon,

96

incubation (not tagged)

nestling 16 May- 3 0 June

fledgling-i

, . , J- 1-24 July

dispersal-1 J

home range

Flights Hours Area

44/44
12/12
56/56

248
117
365

70
78
66

Fig. 13
1975.

Area used by P F ~ 3 o^1, adult male prairie falcon

97

changes in home range from one developmental period to the next
were not always readily apparent in the 1975 data. For example,
there was no increase (N.I.) in the total area used by the PF-1
female (Fig. 11) during the fledgling-dispersal period after the
original 59 km^ covered during the nestling period.

In 1976, four adult falcons in the Intensive Study Site were
instrumented. Prairie falcon home range shape and size changed as
the breeding season progressed as demonstrated through monitoring
of the hunting flights of the Camera (C) female (Fig. 14). Only
one flight by this adult was noted during 107 hours of monitoring
during the incubation period. By 5 May, Camera female began
making several flights a day extending to a maximum of 15 km from
the nest. After the young fledged, the adult female made flights
as far as 19 km from the nest. After the young dispersed, the
female made one extended flight a day to a grassy field 29 km from
the nest. Although the total area utilized for the season was
149 km , the area (home range) used most often (on 34 flights we
followed out of 38 flights made during 377 hours of daylight
monitoring) when the young were present was 65 kirr .

The home ranges of all the falcons studied at the Intensive
Study Site were perpendicular to the canyon but occasionally
flights off of the perpendicular were made. For example, the
Camera (C) male made one flight north to Coyote Butte during the
nestling period and one loop flight to the southeast during the
fledgling period (Fig. 15). The PF-2 female flew out from the
canyon three times during 127 hours of monitoring during the
incubation period (Fig. 16). One flight extended 13 km from the
canyon while the furthest of five flights by other females during
the incubation period was 6 km. The home range of PF-2 in 1975
was 107 km^ versus only 68 km^ in 1976.

The space used by the PF-New (N) male showed a marked increase
from the incubation to nestling period but there was only a minor
change from the nestling to the fledgling period (Fig. 17) .

The relatively small home range (15 km ) of the Spring (S)
female in the Western Study Site may have been the result of our
method of sampling (small number of hours sampled). However, it
is also likely that the home range of this female was influenced
by more land in farming than in the other study sites. The amount
of desert land available over which to hunt was limited and this
may have restricted hunting to a few specific areas (Fig. 18).
Also, two soaring flights were made west of the nest and the area
of these flights was not included in the home range because the
flights were not typical of a hunting falcon. The hunting flights
noted were made to the north out to the grass field and the desert
area. Prior to dispersal between 9 and 12 July, this female made
three flights (one per day) to the northeast into the same area
used by the falcons from the Intensive Study Site.

9 8

Nest
Farm
Grass

incubation
nes tl in
fledgling
dispersal
home range

Fig. 14
1976.

Area used by CJjL, adult female prairie falcon,

99

W

In.ii/if Poi.

J,!

iji;ij.i;j.rt:.i

',¥,(&{ . ! i \n\ i i i i •! i V km 11 i

i 'i i f 1 1 ii i

I'l'l'l'l'l

I i ni i i i

1 i i i i i

im mm ji

— r! I

■I'l'l

lit

■V

i

Jk. Nest

: : : : Farm
|i|i|i Grass
ill!'! field

nes tling
fledgling
dispersal
home range

Period Sample period Flights Hours Area

incubation (not tagged)

nestling 5 May-9 June 43/44 169 18

fledgling 18 June-5 July 15/16 50 40

dispersal 7-8 July 4/4 27 45

home range 62/64 246 30

'ig.15. Area used by Co", adult male prairie falcon, 1976.

100

:j

A

incuba t ion
nestling
fledgling
home range

Only 1
flight

Period
incubation
nes t ling
fledgling
home range

Sample period
2-26 April
5 May-9 June
18-25 June

Flights Hours Area

3/3 127 11

20/24 175 79

4/6 31 90

27/33 333 68

Fig. 16
1976 .

Area used by PF-2,0-, adult female prairie falcon,

101

li'tili!*::-

i.it!: ii

4 m nml'trai! iWtnt

1i i i N if i Pi I J

' ili^i 1 1 1. 'J 1 1 1 1 IT ■ »(■ » • t'l'

m:*piUW"'

t, lil 1 1 1 I , I i I ■ • i Ui ' ■ • 11

I III

incuba t ion
nes t ling
fledgling
dispersal
home range

Period Sample period

incubation 2-26 April

nestling 5 May-9 June

fledgling 18 June-8 July

dispersal 11-18 July
home range

Flights Hours Area
33/33 128 8
46/50 176 35
13/14 75 37
4/5 34 45
96/102 413 37

Fig. 17. Area used by No", adult male prairie falcon, 1976

102

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103

Two mated pairs of falcons from adjacent nests (800 m apart),
instrumented in the Eastern Study Site in 1976, used areas similar
in size to those of the falcons in the Intensive Study Site
(Figures 19-22). From visual observations, the vegetation at the
Eastern Study Site was quite different than at the Intensive Study
Site. No large grass fields were present within the study birds'
home ranges.

Generally, the cumulative area used by adult females remained
small during incubation then rapidly increased with each sample
day beginning a week after hatch. The cumulative area used by
adult males increased rapidly during incubation but later slowed
as no new areas were used. For example, the Humpback (H) female
(Fig. 23) made no flights over 500 m from the nest until 11 May.
Even though the cumulative area used by the Humpback female
increased with each week of our sampling, to a total of 83 km2,
the area flown over on any given day remained between 1-30 km2.
In contrast, cumulative area of the Humpback (H) male increased
from 1 to 19 May but in June no new areas were used (Fig. 23). A
similar behavioral pattern appeared for the Loaf (L) pair but to a
lesser degree. Values referred to as "Minimum daily area" were
recorded and represent the minimum space used by the bird for that
day (Fig. 23). On some days in the Eastern Study Site, we were
forced to monitor from a single location because the automobile
trails were impassible (muddy) or we concentrated observations on
a nearby eagle that remained near the canyon.

In the Intensive Study Site, data indicated that an individual
bird may cover 20 to 40 km2 on several days but only 1 to 5 km2
on other days (Fig. 24).

Composite maps of home ranges of falcons breeding in close
proximity illustrate that in both 1975 and 1976, instrumented
falcons hunted over some of the same areas. The range overlap and
mutual use area for the PF-3 male and the PF-2 female (mated pair)
and the PF-1 female (from an adjacent nest) was 48 km2 in 1975
(Fig. 25) . The area of mutual use for three falcons that nested
adjacent to each other in 1976 was 47 km . This value is essentially
the same as that computed in 1975. The area of mutual use for
four falcons in the Intensive Study Site in 1976 was 9 km (Fig.
26) . Probably this area is smaller than computed for three
falcons because the scrape of one bird (PF-New) was further west
and the hunting flights paralleled those of the other three (Fig.
17). The range overlap of the two adjacent pairs in the Eastern
Study Site was 27 km2 (Fig. 27).

Although falcons did have overlapping home ranges, not all of
the birds were out of the canyon at the same times. Falcons also
appear to maintain "individual distance" as they fly within the
home range. The term individual distance refers to the area

10 4

\

4 Km

3 M

N

'W^

^

:X

r

4

I
1

J 3

jl

;i

i-

*3

!

i

';i

v '"''

|RW

I,

^AH^

ITMf

« >:x": : : >:

a

*T6l_

!Tk^%|M:^B^'*!»^ nestling

home range

Only 1
flight

WWWWWW

— \\\\\

Period

X^t incubation

Period Samp le period Flights Hours Area

incubation 29 April-10 May 18/19 48 30"

nestling 11 May-13 June 43/49 156 40

fledgling 19-25 June 9/9 45 N.I.

home range 70/77 249 35

Fig. 19. Area used by Hd\ adult male prairie falcon, 1976

105

4 Km

* A, '' tf i !]<k_ JKIJ SI P. i ! i: i : : ;

iu:^;iM;';i;>ii;i;i;i|iii^iiiiiM!'i-^-,-i-u

'k'i'i1 i i I i i i ■ •

Fig. 20
1976.

Area used by H$ , adult female prairie falcon

106

■::T^:

Ja&

■* ^i-''" . j> « . i; : : x

i*sr„ ;-■

incubation
nestling
home range

vWWWWWW

Mir. .... .T^

=*• • iV' •> >i

,. . . . ~p . ... ... B.p sV-l;

rr.". . . -. .-: . *■.■ . .. t- ,

H&frc" ' : ' £ ' " •" " Period Sample period Flights Hours Area

:;l;: incubation 29 April-13 May 16/18 76 70

,/V nestling 19 May-13 June 16/34 129 120

laAoMna 1Q-9S .TnnP 11/11 44 N.I.

43/63 249 78

fledgling 19-25 June
home range

Fig. 21. Area used by L<?, adult male prairie falcon, 1976

107

Fig. 22
1976 .

«Q«

incubat ion
nestling
fledgling
(dispersal)
home range

Area used by L0. , adult female

14/21
prairie

227

falcon,

77

108

90-

80-

70-

H?

60-

'J)

;3o-
<

h

20-

10-

0

4

•

i

ft

11 •

0 .

— , — , — w-

r-4 , • , , ,

1 10 20 1 10 20 1 10 20 1 10 20
April Kay (1976) June July

100

80-

70 -

S 40

u

<

30
20
10.

L?

1 i f*-^3*—^ 1 1 1 1 1 T~

1 10 20 1 10 20 1 10 20 1 10 20

April May (1976) June July

O Cumulative area

• Daily area

4 Minimum daily area

30

£20

HO51

— i 1 1 1 r — i — i 1 1 1 r~

1 10 20 1 10 20 1 10 20 1 10 20
April May (1976) June July

130-|
120

110-

80-

30

20.

i u

-i i i l 1 1 1 i 1 1 r—

1 10 20 1 10 20 1 10 20 1 10 20
April May (1976) June July

Fig. 23. Area used by adult prairie falcons, Eastern
Study Site, 1976 breeding season.

109

O Cumulative ares
• Daily area

1 10 20 1 10 20 1 10 20 1 10 20
April May (1976) June Julv

April

May (1976) June July

80 -I

70

50

s$

i 10 20 1 10 20 1 10 20 1 10 20
April May (1976) June July

~r l 1 i 1 1 1 1 — ™ — i r-

1 10 20 1 10 20 1 10 20 1 10 20
April May (1976) June Julv

140-

130-

c?

120-

110-

100-

90-

80-

/ •

70-

'B60-

/ •
/ *•

'50-

/ •
1%

IS

S*o-

<

30-

/•

0

•

20-

•

o

10-

•pr •

•

o

0 .

•

M

0

a*

o

1 10 20 1 10 20 1 10 20 1 10 20
April May (1976) June July

Fig. 24. Area used by adult prairie falcons, Intensive
Study Site, 1976 breeding season.

110

>« n J !!JSjfer:i4?:ii?iii^!i«

Fig. 25. Composite of home ranges of 3 adult prairie
falcons (PF-10., PF-2£, PF-36*) , 1975 breeding season,
Intensive Study Site.

1 11

'|'| i ■ i i i i i U'o.' i iS i ! > ft 1 1,1

i i'i'ii iv • r * • ►..,.....".

'ill!

Fig. 26. Composite of home ranges of 4 adult prairie
falcons (Co* Cg, No*, PF-22), 1976 breeding season,
Intensive Study Site.

112

No . b irds Are

us ine

4 Km

ft^sijiiiiiSrsi'iiaa^!!!!!!

r: : -n: : : : : : l^t-^fV^laHf^"

Fig. 27. Composite of home ranges of 4 adult prairie
falcons (Ho*, H$ , Lcf, L$ ) , 1976 breeding season, Eastern
Study Site.

113

around the falcon which moves with it as the falcon flies over the
hunting range. Individual distance is "an area around a bird,
which moves with it, has no topographical reference and into which
no other individual is allowed to come" (Condor 1949). At present,
it appears that the falcons have enough area so that they can hunt
without intraspecif ic conflict.

Nesting territoriality. From the period prior to egg laying
through dispersal of the young, areas around the scrapes were
defended by the adult pairs against other adult or young falcons.
In the Intensive Study Site, the area defended included favored
perches and air space within 50 m of the 100 m tall cliffs. All
nest cavities were located on a vertical portion of the canyon
wall and the visibility of a perched falcon at an adjacent scrape
entrance was limited or prevented by protruding rocks or ledges.
During the breeding season, the falcons spent considerable time
within the canyon near the nests. The male and female of a pair
often soared, flapped, and glided throughout the territory as if
to advertise the boundaries and to observe the activities of
falcons at adjacent territories which could not be seen well from
perches.

The spatial arrangement between territories in the Intensive
Study Site differed in 1975 and 1976 (Fig. 28). The female from
the Camera territory, the male and female from the PF-2 territory,
and the female from the PF-1 territory in 1976 were the same
falcons that were present at the respective territories in 1975.
The 1976 scrape at the PF-2 territory was 35 m further down the
cliff than in 1975. The territory did not extend as far to the
east in 1976 but did extend further to the west around a vertical
ridge in the cliff. Another vertical ridge separated the territories
of PF-1 and PF-2 in 1976. The Camera Site falcons occupied a
relatively large territory in both years.

In 1976, the territories were not fully established until
early April. For one week in mid-March, an unmarked pair of
falcons perched and courted around a rocky outcrop along the
cliffs above the eastern end of the Camera territory but failed to
establish a nesting territory.

On 3 April, we captured the Camera female for radio-tagging.
While we held her away from the scrape and eggs, the Camera male
repeatedly defended the scrape, literally shoving and bumping an
intruding adult female from the cavity entrance. This intruding
female then flew up to the rim and was chased east past Dedication
Point by the male from the PF-2 site. An unknown number of non-
breeding adults may be available to fill unoccupied nesting sites
but it appears that in some areas of the BPNA, there is no more
room and the cliffs are saturated.

DS

(top of cliff)

rim

DP

L'

FACE

_X b "^v

v """ = d

1975 CLIFF

(bottom of cliff?

talus slope ~— \^

^l

e

Nor thwes t

_^ Southeast

DS

(top of cliff)

rim

DP

19 76 CLIFF FACE

(bottom of cliff)

talus slope

100 m
i i

Scrape locations

a Dedication Site d
b PF-1 e

c PF-2 f

Camera

Dedication Point
Unsuccessful pair

Fig. 28. Prairie falcon nesting territories in the Intensive Study Site, 1975 and
1976. Vertical territory boundaries are outlined, and scrape locations are indicated
by letter. Landmarks are Dedication Site (DS), and Dedication Point (DP).

115

The two breeding pairs studied in the Eastern Study Site
nested in cavities on relatively small vertical cliffs (8 to 14 m) .
The surrounding terrain was rolling, with some draws and ridges;
much different than the sheer cliff habitat in the Intensive Study
Site. In the Eastern Study Site, the territories had more horizontal
dimension and less vertical dimension (Fig. 29). The horizontal
territory was defended against red-tailed hawks and golden eagles
as well as other prairie falcons. The nesting density of falcons
per kilometer of canyon rim in the Eastern Study Site was much less
than in the Intensive Study Site (Study I, 1976 Annual Report).

Territorial defense waned during July and adults that remained
in the territories after the young dispersed showed a noticeable
decrease in vocal responses and aggressive defense against human
intrusion.

Fledglings that floundered in flight and landed in the territory
of another breeding pair were subject to aggressive acts by the
adults defending the area. A visitor to the Intensive Study Site
reported (Study VII, pers. comm. ) a fledgling falcon passing from
one territory to the next. The young intruder was killed in the
air by an adult female (presumably PF-2) .

It is a moot point as to whether adults can distinguish their
own young once the young have fledged. Adults observed during the
1975 and 1976 breeding seasons reacted aggressively to any falcon
that was seen entering the territory. We speculate that if we
placed a fledgling in an adjacent territory when no adults were
around, the young falcon would later be attacked by its parents as
the fledgling attempted to return to the natal scrape.

Hunting behavior. All falcons studied used several hunting
methods. According to our observations and the food habits data
from Kochert (1973), the primary prey for falcons in the study area
is the Townsend ground squirrel. Our study birds hunted while
watching from perches on power line poles, vegetation (sage) and on
the ground. As with eagles and hawks, flights from perches were
either direct to prey or over favored habitat in search of prey.
Low-level searching flights were made out 30 to 80 m from the
perches and back. As many as 20 to 30 flights were made a few
seconds apart until prey was killed. Falcons commonly hunted from
power poles during March and April before vegetation (grasses) grew
in exposed areas. At this time, ground squirrels were exposed in
openings between patches of sage.

The Humpback (H) male and the PF-New (N) male spent a greater
portion of time on power poles than the other falcons (Table 4) .
Some instrumented falcons were never seen on power poles. As with
eagles and hawks, the instrumented falcons that did use the poles
had specific sections of the tall or short power lines that were
used for perching.

116

N

t

500 m

(desert flats)

£ C_ fjHL ^s^

1 /<T>>s rim t l

1 ^^y^^

/ X talus 3
^"^ slope /

(flood plain)

Scrape
location
H Humpback
L Loaf

Territory

size

37 h

24 h

Fig. 29. Prairie falcon nesting territories in the
Eastern Study Site, 1976. Horizontal territory boundaries
are outlined, and scrape locations are indicated by letter

117

Table 4. Average activity of adult prairie falcons per
round trip away from the canyon.

He? HO_ Ld71 L£ Co* C£ No71 2°. S$*

Incubation

No . of flights

s ampled
Avg duration of

trip (min.)
% t ime in air

% t ime on poles

% time on ground

Avg minutes on
ground/ trip

Nes t ling

No . of flights

sampl ed
Avg duration of

trip (min . )
% time in air

% t ime on poles

% time on ground

Avg minutes on
ground/ trip

Fledgling

No. of flights

s ampled
Avg duration of

trip/min. )
% time in air

% time on poles

% time on ground

Avg minutes on
ground/ trip

15

0 16

50

79

69

87

13

1

18

12

9

10

27
42
69
2 3

15 23
56 81

12
64

83
0

18 8 7

0

13 12
7 9

0
13

8 2
47 147
89 82

0

11

5

0
18
26

1 34

-- 49

-- 86

3

-- 11

5

43 18 47
23 46 54

90
0

10
2

14
39

8 82
1 8

11
5

10
63

10
5

23
55
82

5
13

7

85 88

3 2

12 10

5 6

13 4
32 54
81
14

15
35

96 95

2 1

2 4

1 1

Nestling and Fledgling period data have been combined

118

Several methods of hunting while flying through the hunting
range were identified. The majority of the flight time was in
flapping flight at elevations below 30 m and attacks at ground
squirrels were low angle dives. A second method of hunting while
airborne was used over relatively small areas (10 to 20 m across)
of short vegetation (Bromus tectorum, Poa sandbergii, short Eurotia)
among taller vegetation (Artemisia) . Falcons hovered at 5 to 10 m
height, dove and flew across the area and hovered again if the
attack was unsuccessful. Repeated attempts were made until the
prey was captured or the falcon moved on to another area. A third
method of hunting while airborne was during soaring flight. It was
difficult to say whether or not soaring falcons were hunting or
travelling over the area doing something else. Falcons apparently
did not hunt within the canyon.

By late June, many of the Townsend ground squirrels had gone
underground to begin estivation and by 19 July, squirrel sightings
were rare. As a result, the primary food source for falcons was
gone.

Daily activities and rhythms. Individual falcons spent
anywhere from zero percent (Humpback female, incubation period) to
43 percent (Loaf female, fledgling period) of the daylight hours
out of the canyon at least 100 m from the rim (Fig. 30). We
believe most of the time away from the canyon was spent hunting,
based on obvious attacks on prey, the large food demand, and the
relatively small portion of the daylight hours spent away from the
canyon.

The averages of some of the activities of instrumented birds
during the developmental periods are listed in Table 4. The
majority (69 to 96 percent) of the time away from the canyon was
spent in the air. The duration of flights varied. PF-New (N) male
made one flight out 2 km, made a kill, and returned to the canyon
within 11 minutes. In contrast, the Loaf female once stayed out
for five hours.

Typically, after killing a ground squirrel, a falcon either
flew immediately to the canyon or remained on the ground for 5 to
20 minutes, consuming the prey. Sometimes a portion of the prey
was consumed near the kill site with the remainder carried to the
canyon. Most of the falcons spent an average of 1 to 10 minutes on
the ground per trip (Table 4) . Some trips involved several short
periods on the ground.

The speed of 16 flights by instrumented falcons back to the
canyon (presumably with prey) for distances of 8 to 15 km ranged
from 23 to 80 km/hr (14 to 50 mph) . Most flights were between 30
to 53 km/hr (18 to 33 mph) .

He?

9

42%
(3)b

Eastern Study Site

Lrf1

0.0%
(3)b

0.1%
(5)a

3

36%
(5)a

S?

2$

Intensive Study Site

C?

Co1

No"

5%
(9)c

1%
(8)c

(5

21%
(9)c

6(5(53

28%
CH)a

13%
(ll)a

20%
(9)a

38%
(9)a

000(5

13% 8% 10% 25%

(12)a (12)a (12)a (12)a

©030(530(30

10%
(4)b

8%
(4)b

43%
(2)f

27%

22%

(4)b (8)e

39%
(3)d

18%
(5)b

20%
(5)b

12%
(7)b

Sampling interval (M D T )

a. 0530-2200 d. 0700-2030

b. 0600-2130 e. 0800-1900

c. 0630-2100 f. 0800-2100

333

29%
(6)c

37%
(2)c

21%
(8)e

Fig. 30. Percent of the diurnal day spent out of the canyon by adult prairie falcons,
1976. (Maximum number of sample days shown in parenthesis).

120

The average number of round trips made by each bird for each
developmental period is listed in Table 5. The male of each pair
made more flights per day in each developmental period than the
female (except the PF-3 male in 1975) . Although, we cannot say
that prey was carried to the canyon on each round trip, we can say
that our data represents the maximum possible number of prey
carries. The greatest number of trips out of the canyon on a
single day was nine by the Humpback male (10 May) when the young
were 1.5 weeks old.

As the breeding season progressed, the average number of trips
by the males decreased. The average number of trips by the female
increased from the incubation to the nestling period but then
decreased from the nestling to the fledgling period (Table 5). For
the Loaf and Humpback pairs, the combined number of trips for each
pair increased in the nestling period when compared with the
incubation period but then decreased again during the fledgling
period. In some cases, the females were making fewer trips out
during the fledgling period but spending more time out from the
canyon per trip. At this time, it may have been more difficult to
capture prey than during the fledgling period. There may also have
been reduced opportunity for catching prey because of the growth
and maturation of ground cover at this time. Also, the number of
available prey within the hunting range was reduced because of
mortality and estivation (Study V, pers. comm.).

The time and daily rhythm of falcons away from the canyon
(nestling period and first half of fledgling period) is shown in
Figures 31 to 33. Prairie falcons did not begin hunting as early
in the day as eagles or red-tailed hawks. In general, male falcons
first flew out from the canyon between 0700-0800 hours (1 to 2
hours after sunrise) and the females began flying out from 0.5 to 1
hours later. Most individuals had a mid-day lull in flights to the
north. The probability of finding the falcons out over the desert
after the incubation period was greatest in the afternoon. There
was some indication of bimodal activity in some individuals but not
in others.

From an hour before and until sunset, the female falcons
returned to the canyon to night roost. During the weeks of sampling,
sunrise and sunset times shifted by about 30 minutes. These shifts
are indicated on Figures 31 to 33 by the dotted bars at the bottom
of each figure along the "time of day" axis near the 0530 and 2200
hours symbols. Night locations for the males of the Humpback,
Loaf, and Camera pairs were recorded on 44 occasions prior to the
dispersal period. Males occasionally roosted at night away from
the canyon. On 14 occasions when the males roosted away from the
canyon, the roosts were identified nine times. The Humpback male
was found on the same crossbar of the same pole (tall power line)
on three nights and on neighboring poles on three other nights.
The Loaf male roosted on a small rocky ridge 7 km northeast of

121

Table 5. Average number of round trips north of the canyon
per day per bird and per mated pair of prairie falcons, 1976

Study
Subject

Incubation

Nestling

Fledgling

Ld1

L$

Ho71
H?

Cc?
C?

No71
2$

Average d*
Average ?

4.2(5)-r.

0.2(5K4-4

5-5(3)-.
0.0(3)-' 5- 5

0.1(8)
3.7(9)
0.3(9)

4.5 (3 birds,

17 day s )

0.2 (4 birds ,

25 days)

Average pair 4 . 7

4.1(9)-, , ,
2. 2(9)-r '

A . 9 ( 1 1 )-.
2.6(1 I)-""7 -5

±5.5

3.9(12)
1 .6(12)

4.3(12)

2.2(12)

4.3 (4 birds

44 days )

2.2 (4 birds

44 day s )

6.5

3.7(4)n
1.5(2)-T 5-2

2>7(4)T 3 8
0.9(4)-"

3-8(7)"h-s 4
1.6(7)-T5'4

2.6(7)

2.6(3)

3.2 (4 birds ,

20 days)

1.7 (4 birds,

1 4 days )

4.9

Date cutoffs for each developmental period are the same as
listed for each home range figure. The number of days
sampled for each period is shown in parenthesis. The
samping interval was 0600-2130 hours, MDT .

1975 Comparable data:
(13 May-20 June)

PF-1? 2.9

PF-2? 3.0
PF-3cP 2.5

3-5.5

122

06 07 08 09 10 II 12 13 14 15 16 17 18 19 20 21 22

0.50 -
0.40
0.30 -
0.20-

0.10.

0

ud

rO,

~ — r

J~LT

11 May-20 June -

ru

"1 1 1 1 1 r 1 1 1 1 1 1 1 r^; — r

06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22
Time of day (00 hours) M.D.T.

Fig. 31. Daily rhythm (portion of the diurnal day spent
out of the canyon) for adult prairie falcons, Eastern
Study Site, 1976.

123

0

60-

0

50 -

0

40 -

0

30 -

0

20 -

0

10 -

0 -

c

0(1

SO-.

>,

a

oO

40-

<0

30-

UH

°0

20 -

J)

3

°0

10-

+J

c

<u

0

tt

01

.:)

■r|0

40-,

•P

ou

30-

c

So

20-

+j

u

.10-

0

Ncf

5 M;iy-8 July

1

1 I I I I I 1 I 1 1 1 1 1 1 T

06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22

PF-2?

5 May- 2 5 June

M

■P

r^rj1-^

h.

T i i I I i I i i i 1 1 1 1 — r-' "I i

06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22

c?

5 May-5 July

VtJ

3-

i i I 1 1 1 -i 1 1 1 i 1 1 1 V l

06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22

0.30-
0.20

o: 10

Cc?

_n-

5 May-5 July

06 07 08 09

~ i 1 1 1 1 1 1 1 r

10 11 12 13 14 15 16 17 1!

Time of day (00 hours) M.D.T.

~\

i 1 ' .J... I

19 20 21 22

Fig. 32. Dailv rhythm (portion of the diurnal day spent
Out of the canyon) for adult prairie falcons, Intensive
Study Site, 1976.

124

1 .00
0.90
0.80
0. 70
0.60
0.50 -
0.40 -
0.30 -
0.20

0. 10

0

16 May-20 June

PF-3C?

L

ru

If

U

c

0

§0.70
u

£0.60
•p

O0.50

+J

O0.A0

jj

go. 30 H

D-
0)

0)0.20-1

e

-H

0.10

o

c ° -
o

■ri

£o.60
o

0.50
0.40
0.30 H
0. 20
0.10 -

-i r

-I 1 1 1 r

— 1 1 1 1 1 1

06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22

PF-2$

15 May-20 June

L^LI

Or

I I

— i 1 — — i 1 1 1 1 1 1 1 1 i r

06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22

PF-IO.

r

13

u

May-20

June

^-|

■

r—LA

Ln,

-

J"^

u

r

h^

-

06 0

7 0

8 0

9

1
10 1

1 12

13 14

1

5 16 1

7 1

8 19

20 2

1 22

Time of day (00 hours) M.D.T.

Fig. 33. Daily rhythm (portion of the diurnal day spent
out of the canyon) for adult prairie falcons, Intensive

Study Site, 1975.

125

the nest on two nights. The Camera male roosted 1 to 3 km north-
east of the nest on one night apparently perching on the ground.
Departure times from the canyon out to a night roost varied from
90 minutes before to 40 minutes after sunset. Times for returning
to the nest varied from 40 minutes before, to 140 minutes after
sunrise. Neither the PF-New male, nor any of the female falcons
were noted night roosting away from the canyon.

Activities of the young. Nestling prairie falcons remained
in the nest cavity until five to six weeks of age. Hungry nestlings
gave a "chitter" call and were fed by the adults. As the young
developed, they fed themselves. Food was deposited in the nest
cavity by the adults. The young then fought and scrambled for
portions of the prey item. At four and five weeks of age, the
young climbed out of the scrape cavity entrance and onto adjacent
ledges and rocks. During the day, the nestlings loafed and
practiced flapping, and in the evening returned to the nest
cavity. At fledging, young made flights from the nest cavity area
to rock perches within the adult nesting territory. Until dispersal,
fledglings remained within 100 to 200 m of the scrape and did not
follow the adults out over the desert flats on hunting forays.

Dispersal. The young from a nest dispersed from the home
range before the adults of that nest. Individual dispersal dates
are listed in Table 1. Two patterns of dispersal of young were
noted in 1975 and 1976. From the nesting territory, some fledglings
wandered east or west along the rim for 2 to 10 km and remained
there for one to four days before disappearing. Other fledglings
remained near the nest then dispersed from the BPNA in one flight.

Beginning on 3 July 1976, one tracking vehicle, monitoring
for migrating birds, was positioned at Bonneville Point at the
base of the foothills north of 1-80 (Fig. 34). The other Study
III tracking vehicle and a portable system operated by Study VII
personnel, monitored the locations of marked falcons near the
canyon. Nine aerial searches with telemetry equipment were flown
between 3 and 28 July to locate birds that had shifted out of the
home range. Additional airplane flights were made over an area 15
km to the south and 25 km to the north of the canyon in search of
missing marked birds.

Three adult and three young falcons were tracked on their
dispersal flights out of the BPNA Study Area and to distances at
least 45 km from the nest (Fig. 34).

The young falcon H-12, from the Humpback nest, disappeared
from the nesting territory between 28 June and 1 July. It was
found 9 km west of the nest on 3 July where it remained until
making a long flight east on 7 July (Fig. 35). Radio contact with
H-12 was lost again at 1025 on 8 July 127 km (79 mi) from the
nest .

126

□

BANKS

D4

16 July

10

20 KM

A

-r- — i

10 15 Ml

mmk

IDAHO
D CITY

SHAFER
0 BUTTE
1.0

BOISE

->

KUNA

a

V H'". BONNE ViUE

=; _\< I POINT .....-•

t \

M

MEIBA
□

4

FEATHERVIUE
O

|k'3j0fy

>»„(ri,-,iV 8ENNETT

''«% o i.o,

'^fet'ii,

MOUNTAIN "-*'-'

HOME

GRANDV/EW

GLENNS
FERRY

Fig. 34. Dispersal flights by instrumented prairie
falcons, 19 76.

^^^■P""^

10 20 KM

10

15MI

GRANDVIEW

MAGI Q)V»#S^&_

O
SHOSHONfE

ho

Fig. 35. Dispersal flight of H-12, immature prairie falcon, 1976

128

By monitoring the dispersal flights of six falcons, we have
learned that the rate of travel ranged from 18 to 38 km/hr (11 to
24 mph) . The time of day for the initiation of departure flights
ranged from 0958 to 1430 hours and the falcons reached the foothills
between 1203 and 1639 hours. Four of the six flight paths were
within 90 degrees of the prevailing wind direction. We did not
see at what elevations our marked birds flew but two non-tagged
falcons seen flying into the foothills at Bonneville Point were
less than 20 m above the ground. Once the instrumented falcons
reached the foothills, they kept on flying further north or east.
Our method of intercepting dispersal flights favored flights to
the north or east and it is possible that some falcons migrated
west or south. We restricted our monitoring from the foothills to
approximately 0900 to 1800 hours. Therefore, birds dispersing in
early morning or late evening would have been missed. Once the
falcons left the canyon, the flights to the foothills were non-
stop, lasting two to three hours before the signals were lost.

The first of the instrumented adult falcons to leave the
study area was the PF-2 female on 28 June. Apparently, the young
from this nest had been killed or dispersed and no young were seen
in the territory after this date. The Humpback male did not leave
his home range until after 10 October when the radio signal was
last heard. It is probable that the radio stopped running after
182 days of broadcasting.

Two instrumented falcons were found dead. The young falcon
(band 987-21955) from the Finger (F) nest was found dead near the
nest by Study VII personnel on 17 July. This bird was last
detected alive on 14 July. The Loaf (L) female disappeared from
the home range between 21 and 23 June. On 10 July, her carcass
was found 10 km southeast of the nest (Fig. 34) . The carcass was
intact and had no feathers missing, no blood stains, nor noticeable
holes. The carcass was x-rayed and no broken bones were seen.

Antagonistic Interactions

Golden eagle. During 1976, few antagonistic interactions
were noted between breeding or non-breeding golden eagles of
either sex or age class.

Mated pairs were often seen together perched or flying during
winter and unmated individuals wandered into and out of the hunting
ranges of the breeders. We did not see adult non-mated eagles
winter near nests of resident breeding eagles. However, on several
occasions, eagles in immature plumage were seen passing near
eyries that had been active in 1975 where breeding adults remained.
During winter, eagles of various age classes used the same
perching sites but not at the same time.

129

During the breeding season, non-breeding adults were seen
perched and flying outside the home range of breeding eagles.
Apparently there is a reserve of non-breeding adults ("floaters")
within the study area. Some support for this statement is that
within three days after the disappearance of the adult female from
the Black Butte eyrie, two different adult eagles (larger than the
male and presumably female) were seen near the eyrie and the male
was courting. The courting continued for about a week and an
adult female has been seen with the male within the home range
since that time.

A perched or flying eagle was occasionally harassed by a
prairie falcon or common raven but no physical contact was made.
By 19 July, many insects (Cimex spp., and Oeciacus spp.) accumulated
in the Cabin Draw and Black Butte eyries and parasitized the
nestlings. No mortality was noted but the young did scratch and
bite, apparently in response to the insect activity.

Interactions with humans other than project personnel were
not common. Fur trappers walked close to the eyries in winter and
spring. A golden eagle toe was found torn off and in a trap at an
illegal set within the BPNA.

Red-tailed hawk. No antagonistic interactions between red-
tailed hawks were noted. The spatial arrangement between nesting
pairs seemed to be adequate. Wandering immature red-tailed hawks
were allowed to pass through the home ranges of breeding pairs.
On several occasions during the dispersal period, "foreign"
immatures were seen soaring with our adult study birds. Interactions
with other birds or mammals other than prey were minimal. By
10 June, insect parasites accumulated in hawk nests and in the
case of the Black Butte Site, prompted the young to jump from the
nest prematurely.

Interactions with humans were not common. Fur trappers were
not in the area during the red-tailed hawk nesting season.

Prairie falcon. In the nesting territories, aggressive
interactions between adult falcons were common. Any breeding
adult that crossed into the nesting territory of another was
chased until it left the territorial boundaries. Falcons frequently
harassed golden eagles and red-tailed hawks by diving and swooping.

Humans that walked the canyon rim did flush breeding adults
perched near the rim. If humans were to flush fledglings into an
adjacent falcon's territory, the results could be lethal to the
young falcon. Adult falcons from the Eastern Study Site often
flew within the boundaries of the impact area for a National Guard
artillery range but not at times when firing was taking place.

130

ACKNOWLEDGMENTS

Various forms of assistance to Study III by the following
persons is acknowledged:

Michael Kochert and Albert Bammann for assistance with
contractual and field problems. BLM personnel provided food
habits data.

Morlan Nelson for assistance in providing lure birds and
information pertinent to Study III.

John Messick, Lowell Diller, Gayle Sitter, Jon Montan,
Eb Evans, Richard Howard, Brad James and Charles Schwartz for
field assistance.

Robert Montoya for preparing radio backsacks and other
field gear when needed.

Frank Craighead, Jr. and Bernd Meyburg provided helpful
comments about the study while visiting the area.

Erik Stauber provided a lure bird for trapping.

Morton Awes volunteered pilot time during aerial searches.

Clifford Code x-rayed the falcon carcass.

LITERATURE CITED

Condor, P.J. 1949. Individual distance. Ibis 91:649-655.

Dunstan, T.C. and J.F. Harper. 1975. Activity, hunting patterns,
territoriality, and social interactions of birds of prey
in the Birds of Prey Natural Area. Pages 51-96 in Snake
River Birds of Prey Research Proj . Ann. Rep. 1975. U.S.
Dept. of Interior, Bureau of Land Management, Boise.
193 pp.

Kochert, M.N. 1973. Density, food habits, and reproductive
performance of raptors in the Snake River Birds of Prey
Natural Area. USDI, Bureau of Land Mgmt. , Boise District
Annual Report. 54 pp.

131

Notes :

132

Jon Montan, Study IV-A, conducts a trapping program to determine
rodent species composition, density, and distribution in the
Study Area (BLM photo by Dunstan) .

133

STUDY IV- A:

CONTRACTOR:

Density and species composition of small
mammals and birds in the Snake River Birds
of Prey Natural Area, Idaho.

Ecology Center, Utah State University,
Logan, Utah 84322

INVESTIGATORS

PROJECT SUPPORT:

Dr. Michael Wolfe, Principal Investigator.
Jon R. Montan, Jr. , Research Associate.
Kathleen Montan, Field Aid.
Jan Wassink, Field Aid.
Martha Wakefield, Field Aid.
Brad James, Field Aid.

U.S. Department of the Interior, Bureau of
Land Management, Contract No. 52500-CT5-1003.

OBJECTIVES:

1. Stratify the study area into major vegetation types
and land-use patterns from existing aerial photographs
and other means.

2. Sub-stratify the major vegetation types into basic
land-use types, and sample the major prey species by
acceptable statistical and censusing techniques.

3. Specifically, sample for prey species in those areas
identified by radio telemetry efforts (Study III) to

be intensively hunted by golden eagles, prairie falcons,
and red-tailed hawks. Aspects of vegetation composition,
cover, physiography, and any other factors will be
studied to determine the importance of prey vulnerability
to predation.

4. Assess and compare prey densities in the different
habitat and land-use types to ascertain the possible
effects of different land uses upon prey densities.

5. Ascertain seasonal and yearly changes in prey species'
composition and densities.

6. Calculate annual prey population densities in terms
of biomass within each strata of the total study area
and in areas intensively hunted by raptors.

ANNUAL SUMMARY

Relative rodent densities were determined at 34 sites within
the study area during spring, summer, and fall, 1976. Relative

134

densities were also determined in areas identified by Study III as
actively hunted by raptors. Cottontails (Syvilagus nuttali) were
trapped at eight sites, including two range-agriculture ecotones.
Jackrabbit (Lepus calif ornicus) flushing transects were run in
spring and fall with the cooperation of Study I personnel. In
addition, 20 transects were walked in June in areas identified by
Study III as actively hunted by golden eagles. Marmots (Mar mot a
f laviventris) were trapped at Evans Ranch, Melba during spring in
an attempt to measure density and biomass at a typical colony.
During June, marmot distribution along the canyon wall was recorded
by means of a walking survey. Censuses of pheasants (Phasianus
colchicus) and passerines were conducted during spring. Chukar
partridge (Alectoris graeca) were censused using the recorded
rally call method. A vegetation map of the study area was completed
and distributed to research personnel.

METHODS

Vegetation Map

The completed vegetation map (1 cm = 0.64 km) was photo-
graphically reproduced and distributed to research personnel.
Percent ground coverage by major vegetation types and other areas
was determined by cutting cut areas of map and weighing them with
a Mettler balance.

Determination of Prey Densities and Biomass

Rodents. Relative rodent densities at 34 sites in the study
area were determined by operating kill-trap lines of 25 traps for
two nights at each site. The kill-trap index for a particular
species and site was calculated as:

(captures) (100)

(trap nights) - (sprung traps & missing traps)

One trap-night was one trap open for one night. Analysis of
variance was used to test for differences among habitat types and
seasons. If the analysis of variance showed significant differences,
then a series of t-tests were used to identify specific habitats
with higher numbers of prey. All statistical tests were made at
the 0.05 level.

Marmots were live-trapped between 23 February and 26 March
along a 0.5 km-section of cliff and at a 0.1 ha^ rocky outcrop at
Evans Ranch, Melba. Live-trapping provided an estimate of density
at a typical colony. In addition, marmot colony distribution was
mapped by Martha Wakefield by walking along the base of the canyon
wall.

135

Lagomorphs. Jackrabbit flushing transects were walked with
the cooperation of Study I personnel during spring and fall.
Gates (Gates et al. 1968) transect indices were converted to
density estimates (hares/km2) by multiplying by 1.43, the conversion
factor applied in northern Utah by Gross et al. (1974). This
factor may not apply to this area, thus drive counts would be very
desirable to establish the correct factor for this area. Drive
counts should be made immediately before spring transects or
immediately after fall transects.

Cottontail rabbits were live-trapped at eight sites, but not
all sites yielded enough captures to permit a Schnabel density
estimate. Relative densities were expressed as captures/100 trap-
nights.

Avian prey species. Pheasants were censused in the major
blocks of agriculture within the study area by the crow-count
method of Kimball (1949). Spring censuses of passerines were
conducted by walking Emlen (1971) transects between sunrise and
0900. Individual transects comprised a quadrat, 1.6 km on a side,
with a total linear length of 6.4 km. Chukar partridge distribution
was determined during August by floating and/or driving along
portions of the Snake River through the study area and playing
recorded rally calls at 1.6 km intervals. The number of responses
following a call during a 10 minute period can be correlated to
flock size (Oeklaus 1976) .

RESULTS

Vegetation Analysis

Percent ground coverage by major vegetation and land-use
types is presented in Table 1. Big sagebrush (Artemisia tridentata)
associations are grouped in the table because it was often difficult
to differentiate between "pure" and "mixed" associations. The map
in Figure 1 represents a simplification of the original vegetation
map, in which only the major vegetative associations are represented.

Rodents

Limitations of kill-trap indices. It must be emphasized that
a reliable kill-trap index has only been demonstrated for deer
mice (Montan 1977). Other rodent species were not captured in
sufficient numbers during 1975 to calculate densities. The kill-
trap index can only resolve differences among seasons or trapping
sites. It bears no relation to kill-trap values for other rodent
species. For the purposes of the subsequent discussion, however,
kill-trap indices will be assumed valid for other rodent species.

136

Table 1. Estimated percent ground coverage by major vegetative
associations of the Snake River Birds of Prey Study
Area, 1 November 1976.

2
Description % Coverage km

Greasewood-cheatgrass

Big sagebrush-cheatgrass

Big sagebrush-winterfat 22.3

Big sagebrush-Sandberg bluegrass

Farms

Shadscale/budsage

Snake River and reservoirs

Spiny hopsage/shadscale

Shadscale-winterfat

Mountain Home Air Force Base

Cheatgrass

Winterfat

Cheatgrass-shadscale/greasewood

Bruneau Sand Dunes

Crested wheatgrass

Burn (previously big sagebrush-winterfat)

Sandberg bl uegrass

Totals 100.0 1937

a

Hyphen indicates former is numerically or physically dominant
and latter is less frequent or exists as an understory.

Slash indicates co-dominance or patches of pure stands of either
vegetation type.

30.6

591

8.6

166

7.7

149

6.0

117

18.6

360

18.2

352

2.7

51

2.2

43

1.9

37

0.9

17

0.7

14

0.6

11

0.6

11

0.3

6

0.3

6

0.1

3

0.1

3

Initial Point

Murphy

5j) Agriculture

72 Big sagebrush-cheatgrass-
Winterfat association
Spiny hopsage-shadscale
association

0 Shadscale

^ Grease wood

Mountain Home

Intensive Study Area

Birds of Prey Study Area

5 10

5 10

I I I l| I I III

■>«j

Fig. 1. Major vegetative associations of the Snake River Birds of Prey Study Area, 1976.

138

Assessment of habitats as contributors of prey. The number
of rodent species caught or observed at the 34 sites (Fig. 2)
ranged from a high of 12 in the riparian zone along the Snake
River to three in an irrigated wheat field (Table 2). The only
diurnal rodent species, other than the Townsend ground squirrel
(Spermophilus townsendi) , are the white-tailed antelope squirrel
(Ammospermophilus leucurus) and the least chipmunk (Eutamius
minimus) . Occasionally, kangaroo rats (Dipodomys spp.), meadow
voles (Microtus spp.), and woodrats (Neotoma spp.) have been
observed during the daytime, but their time spent exposed was
brief compared to the Townsend ground squirrel.

While Larrison and Johnson (1973) found least chipmunks most
abundant in depleted shadscale stands, the results of this study
indicate as Fautin (1946) found, that they were restricted to big
sagebrush. Chisel-toothed kangaroo rats (Dipodomys microps) were
confined to shadscale (Atriplex confertif olia) . Kenagy (1972)
showed that this species, because of its tooth morphology, is able
to excise hypersaline tissue and feed on the less-saline mesophyll
of shadscale leaves. Ord's kangaroo rats (Dipodomys or-lii) were
widely distributed but were most common on sandy substratts.
Bushy-tailed woodrats (Neotoma cinerea) were found within the
canyon proper, on rocky buttes in the Kuna Desert, and in dense
greasewood (Sarcobatus vermiculatus) stands along Rabbit Creek
near Murphy. Desert woodrats (Neotoma lepida) were found only
along the canyon rim and in the dense greasewood of Rabbit Creek.
Western harvest mice (Reithrodontomys megalotis) , house mice (Mus
musculus) , and meadow voles (Microtus montanus) were only caught
on very wet sites, such as riparian habitats or along irrigation
ditches. Canyon mice (Peromyscus crinitis) were specific to
canyon talus slopes. Deer mice, grasshopper mice (Onychomys
leucogaster) , and Great Basin pocket mice (Perognathus parvus)
were widely distributed, although deer mice were captured most
often.

The Snake River represents a zoogeographical barrier to
rodents (Davis 1939) . The number of rodent species was lower on
the south side of the river except in greasewood habitat. There
were no least chipmunks caught or seen on the south side. This
species was replaced by the white-tailed antelope squirrel, which
did not occur on the north side except near Grand View, where it
is presumed to have crossed a bridge. The river has apparently
also affected the distribution and speciation of the Townsend
ground squirrel. Spermophilus townsendi idahoensis has a continuous
distribution north of the river, but is absent on the south side.
jS.jt. mollis is found on the south side, but only in a small, local
population near Fossil Butte. On the north side, shadscale seemed
to support fewer Townsend ground squirrels than other vegetation
types. A lack of food grasses and shallow soil was probably
responsible.

Fig. 2. Distribution of rodent kill-trap sites on the Snake River Birds of Prey study area, 1976.

140

Table 2. Number of rodent species at 34 sites in and around the
Snake River Birds of Prey Study Areas, 1976.

Species

Other

Total

Site

Description

caught

species
seen

1

Burn (North of river)

4

1

5

2

Winterfat (N)

5

5

3

Big safebrush (N)

5

5

4

Cheatgrass (N)

4

1

5

5

Shadscale (N)

2

2

4

6

Greasewood (South of river)

8

8

7

Big sagebrush (S)

3

3

8

Shadscale (S)

3

3

9

Cheatgrass (S)

2

1

3

10

Big sagebrush (Mtn. Home area

, N)

4

1

5

11

Shadscale (Sirnco Rd. , N)

2

1

3

12

Spiny hopsage (S)

4

4

13

Big sagebrush-cheatgrass (N)

5

5

14

Big sagebrush-cheatgrass (S)

3

3

15

Big sagebrush-shadscale (N)

2

2

4

16

Shadscale-winterfat (N)

3

3

17

Big sagebrush-winterfat (N)

5

5

18

Big sagebrush-shadscale (S)

4

4

19

Greasewood-cheatgrass (S)

3

3

20

Bi g sagebrush/winterfat-farm

(N)

3

3

21

Big sagebrush-farm (Mtn. Home

, N)

4

1

5

22

Shadscale-farm (N)

3

3

23

Winterfat- farm (N)

3

3

24

Big sagebrush-farm (S)

4

4

25

Greasewood- farm (S)

3

1

4

26

Siberian wheatgrass (1st spring

in 1976, N)

3

1

4

27

Crested wheatgrass-wi ldrye (2

nd

spring in 1976, N)

3

1

4

28

Crested v/heatgrass (Old seeding.

10 yrs., N)

3

3

29

Canyon talus (N)

4

1

5

30

Canyon flats (N)

3

1

4

31

Canyon riparian (N)

8

4

12

32

Road effect in big sagebrush

(N)

4

1

5

33

Marsh (N)

4

1

5

34

Wheatfield (S)

2

1

3

1-8 =

1975 Live-trapping sites

26-

28 = B.L.M.

seedings

9-12=

Other range vegetation

29-

31 = Canyon

sites

13-19=

= Range ecotones

32-

34 = Mi seel 1

aneous

20-25= Range-farm ecotones

141

Other rodent species observed, though not systematically
trapped were pocket gophers (Thomomys townsendii) , yellow-bellied
marmots, beaver (Castor canadensis), and muskrat (Ondatra zibethica)
Pocket gophers occurred in places of adequate soil moisture and
with a texture friable enough to allow tunnelling. Irrigated
fields, irrigation ditch banks, alluvial soils of the canyon
bottom and side streams and certain north-facing slopes in Con
Shea Basin and on Sinker Butte harbored these animals. Marmots
were found exclusively in close proximity to rocky areas. Talus
slopes, boulders on the canyon floor, and lava outcroppings,
especially near alfalfa fields supported populations of these
sciurids. Beaver and muskrat were restricted to aquatic and
riparian habitats along the Snake River. Neither of these species
were particularly abundant.

an

Seasonal density changes. To assess differences among seasons
aud sites, the kill-trap values for each species (Tables 3-5) were
subjected to an analysis of variance. Kill-trap values for deer
mice were converted to densities before the analysis because of
the observed disparity between index values for big sagebrush
versus other areas (Wolfe and Montan 1976). Deer mice, Ord's
kangaroo rats, and chisel toothed kangaroo rats all showed signifi-
cant declines as the year progressed. Other rodent species showed
no statistically significant seasonal changes. However, one
species which obviously became unavailable was the Townsend ground
squirrel, due to its July estivation.

Effect of native range types and agricultural ecotones.
Several species exhibited significant differences among trapping
sites (Table 6). Most of these differences have' been previously
explained as strong habitat specificity. Deer mice, Ord's kangaroo
rats, and Great Basin pocket mice were more eurytypic. For deer
mice, big sagebrush and associated ecotones supported significantly
higher densities than all other sites. Ecotones between big
sagebrush and other range vegetation did not significantly concen-
trate deer mice in comparison to pure stands of big sagebrush, nor
did big sagebrush-agriculture ecotones when compared to adjacent
big sagebrush. Great Basin pocket mice occurred in higher densities
at sites within the canyon when compared to all other sites. It
is not known why the canyon floor proved so favorable to this
species. Ord's kangaroo rats had significantly higher densities
along range-agriculture ecotones. It is probable that disturbed
soil from roads (around every farm sampled) plus dunes of wind-
eroded soil from the fields created favorable burrowing conditions
for kangaroo rats (Johnson 1961). Neither the important Townsend
ground squirrel nor the potentially important white-tailed antelope
squirrel showed significant concentrations in any particular
habitat. Capture rates were highly variable, however, and more
extensive sampling may have revealed differences. It appears from
the data that any concentrations of these species that did occur
were strictly sporadic, local phenomena and not typical of a
particular habitat.

Table 3. Captures/100 trap nights3 at 34 sites
23 March-29 April 1976.

in

and around the Snake River Birds of Prey Study Area,

Site

Pm

Pp

Do

St

Em

01

Nc

Nl

Al

Dm

Rm

Mm

Mim

Pc

1

16.3

1.3

17.5

2

19.2

1.1

2.1

3.1

3

17.6

1.1

2.2

4

14.6

2.4

5

23.3

6

32.7

2.0

4.0

7

30.0

2.5

8

17.5

9

R.9

2.2

10

19.5

11

38.1

12

19.5

9.8

13

6.3

2.1

2.0

11

26.7

15

8.3

16

12.2

2.0

17

29.3

4.9

2.4

4.7

18

2.1

4.3

19

14.9

29.8

20

7.0

25.6

2.3

21

20.5

2.3

2.2

22

33.3

2.0

23

16.2

16.2

14.0

24

20.5

43.6

25

30.2

4.7

26

8.7

2.2

27

16.3

2.3

28

26.2

6.7

29

35.3

11.8

30

23.7

7.9

2.6

31.

31.7

32

23.4

2.1

2.1

33

12.8

34

2.2

2.1

2.5

2.4

2.0

2.0

6.3

5.0

2.4
4.2
4.3

K>

8.8

2.6

2.6

Means

19.5

1.1

5.1 1.2

0.1

0.3

0.1

0.0 0.2

0.5 0.1

0.0

0.3 0.3

See text for definition.

See Table 2 for site names.

See Appendix for common and specific names.

Table 4. Captures/100 trap nightsa at 34 sites in and around the Snake River Birds of Prev Studv Area
28 June-29 July 1976.

Siteb Pmc Pp Do St Em 01 Nc Nl Al Dm Rm Mm Mim Pc

16.7

2.3
2.1 4.4

2.1 2.1
2.8

11.9 2.2 6.5

2.9 3.0

2.3 9.1

2.0
8.3 2.0

2.9 16.7 2.9

2.2 6.1

1

27.1

4.2

2

18.2

4.6

3

6.5

8.7

4

2.1

4.3

5

4.2

6

7

9

9

10

29.6

4.6

11

6.1

12

13

8.6

14

15

8.5

16

2.1

17

16.0

18

19

20

2.2

21

23.9

22

10.9

23

6.8

24

2.9

25

26.8

26

8.5

27

2.2

28

8.2

29

26.2

4.8

30

4.4

31

28.6

32

12.5

33

34

10.3

2.0

2.1
13.0

13.9 3.2

2.2

2.2

2.3

5.7 16.7

2.2

8.3

2.4

4.1

2.4 H.9

2.4 2.4

Means 8.7 1.1 2.4 0.6 0.5 0.5 0.1 0.1 1.4 0.2 0.1 0.1 0.0 0.4
See text for definition. See Table 2 for site names. cSee Appendix for common and specific names.

Table 5. Captures/100 trap nights3 at 34 sites in and around the Snake River Birds of Prey Study Area,
7 September-30 September 1976.

Site

Prri

Pp

Do

St

Em

01

Nc

Nl

Al

Dm

Rm

Mm

Mi'm

Pc

1

2

4.0

3

8.5

4

5

8.0

6

17.8

7

2.2

8

9

2.0

10

22.9

11

4.2

12

13

14

15

4.2

16

2.0

17

18

19

6.3

20

4.4

21

20.4

22

8.2

2 3

6.5

24

6.4

25

38.7

25

2.0

27

28

8.0

29

4.4

30

5.3

31

7.7

32

7.1

33

34

20.0

2.2

6.3

4.0

2.1

2.1

2.0

2.2

4.8

6.7

4.4

2.2
2.1

4.2

2.0
2.1

2.0

2.0

2.4

4.3
4.2

4.1

4.3
2.0

2.6

7.7
4.2

2.2

-P-
-P-

2.2

Means

6.5

0.7

0.5

0,0

0.1

0.3

0.0

0.0

0.6

0.0

0.1

0.4

0.1

0.1

See text for definition.

See Table 2 for site names.

'See Appendix for common and specific names.

145

Table 6. Analysis of variance F-values for rodent ki 11 -trapping
data from 34 sites in and around the Snake River Birds
of Prey Study Area, 1976.

Species Season Site

Peromyscus maniculatus
Dipodomys ordii
Perognathus parvus
Spermophilus townsendii

Eutanii us minimus

Onyrhomys leucogaster

leotoma cinerea

leotoma lepida

Ammospermophi 1 us leucurus
Dipodomys mi crops
Reithrodontomys megalotis

Mus musculus

Microtus montanus

Peromyscus crinitis

14. 8a

3.9b

6.la

1.9b

0.5

1.9b

2.4

1.1

2.0

2.7b

0.9

1.6

1.2

0.9

1.0

1.0

2.5

1.4

3.6a

2.5b

0.0

2.3b

1.5

1.0

0.5

1.0

1.0

7.1b

aValues larger than 3.15 (p = 0.05; df = 2,66) indicate a signifi
cant difference among seasons.

Values larger than 1.6 (p = 0.05; df = 33,66) indicate a signifi
cant difference among trapping sites."

146

Effect of livestock grazing. Due to the lack of non-grazed
control sites, the experimental design did not include an evaluation
of grazing effects. Work by Larrison and Johnson (1973), Phillips
(1936) , and Quast (1948) have shown that range depletion tends to
diminish the numbers of western harvest mice and Great Basin
pocket mice, and increase the number of deer mice. Grazing in wet
sites limits the distribution of meadow voles by eliminating the
thick grass cover under which runways are constructed. Linsdale
(1946) and Howard (1953) found that heavy grazing favors increased
numbers of ground squirrels in California. Johnson and Melquist
(1976) have noted similar effects in the Birds of Prey study area.

Miscellaneous effects. Several special effects could not be
substantiated statistically, but nevertheless showed results worth
noting. Roads constructed in big sagebrush created suitable
burrowing conditions for Ord's kangaroo rats (Table 4), providing
travel lanes across unfavorable habitats. This observation has
also been made by Johnson (1961) . Range-fire burns seemed to
attract higher numbers of deer mice during the summers of 1975 and
1976 than adjacent big sagebrush and winterfat (Eurotia lanata)
stands. BLM range seedings following range fires (sites 26-28)
seemed to support more deer mice and Townsend ground squirrels as
the seedings aged. As summer progressed, these differences became
less pronounced. A planted wheat field south of the river did not
harbor any rodents until the wheat grew high enough to provide
cover. The ubiquitous deer mice then moved into the interior of
the field. Later in summer, meadow voles colonized the field
under large-diameter irrigation feeder pipes. By fall, deer mice
had established a large breeding population. When the wheat was
cut, the rodents' vulnerability increased, and raptors, particularly
red-tailed hawks, were seen hunting the field extensively.

Rodent abundance in intensively hunted areas. During June,
Martha Wakefield ran eight rodent trap lines in areas Identified
by Study III as intensively hunted by raptors (Table 7). More
Townsend ground squirrels were caught in these areas than at 23
sites in different habitats north of the river. Grasshopper mice
were also caught more often in raptor hunting areas, however,
these mice seldom appear in raptor diets.

Marmots. Yellow-bellied marmots occurred exclusively on or
near rocky areas. Talus slopes, boulders on the canyon floor, and
lava outcroppings, especially near alfalfa fields, supported
colonies of these sciurids. Colony distribution was mapped by
walking along the base of the canyon wall. The route surveyed
extended from Evans Ranch to a point 3.2 km upstream of Swan Falls
Dam on the north side of the river, and from Swan Falls Dam to the
Priest Ranch on the south side. The only places marmots were seen
were near Evans Ranch, along the base of Jensen Cliff, near Swan
Falls Village, and in the old fields of Priest Ranch. All of
these sites are adjacent to irrigated fields or other sources of
succulent vegetation.

147

Table 7. Relative rodent densities (captures/100 trap-nights) in
raptor hunting areas. 1976. Snake River Birds of Prey
Studv Area.

Dates

Site

Grid Location Pm St 01 Em Do

June 3-4

Prairie falcon
hunting area
(Sand Creek)

5-6

Near Swan Falls R

10-11

Trio Butte area

10-11

Trio Butte area

12-13

Coyote Butte area

15-16

Chattin Flats

15-16

Little Baja

15-16

Dorsey Butte

(37.5,96.0) 23.3 6.5

(31,3,92.0) 8.9 4.3

(31.55,92.4) 11.9 2.3 2.4

(32.5,92.5) 10.6 2.1

(32.3,93.5)

(46.5,75.0)

(46.6,75.8)

(48.2,77.25)

7.3 4.7 2.4 2.4

4.3 8.5

8.5 8.5

4.2 2.1 2.1

Means

Spring, 1976 means for rodent transects
north of Snake River

9.9 2.5 1.9 0.3 1.3

20.0 1.8 0.5 0.3a 3.2

Seven big sage transects with ecotones (only habitat in which
Eutamius minimus is found).

148

A colony assumed to be typical was live-trapped between
23 February and 26 March at Evans Ranch, Melba. In a 2.5 ha area
a Schnabel population estimate showed a population of 40 marmots,
or about 15.9 animals/ha. The sex ratio of the catch approximated
unity (14 males and 13 females) . Trapping was purposely terminated
before the juveniles emerged from their natal burrows to avoid
biasing capture data with a preponderance of trap-pone juveniles.
Mr. Evans reported that a particular marmot was seen repeatedly
throughout the winter at the ranch, however, the first observation
by field personnel was on 29 January. If one arbitrarily chooses
1 February as the adult emergence date, then 1 April was the
approximate date of juvenile emergence from natal burrows, according
to the breeding schedule determined by Nee (1969) . Marmots were
last seen in early August, although most began estivation during
July.

Voles (Microtus montanus) . Meadow voles were removal trapped
for five days in a 53 m^ area along an irrigation ditch at Evans
Ranch, Melba. Captures leveled off at four animals on the third
day but rose to eight voles on the fifth day, perhaps a result of
new individuals occupying areas left by trapped voles. In any
case, the vole density is potentially great; on the order of 750-
1500 voles/ha. Using an average weight of 29.3 g, this yields an
estimate of 21.9-43.9 kg/ha. It should be emphasized that these
figures refer to a small area of favorable habitat, and not to a
large, continuous vegetation type. Voles seem to require a growth
of grass high enough to lodge and form a mat under which runways
are constructed. Areas heavily cropped by cattle near the ditch
showed little or no vole activity. The presence of dead Russian
thistle (Salsola kali) in the ditch also provided cover for voles.

Lagomorphs

Cottontails. Table 8 summarizes cottontail live-trapping
efforts. At some sites, the number of animals caught was insuffi-
cient to permit a density estimate, therefore, captures/100
trapnights have been included to show relative densities. Woodrats
(Neotoma spp.), marmots, and jackrabbits were also caught (Table
9) . Rarely were there enough captures of these species to calculate
densities. Of particular significance is the fact that the
greasewood vegetation type can support significant numbers of
cottontails. Big sagebrush does not harbor many cottontails. The
few that were caught in big sagebrush were near small rocky outcrops.

Jackrabbits. Flushing transects walked in the spring and
fall produced ambiguous results (Table 10). In theory, the index
should have been lower in spring 1976 than fall 1975, and higher
in fall 1976 than spring 1976. That it was not may indicate a
flaw in the method itself. One problem is that not all of the 61
transects are in good jackrabbit habitat. Therefore, during years

Table 8. Lagomorph live-trapping summary. Snake River Birds of Prey study area, 1976.

Grid location

Description

S.n.

L.c.

c ■ 1

Spnr. j

Capt./lOO t-n
S.n. L.c.

Density
(animals
(S.n. or

/ha)

iy)

S.n.

L.c.

2

Summer

Capt./lOO t-n
S.n. L.c.

Den s i ty

S.n.

L.c.

lm - 3
Winter

Capt./lOO t-n

S.n. L.c.

Density

(32.5,92.4)

Trio Butte

5

0

3.3

0.0

5

0

7.1

0.0

0.5

1

3

1.2

3.6

(32.1,87.1)

Canyon talus

15

0

10.0

0.0

1.9

11

0

11.0

0.0

1.1

8

0

5.2

0

2.0

(30.3,90.8)

Canyon riparian

16

0

10.7

0.0

2.3

9

0

9.0

0.0

1.1

13

0

10.5

0

3.6

(32.3,88.5)

Canyon rim

3

0

1.9

0.0

8

1

8.0

1.0

0.8

2

0

1.9

0

^3

(29.3,93.6)

Big sagebrush

1

1

0.6

0.6

1

1

1.4

1.4

0

0

0

0

(29.1,93.1)

Big sagebrush-
farm ecotone

1

. 1

0.6

0.6

2

6

2.9

8.6

3

1

3.2

1.1

(20.4,95.4)

Greasewood-
farm ecotone

9

1

4.5

0.5

7

0

4.7

0.0

1.8

2

0

2.1

0

(23.2,86.9)

Greasewood

4

3

2.0

1.5

0

2

0.0

1.3

0

0

0

0

Spring: 8 May-26 June
Summer: 4 August-10 September

Winter: 2-31 December

Table 9. Marmot, woodrat, and skunk capture summary. Snake River Birds of Prey study area, 1976.

— ■

Grid location

Description

M.f.

N.c

Spri
N.l

1

ig

. S.p.

M.m.

M.f.

N.c

Summer
N.l. S.p.

M.m.

M.f.

Win
N.c.

ter3

N.l.

S.p.'

(32.5,92.4)

Trio Butte

0

0

0

0

0

0

1

0

1

0

0

3

0

0

(32.1,87.1)

Canyon talus

3

3

0

0

0

0

4

0

0

0

0

0

0

0

(30.3,90.8)

Canyon riparian

3

1

0

0

0

0

0

0

0

0

0

0

0

0

(32.3,88.5)

Canyon rim

n

0

0

0

0

0

0

0

0

0

0

1

0

0

(29.3,93.6)

Big sagebrush

0

0

0

0

0

0

0

0

0

0

0

0

0

0

(29.1,93.1)

Big sagebrush-
farm ecotone

0

0

0

0

0

0

0

0

0

0

0

0

0

0

(20.4,95.4)

Greasewood-
farm ecotone

0

0

0

0

0

0

0

0

0

1

0

0

0

0

(23.2,86.9)

Greasewood

0

1

2

0

0

0

0

2

0

0

0

0

0

0

Spring; 8 May-26 June

?

Summer: 4 August-10 September

3
Winter: 2-31 December

o

Table 10. Jackrabbit population indices
study area, 1972-1976.

Snake River Birds of Prey Natural Area

Year Season

No.
transects

No. jackrabbits
observed (n)

index (N)

Jackrabbits/
km2

1972 Fall

1973 Fall

1974 Spring

1974 Fall

1975 Spring

1975 Fall

1976 Spring
1976 Fall

26
36
34
59
60
60
61
61

9
6
4

12
7

11

21

18

43.8
19.0
12.3
31.2
12.5
19.4
57.7
34. 0a

62.6
27.2
17.6
44.6
17.9
27.7
82.5
48. 6a

Includes 2 transects where only 3 animals were seen.

152

of low jackrabbit numbers the effective sampling ability of the
transects is so low that the transects fail to document population
changes.

Martha Wakefield walked 20 transects during June in areas
identified by Study III as intensively hunted by golden eagles
(Table 11) . Jackrabbit densities were significantly lower in
these areas than the average density for the study area. The
t-value obtained when the mean index value (4.5) for the intensively
hunted areas was compared to the spring index value (34.0) for the
entire study area was 4.46. Perhaps golden eagles do not respond
to hare density but rather hare vulnerability. Areas (such as
short grass or winterfat) where hares are most vulnerable are also
areas where the fewest hares will be found. In contrast, big
sagebrush and greasewood are good hare habitats but vulnerability
to avian predators is relatively low. The management significance
for golden eagles is that perhaps, as a mitigating practice, the
BLM should seek to increase the amount of ecotone between big
sagebrush (or greasewood) and grass. Further prey vulnerability
studies should shed more light on this question.

Avian Prey Species

Pheasants. Pheasant crow count routes were driven by
Jan Wassink between 1 and 14 May. Two measures of pheasant abundance
are provided: no. calls/station and no. birds seen/linear kilometer
driven (Table 12). Kimball (1949) described a method of calculating
densities from crow count indices based on the distance a crowing
cock can be heard. The problem with this method in the study area
is that topography and vegetative cover differ too greatly to
assume that crowing cocks can be heard within the same radius at
each station. Pheasants showed relative densities similar to
those observed during the previous field season. The mean number
of birds per kilometer observed on all routes in the spring of
1975 (0.34) was not significantly different (t = 0.82; df = 13;
p <L 0.05) from the comparable figure for 1976 (0.45). Older
agricultural areas with a certain amount of weedy cover contained
more pheasants than new desert land entries. This is shown by the
results in Table 12. The unweighted mean values for the number of
calls per station were 3.2 and 1.2 in old and new agricultural
tracts, respectively. This difference is not significant
(t = 1.91, d.f = 6) at the 0,05 level but is significant at the
0.10 level. The observed differences in the number of birds seen
per kilometer (0.7 versus 0.2) are significant (t = 2.53) at the
0.05 level.

Chukar partridge. The recorded-call method was only partly
successful. Chukars were largely restricted to the main canyon
and Sinker Creek Canyon. A map of flock locations and census
routes has been filed at the Birds of Prey Office. The use of a
dog to flush flocks seen (but not heard) would complement the
audio census.

Table 11. Jackrabbit transects in eagle hunting areas. Snake River Birds of Prey Natural Area
29 May-14 June, 1976.

Density
(1.43 (n)
Grid Location Description No. transects No. jackrabbits (N) jackrabbits/km„

(45.0,77.0) Section 9

Pole 60, Big Baja 5 0 0 "

Spring, 1976

Fall, 1976 Entire Study area 61 18 34.0

17.5

7.7

(48.0,75.0) Section 24

Pole 33, Big Baja 5 1 12.5

(Area from

30.5,92.7 to near pole 241 5 2 5.4

30.8,92.3 to w

31.9,93.0 to

31.5,93.5)

48.0,77.0 Section 12

Dorsey Butte 5 0 0

Totals and Means

for hunting areas 20 4 4.5 6.4

Study area
transects: Entire Study area 61 21 57.7 82.5

48.6

Table 12. Indices of pheasant abundance on the Snake River Birds of Prey study area, May 1976.

Route

Km.

Area
sampled
(Km2)

No. of
stations

No. of
call s

Calls/
station

Birds
seen

Birds/
Km

Older agricultural areas

Melba

37.0

7.4

12

56

4.6

32

0.9

Grandview

16.1

3.2

5

5

1.0

14

0.9

Chattin Flats

17.7

3.5

7

23

3.3

5

0.3

Bruneau

25.4

5.1

11

45

4.1

15

0.6

Unweighted means

24.1

4.8

8.8

32.2

3.2

16.5

0.7

New agriculture

Castle

9.7

1.9

5

16

3.2

4

0.4

Sinker

19.3

3.9

7

2

0.3

5

0.3

Flying H

24.9

5.0

9

12

1.3

8

0.3

Mountain Home

12.9

2.6

5

0

0

0

0

Unweighted Means

16.7

3.4

6.5

7.5

1.2

4.2

0.2

155

Passerines. A series of 18 sites, including the eight small
mammal live-trapping grids (1975), were sampled by Jan Was sink
between 16 May and 16 June (Table 13). Passerines were grouped
into three size classes: horned larks, meadowlarks , and "sparrows'1.
Brad James later sampled six small mammal grid sites between 18-20
August (Table 14). Passerines seemed to have higher biomass/ha
than rodents (excluding Townsend ground squirrels) (Wolfe and
Montan 1976). Higher biomass combined with diurnal activity makes
passerines a potentially important prey source. Raptors probably
have a more difficult time capturing birds than small mammals,
however. Fall densities were low.

DISCUSSION
Rodents

Agriculture. Although range-agriculture ecotones supported
concentrations of Ord ' s kangaroo rats, these ecotones have not
been shown to concentrate any other rodent species. It is evident,
therefore, that the increased supply of nocturnal Ord ' s kangaroo
rats around the periphery would not offset the loss of the diurnal
Townsend ground squirrel when a native range vegetation type is
converted to an agricultural development.

The yearly schedule of prey availability is also an important
consideration. Although irrigated wheat fields will attract
breeding deer mice and meadow voles, by the time this happens the
wheat has grown to a height which effectively renders them invulner-
able to predation. By then the critical raptor breeding season
has passed before the interior of these fields can yield prey.
After the wheat is cut in the fall, the prey become vulnerable,
benefiting resident raptors. Fields, which in early spring
supported no rodents, support more vulnerable prey in the fall
than almost any other habitat in the study area (Table 5).

Native range types, ecotones, and burns. The riparian zone
had the greatest number of rodent species, but because of its
small size relative to other habitat types, lack of Townsend
ground squirrels, and low prey vulnerability, it was not considered
the most important contributor of prey. The greasewood vegetation
type south of the river had the next highest number of species,
but it was the presence of an extensive Townsend ground squirrel
population north of the river which caused raptors to preferentially
hunt the big sagebrush-winterfat-cheatgrass complex on the north
side (Dunstan, personal communication I/). The big sagebrush type
north of the river ranked overall as the most important contributor

1/ Personal communication from Dr. T. Dunstan, 19 Oct. 1976,
Boise, Idaho.

Table 13. Passerine density in the Snake River Birds of Prey Study Area. 16 May-16 June, 1976.

Site

Horned

lark

Meadowlark
(b/ha) (kg/ha)

"Sparrows"

Total

(B

Lrds/ha)

(kg/ha)

(b/ha)

(kg/ha)

(b/ha)

(kg/ha)

Grid 1 (32.3,88.0)

7.0

0.2

0.5

tr

0.0

0.0

7.5

0.2

Grid 2 (31.7,89.8)

7.0

0.2

0.0

0.0

0.0

0.0

7.0

0.2

Grid 3 (30.3,93.3)

0.0

0.0

6.2

0.6

4.8

0.1

11.0

0.7

Grid 4 (35.4,88.1)

5.0

0.1

1.1

0.1

4.3

0.1

10.4

0.3

Grid 5 (36.0,83.2)

23.8

0.7

0.0

0.0

0.0

0.0

23.8

0.7

Grid 6 (23.5,87.2)

9.6

0.3

27.0

2.6

0.0

0.0

36.6

2.9

Grid 7 (29.3,85.1)

16.0

0.4

25.0

2.4

23.7

0.6

64.7

3.4

Grid 8 (29.0,78.0)

17.4

0.5

0.0

0.0

18.6

0.4

36.0

0.9

Burn (31.5,88.5)

5.1

0.1

0.5

tr

0.0

0.0

5.6

0.1

Shadscale-spiny hopsage

(27.4,78.8)

21.4

0.6

0.0

0.0

6.7

0.2

28.1

0.8

Spiny hopsage (25.0,89.5)

15.7

0.4

0.0

0.0

5.0

0.1

20.7

0.5

Big sagebrush-shadscale

(36.0,83.2)

13.1

0.4

14.9

1.4

8.3

0.2

36.3

2.0

Big sagebrush-farm

(26.7,84.1)

4.6

0.1

10.1

1.0

3.0

0.1

17.7

1.2

Wheat field (27.5,84.0)

1.9

tr

0.0

0.0

0.0

0.0

1.9

tr

Shad scale- farm

(50.0,74.0)

15.7

0.4

3.4

0.3

0.0

0.0

19.1

0.7

Prairie falcon hunting area

(38.9,97.9)

17.9

0.5

1.0

0.1

0.0

0.0

18.9

0.6

Shadscale (52.4,75.0)

20.6

0.6

0.0

0.0

5.0

0.1

25.6

0.7

Golden eagle hunting area

(49.2,58.2)

11.0

0.3

1.4

0.1

0.0

0.0

12.4

0.4

in

Table 14. Passerine density on six of the small mammal grid sites. Snake River Birds of Prev
study area. 18-20 August, 1976.

Grid

No.

Meadowl

ark

"Sparrows"

Mourn ir
(b/ha)

lg Dove
(b/ha)

Total

(birds/ha)

(kg/ha)

(b/ha)

(kg/ha)

(b/ha)

(kg/ha)

1

0.0

0.0

0.2

tr

0.0

0.0

0.2

tr

i— ■

2

0.0

0.0

0.3

tr

0.0

0.0

0.3

tr

^l

3

0.1

tr

0.1

tr

0.0

0.0

0.2

tr

4

tr

tr

0.2

tr

0.0

0.0

0.2

tr

5

0.0

0.0

0.1

tr

0.0

0.0

0.1

tr

6

0.3

tr

1.1

tr

tr

tr

1.4

0.1

158

of prey. Not only did it have a relatively large number of rodent
species (five), including the Townsend ground squirrel, but it
also supplied the diurnal least chipmunk. Greasewood on the south
side of the river ranked second in importance because of the less
abundant white-tailed antelope squirrel. Shadscale seemed to be
relatively poor in its ability to support Townsend ground squirrels.
Winterfat and cheatgrass appeared to be good habitats for Townsend
ground squirrels, but poor for other rodents. Range ecotones had
no significant ability to concentrate rodents. Burns consistently
seemed to attract deer mice during the summers of 1975 and 1976,
and prescribed burning may have some potential as a management
tool. However, extensive destruction of big sagebrush might be
seriously detrimental to the jackrabbit population.

BLM seedings generally do not support rodent densities as
high as previously existed in the burned vegetation type with the
exception of the Townsend ground squirrel, which prefers grassy
sites. Dirt roads created disturbed soil which allowed easier
burrowing for kangaroo rats. Roads serve as bridges over unsuitable
habitats, permitting kangaroo rats to extend their range.

Lagomorphs

Cottontails were most abundant in or near the canyon where
they find cover in rock crevices. Rabbit numbers seem to fluctuate
least at these sites. Of secondary importance is dense greasewood.
It is the only vegetation type which provides enough cover to
sustain cottontail populations. Proximity of greasewood or rocks
to agriculture resulted in especially high densities. Big sage-
brush, while good habitat for jackrabbits, is poor for cottontails.

Jackrabbits need more work in the next two years. Trapping
is not an efficient method of estimating densities. If the BLM is
interested in actual hare densities, drive counts must be made and
correlated to walking transects. Additional transects in big
sagebrush and greasewood should be run in the spring and fall, but
should be analyzed separately from the 61 already existing to
avoid biasing the data. The relationship of ground squirrel
habitat to jackrabbit habitat and their respective importance to
prairie falcons and golden eagles needs to be resolved. At present,
it appears that efforts should be directed toward determining
minimal habitat necessary to supply enough vulnerable jackrabbits
to golden eagles. The reason for this is that the jackrabbit is
known to fluctuate cyclically, while the ground squirrel population
has not been shown to cyclically fluctuate. Therefore, lower
jackrabbit populations than normally exist in the study area are
not desirable, but lower ground squirrel populations might be
permissible. Essentially, what this reduces to is a question of
how much "grassy" versus '"big sagebrush" habitat is necessary to
supply prey for current numbers of prairie falcons, red-tailed
hawks, and golden eagles.

159

LITERATURE CITED

Davis, W.B. 1939. The recent mammals of Idaho. Caxton Printers
Caldwell, Idaho. 400 pp.

Emlen, J.T. 1971. Population densities of birds derived from
transect counts. Auk 88:323-342.

Fautin, R.W. 1946. Biotic communities of the northern desert
shrub biome in western Utah. Ecol. Monogr. 16 (4) : 251-310.

Gates, C.E., W.H. Marshall, and D.P. Olson. 1968. Line transect
method of estimating grouse population densities. Biometrics
24(1): 135-145.

Gross, J.E., L.C. Stoddart, and F.H. Wagner. 1974. Demographic
analysis of a northern Utah jackrabbit population. Wildl.
Monogr. 40. 68 pp.

Howard, W.E. 1953. Rodent control on California ranges.
J. Range Manage. 6:423-434.

Johnson, D.R. 1961. The food habits of rodents on rangelands
of southern Idaho. Ecology 42 (2) : 407-410.

, and W.E. Melquist. 1976. Population ecology and habitat

requirements of Townsend ground squirrels. Pages 156-176

in Snake River Birds of Prey Ann. Rep. 1975. (U.S. Bur. Land

Manage., Boise, ID.).

Kenagy, G.J. 1972. Saltbush leaves: excision of hypersaline
tissue by a kangaroo rat. Science 178:1094-1096.

Kimball, J.W. 1949. The crowing count pheasant census. J. Wildl.
Manage. 13:101-120.

Larrison, E.J., and D.R. Johnson. 1973. Density changes and
habitat affinities of rodents of shadscale and sagebrush
associations. Great Basin Nat. 33.(4) :225-264.

Linsdale, J.M. 1946. The California ground squirrel: a record
of observations made on the Hastings Natural History
Reservation. Univ. Calif. Press. 475 pp.

Montan, J.R. 1977. Rodent density and species composition in
the Snake River Birds of Prey Natural Area, Idaho.
M. S. Thesis. Utah State Univ., Logan. 44 pp.

Nee, J. A. 1969. Reproduction in a population of yellow-bellied
marmots (Marmota f laviventris) . J. Mammal. 50 (4) : 756-765.

160

Oelklaus, W.F. 1976. Chukar partridge dispersion along the

middle and lower Snake and Columbia Rivers. M. S. Thesis.
Univ. Idaho Coop. Wildl. Res. Unit. Moscow, ID. 56 pp.

Phillips, P. 1936. The distribution of rodents in overgrazed

and normal grasslands of central Oklahoma. Ecology 17:673-679.

Quast, J.C. 1948. Habitat preferences of rodents on grazed and
ungrazed foothill pastures in California. M. S. Thesis.
Univ. Calif., Berkeley. 52 pp.

Wolfe, M.L., and J.R. Montan. 1976. Density and species

composition of birds and mammals, Snake River Birds of Prey
Natural Area, Idaho. Pages 98-126 in Snake River Birds of
Prey Res. Proj . Ann. Rep. 1975. U.S. Dept. of Interior, Bur.
of Land Manage., Boise, ID. 193 pp.

161

APPENDIX 1

Common

and scientific names of animals

in Tables 8-10.

Species

code

Common name
Deer mouse

Specific name
Peromyscus maniculatus

■

Pm

Pp

Great Basin pocket mouse

Perognathus parvus

Do

Ord kangaroo rat

Dipodomys ordii

Dm

Chisel-toothed kangaroo rat

Dipodomys microps

St

Townsend ground squirrel

Spermophilus townsendii

Em

Least chipmunk

Eutamius minimus

01

Grasshopper mouse

Onychomys leucogaster

Nc

Bushy-tailed woodrat

Neotoma cinerea

Nl

Desert woodrat

Neotoma lepida

Al

White-tailed antelope

squirrel

Ammospermophilus leucurus

Rm

Western harvest mouse

Reithrodontomys megalotis

Mm

House mouse

Mus musculus

Mim

Mountain meadow vole

Microtus montanus

Pc

Canyon mouse

Peromyscus crinitus

Sn

Mountain cottontail

Sylvilagus nuttalli

Lc

Blacktailed jackrabbit

Lepus californicus

Mf

Yellowbellied marmot

Marmota flaviventris

Nc

Bushytailed woodrat

Neotoma cinerea

Nl

Desert woodrat

Neotoma lepida

SP

Spotted skunk

Spilogale putorius

Mm

Striped skunk

Mephitis mephitis

162

Appendix 2. Mean Biomass values (in kilograms) for various prey
species.

■

f

N

2

N

Mean weight

Sylvilagus nuttalli

0.59

45

0.72

47

0.65

Lepus cal ifornicus

1.63

3

1.46

18

1.48

Marmota flaviventris

1.80

21

2.08

24

1.94

Neotoma cinerea

0.30

2

0.32

5

0.31

Neotoma lepida

0.19

1

0.13

2

0.15

163

Notes

164

Lowell Diller, Study IV-A-S , examines a rattlesnake as part of
a study on reptiles in the Birds of Prey Study Area.

165

STUDY IVA-S: Ecology of reptiles in the Snake River Birds
of Prey Natural Area.

SUBCONTRACTORS: Department of Biological Sciences, University
of Idaho.

INVESTIGATORS: Donald R. Johnson, Major Professor.
Lowell Diller, Research Assistant.

PROJECT SUPPORT: U.S. Department of the Interior, Bureau of

Land Management, Contract No. 52500-CT5-1003
(To Utah State University) .

OBJECTIVES:

1. Determine the species composition and biomass of lizards
and snakes in specific habitat types within the study
area.

2. Determine significant population parameters for these
reptile populations including productivity, sex and
age ratios, survivorship, and mortality factors.

3. Assess the availability of reptiles as prey for raptorial
birds.

A. Determine the impact of reptiles on prey populations
utilized by raptorial birds.

5. Determine the effects of habitat alteration on reptile
densities.

ANNUAL SUMMARY

Lizard densities continued to be observationally censused in
1976 with new transects established on the south side of the Snake
River. Efforts were increased to assess density, population
parameters, growth rates, food habits and prey consumption of
gopher and rattlesnakes in the BPNA. Densities as high as 11
rattlesnakes per hectare were found in certain locations with
gopher snakes existing in lower, more uniform densities. Pour age
classes were identified for both snake species. Searches usually
fail to detect the younger age classes for both species. Reproduc-
tive data is still preliminary on gopher snakes, but rattlesnakes
were shown to have a low reproductive rate. Adult rattlesnakes
feed almost exclusively on Townsend ground squirrels while gopher
snakes consume a variety of prey items. Consumption rates calculated
from feeding experiments and growth rates indicate that gopher and
rattlesnakes consume about 3 to 4 times their body weight of prey
each year .

166

METHODS

Lizard densities continued to be censused in 1976 employing
an observational technique described in the 1975 annual report.
During the summer of 1976, certain transects previously established
in 1975 were recensused and other new transects were established
with more emphasis shown to the south side of the Snake River. In
addition, more lizards were collected for life history data.

In 1976, more emphasis was placed on snake related studies
than lizards. There was an increased mark-release effort to
determine snake densities. Mark-release work was conducted at
Coyote and Trio Buttes, the east rim of "Tick City Basin," the
north rim of "Tom Draw" and the canyon rim from the Swan Falls
substation to "Balls Point." In addition, a removal-type census
of rattlesnakes was done along the rim from Dedication Site to
Dedication Point, where all possible snakes were located and
removed during a series of successive searches.

As described in the 1975 annual report, all snakes encountered
are captured to obtain relevant life history data, and most of
these are also marked with a heat brand to facilitate later
identification. In addition, in 1976 all of the gopher and
rattlesnakes captured were palpated to determine the presence of
prey items and held in collecting bags to obtain scats. Also,
snake scats were collected in the field for later identification
of the prey items consumed.

Outdoor enclosures were constructed, and captured rattlesnakes
and gopher snakes were fed known amounts of natural prey items
to determine growth efficiencies. Before each feeding, the snake
was weighed and placed in an isolated cage with several prey
items. After the snake had fed, it was again weighed and the
number of prey items ingested were noted. This was repeated at
intervals of 6 to 8 days during the summer. In addition, nine
gravid rattlesnakes were kept in captivity to obtain data on
clutch size and growth of newborn snakes.

RESULTS AND DISCUSSION
Body Weights of BPNA Reptiles

Cumulative data on body weight measurements for 1975 and 1976
are listed in Table 1. Since virtually every snake encountered
was weighed, the N value for each snake species also reflects its
relative abundance in the BPNA, allowing for certain biases due to
activity patterns and detectibility. The N value for the lizard
species does not reflect their relative abundance, since lizards
are captured for measurement in an arbitrary manner.

167

Table 1. Mean body weights of snakes a.nd lizards collected in the BPNA.

All Age Classes Excluding Young
sPecies ^ n x_ SD n x SD

Lizards :

Western whiptail

(Cnemidophorus tigris) 13 15-35 6.96 11 17.1+7 5.10

Collared

(Crotaphylus collaris) 17 26. U8 ik . 58 12 3U . 63 J ,6k

Leopard

(Crotaphytus wisiizenii) 30 22.63 7.^3 2k 25.53 L . 90

Desert horned

(Buaoiasoma -platyrhinoa) 77 17.81 9.08 i+2 23.62 6.12

Western fence

(Sceloporus occidentalis ) 29 16.75 h.77 25 18.03 3.29

Side-blotched

(Uta stansburiana) 37 3.66 0.8.1

Snakes :

Western rattlesnake

(Crotalus viridis) UtO U68.15 213-91 128 502.1+2 189.87

Gopher

(Pitupphis melanoleucus) 88 232.11 165.2U 73 276.03 1U6.77

Striped whipsnake

(Masticophis taeniatus) It 3 97. ill U5 . 93 l+i 100.92

Western ground

(Sonora semiannulata) 12 8.06 2.68

Racer

(Coluber constrictor) 2 67-5

Western terrestrial garter

(Thamnophis elegans) 2 70.2

Night

(Hypsiglena torquata) 2 13-9

Long-nosed

(Rhinocheilus lecontei) 2 51. 1+

168

Lizard Density and Biomass

Observational census data (Table 2) indicates that the side-
blotched lizard is the most abundant lizard in most habitat types.
Because of their larger sizes, however, the western whiptail and
the leopard lizard comprise a greater biomass at most sites with a
loose or sandy substrate. At most sites with a rocky substrate,
the western fence lizards make up the greatest biomass. Winterfat,
big sage, and greasewood habitats supported less than half the
lizard biomass found in other habitat types. No lizards were
found on the Sand Creek grasslands. With the exception of the
greasewood habitat, all low density lizard habitats possess
reduced amounts of open ground, while all areas of high lizard
density have considerable open ground with extensive areas of rock
or reduced amounts of vegetation. The data from the greasewood
habitat are preliminary and in fact may be reversed with additional
censusing next field season.

Snake Densities

Because of unequal sampling effort and differences in activity
patterns and detectibility, the data on snakes encountered in
relation to habitat types (Table 3) cannot be used quantitatively
to assess relative densities of the three major snake species in
the BPNA. However, the totals for the number of snakes encountered
indicate that the western rattlesnake is the most abundant snake
in the study area followed by the gopher snake and striped whip-
snakes. Rattlesnakes and whipsnakes tend to be found in rocky
substrates while gopher snakes occur in a variety of habitat
types.

Rattlesnakes were the only species that were recaptured
frequently enough to determine density. After repeated searches
of an area, we reached a point where all rattlesnakes captured had
been previously marked and thus the total number marked should
equal the population size in that area. In this manner density
estimates of 6.6, 5.6, and 4.5 snakes per hectare were determined
for Trio Butte, the north rim of "Tom Draw" and the east rim of
"Tick City Basin" respectively. Other areas censused by mark-
release have not been adequately sampled to determine density
figures yet.

The removal census of rattlesnakes along the rim between
Dedication Point and Dedication Site yielded 27 snakes in seven
successive searches over a nine day period. This gives a density
estimate of 11.0 snakes per hectare. During the entire field
season a total of 35 rattlesnakes were removed from this location,
but this figure was not used to calculate density since snakes may
have immigrated into the area. It is important to note that these
density estimates only reflect the density of rattlesnakes in the
habitat types where they were censused and not the BPNA in general.

169

Table 2. Indices of lizard density and biomass. C.t. = western whiptail
(Cnemidophorus tigris) ; C.w. = leopard lizard (Crotaphytus wislizenii)

C.c. = collard

lizard (Crotaphytus cc

llaris) ; U

s. = side-
(Phrynosoi

-blotched lizard

(Uta stansburiana) ; P.p. -

iesert horned lizard

ia platyrhinos) ,

S.o. = western

fence lizard

(Sceloporus occidentalis) .

Habitat Type

Species

Density

Biomass

Transect

Mean

(grid coordi-

(n/ha)

(g/ha)

Total

Habit

nates)

Biomass

Winterfat (a)

C.t.

0.7

10.3

(328875)

C.w.

0.7

15.2

U.s.

3.3

12.3

37.7

Winterfat (b)

C.t.

1.0

14.9

(314896)

C.w.

1.0

21.9

U.s.

15.3

56.0

92.8

65.3

Big Sage (a)

C.w.

1.8

41.9

(320924)

U.s.

2.8

10.2

52.0

Big Sage (b)

C.t.

1.0

15.8

(297932)

P.p.

1.0

18.3

34.2

Big Sage (c)

C.t.

1.6

23.8

(308868)

p.p.

2.5

9.1

32.9

39.7

Shadscale (a)

C.t.

1.2

19.2

(371825)

C .w.

0.8

18.8

P.p.

0.6

9.8

U.s.

13.8

50.6

98.4

Shadscale (b)

C.t.

6.8

103.8

(385796)

C .w.

0.5

10.9

P.p.

1.9

34.4

U.s.

17.7

64.9

213.9

Shadscale (c)

C.t.

2.5

38.1

(303780)

C .w.

1.9

42.1

P.p.

2.5

44.2

U.S.

9.9

36.4

160.7

157.7

Canyon rim (a)

C.t.

2.8

43.0

(348834)

S.o.

2.4

40.7

U.S.

12.6

46.2

129.9

Canyon rim (b)

C.c.

1.5

39.7

(348834)

C.t.

2.2

34.5

p.p.

1.0

17.8

S.o.

4.0

67.0

U.S.

6.5

23.8

182.9

Canyon rim (c)

C.c.

0.6

27.0

(323873)

C.t.

1.8

15.6

C .w.

1.2

26.7

S.o.

7.6

126.6

U.s.

8.2

30.1

226.1

179.6

Talus slope (a)

S.o.

11.4

190.3

(278921)

U.s.

11.9

43.6

233.8

Talus slope (b)

S.o.

8.5

142.9

(307904)

U.s.

8.7

28.4

171.2

202.5

Riparian (a)

C.t.

6.4

98.8

98.8

(370795)

Riparian (b)

C.t.

7.1

108.7

(300913)

S.o.

6.2

22.7

131.3

115.1

Greasewood (a)

C.t.

1.7

26.7

(224897)

U.s.

1.4

5.1

31.8

Greasewood (b)

C.t.

2.8

43.6

(343752)

U.s.

3.0

11.1

54.7

43.2

Grass

0

0

0

0

0

(Sand Creek)

170

Table 3- Numbers of snakes encountered in relation to habitat and substrate.

Vegetation/
Habitat

Substrate

Snake

Species

Habitat
Totals

Western
rattlesnake

Gopher
snake

Striped
whipsnake

Winter fat

loess
rocky

5

1

16

6

27

1

Big Sage

loess

rocky

13

:■"'

19

1

3

2

3^
5

Shadscale

loess
rocky

2
2

2

1

l

5
3

Winterfat-
Big Sage

loess
rocky

12

Ifi

1*

2

18

22

Grass

loess

rocky

29

9
7

3

17
36

Canyon Rim

rocky

65

7

30

102

Talus

rocky

3

7

1+

lit

Riparian

mixed

11

29

f.

k6

Greasewood

sand/loess

2

1

3

Substrate Totals

rocky
loess
other

120
37
13

23

50
30

Uo

15
6

183

102

k9

Species Totals

170

103

61

33U

171

It would appear from Table 3 that there are about 70 percent
more rattlesnakes than gopher snakes on the BPNA. However, there
are two factors which bias this ratio. First, gopher snakes tend
to be scattered over a wide variety of habitat (not clumped in
areas of preferred habitat like rattlesnakes) and though they mav
not exist in high densities anywhere, their total numbers may be'
higher than expected. Second, gopher snakes are seldom recaptured.
Out of 12 gopher snakes that were captured near our summer camp
site and another seven that were accidentally released there, only
two were ever recaptured. For this reason, gopher snakes are not
amenable to a mark-release estimate of density.

Western Rattlesnake and Gopher Snake Life History Data

Data from 1975 and 1976 on size classes, age structure and
sex ratios are given in Table 4 and Figure 1. Since the snakes
are continually growing during the field season, those collected
late in the summer would tend to obscure distinct size classes and
increase the variance in the means for the size classes. In spite
of this problem there appear to be four identifiable size classes
of both male and female rattlesnakes. The first includes snakes
that had been born the previous September and would be close to
one year old. The second and third size classes consists of
snakes nearly two and three years old respectively. The adult
snakes include individuals that are four years old or older (possibly
as old as 15 to 20 years, Fitch, 1949). It is of interest to note
that there were more snakes found in each successive size class
for both males and females. It would be expected that the adult
age class be the largest since it includes many age cohorts in a
long-lived species. However, if there is any mortality at all and
the population is stable, there should be a decrease in the number
of snakes from the one to the three year old age classes. The
apparent explanation is that either younger age classes exhibit
different activity regimes, or they may be less conspicuous.

There are obvious size differences between male and female
snakes in the older age classes, with the males being significantly
larger. The newborn snakes show no significant size differences,
so apparently the males grow more rapidly. Also, there were
significantly more adult males found than females. This may
result from a higher mortality among adult females, but we believe
that the difference is due to the males being more aggressive and
conspicuous .

Sufficient numbers of young (one year old ) rattlesnakes were
not found to compare the growth race between newborn and young and
juveniles (two year olds). However, the mean increase in weight
between juvenile males and subadult (three year old) males is 168
grams with the females showing a slightly lower increase. Between
the subadult and adult males, there was a mean increase of 263
grams, while the females only showed an increase of 206 grams.

172

Table k. Western rattlesnake size classes, age structure and sex ratios.
Snout-vent lengths(mm)

Males

Females

n

X

SD

n

X

SD

newborn*

30

277.2

lit. 8

22

280.5

15. T

young (l yr old)

1

U60

—

6

502.5

9^.1

juveniles (2 yrs old)

12

TOT. 9

50.3

10

681.8

32.6

subadults (3 yrs old)

22

828.8

33.8

2k

T83.8

3T.6

adults (>k yrs old)

k6

971 . 5

U2.T

21

900. k

39-0

Body weights(g;

Males Females

n

X

SD

n

X

SD

newborn*

30

19.23

3.2li

22

19.19

3.1+8

young (l yr old)

1

50

—

<>

69.6

33.1*

juveniles (2 yrs old)

12

235.5

67.8

10

211.2

70.7

subadults (3 yrs old)

21

1+03.8

9I+.2

23

371.3

130.7

adults (>h yrs old)

k6

667.1

109. T

21

577-5

123.1+

*born in captivity

Ik

12
10

8
6
h

1+6 50 51* 53
Young

10

It

2

173

MALE

62 66 TO T'i
Juvenile

78 82 t

Subadult

_! I 1 I

90 9^ 96 102 106

Adult

Snout-Vent Length in Centimeters

FEMALE

50 5k 58
Young

62 66 TO
Juvenile

7h T8 82
Subadult

86 90 9^
Adult

Snout-Vent Length in Centimeters

102

Fig. 1. Western Rattlesnake size class histograms.

174

This would tend to indicate that the greatest difference in growth
rate between males and females occurs from the third to fourth
year. Growth after reaching maturity is minimal (Fitch, 1949;
Fitch and Glading, 1947).

As indicated in Figure 1, female rattlesnakes reach their
mature size in four years. However, two of the nine captive
gravid females were three year olds as indicated by their snout-
vent lengths. This would indicate either that some females reach
sexual maturity in three years or that some mature females are
stunted in growth. Nine captive females produced a mean clutch of
7.5 (range 4 to 10) with about 90 percent of the newborns surviving
long enough to be released on the study area. This would indicate
that the western rattlesnake in the BPNA has a low reproductive
rate, because along with few young per female, they also only bred
every other year (Klauber, 1972).

Size classes and age structure for gopher snakes are indicated
in Table 5 and Figure 2. Although it appears there is a slight
difference in size between males and females, with the males again
being larger, the difference is not great enough to separate the
two sexes in a size histogram. Once again four size classes can
be identified. The size class histogram for gopher snakes follows
a logical pattern with the exception that subadults outnumber
both young and juveniles. Again this difference can probably be
explained in terms of activity patterns or detectibility. It is
of interest to note that there were fewer adults found than sub-
adults, which might indicate that adult gopher snakes have a
higher mortality rate than that indicated for adult rattlesnakes.

Gopher snakes demonstrated a balanced sex ratio for each age
class with the exception of adult size class where males outnumber
females 3:1. This difference may be due to unequal mortality
rates, or merely differences in detectibility or activity patterns.

Growth rates as indicated by the mean body weights of the
different age classes (Table 5) gives a mean increase of 68, 121,
and 129 grams between each successive age class. The corresponding
values calculated for rattlesnakes are greater, although the
percentage increase in rattlesnakes is less. Few reproductive
data on gopher snakes are available as yet.

Snake Food Habits

Food habits for gopher and rattlesnakes are indicated in
Table 6. All the field collected scats were included under rattle-
snakes because they were collected in areas of high rattlesnake
density and their size and shape would indicate they were deposited
by rattlesnakes. A few of these scats may have been deposited by
large gopher snakes. It is of interest to note that 51 out of

Table S. Gopher snake size classes and age structure

young (l yr old)
juveniles (2 yrs old)
subadults (3 yrs old)
adults {>h yrs old)

Snout-vent lengths (mm) Body Weight (grams )
n x SD n x SD

13

386.5

39-9

13

18. T

5-7

12

676.7

72. U

11

86.5

38.9

37

89'k6

82.3

35

207.8

78.7

27

111^.7

5^.2

25

U36.U

76.O

Ik

12

J

_| l_

30 35 k0 1*5 50

1 1 1 !_

55 60 65 TO 75

i i i i i

30 85 90 95 100

_j i i i i

105 110 115 120 125 130

Snout-Vent Length in Cent ir-eters .

-4

Fig. 2. Gopher snake size class histogram.

177

Table 6. Snake food habits

Prey Item

Townsend ground
squirrel

Nuttall ' s cotton-
tail rabbit

Deer mouse

Western Harvest
Mouse

Great Basin
Pocket Mouse

House mouse

Antelope Ground
Squirrel

Muskrat

Whiptail lizard

Side-blotched
lizard '

Western Rattlesnake

Scat

Palp. Anal. Total

Gopher Snake

13

f38

51

Scat
Palp.. Anal. Total

3

1

6

1+

Grand
Total

'■'.

7
1+

2

1

1
1

Great Basin
spadefoot toad

Unidentified
bird egg

*includes 2h field collected scats

178

59 prey items identified for rattlesnakes were Townsend ground
squirrels. Also, with only one exception, adult rattlesnakes
exclusively took Townsend ground squirrels. In comparison, gopher
snakes took a variety of prey items with no clear prey selection
evident. The size of the ground squirrels (palpated from snakes)
ranged from 70 to 200 grams, while the cottontail rabbits ranged
from 115 to 190 grams. Snakes were found to ingest prey as large
as their own body weight, but most prey items were about half
their own body weight.

Snake Growth Efficiency and Prey Consumption

The growth efficiencies given in Table 7 are calculated in
terms of increase in snake weight in relation to whole weight of
the prey consumed. Although the values are still preliminary, it
is possible to calculate the estimated prey consumption for each
age class of gopher and rattlesnakes that was discussed earlier
(Table 4 and 5). Thus between their second and third year rattle-
snakes would consume a mean of 840 grams of prey. Between their
third and fourth year, male rattlesnakes would consume 1,315
grams, while females would consume 1,030 grams. Gopher snakes
would consume a mean of 400, 712, and 759 grams of prey between
their second, third, and fourth years respectively. Although they
are preliminary these data compare favorably with the scanty data
in the literature where estimates of annual consumption rates
range from two times the snake's weight (Fitch, 1948) to 4.5 times
the snake's weight (Klauber, 1972).

LITERATURE CITED

Fitch, H.S. 1948. Ecology of the California ground squirrel
on grazing lands. Am. Midi. Nat. 39(3) :513-596.

. 1949. Study of snake populations in Central

California. Am. Midi. Nat. 41(3) :513-579.

Fitch, H.S., and B. Glading. 1947. A field study of a
rattlesnake population. Calif. Fish and Game.
33(2) :103-123.

Klauber, L.M. 1972. Rattlesnakes: their habits, life history
and influence on mankind. 2nd' Ed. Univ. of Calif. Press.
2 Volumes.

179

Table 7. Consumption rates and growth efficiencies of captive snakes.

Species

Feeding

Period

(days)

Body
Start

Weight (g)
Finish

Prey

Consumed

(g)

f

wt(g)/
day

'on sump,
g/day Growth

Efficiency

Gopher snake 51

52

71

^3

0.13

0.8U

.21

97

122

175

20U

0.55

2.10

.26

kk

21U

253

107

0.39

2.U3

.36

106

1U5

168

225

0.22

2.12

.10

65

15I*

181*

260

0.^6

U . 00

.12

100

190

199

211*

0.09

2.1k

.0k

137

319

3U6

205

0.19

1.50
Me

.13

an

.17

Western
Rattlesnake

117
105

151+
21k

220
216

273
9h

O.56
0.02

2.33
0.90

.23
.02

9 ;

170

172

85

0.02

0.91

.02

JO

100

126

85

0.87

2.83

■ 31

32

208

232

59

0.75

1.31+

.1*1

Me

an

.20

180

-W-;

-33**l|j(P

jyiMi:

Mwi

'Jj^lpilpw*1^'

■«m

John Messick, Study IV-B, records movements and activity patterns
of one of the radio-collared badgers on the Study Area.
(BLM photo by Dunstan)

181

STUDY IV-B:

Dynamics of predation upon a raptor prey base
in the Snake River Birds of Prey Natural Area,
Idaho.

CONTRACTOR:

INVESTIGATORS :

PROJECT SUPPORT:

Idaho Cooperative Wildlife Research Unit,
University of Idaho, Moscow, Idaho 83843.

Maurice G. Hornocker, Principal Investigator.
John P. Messick, Research Associate.
Jackson S. Whitman, Randall M. Olson, and
Graham W. Smith, Research Aids.

U.S. Department of the Interior, Bureau of Land
Management, Contract No. 52500-CT5-1004.

OBJECTIVES:

2.

Attempt to ascertain the density, sex and age structure,
and other population parameters of badgers (Taxidea taxus)
on the Snake River Birds of Prey Natural Area, Idaho.

Explore the strategy of habitat use and describe the
social organization of the badger population.

3. Gather information on the food habits of badgers as an aid
to estimating the impact of this predator.

4. Collect information on the food habits of other important
carnivores, coyotes (Canis latrans) and bobcats (Lynx
rufus) .

ANNUAL SUMMARY

By the end of 1976, the second year of this project, 8,023
trap days had produced 234 captures and recaptures representing
154 different badgers. Eighty-one were males and 73 were females.
Fifty-two were identified as young-of-the-year , while 22 of 47
adult females were pregnant, lactating, or showed evidence of
recent pregnancy. Of five females captured in both 1975 and 1976,
two were probably not productive either year, three were productive
in only one of the years, and one animal appeared to have raised
young during both 1975 and 1976. Part of all of seven different
family groups were captured and recaptured. Captures were made in
an area of 35 km2, but were concentrated in 13 km2. This suggests
a density of over four per km2. Young moved as far as 30 km from
their natal area while adults confined movements to areas usually
less than 1 km2. Radio-tracking of 27 different badgers for
periods of up to 22 weeks revealed that badgers often returned to
previously used dens and decreased their activity in November and
December. Townsend ground squirrels (Spermophilus townsendi)
occurred in 191 of 231 scats and were more important than smaller

132

rodents, reptiles, and various insects. Analysis of 46 coyote
scats revealed jackrabbits (Lepus calif ornicus) and Townsend
ground squirrels to be the most important foods for this species.
Carcass collections for reproductive, age, and food habits data
now total 115 badgers, 73 coyotes, and five bobcats. We have
collected 289 badger, 173 coyote, and five bobcat scats. Field
work will continue for nine additional months.

RESULTS

This report marks the end of the second year of field work on
this project. Our effort has been directed toward the American
badger because of the animal's importance as a predator and apparently
high population density. The badger is the major competitor with
the prairie falcon (Falco mexicanus) in predation on Townsend
ground squirrels. Furthermore, research on the American badger is
meager compared to its European counterpart and other carnivores
of similar size and distribution.

Work on coyotes and bobcats has largely been limited to
qualitative food habits, although we will increase attention to
coyotes during 1977, utilizing extensive methods and survey
techniques to estimate population trends.

Our methods were reported in the Annual Report for 1975
(Messick et al. 1975). Much data remains to be analyzed, so this
report includes only part of the information which will be available
later. Field work will continue through September 1977.

Badger Segment

Population structure and density. In Tables 1 and 2, I
summarize the results of our capture and marking efforts for the
past year and also for 1975. Thus far, 8,023 trap days have
produced 234 captures and recaptures representing 154 different
badgers. Trapping was supplemented by spotlighting, daytime
netting, and various other methods. We have frequently used more
than one trap at a set, so the ratio of trap days to captures
actually underestimates trapping success, especially during 1976
(Table 2).

Essentially all animals marked and released in this project

have been taken in an area of less than 35 km2 (Fig. 1), but

approximately 80 percent of the capture effort and captures have

occurred in a much smaller area of 13 km2 . One hundred fifty-four

2 9

captures xn 35 km suggests four marked animals in each kmz . If

we use 123 captures (i.e., 80 percent of 154) in 13 km2, the

density is over nine per km2. Such elementary arithmetic ignores

births, deaths, emigration and immigration, but does give a rough

estimate of badger numbers in the intensive study area. The

population appears to be stable, so the annual population increase

Year

Table 1. Numbers and classification of badgers captured by various methods during
1975 and 1976, on and near the Snake River Birds of Prey Natural Area, Idaho.

Adults Young-of-

the-year luwlb

$f's 99' s (5&*'s 991 s Adults YOY d8% 991 s I

CD

1/1

4- c/>

>

QJ

CD

i— to

</)

S_

c/1 S.

■*-> -

CD

=5

CD 3

S8t

-M

CX

3 Q_

+J

CO

-M CD

o

Q_

u

CL O

tO

CD

rO O)

CJ

d;

C_) S-

1975 35 28 6 6 63 12 41 34

75 24 99

19?6 20 19 20 20 39 40 40 39 14 79 56 135
Total 55 47 26 26 102 52 81 73 22 154 80 234

OO

184

Table 2. Method of capture for badgers during 1975 and 1976
on and near the Snake River Birds of Prey Natural Area, Idaho.

Other

Year Trap Trap days Spotlight K]?! (flooding,

netting dens, etc.)

1975

68

2,793

18

11

1976 106

5.230

15

Total 174

8,023

27

26

NOTE: One hundred fifty-four badgers captured 234 times.

135

G Melba

N

A

Fig. 1. Intensive study area for badger population studies,
on and adjacent to the Snake River Birds of Prey
Natural Area, Idaho.

136

created by young is probably balanced by higher mortality in this
age class. Similar calculations considering only the adult,
resident segment of 102 marked animals will yield values of two
and six badgers per km^ for the larger and smaller areas, respectively.

Few estimates of badger densities exist, but Lindzey (1971)
thought that Curlew Valley, Utah and Idaho, might support in
excess of one badger per mi (one per 0.4 km ) . The intensive
study site (Fig. 1) has more badgers per unit area than nearly any
other area on, or adjacent to, the Snake River Birds of Prey
Natural Area. Indeed, our observations suggest quite low badger
densities on the south side of the Snake River and in certain
other areas. We plan to use counts of badger digs as an index to
relative numbers on these sites.

The population is composed of similar numbers of males and
females (Table 1). This is consistent with Crowe's data (Crowe
1975). He obtained carcasses from hunters, trappers, and fur
buyers in Wyoming.

Lindzey (1971) had 22 males and 29 females in his carcass
collection. Eighteen, or 35 percent, were young-of-the-year . Our
results show that young-of-the-year contribute 34 percent to our
marked population. After tooth sections are completed and inter-
preted, the age structure given in Table 1 should be more detailed
and perhaps the ratio of young to adults will change slightly.

Reproduction and dispersal of young. One-half of the mature
females were judged productive, that is, they were pregnant,
lactating, or showed evidence of recent pregnancy (Table 1). Of
five females captured in both 1975 and 1976, two were probably not
productive either year, three were productive in only one of the
years, and one animal appeared to have raised young during both
1975 and 1976.

During 1976, part or all of seven different family groups
(female with young) were captured and recaptured. Females No. 19
and No. 98 (both with young) were radio-tracked during the maternal
period. Additional females (Nos. 21 and 76) were radio-tracked
during the spring of 1975 and 1976, but we did not capture their
young if they raised families (Table 3) .

Litter size was usually two or three, and births occurred in
March and April. We are uncertain of exact parturition dates.
The families we monitored utilized a series of maternal dens. For
example, No. 19 utilized only one den from 14 March through 8
April 1976. She emerged, presumably to hunt, approximately every
other day, although she frequently was away from this den. She
never spent more than two days underground without emerging. Most
above-ground activity occurred at night. Later in April and May,
No. 19 and her young utilized other dens for spans of four to
seven days.

'"» ■ ■ >'''-'—**mmmmmmim^i~i^mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm

Table 3. Summary of badgers radiotracked during 1975 and 1976 on and near the Snake River Birds of Prey Natural

Area, Idaho.

0) 41 d

u 4-> r. -a cq o> a>

ni «Q -h nj cu 3 t3 cq jj

ai a) roa) co a> • <d ■ 3 • :3

>h 4J u DVjO>njoa>oo

i C a) t3 TdTStfl C-h CMdCxla)

i-i m-i a) -Ma) cocdoj en w -u M cu-uo

oo S ts i-i £*, M *J (B S CBSOgM-ixlE-HC w

I 3 -ao njocdg a) a) 3 u m 3 o 3S<u Jd

J-> M M 4-) 13-Mgo xi T3B0J E ctj B M M

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5 M A 09 Mar 75 04 Jun 75 70 1.0 25 12 12 12 — Radio recovered

8 F A IS Mar 75 05 Jun 75 74 0.3 24 12 16 23 — Radio recovered both
23 Jun 76 16 Jul 76 18 0.6 11 2 2 2 1 years. With young in

75? Used same area
both years.

6 — 5 12 2 2 Radio failed both

6 2 2 2 2 2 years; recovered in 75

5 10 5 — 5 Died; radio recovered

4 4 0 1 — 1 Radio failed

12 015 13 1 1 2 1 Radio recovered

54 0.7 44 7 3 6 3 Radio failed in 75.
Data included with 1975 Radio recovered in 76

11 2 2 5 5 5 Radio recovered

1 1 — — — — Radio failed

1 1 — — — — Radio failed but

recovered

53 2.6 14 7 32 32 12 Radio recovered.

With young?

65 0.6 12 9 14 38 2 Radio recovered.

With young

7

M

A

12

Mar

75

17

Mar

75

08

Apr

75

26

Apr

75

15

F

A

01

Jun

75

05

Jun

75

3

F

A

19

Jun

75

21

Jun

75

0

V

A

03

Aug

75

14

Aug

75

20

M

A

31

Aug

75

17

Nov

75

18

Jul

76

20

Jul

76

75

F

A

08

Nov

75

19

Nov

75

73

A

A

26

Nov

75

26

Nov

75

65

F

A

16

Feb

75

16

Feb

75

76

F

A

27

Feb

76

12

May

76

19

F

A

06

Mar

76

09

May

76

Tabl

e 3

(Continued)

98

F

A

11

May

76

10

Jun

76

37

22

Jul

76

06

Aug

76

Dat;

21

F

A

13

May

76

20

Jul

76

26

42

F

A

22

May

76

29

May

76

5

116

M

A

09

Jun

76

05

Nov

76

78

37

M

YOY

24

Jun

76

01

Sep

76

20

128

F

YOY

02

Jul

76

14

Jul

76

5

138

"'7

YOY

17

Jul

76

18

Jul

76

1

148

F

YOY

05

Sep

76

18

Sep

76

10

141

M

YOY

10

Sep

76

08

Oct

76

21

150

M

A

20

Sep

76

25

Sep

76

5

152

F

A

13

Oct

76

67+

110

M

YOY

24

Oct

76

09

Nov

76

15

154

M

YOY

11

Nov

76

22

Nov

76

5

139

M

YOY

16

Nov

76

17

Dec

76

24

156

M

YOY

25

Nov

76

28

Nov

76

1

2

2

2

2

1

50

14

9

9

7

16

2

2

2

1

?

1

'J

2

2

0.5 10 5 7 16 6 Died. Radio
Data included in above. recovered. With young

0.6 10 2 10 14 9 Radio recovered.

With young?

Radio recovered

Radio failed.

Radio recovered

Radio recovered

Lost contact

Lost contact

1.2 4 4 3 Collar recovered

Radio failed?

Still operating

b d 'j Collar recovered

Lost contact

2.1 16 5 2 2 Shot by hunter.

Radio recovered

Lost contact

o

0

1

—

—

12

4

4

4

3

1

i

4

4

3

24

13

30+

30+

304

8

3

5

3

4

0

1

—

16

5

2

2

2

CO

00

189

Females may move their young from den to den. On 22 May
1976, G. Smith observed an unmarked female carrying her male
offspring (No, 109) by the scruff of the neck. This young badger
weighed 2 . 3 kg and was capable of considerable mobility on his
own.

Badgers weighing approximately 2 kg (probably some six weeks
old) may venture from the den. As time progresses, the mother
spends more time away from the litter. Litters seem to remain
together. Radio-tracking information from No. 98 during mid-May
revealed that she. was seldom more than a few hundred meters away
from her young.

Females may carry food to the natal den. Dead Townsend
ground squirrels (Spermophilus townsendi) were found at two natal
dens by R. Olson and J. Messick on 28 April and 31 May. Messick
observed a family group playing "cat and mouse" with a dead ground
squirrel on 25 May 1975.

While most adult badgers return to previously used dens, the
pattern of den use and activity established by radio-equipped
females Nos. 19 and 98 is unique to productive females.

Individual adult badgers captured on more than one occasion
show constant body weights, although size in this species is quite
variable. Adult females range from approximately 5.5 to 9.5 kg
while males weigh 2 to 5 kg more. Lactating females often loose
more than 0.5 kg while raising their litters.

Weight and weight gains of young-of-the-year animals are
highly variable (Fig. 2), and weight is probably not a reliable
indication of age. Our recapture data suggest that young dispersing
from their home area may have difficulty maintaining their body
weight (Fig. 2).

Family groups break up in late May or June, and this coincides
with the onset of the breeding season. Most young weigh 4 to 6 kg
at this point. Young badgers frequently show wounds obviously
made by teeth. This suggests that adults may encourage young to
leave their natal area.

Wright (1966) suggests that female badgers may sometimes
breed during their first summer. While it is not reflected in the
raw data (Fig. 2), my impression is that females reach their adult
weights faster than males, perhaps supporting Wright's contention
that females mature faster than males.

Distances moved by young badgers are greater than those of
adults. Three young males were killed after leaving their natal
area. No. 88 was first trapped at his maternal den on 5 May and
weighed 1.6 kg. In early July, a local farmer killed this badger

O

Fig. 2. Body weights and weight gains of young-of-the-year badgers captured or collected

during 1975 and 1976, on or near the Snake River Birds of Prey Natural Area, Idaho,
Data were from both marked animals and carcasses. Solid lines connect weights for
individuals captured more than once in a given year.

191

after the animal was discovered undermining an irrigation ditch.
No. 88 had moved nearly 12.8 km (straight-line).

His litter mate, No. 89, weighed 1.8 kg at the maternal den
on 1 May. We recaptured him on 3 August, 3.2 km from the initial
capture site. He weighed 6.3 kg. A trapper killed this badger on
22 December, 4.4 km from the natal den. His carcass, minus the
pelt, weighed 8.3 kg. Directions moved by Nos. 88 and 89 differed
by 90°.

Badger No. 108 weighed 2 . 3 kg when G. Smith captured him on
22 May. This animal was found dead in the road in early July more
than 30 km (straight-line) from the original capture location.

This summer and fall, we fitted nine young-of-the-year badgers
with radio transmitters designed to expand with growth (Table 3) .
In general, young animals move erratically and cover several times
the area of an adult's range. Most adults confine movement to 1
km2 (Table 3). The movement pattern of adult male No. 20 (Fig.
3) is typical of non-lactating adult badgers. Contrast this with
the movement of male young-of-the-year No. 87 (Fig. 4) .

Number 87 weighed 3.2 kg when flooded from his natal den on
29 April. J. whitman trapped this animal on 22 June a few hundred
meters from the initial capture site. His weight had increased to
8.3 kg and he was fitted with an expandable radio collar. We
monitored No. 87 intermittently until he lost his collar on 1
September. On 3 August this badger was recaptured, and inspection
revealed that the collar was causing absolutely no irritation,
although his weight had decreased to 6.9 kg.

The relatively great distances moved by No. 87 and other
young-of-the-year made it difficult for us to regularly locate
them. To find missing animals, we resorted to radio-tracking from
a Cessna 172. We appreciate the help of T. Dunstan, J. Harper,
and Gem State Aviation in this venture.

The behavior of certain young-of-the-year badgers contrasts
sharply with that of adults. It is uncommon to encounter adult
radio-equipped badgers above ground more than perhaps 10 percent
of the time. Number 148, a young female, was radio-tracked during
September and was often located above ground, even during inclement
weather and in daylight. Furthermore, this badger seemed fearless
and careless compared with other individuals.

We do not know when young badgers and their dispersal and
settle into a home area. The movement and activity of males Nos.
139 and 141 during the fall seemed more like that of adults, so it
may be that dispersal ends for some young after their first summer.

Movement and activity of adult badgers. Distances between
capture and recapture sites was seldom greater than 1.5 km, and we

192

N

o radio fix

&/or den

recapture
28 Aug

e capture
17 Jun

Fig. 3. Movement of adult male badger No. 20, as determined

by trapping and radio-tracking, from 31 August through
11 October 1975.

193

,29Jun

radio fix
&/or den

• capture site

with date

1 km

N

A

5 Aug

radio-^
lost

S Sei

Fig. 4. Movement of male young-of-the-year badger No. 87, as

determined by trapping and radio-tracking, from 29 April
through 1 September 1976.

194

have trapped representatives of all age and sex classes within a
few meters of each other, and in time spans of a few hours. Adult
males have even been trapped at active maternal dens. Unrelated
adult female No. 155 and young-of-the-year male No. 154 occupied
the same den on 22 November 1976.

During 1975 and 1976, 27 badgers (12 males and 15 females)
were equipped with radio transmitters (Table 3) . Radios operated
an average of 5.3 weeks with extremes from a few hours to 22
weeks. In Table 3, I present information on the number of burrows
used and reused, maximum time underground, etc. Home areas were
subjectively estimated for badgers for which we have ample data.

The configuration and movement pattern for adults (Fig. 3) is
fairly consistent and has been reported in detail in last year's
Annual Report (Messick et al. 1975). Home areas of the same and
opposite sexes may overlap. Badgers are more active at dawn,
dusk, and during the night, but they are certainly not exclusively
nocturnal.

Estimated home areas of badgers in this study (Table 3) are
smaller than data reported by other investigators. A female
radio-tracked in Minnesota for 165 days moved over 2,100 acres
(8.4 knn), with more restricted activity during the fall and
winter (Sargeant and Warner 1972) . Home ranges of two males
monitored in Curlew Valley, Utah and Idaho, averaged 1,440 acres
(5.8 km^) , more than twice the size of four females utilizing the
same area (Lindzey 1971) .

Testes size and condition of females suggest that badgers in
this area breed in late May and through June. J. Whitman observed
copulation at 1520 on 22 June 1976. Badgers are more active and
mobile, especially in the daytime, during the breeding season.
Scent glands become obvious, but we do not know if badgers mark
with scent, urine, or feces. We do find more scats on the surface
during this period.

Fighting does occur (observation by G. Smith on 28 June
1976), but we do not know if it regularly happens between males,
or is part of copulatory activity. A radio collar lost by female
No. 8 on 16 July 1977 (Table 3) was marked by teeth, and we believe
that a male was in the vicinity of No. 8 when she lost her radio.

My current impression is that badgers have a promiscuous
mating system.

Badgers are less active during periods of cold weather,
although it is not uncommon for badgers to be above ground in
blizzards with temperatures below -10°C. J. Messick found fresh
badger sign on 9 January 1977 when the temperature was below
-22°C. However, radio-tracking data from adult female No. 152

195

indicate that some badgers will remain mostly inactive during the
winter. Number 152 has used one burrow continuously since 3 December
1976 with only one recorded emergence (Table 3). In addition, No.
116 (adult male) began remaining underground for longer periods
during October and November. Observations of dens known to be
occupied by badgers also indicate longer stays underground during
the winter than in other seasons.

Food, habits, feeding studies and predation. The Townsend
ground squirrel is by far the most important item in the badger's
diet (Table 4). Seasonal variation in the utilization of ground
squirrels does occur, but Table 5 and Fig. 5 demonstrate that
badgers are effective predators on hibernating ground squirrels.
Scats analyzed for this study were obtained from carcasses collected
outside the study area, but in similar habitats, or from badgers
confined for 24 or more hours after capture. Some were picked up
near dens, etc., but these usually appeared no older than several
days, so the chronology in Table 5 and Fig. 5 should be reasonably
accurate.

Badgers consumed a wide variety of other foods (Tables 4 and
5), but the caloric contribution of alternate prey is minor compared
to that of Townsend ground squirrels. Badgers seem to be opportunistic
in selecting prey.

Scats and stomachs yield only qualitative food habits information.
Quantitative consumption rates of free-roaming carnivores that
leave no detectable prey remains has not been obtained. Feeding
studies under caged conditions can help overcome this void, but it
is still necessary to extrapolate from the maintenance diet to
account for the animal's activity under wild conditions. Jense
(1968) determined that slightly more than 70 g of digestible
ground squirrel (Citellus sp.) were required to meet the maintenance
requirements of an 8.25 kg female badger. Since he found that
ground squirrels contained over 60 percent water and were 90
percent digestible, the 70 g would have to be converted for comparisons
with live prey.

We have initiated feeding studies in two, 3 by 4.5 m pens.
These pens allow limited movement and digging because each contains
1.5 m of soil. Number 155, an adult female, was placed in one of
these cages in early December. This animal consumed 2,500 g of
mostly Townsend ground squirrels during one month. In an earlier
trial, No. 8, also an adult female, used 3,086 g of assorted rodents
and reptiles in 15 days.

Data from these and other feeding studies, coupled with our
radio-tracking information on home range use and activity, should
allow us to roughly estimate the biomass of ground squirrels removed
by badgers on the Snake River Birds of Prey Natural Area.

196

Table 4. Items encountered in 231 badger scats collected on
or near the Snake River Birds of Prey Natural Area, Idaho.

Classification No. scats

Mammals

Aves

Townsend ground squirrel (Spermophilus townsendi ) 191

Blacktail jackrabbit (Lepus cal ifornicus) 10

Kangaroo rat (Dipodomys sp.) 12

Deer mouse (Peromyscus maniculatus) 18

Other mammals (identified but not listed here) 10

Unknown mammals 9

All mammals 250

Unknown bird 1

Unknown bird eggs 2

All aves 3

Reptiles

Great Basin gopher snake (Pituophis melanoleucus) 7

Collared lizard (Crotaphytus collaris) 4

Whiptail lizard (Cnemidophorus tigris) 3

Side-blotched lizard (Uta stansburiana) 1

Unknown reptiles 6

Reptile eggs 1 1

All reptiles 32

Insects and Arachnids

Scorpions 72

Beetles 55

Grasshoppers 22

Other insects and spiders 3

All insects and arachnids 152

197

Table 5. Monthly summary of items encountered in 231 badger
scats collected during 1975 and 1976 on or near the Snake
River Birds of Prey Natural Area, Idaho.. Individual species
within each taxonomic grouping d.re given in Table 4.

-a

to

r—

-1 —

(13 CO

■p

ooho

T3

CO

-a

rO

</i

cy>

SZ

C

CD

CO -i-

U

, —

! ■

+->

o>

oj

1 —

+J c

jr

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2

n3

Q.

CO

CO

■r—

o x:

-p

E

E

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c

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CD o

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E

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3

a>

CL

CO rd

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o

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cu

c: s-

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-z.

<c e

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CD

+->

oo|+->

<=c

a:

i—i ro

Jan

0

--

--

--

—

--

--

Feb

6

6

3

3

0

0

0

Mar

7

6

3

3

0

0

0

Apr

6

5

2

3

0

0

0

May

23

24

8

16

0

2

5

Jun

47

48

n

37

0

7

18

Jul

99

94

17

77

1

9

39

Aug

17

19

7

12

0

3

18

Sep

14

9

2

7

1

0

18

Oct

4

5

2

3

0

0

6

Nov

5

5

1

4

0

0

3

Dec

3

1

0

1

0

1

0

19!

20 %

No Samples

M

m

23

47

99

17

14

l;

Fig. 5. Monthly summary of the percentage of badger scats containing
Townsend ground squirrels (Spermophilus townsendi) . The
total number of scats analyzed for each month appears to
the right of the bars.

199

With Townsend ground squirrels so common in the diet of
badgers, we wonder if badger predation influences the density of
this prey. We considered devising a predator or prey removal
experiment to answer this question. However, it does not seem
worthwhile to approach the problem in this way for several reasons.
Perhaps central to this argument is that Townsend ground squirrel
populations are probably regulated by a mix of intrinsic and
extrinsic factors. Under the constraints of limited time and
manpower, any attempt to sort out the relative importance of avian
and coyote, versus badger predation would probably end in failure.
Complicating this are various intrinsic population control mechanisms
such as disease, dispersal, and physiological changes in natality.
Predation is a density dependent process, so that any results from
one area with a given population of badgers and squirrels might
not be valid for other habitats with different numbers of predators
and prey.

D. Johnson, G. Smith, and the other workers on Study V have
recorded badger hunting activity on their 1 ha grids. This information,
pooled with data on badger food habits, home range use, activity,
etc., may allow us to at least describe the qualitative aspects of
badger predation on ground squirrels. Furthermore, we might be
able to design a useful experiment for future testing which would
determine the effects badgers exert on populations of Townsend
ground squirrels. The basic variables of density, sex and age
structure, natality, and mortality we are delineating for the
badger population must be the initial step in any population
study. These data are essential before any hypotheses about
predation or other regulatory mechanisms can be tested.

Other activities: Monthly carcass collections now total 115
badgers. These reproductive tracts, stomach samples, and teeth
(for aging) have only been partially studied.

Blood samples are regularly taken from badgers and tested for
antibodies to plague (Yersinia pestis) . D. Johnson and G. Smith
report these results in their Annual Report for Study V.

J. Messick gave presentations on this project to elementary
and junior high students at Melba, Idaho, and to some of the
biology classes at Borah High School, Boise, Idaho.

R. Olson, J. Whitman, and J. Messick discussed this work with
several different groups associated with, or sponsored by the BLM.

In late September, J. Messick met with his research committee
at the University of British Columbia.

200

Other Predators

Coyote food habits: Scats were collected from carcasses, or
picked up along roads and trails on and near the Snake River Birds
of Prey Natural Area. These reveal that jackrabbits (Lepus
californicus) were most important in the diet of coyotes from a
biomass standpoint (Table 6). Townsend ground squirrels would only
be available during part of the year. Squirrels are probably
valuable to young coyotes and females with litters. Analysis of
the collection of coyote scats is incomplete, and most of the
samples in Table 5 were acquired from April to October.

Carcass collections: Monthly carcass collections, a rich
source of age, reproductive, and dietary information, now total 73
coyotes and five bobcats. We have also collected 173 coyote and
five bobcat scats. Analysis is only partially complete.

ACKNOWLEDGMENTS

We thank L. Guilliams, H. Fannon, K. Leavitt, J. Amos,
J. Gahl, J. and S. Cowger, L. Noe, S. Greenfield, the E.T. Evans
family, K. Brant, and many of the contractors and staff on the
Snake River Birds of Prey Research Project for help with carcass
collections, tag recoveries, etc.

LITERATURE CITED

Crowe, D.M. 1975. Population structure of some mammalian
predators in southeastern Wyoming. J. Wildl. Manage
39(2):449-450.

Jense, G.K. 1968. Food habits and energy utilization of badgers.
M.S. Thesis, South Dakota State Univ., Brookings. 39 pp.

Lindzey, F.G. 1971. Ecology of badgers in Curlew Valley, Utah
and Idaho with emphasis on movement and activity patterns.
M.S. Thesis, Utah State Univ., Logan. 50 pp.

Messick, J. P., M.G. Hornocker, I.T.McT. Cowan and J. Whitman.

1975. Dynamics of predation upon a raptor prey base in the
Snake River Birds of Prey Natural Area pages 136-154 In Snake
River Birds of Prey Res. Proj . Ann. Rep., 1975. U.S.- Dept.
of Interior, Bureau of Land Management, Boise, Idaho.
193 pp.

Sargeant, A.B. and D.W. Warner. 1972. Movements and denning
habits of a badger. J. Mammal. 53 (1) : 207-210.

Wright, P.L. 1966. Observations on the reproductive cycle of
the American badger (Taxidea taxus) . Comparative biology
of reproduction in mammals. Symposia, of the Zool. Soc.
London. 15:37-45.

201

Table 6. Items encountered in 46 coyote scats collected
during 1975 and 1976 on or near the Snake River Birds of
Prey Natural Area, Idaho.

Classification No. scats

Mammals

Blacktail jackrabbit (Lepus call fornicus) 21

Townsend ground squirrel [Spermopni lus" townsendi) 22

Whitefooted mice (Peromyscus sp.) 15

Kangaroo rat (Dipodomys sp.) 4

Mountain cottontail (Sylvilagus nuttali) 3

Woodrat (Neotoma sp.) 2

Townsend pocket gopher (Thomomys townsendi ) 1

American badger (Taxidea taxusj 1

Unknown vole (Microtus sp. ) 1

Unknown lagomorph 2

Domestic cow 1

All mammals 73

Aves

Raven (Corvus corax) 1

Unknown passerine 1

All aves 2

Reptiles

Unknown lizard 1

All reptiles 1

Insects and arachnids

Beetles 49

Grasshoppers 43

Scorpions 6

All insects and arachnids 98

Other

Various vegetation 24

202

r I

Graham Smith, Study V, records information from one of 472
Townsend ground squirrels trapped in 1976 (Photo by L. Oftedahl)

STUDY V:

CONTRACTOR:

INVESTIGATORS:

PROJECT SUPPORT:

OBJECTIVES:

203

Population ecology and habitat requirements
of Townsend ground squirrels.

Department of Biological Sciences, University
of Idaho, Moscow, Idaho 83843.

Donald R. Johnson, Principal Investigator.
Graham W. Smith, Research Associate.
Randall M. Olson, Research Aid.

U.S. Department of Interior, Bureau of Land
Management, Contract No. 52500-CT5-1002.

1. Describe population variables of ground squirrel
populations including productivity, recruitment, age
and sex ratios, and turnover rates.

2. Ascertain population density (a) in representative
habitat types, (b) in relation to land use patterns
including the effects of cultivation on density, and,
(c) in hunting areas of those raptors outlined by
the radio-telemetry study (Study III).

3. Ascertain food habits and habitat requirements in the
major vegetation types.

4. Isolate factors (predation, disease, parasites, etc.)
which may limit ground squirrel density and distribution.

ANNUAL SUMMARY

Populations of Townsend ground squirrels (Spermophilus
towns end i) were studied from January through July, 1976, the
second year of a four year study of squirrel ecology and habitat
requirements. Squirrels occurred at peak densities in vegetation
types disturbed by fire and heavy grazing where annual grasses had
replaced native vegetation. Densities of resident squirrels
(adults and yearlings) ranged from 8 to 20/ha. The overall sex
ratio was balanced but that of adults and yearlings favored
females, indicating a higher mortality in males. Over winter
survivorship of juvenile females exceeded that of the adults of
either sex. Litters at birth averaged 7.7 young and at emergence
5.2 young per female. All adult females bred, and at least 62
percent of the yearling females produced litters. Growth rates,
based on weight changes of free-living animals, were calculated.
Adult males entered torpor in late May and early June, adult
females throughout June and juveniles from mid-June to mid-July.
In early spring, squirrels utilized green forage, primarily the
leaves of cheatgrass. Forb seeds and the leaves of shadscale

204

and winterfat comprised most of the diet later in the activity
season. Six squirrels have been found positive for plague
(Yersinia pestis) within the study area. Badgers show elevated
titers of plague antibodies indicating intermittent exposure.
Prairie falcons, badgers and rattlesnakes exert a sustained
predation pressure on these ground squirrel populations, particu-
larly the young-of-the-year .

INTRODUCTION

The Townsend ground squirrel is the primary prey item of
prairie falcons (Ogden 1973), red-tailed hawks and ravens (Kochert
1973) which nest within the Birds of Prey Natural Area (BPNA) .
Ground squirrels comprise a minor portion of the diet of golden
eagles (Beecham 1970; Kochert 1972, 1973). Badgers, which occur
in large numbers within the BPNA, and rattlesnakes also prey
heavily on ground squirrels.

Recognizing the importance of the Townsend ground squirrel in
this ecosystem, the Bureau of Land Management has contracted a
four year study of its ecology.

STUDY AREA

Five intensive study sites were established in 1975, and two
additional sites were established in 1976. These sites are
representative of the vegetation types found within the BPNA (Fig.
1). Each site is 1 ha in area (100 x 100 m) with grid stakes at
10 m intervals. Tables la and lb summarize the plant coverage at
these sites for 1975 and 1976.

The intensive study sites are:

1. Bedspring Site. This site is used yearly as a sheep camp
during the winter grazing season and as a result is richly fertilized
with droppings. Its vegetation consists primarily of a dense lawn

of cheatgrass (Bromus tectorum) .

2. Eurotia Site. This site supports a stand of winterfat
(Eurotia lanata) with an annual fescue (Festuca octoflora) and
cheatgrass understory.

3. Burn Site. This site was burned during a range fire

24 June 1974. Its vegetation consists primarily of cheatgrass and
Sandberg bluegrass (Poa sandbergii) . Before the burn this site
supported a vegetation similar to the Comparison Site.

4. Comparison Site. This site is dominated by big sage-
brush (Artemisia tridentata) , hop sage (Atriplex spinosa) and
winterfat. Sandberg bluegrass occurs as an understory.

205

Intensive Study Sites

Fig. 1. Location of intensive sites in the Snake River Birds
of Prey Study Area.

Table la. Plant coverage [%) , May, 1975 and 1976, on intensive sites, Taxa with coverage less than
1% excluded.

Winterfat

Healthy

1975 1976
(Eurotia)

Shrubs

Eurotia lanata

Artemisia tridentata
Atriplex spinosa

lit. 5

12.2

Depleted
1975

1976
Bedspring)

<1

1.3

Shrub-grass

Unburned

1975 1976
(Comparison)

9.8

8,5

5-i

U.3

2.8

l.l

Burned

1975 1976
( Burn )

<1

<1

Forbs

w

Cruciferae

<1

<l

2.0

<1

1.1

<1

<1

o

Grasses

Bromus tectorum

1.5

<1

60.2

66.3

<1

<1

U.O

6.1

Poa sandbergii

3-2

k.O

It. 6

1.7

3. ^

3. it

5.8

8.9

Festuca octoflora

20.3

l.h

<1

Bare ground

6i.i

76.2

35-2

30.0

79. 8b

82.5

89.1*

8U. 5

Burned 24 June 197^

Corrects error in 1975 annual report

207

Table lb. Plant coverage (<), May, 1975 and 1976, on the intensive
sites. Taxa with coverage less than 1% excluded.

Site

Hole

Count

Artemisia
1976

Shadscale

1975

1976

1976

Shrubs

Artemisia tridentata

k.l

Artemisia spinescens

2.5

Atriplex confertifolia

3.5

Tetradvmia elabrata.

l.U

Forbs

Lomatium sp.

1.5

Astragalus sp.

1.1

Grasses

Bromus tectorum

11.5

19.8

22.0

2.0

Poa sandbereii

<1

<1

7.1

Bare Ground

87. k

77-9

67.0

95-0

:08

5. Artemisia Site. The vegetation at this site consists of

a stand of big sagebrush with a cheatgrass and bluegrass understory.

6. Shadscale Site. Shadscale (Artiplex conf ertifolia) and
bud sage (Artemisia spinescens) are the dominant shrubs at this
site located in Section 36, Township 4 S. and Range 3 E. (Boise
Meridian). Much of the site is bare ground.

7. Hole-Count Site. This site was intensely burned during
the 1974 range fire and now supports cheatgrass.

METHODS

Field work began when ground squirrels emerged in January and
continued until most squirrels had entered torpor in July.

Squirrels were live-trapped on all intensive sites except the
hole-count area. A minimum of 20 live traps at each site were set
in the morning and checked periodically throughout the day. Apple
was used as bait. Trapped squirrels were anesthetized with ether
and ear-tagged using self-piercing monel tags (National Band and
Trap Co., Newport, KY) . Squirrels were permanently marked by toe-
clipping. Capture location, body weight and reproductive condition
(testes position or nipple visibility) were recorded. Litter
counts were made soon after young squirrels appeared above ground.
The locations of maternal burrows were marked with stakes. Density
was determined by recording the number of home burrows occupied by
adults and yearlings (suspected residents) . Home burrow locations
were identified from trapping results.

The squirrels visible within a 360° sweep were counted
periodically from May through July at index points. These points
were established at locations hunted by birds of prey as determined
by Tom Dunstan (Study III). Plant coverage within these raptor
hunting areas is summarized in Table 2.

Squirrels were collected using a .22 caliber rifle. Their
reproductive tracts were examined, and their stomachs were removed
for food habits analysis. Stomach contents were examined micro-
scopically using the techniques described by Johnson and Groepper
(1970).

Plant coverage was estimated on 0.25 m circular plots on
intensive sites (minimum 66 plots) and index points (minimum 20
plots). Coverage of mosses, litter, and rocks was included in the
bare ground estimates. Taxa with less than one percent coverage
were ignored. Total coverage often exceeded 100 percent because
of stratification.

Table 2. Plant coverage (%) at index points. Taxa with coverage less than 1% omitted.

Grasses

Bromus tectorum

Poa sandbergii
Sitanion hystrix

Forbs

Salsola kali

Cruci ferae

Ranunculus testiculatus

Shrubs

Artemisia spinescens

Artemisia trident at a

Atriplex confertifolia

Atriplex spinosa

Eurotia lanata

Bare Ground

HLP(l) HLP Cairn Sna.ke Rock Dry Coyote Trio Pole
#1 #2 Ridge Outcrop Triang. Butte(2) Butte 233

21.0

15-7

16. h

k.h

3.2

2.2

12.7

21.2

U0.8

36.9

U.l

71.3

2.3

3.1

65-9

,l) Halverson Lake Pair (Prairie Falcons)
.2) Mean of Eurotia and Artemisia habitats

2.6
2.1

57-7 55.0

3.1

36.5

56.7 5^.3

1.0
20.7

19.9
1.9

5-3

3.0

Cinder Pole
Road 36l

3-5

5-7
2.5

10.6

h.5

to

o

1X1

7.2

2.0

6.2

2.7

15.6

2.5

6U.8

73.6

75-8

79. h

85.9

210

Counts of fresh badger digs on the intensive sites were
recorded monthly.

Spleens and livers of moribund and dead squirrels found on
the study area, together with blood samples from badgers collected
by John Messick (Study IVB) , were analyzed by the Public Health
Service, Center for Disease Control, Fort Collins, Colorado, for
the presence of plague organisms.

RESULTS AND DISCUSSION
Population Parameters

A total of 472 ground squirrels were live-trapped, marked and
released on the intensive study sites in 1976 (Table 3). The
Shadscale Site was not trapped early enough in the season to
accurately determine its resident population.

The effect of habitat alteration on squirrel density can be
evaluated by comparing the number of residents (adults and year-
lings) captured at the Eurotia Site (N = 41) with that on the
heavily grazed Bedspring Site (N = 45) and that of the unburned
Comparison Site (N = 25) with that of the Burn Site (N = 32).
These represent 10 and 28 percent increases respectively on the
disturbed sites.

The overall sex ratio was balanced, but that of adults and
yearlings favored females (57 percent), an indication of higher
mortality among juvenile males. Juveniles composed 66 percent,
adult and yearling females 19 percent and adult and yearling males
15 percent of the squirrels captured.

The total number of adult and yearling squirrels trapped on
four intensive study sites in 1975 and 1976 was similar indicating
a stable population of residents. In 1975, 61 resident males and
81 resident females were trapped, a total of 142 squirrels. At
these same sites in 1976, 63 resident males and 81 resident
females were captured, a total of 144 squirrels. The increased
catch in 1976 (N = 472) over that of 1975 (N = 246) results from
an intensive trapping effort to capture juvenile squirrels prior
to their disappearance through death and dispersal rather than a
true population increase.

Densities varied throughout the active season because of
dispersal, recruitment and mortality. Those reported in Table 4
represent the spring population of resident animals before emergence
of the young.

Age-specific survivorship from 1975 to 1976 differed somewhat
between sites (Table 5). That of juvenile to yearling females

211

Table 3. Sex

and age of

ground

squirrels

captured on

study sit

es in

1975 and 1976

Percent in parentheses.

Site

adults /yearlings
1975 1976

juveniles

total

1975

1976

1975

1976

Bedspring

male

28

20

30

50

58

70

female

U3

25

9

56

52

81

Eurotia

male

13

21

12

2k

25

^5

female

19

20

6

2k

25

1+1+

Burn

male

9

10

18

35

27

k5

female

7

22

2k

29

31

51

Comparison

male

11

12

2

18

13

30

female

12

13

3

13

15

26

Artemisia

male

-

IT

-

2k

-

29

female

-

10

-

23

-

33

Shadscale

male

-

1

-

11

-

12

female

—

0

-

6

-

6

Total

male

61(43)

69(^3)

62(60)

162(52)

123(50)

23l(!i9)

female

81(57)

90(57)

1+2(^0)

151(1*8)

123(50)

21+1(51)

lit 2

159(3*0

id

313(66)

2^6

1+72

1

212

Table h. Estimated population density (n/ha) of adult and yearling ground
squirrels on the intensive study sites.

Site

Bedspring
Eurotia
Burn
Comparison

Artemisia

Male

k-6
k-9
3-fc

2-3

Female

10-14

7-9

8-9
5-7
6-8

Total

lU-20

11-18

11-13

8-ll

8-11

Table 5. Age-specific survivorship from 1975 to 1976. Percent in parentheses,

Site

Male

Female

Adult

Juvenile to Adult
Yearling

Juvenile to
Yearling

Bedspring k of 27 (15) k of 32 (13) 12 of 43 (28) h of 9 (hh)

Eurotia 7 of 19 (37) *i of 6 (67) 6 of 13 (46) U of 12 (33)

Burn 4 of 9 W) 3 of 18 (17) k of 8 (50) 11 of 23 (48)

Comparison 6 of 13 (U6) 2 of 6 (33) h of 11 (36) 2 of 2 (100)

Total

21 of 68 (31) 13 of 62 (21) 26 of 75 (35) 21 of ^6 (k6)

213

(46 percent) exceeded that of adult females (35 percent), adult
males (31 percent) and that of juvenile to yearling males (21
percent) .

Reproduction

Adult males emerged with scrotal testes. Regression to the
inguinal position appeared to occur during the second and third
weeks of March. By the end of the third week of March all males
had abdominal testes. One yearling male also possessed scrotal
testes during the breeding season.

Females bred soon after emergence. All known adult females
(2+ years old) became pregnant. The nipples of gravid females
enlarged by the second week of March. Parturition began in the
second week of March, peaked during the third week and extended
throughout the month. At least 62 percent of the yearling females
on intensive sites bred in 1976 (Table 6) with their pregnancies
lagging those of adult females one or two weeks.

Reproductive tracts were obtained from 27 Spermophilus
townsendi idahoensis females. Litter size based on embryo counts
(N = 9) and placental scar counts (N = 18) both averaged 7.7 young
per female. Above ground litter counts averaged 5.2 (N = 7)
indicating the amount of mortality between parturition and emergence.

Reproductive tracts were obtained from nine Spermophilus
townsendi mollis females. The placental scar counts in this
subspecies averaged 10.4, a value similar to that found by Alcorn
(1940) in Nevada populations.

The above ground count of litter size was used to calculate
the number of young produced on each site (Table 7).

Growth and Development

Adult males lost weight during the breeding season and gained
weight thereafter. Many males became so obese prior to torpor
that their mobility was impaired.

All females gained weight after emergence, particularly those
which were gravid. Lactating females maintained body weight.
After mammary regression, they rapidly gained weight until sub-
mergence. Most juveniles weighed 35 to 40 g at emergence above
ground in early April. They rapidly gained weight so that juvenile
weight differed little from that of adult and yearling females by
June. Juvenile males weighed about 200 g and juvenile females
slightly less at the time of submergence.

214

Table 6. Reproductive status of yearling females on study sites. Percent
in parentheses.

Site Gravid Nongravid Unknown Total

0 1 h

3 0 U

2 1 11

10 2

Total 13 (62) 6 (29) 2 (9) 21

Bedspring

3

Eurotia

1

Burn

H

Comparison

1

Table 7. Estimated number of young produced on the intensive study sites,
assuming a litter of 5-2 young per adult female.

Site Number of Number of

Litters Young

Bedspring 10 52

Eurotia 7-8 36-1+2

Burn 8 k2

Comparison 5-6 26-31

Artemisia 6-7 31-36

215

The post-emergence growth rate (In g/ day) of juvenile males
was 0.019 + .002 (N = 15) and that of juvenile females 0.017 +
.002 (N = 13). Both were significantly greater than that of adult
males (0.007 + .001; N - 14) and that of adult non-gravid females
(0.009 + .001; N = 7).

Seasonal Activity

The earliest sighting of a ground squirrel occurred on
11 January. Adult males emerged during January and early February.
Emergence of adult females peaked in mid-February. Yearling
ground squirrels emerged during the last half of February and the
first part of March, particularly the latter period.

The first evidence of litters above ground occurred on
26 March on the Artemisia Site. Most juveniles emerged from the
natal burrow during early April.

Females nursed their young after their emergence but by late
April and early May mammary regression was complete. Juveniles
dispersed from natal burrows during early May. No hostility by
the female to drive off young was observed.

Based on trapping results, adult and yearling males entered
torpor from mid-May to mid- June. Adult and yearling females
entered torpor throughout June. Dormancy began in juveniles from
mid-June to mid-July. A few individuals were active outside these
time periods. The last squirrel was seen by John Messick (Study
IVB) on 28 July. Some above ground activity, probably by juveniles,
occurred intermittently during mild fall weather.

During the late winter and early spring, squirrel activity
was confined to the warmer mid-day hours. Squirrel activity above
ground became bimodal as the season progressed and mid-day tempera-
tures rose.

Temperatures in the Boise area during June averaged 1.2° C
below normal and total rainfall was 4.2 cm, 1.5 cm above normal
(E. Elser, pers. coram. ). This unseasonably cool weather may have
reduced above ground feeding which resulted in a weight loss in
both adult and juvenile squirrels (Fig. 2).

Food Habits

Based on field observations, ground squirrels fed preferen-
tially on the fresh growth of cheatgrass in the spring. Little
use of Sandberg bluegrass was observed. Ground squirrels also fed
on winterfat and big sage, especially during periods when snow
covered the grasses. Late in the season, after the cheatgrass had
cured, ground squirrels were observed feeding on tansy mustard
(Descurainia pinnata) .

300-

230-'

UJ

* 150-

100-

50-

APRIL
10 20

MAY
9 19

JUNE
8 18

30 9 19 29 8 |8 28

Fig. 2. Weight changes in free-living ground squirrels.

217

Ground squirrels living next to grainfields appeared to
forage extensively along their edges. No squirrel use of potato
or sugar beet fields was observed. Cultivated fields do not
support ground squirrel populations. Farmers eliminate squirrels
from fields by poisoning them.

Based on an analysis of stomach contents, ground squirrels
subsist primarily on grass leaves and stems during the late winter
and early spring months (Table 8) . Grass seeds became an important
dietary component in late May and June. The seeds of tansy mustard
were heavily utilized at Sand Creek by both adults and juveniles
in May and June and by juveniles in July after adults had entered
torpor. Ground squirrels ate shadscale leaves in habitats where
shadscale was abundant. The diets of adults and juveniles were
similar in composition.

Cannibalism was observed twice in the study area. In addition,
squirrels were seen feeding on a road-killed gopher snake (Pituophis
melanoleucus) .

Mortality

Badger digs, which were recorded on a monthly basis, increased
on the intensive sites as ground squirrel litters matured and
reached a peak in July after all squirrels had entered torpor
(Table 9) . Several instances were observed where badgers plugged
secondary exists before excavating the primary burrow entrance
just as Knopf and Balph (1969) have described.

Although there was a significant positive relationship
between badger hunting attempts and ground squirrel density based
on last year's data (1975 annual report), this relationship only
approached significance (.05 P < .10) when 1976 data are combined
with those of last year. Badger appear to localize their hunting
efforts (Table 9) and then move elsewhere. We continue to believe
that badgers concentrate their hunting efforts on sites supporting
dense squirrel populations. The high counts of badger digs on the
Bedspring and Burn Sites support this contention.

Six squirrels collected in 1975 and 1976 have been reported
positive for plague by the Public Health Service, Fort Collins,
Colorado. The location and dates of collection of positive
specimens are given in Fig. 3.

An analysis of antibody titers in badgers is a useful method
of monitoring for plague in ground squirrel populations. Badgers
develop antibodies to the plague bacillus after exposure to
infected prey. Plague titers in blood from badgers collected in
1976 by John Messick (Study IVB) are reported in Table 10. Titer
levels were highest during the middle and later parts of the
squirrel activity season in both adult and juvenile badgers.

21*

Table 3 . Estimated volume {%) of food items in stomach contents. T = trace;
less than 1%.

Site/ Month(s) Sample Grass Forb/ shrub Insect

Size Seed Leaf/stem Seed Leaf/stem

ADULTS

Big sage-grass

(Feb-Mar) 8 98 T 1 1

(Jun) 3 1+7 1+9 t 3

Sand Creek
(May- Jun)

2

kl

5U

1

Shadscale
(Mar)

8

38

62

1

( May- Jun )

3

7

^0

17

37

T

JUVENILES

Big sage-grass
(May)

7

91

k

5

(Jun)

5

39

hk

1

T

Sand Creek
(Jun)

8

U6

32

lit

9

(Jul)

9

13

1

59

26

T

Shadscale
(Apr-May)

3

35

58

7

(Jun)

9

21

50

2

21+

2

Table 9. Badger hunting attempts on intensive sites, 1975 and 1976. One attempt may include more than
one dig.

Month

Bed

spring

iCurotia

Burn

Comparison
1975 1976

Artemisia
1976

1975

1976

1975

1976

1975

1976

January

-

0

-

0

-

2

_

0

_

February

-

0

-

0

-

0

-

0

0

March

h

0

1

2

0

0

0

0

0

April

5

h

2

3

2

5

3

It

0

May

11

15

k

k

2

6

0

'■'

June

5

11

2

2

10

2

9

3

2

l— ■

to

July

32

91

6

22

12

7

5

3

h

August

15

197

3

17

16

21

6

1

5

September

3

57

5

5

15

30

2

2

28

October

18

18

5

2

5

25

10

It

15

November

7

3

k

1

7

11

0

l|

5

December

1

1

5

0

0

3

1

1

2

Total attempts

101

398

37

58

69

102

36

23

6l

Victory lane

220

r\ Coyote Butte

\J) Tr 10 But

3- 12- 75

6-16-76

3-8-76

Fig. 3. Locations and dates of collection of plague-positive
squirrels .

Table 10. Agglutination titers of Yersinia pestis in blood samples from badgers, 1976. Samples

collected

by John

Messick

(Study IVb) .

Numerator

= number

of adults; demoninator=

no. of juveniles.

Month

Neg.

1/8

1/lb 1/32

1/6U

1/128

1/256

1/512

1/0

1/1021+
1/0

1/201+3 1A096 1/8192

February

1/0

1/0

1/0

March

2/0

1/0

1/0

April

1/0

2/0

2/0

0/1

1/0

1/0 1/0

May

1/0

1/5

1/5

1/1

6/0

3/0

1/0 k/0 1/0

re

June

1/0

0/2

1A

1/0

1/0

2/0

5/0 2/0 2/1

1 — 1

July

0/1

0/5

2/3 0/1

2/3

0/1

3/0

1/0

2/0

1/0

August

0/1

iA

September

0/3

1/1

1/1

October

1/0

222

Vulnerability to Avian Predation

Ground squirrel counts at index points are reported in Table
11. These sites (Fig. 4) were identified by Tom Dunstan (Study
III) as hunted by radio-equipped raptors, particularly prairie
falcons. Squirrel vulnerability at these sites varied daily and
seasonally because of changes in above ground activity. Counts at
index points usually increase during the morning hours, remain
at low levels during the warmest part of the day and increase
again in the late afternoon. Prey deliveries, mostly ground
squirrels, to prairie falcon nests show a similar bimodal distribution
(G. Sitter and S. Peterson, pers. coram. ) . Vulnerability declined
as the season progressed, particularly at sites of high squirrel
density (Table 11) . The initiation of torpor by adults and heavy
juvenile mortality decreased the availability of ground squirrels
to low levels by 1 July.

Areas supporting a low, sparse vegetation such as the Sand
Creek grassland (Cairn, Snake Ridge, Rock Outcrop and Dry Triangu-
lation index points, Table 2) are favored as hunting areas by
prairie falcons (Tom Dunstan, pers. comm. ) . These index points
also supported the highest squirrel densities (Table 11) .

Juvenile ground squirrels are less wary and alert than adults
and thus they sustain much of the predation pressure. The emergence
of juveniles above ground in early April and their subsequent
availability coincides with the increased energy demands for
growth and development by nestling raptors. Juveniles are also
available as prey for newly fledged raptors well into July, two to
four weeks after adult ground squirrels have entered torpor.

LITERATURE CITED

Alcorn, J.R. 1940. Life history of the Piute ground squirrel.
J. Mammal. 21:160-170.

Beechman, J.J. 1970. Nesting ecology of the golden eagle in
southwestern Idaho. M.S. Thesis. Univ. of Idaho.
48 pp.

Johnson, D.R. and K.L. Groepper. 1970. Bioenergetics of North
Plains rodents. Amer. Midland Nat. 84:537-548.

Knopf, F.L. and D.F. Balph. 1969. Badgers plug burrows to
confine prey. J. Mammal. 50:635-636.

Kochert, M.N. 1972. Population status and chemical contamination
in golden eagles in southwestern Idaho. M.S. Thesis.
Univ. of Idaho. 102 pp.

Table 11. Maximum squirrel counts at index points.

May

June

July

Site

21

2k 26

30

1

r;

6

11

15

16

17

23

28

1

HLP if ]a

0

20

HLP a 2

7

15

Cairn

56

kl

51

21

Snake Ridge

33

52

22

5

Rock Outcrop

52

53

'kk

8

Dry Triang.

25

3^

8k

13

Coyote Butte

18

9

2

Trio Butte

16

0

2

Pole 233

IT

k

12

2

Cinder Road

7

7

Pole 36l

6

5

6

2

NO
00

224

Fig. 4. Location of index points in the Snake River Birds of
Prey Study Area.

225

Kochert, M.N. 1973. Density, food habits and reproductive
performance of raptor in the Snake River Birds of Prey
Natural Area. Ann. Rep. U.S. Dept. of Interior, Bur.
of Land Manage., Boise, ID. 54 pp.

Ogden, V.S. 1973. Nesting density and reproductive success
of the prairie falcon (Falco mexicanus) in southwestern
Idaho. M.S. Thesis. Univ. of Idaho, Moscow. 43 pp.

226

^■f'SPSS:?"'

Gayle Sitter, Study VII, surveys the canyon for post-fledging
prairie falcons (BLM photo by Berglund) .

STUDY VII:

CONTRACTOR:

INVESTIGATORS:

PROJECT SUPPORT:

OBJECTIVES:

227

Feeding activity and behavior of prairie
falcons.

Department of Wildlife Resources.
University of Idaho.

Steven R. Peterson, Principal Investigator.
Gayle M. Sitter, Research Assistant.
Arn Berglund, Research Aid.
Brad James, Research Aid.

U.S. Department of the Interior, Bureau of
Land Management Contract No. 52500-CT5-1-64.

Measure the quantity of food necessary to fledge prairie
falcons in Birds of Prey Natural Area. As subsidiaries
of this objective, we will:

a. Compare methods of analyzing species and age of
prey brought to the scrape.

Automatically set time-lapse cameras
Radio-controlled time-lapse cameras
Visual observation
Pellet analysis

b. Compare the daily number of feeding visits and
the quantity of food brought to and consumed at
the scrape with 1, 2, 4, or 6 young.

c. Investigate prey-caching by prairie falcons.
Examine nesting density in relation to cliff relief.
Investigate post-fledging mortality.

ANNUAL SUMMARY

Field work for 1976 was initiated two weeks before the young
hatched. We located six scrapes containing various brood sizes
for intensive study and placed cameras in four of these. We used
dawn-to-dusk observations and periodic nest visits to complement
the camera records. We conducted periodic cliff searches to
locate dead birds and gain further information on mortality.
Strong aggressive attacks were observed between adults and young
from adjacent territories. This aggression resulted in death to
at least three juveniles in a densely populated portion of the
canyon. Other mortality was attributed to the disappearance of
eggs or young at time of hatching and later loss of young from

223

the scrapes. Due to camera malfunctions, film records were of
limited value. Comparisons of young from broods of various sizes
showed no significant difference in growth rates. There is a
linear relationship between weight and age up to 25 days and wing
length and age after 20 days. We are presently conducting a
feeding study to check discrepancies between prey items fed and
pellets received.

PLANS FOR NEXT SEASON

We plan to conduct close observations of two or three scrapes
during the 1977 season. These data along with the information
from 1975 and 1976 should enable us to quantify the prey items
used. The data from the feeding study will be analyzed and
included in the final report. Sitter plans to submit the final
report in 1977 to fulfill requirements for a M.S. degree.

METHODS

Field work started in mid-April. Prior to this time, we
worked on literature searches and analysis of the 1975 data. Two
preliminary trips were made to the study area to arrange for the
1976 season and to observe prairie falcon (Falco mexicanus)
behavior during territory establishment and courtship. We located
most of the scrapes in our intensive study area. A special effort
was made to locate easily accessible scrapes in which we could use
a camera. We located these scrapes by bumping the birds or through
stationary watches. All scrapes were recorded and deemed accessible
or inaccessible. We entered accessible scrapes, checked for eggs,
and measured the scrapes for camera placement.

The cameras were described in the Snake River Birds of Prey
Research Project Annual Report for 1975 (Study I) and require a
scrape that will accept an aluminum box, 27.6 cm high by 20.3 cm
wide by 29.2 cm long with a 40.6 cm antenna on top. Optimum focal
distance is >_ 45.7 cm from the front of the camera box. It was
preferable to have the scrape small enough so feeding activity is
localized and the young cannot move out of the field of view.
Also, we attempted to locate scrapes with different numbers of
young. The above requirements required checking many scrapes
before we selected four that would be acceptable for camera
placement. We also selected four other nests for comparison
checks on growth of young. All selected scrapes were checked
every four days to determine the exact hatching date, to measure
the growth rate of the young, and to collect prey remains.

After hatching, the cameras were inserted in four scrapes
containing 2, 3, 5 and 7 young. All selected nest were entered
every three or four days to change film, collect prey remains and
measure young. On the first visit to a scrape after hatching, we
individually marked each nestling by coloring the down on top of
the head with a felt-tip marking pen. This technique worked

229

well with only periodic touch-ups needed for fast and easy identi-
fication of the young. We continued using this technique after
the young were banded to aid identification and minimize the time
for nest visits.

After hatching and camera placement, we conducted dawn-to-
dusk observations on selected camera scrapes. The one camera we
compared with intensive observations was remotely triggered during
each feeding visit. We set all other cameras to take one frame
every thirty seconds during daylight hours. Most observations on
one of the camera scrapes were correlated with personnel from
Study III since both the adult birds were instrumented with radios.
In this way we had a check on the number of feeding visits, type
of habitat utilized to secure prey, and the density of prey in
feeding areas. During our dawn-to-dusk observations, we noted all
feeding visits, the behavior associated with feeding, and any
aggression displayed with prairie falcons in adjacent territories.

As the season progressed, we conducted cliff searches to
locate dead or dying young prairie falcons. This was especially
important to determine types and causes of mortality during and
after fledging.

Just prior to fledging, we placed radio packages on six
nestlings from four different scrapes. These radio packages were
described by Dunstan (Study III) in the 1975 annual report. We
used these birds to record movement of fledged young prior to
dispersal. Study III personnel followed these radioed young
during dispersal from the canyon. Our field work ceased when all
young from the selected scrapes had dispersed or disappeared.

After the field season, we obtained two rehabilitated adult
prairie falcons from Morley Nelson to use in a feeding study.
This study was designed to compare remains in pellets with known
types of food fed to the falcons at specific intervals. The prey
items are weighed before feeding and each day the pellets and prey
remains are collected, described and weighed. Daily weight changes
of the birds are recorded. Once each week all excrements and
molted feathers are collected and weighed.

The prey items are segregated so that a diet of mammals will
be fed for a certain period, followed by a diet of exclusively
avian prey. In this way, we can see what effects each diet has on
consumption rates, wastage, and weight of the birds.

RESULTS

Raptor Density

Our intensive study area, a 7 km portion of the BPNA near
Swan Falls Dam, has a high density of nesting prairie falcons.

230

We surveyed only the north side of the river and found 21 nesting
pairs of prairie falcons. This area also included four raven
(Corvus corax) nests, one red-tailed hawk (Buteo jamaicensis) , one
great horned owl (Bubo virginianus) , one kestrel (Falco sparverius) ,
and one turkey vulture (Cathartes aura) . The vulture nest was the
first to be located within the BPNA. Cliff structure and placement
of scrapes indicated breeding pairs at scrapes had limited or no
visibility of adjacent scrapes due to canyon wall projections or
the stepped nature of the wall. These projections are obvious to
the human observer when viewed from the rim or from beneath the
scrape. In one area of high cliffs, a staggered system of high-
low scrape locations appears to help shorten the average distance
between scrapes. In most cases, both scrapes and favorite perches
had a limited view of an adjacent bird's territory.

Assessment of Film Records

Forty-eight rolls (2,400 feet) of film were taken in four
different camera scrapes. Only 40 rolls contained any information
at all. Even these rolls were plagued by large portions of over-
exposed or blank portions of film. This failure was attributed to
the temperature extremes characteristic of the exposed cliffs in
this area and failure of the camera's automatic metering system.
We are still working on these films and will incorporate this
information in a later report, but the data contained in these
records appears to be of limited value. We should be able to
extract information on prey items used.

Prey Caching

Last year we observed prey caching in 16 of 84 feeding visits
(19 percent) . We noted additional incidences this year among
seven different nesting pairs. This behavior does not appear to
be isolated to certain individuals or depend on the number of
young in the scrape. One pair which had two young last year
continued to cache prey even though they had seven young this
year. One adult female cached prey immediately after taking it
from the male and then returned to the same cached item six hours
later and consumed it. This particular prey item was never taken
to the scrape.

Food Habits

Analysis of periodic pellet and prey item collections revealed
12 species of mammals, birds and reptiles (Table 1). Townsend
ground squirrels were the most common prey item, both in total
number identified and percentage of occurrence (Table 2). When
these data are segregated by two week intervals, there is an
increasing percent occurrence of birds and lizards through the
nesting period. Personnel from Study V (D. Johnson, pers. comm. )
documented a weight loss in Townsend ground squirrels during

231

Table 1. Prey items observed in pellets or remains found in
prairie falcon scrapes.

Common Name

Scientific Name

MAMMALS

Townsend Ground Squirrel
White-tailed Antelope Ground

Squirrel
Coyote

Spermophilus townsendi

Ammospermophilus leucurus
Canis latrans

BIRDS

Horned Lark
Western Meadowlark
Sage Thrasher
Cliff Swallow
House Finch
Chukar

Erpmophila alpestris
S1 urnella neglecta
Oreoscoptes montanus
Petrochelidon pyrrhonota
Carpoda cus mexicanus
Alectoris graeca

LIZARDS

Desert Horned Lizard
Western Whiptail Lizard
Leopard Lizard

Phrynosoma platyrhinos
Cnemidophorus tigris
Crotaphytus wislizenii

232

Table 2. Number and percent frequency of mammals, birds and lizards
in pellets and prey remains found in prairie falcon
scrapes, 1976. *

Total Total
1-15 16-31 1-15 16-30 Collec- Indiv.
May May June June tions Ident.

No. of Collections 20 24 24 9 77

Collection Containing

Townsend Ground Squirrels 20 24 24 8 76 105

Collections Containing
Other Mammals

% Frequency for Mammals

Collections Containing

Birds

% Frequency for Birds

Collections Containing
Lizards

% Frequency for Lizards

Data from six intensively studied scrapes plus a few collections from
other scrapes visited for incidental reasons.

0

0

0

1

1

100

100

100

89

99

1

5

11

4

21

5

28

46

44

27

0

1

3

3

7

0

4

13

33

9

233

June. This was attributed to inclement weather which reduced the
foraging time of the ground squirrels. The increase in avian and
reptilian prey suggest: (1) decreased vulnerability of the Town-
send ground squirrels because they were underground, and (2) the
importance of birds and lizards as prey items for fledged prairie
falcons. This trend is not apparent in individual pairs. One
pair fed exclusively on Townsend ground squirrels and another pair
fed on birds in early June, but then fed mainly on ground squirrels
later in the nesting season.

It was interesting to record the white-tailed antelope ground
squirrel in the prey collections. This means either the birds are
crossing the river canyon to hunt, or antelope ground squirrels
are present on the north rim of the canyon. Lowell Diller (Study
IV, pers. comm. ) found antelope ground squirrels in rattlesnake
scats on the north rim.

A piece of coyote skin was found in one scrape. This incidence
may be the result of wood rat activity, carrion eating by the
falcon, or predation on a young coyote pup.

Aggression

Prairie falcons are very tenacious when defending their
territories against other intruding prairie falcons or other
raptor species. One or both of the territorial pair will attack
the intruder with repeated stoops and vocalizations until the
intruder is completely out of the territory. This tenacity
lessens as the nesting season progresses. The last territorial
aggression involving contact between adjacent pairs that we
observed in 1976 occurred on 20 May, two weeks before young from
one nest fledged. Defenses shortly before and after this time
were limited to brief chase flights or strong vocalizations.

Territorial aggression was renewed when young fledged from
adjacent territories. At this time all aggression was displayed
against young from adjacent territories with almost complete
tolerance of other adults. This aggression was strong and often
the contact was audible. Adults extended their territories up to
200 m from the cliff face to attack young, and several times were
joined by additional adults in attacking young birds. The young
birds were attacked repeatedly until driven off or forced to land.
On several occasions these attacks resulted in such a hard blow to
the young bird that it dropped several meters before recovering.
On 13 June, a tourist witnessed an attack which actually killed
the young bird. Examination of the dead bird revealed a crushed
skull. During our cliff searches, we found two other birds which
also had crushed skulls. Their condition and position in sage
clumps or on cliffs indicated they also were victims of this type
of aggression. Cliff searches also revealed seven other fledglings
that had been partially consumed by some predator. It is possible

234

that some of these birds may have been killed by aggression and
later scavenged by another predator.

First flights of young birds are unpredictable, and erratic.
When the population density is high, these flights may take them
into adjacent territories. The aggression related mortality may,
therefore, depend on the density of the population in that partic-
ular area. Radio-tracking of young birds showed a definite
affinity to their home area. This may be the result of repeated
attacks in adjacent territories.

Other Mortality

In conducting this aspect of the study, we entered scrapes
both before and after hatching to document the disappearance of
eggs or young during hatching. Loss of eggs or young occurred in
two out of six scrapes (total of three eggs lost) which we had
visited shortly before and after hatching. The elapsed time
between visits was 3 to 5 days. Infertile eggs may have broken,
but it is quite common to find unhatched or infertile eggs still
in the scrape even as late as fledging. The simultaneous breaking
of these eggs at hatching seems unlikely. In 1975, we found one
dead day old prairie falcon chick in a cache. In 1976, we docu-
mented a scrape that had five eggs before 10 May, but on that date
it contained one young and two eggs in the process of hatching.
There was no sign of the other eggs or young. Some chips from
hatched eggs were present, but we were unable to determine which
eggs they came from or how many eggs they represented. The size
and shape of the scrape prevented any accidental expulsion. No
evidence of eggs or young were present beneath the scrape. It is
possible that the other two young died during hatching, were
carried from the nest, and deposited in a cache similar to that
recorded in 1975.

Additional mortality occurred later in the nesting cycle.
This mortality occurred while the young were still downy, but
large enough to move around the scrape. These individuals were
usually found beneath the scrape. Fratricide would be a logical
explanation, except in cases where the entire clutch disappears.
Similar cases were noted in two different scrapes this year.
Both of these scrapes had heavy infestations of swallow bedbugs
(Oeciacus vicarius) . In these scrapes there may have been a
voluntary abandonment of the scrape by the young falcons. Such
abandonment may not necessarily result in death. On two occasions,
we found young that had disappeared from scrapes but were being
fed and protected on the ground by the adults. In one case, the
young had been outside the scrape for 12 to 17 days. In both
cases, we observed feeding visits or found prey remains with the
vagrant young even though as many as six other young remained in
the original scrapes.

235

Our cliff searches were conducted along 5.6 km of cliff which
contained 14 nesting pairs. If we assume 3.34 young fledge per
attempt, as we found for the entire BPNA in 1975, then the ten
dead juveniles found represent 21 percent of the total fledglings.

Ten of 17 attempts (59 percent) were monitored sufficiently
to determine a fledging rate of 2.7 young per attempt. If this is
expanded to the four nests of unknown success, then post-fledging
mortality could be 26 percent.

Growth Comparisons

During the year we measured the growth of young falcons to
see if the number of young in a scrape affected development or
fledging time. Clutch sizes of study scrapes varied from 2 to 7
young per scrape. There was no significant difference in fledging
times of these scrapes (Table 3) .

Weight was correlated with age (r > 0.98; Figures 1 and 2).
All known age birds were plotted for the first 25 days at which
time maximum weight was attained. The birds were from two different
clutch sizes and each sex was plotted separately. Individual
birds often lost weight before fledging.

Wing length was correlated with age after 20 days
(r > 0.97; Figures 3 and 4). This wing length was measured from
the notch in the wrist joint (antebrachium and manus) in a straight
line distance to the end of the 8th primary.

Feeding Study

This part of our study is still in progress and a more complete
analysis will be reported later. For the past six months we have
been feeding two captive prairie falcons selected prey items
collected on or near the Birds of Prey Natural Area. In this way
we can see what parts are eaten and what items appear in the
pellets. At the same time, we are conducting some basic bioener-
getics studies.

236

Table 3. Fledging dates of clutches with different numbers of young.

Scrape

No

. Y

oung

Days to

Name

Per S

crape

Fledging

Camera 1

7

38-41

Dam Rd.

5

35-40

Black Rock

3

36-38

B.B. Powerl

ine

2

38-40

237

800-

700-

600-

_j FROM CLUTCH OF 5
= - FROM CLUTCH OF 3
r = 0.99

CO

<

CD

500-

— 400-

I-
X
C£
UJ 300-

200-

100-

1 1 1

10 15 20

AGE IN DAYS

t

25

- 1-
30

Fig. 1. Age in days vs. weight in juvenile female prairie
falcons .

238

• " ? FROM CLUTCH OF 5

600'

■ "2 FROM CLUTCH OF 3

#

r = 0.98

• /

y' 0

o

0 •

w>

500-

2

<

•

/ O

tr

(3

400'

■ ° /

• /

■z.

/v

t-
X

2

300-

uu

/a

<z

200«
100-

/ a

/* -
1 1 t 1

10

15

AGE IN

20

DAYS

25 30

Fig. 2. Age in days vs. weight in juvenile male prairie falcons.

239

T-
35

*j FROM CLUTCH OF 5
J? FROM CLUTCH OF 3
r = 0. 98

-T—
40

45

— T—
50

AGE IN DAYS

Fig. 3. Age in days vs. wing-length of male prairie falcons,

240

25

24-

25

CO

o
z

22

21-

20<

C-
(5

-J

(

CD

? 19

18-

17-

30

*j FROM CLUTCH OF 5
s FROM CLUTCH OF 3

r= 0.97

35

40

45

50

AGE IN DAYS

Fig. 4. Age in days vs. wing-length of female prairie falcons,

'& U.S. GOVERNMENT PRINTING OFFICE:1977— 781-803 / 174 REGION NO.8

_ Library

Dfmw *"*• center

Date

Loaned

—

Borrower's Cs

QL 1
684 Snake River bird
#j2 project: Annual

L35
1976

Borrower