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BOSTON PUBLIC LIBRARY 



3 9999 06317 643 
V 



THE SCREECH OWL: 
ITS LIFE HISTORY AND 
POPULATION ECOLOGY 

IN NORTHERN OHIO 




NUMBER 71 



UNITED STATES 
DEPARTMENT OF THE INTERIOR 

FISH AND WILDLIFE SERVICE 



NORTH AMERICAN FAUNA 

This publication series includes monographs and other reports of scientific 
investigations relating to birds, mammals, reptiles, and amphibians, for profes- 
sional readers. It is a continuation by the Fish and Wildlife Service of the series 
begun in 1889 by the Division of Ornithology and Mammalogy (Department of 
Agriculture) and continued by succeeding bureaus — Biological Survey and the 
Bureau of Sport Fisheries and Wildlife. The Service distributes these reports to 
official agencies, to libraries, and to researchers in fields related to the Service's 
work; additional copies may usually be purchased from the Division of Public 
Documents, U.S. Government Printing Office. 

Reports in NORTH AMERICAN FAUNA since 1950 are as follows (all sale stock 
is exhausted): 

60. Raccoons of North and Middle America, by Edward A. Goldman. 1950. 153 pp. 

61. Fauna of the Aleutian Islands and Alaska Peninsula, by Olaus J. Murie; 

Invertebrates and Fishes Collected in the Aleutians, 1936-38, by Victor B. 
Scheffer. 1959. 406 pp. 

62. Birds of Maryland and the District of Columbia, by Robert E. Stewart and 

Chandler S. Robbins. 1958 401 pp. 

63. The Trumpeter Swan; Its history, habits, and population in the United States, 

by Winston E. Banko. 1960. 214 pp. 

64. Pelage and Surface Topography of the Northern Fur Seal, by Victor B. Scheffer. 

1961. 206 pp. 

65. Seven New White-winged Doves from Mexico, Central America, and South- 

western United States, by George B. Saunders. 1968. 30 pp. 

66. Mammals of Maryland, by John L. Paradise 1969. 193 pp. 

67. Natural History of the King Rail, by Brooke Meanley. 1969. 108 pp. 

68. The Sea Otter in the Eastern Pacific Ocean, by Karl W. Kenyon. 1970. 352 pp. 

69. Natural History of the Swainson's Warbler. 1971. 90 pp. 

70. The Distribution and Occurence of the Birds of Jackson County, Oregon, and 

Surrounding Areas, by M. Ralph Browning, 1975. 69 pp. 

Library of Congress Cataloging in Publication Data 

VanCamp, Laurel F 
The screech owl. 



(North American fauna; no. 71) 

Bibliography: p. 

Supt. of Docs, no.: I 49.30:71 

1. Screech owls. 2. Birds— Ohio. 3. Bird populations. I. Henny, Charles J., joint 
author. II. Title. III. Series. 
QL155.A4 no. 71 [QL696.S83] 596'.0097s [598.9'7] 75-619387 



THE SCREECH OWL: 
ITS LIFE HISTORY AND POPULATION 

ECOLOGY 
IN NORTHERN OHIO 



By 

Laurel F. VanCamp, Wildlife Naturalist 

Ohio Department of Natural Resources 

Division of Wildlife 

Charles J. Henny, Research Biologist 

Migratory Bird and Habitat Research Laboratory 

U.S. Fish and Wildlife Service 




NUMBER 71 



UNITED STATES 
DEPARTMENT OF THE INTERIOR 

FISH AND WILDLIFE SERVICE 




^■Vd wiU*-* 



North American Fauna, Number 71 



Published by 

FISH AND WILDLIFE SERVICE 

1975 



Contents 



Page 

Introduction 1 

The Study Population 3 

The Study Area 4 

Accumulation of Data 6 

Taxonomy 8 

Food Habits 9 

During the Nesting Season 10 

In Autumn and Winter 13 

Fish in the Diet 15 

Seasonal Variation 16 

Migration and Dispersal 18 

Dispersal Related to Age and Season 19 

Dispersal Distance 20 

Directional Pattern of Dispersal 22 

Records of Individual Birds 24 

Breeding Biology 25 

Breeding Season 25 

Clutch Size 26 

Nesting Success and Fledging Rates 27 

Causes of Nest Loss 32 

Population Dynamics 33 

Sex Ratio in Population 33 

Mortality Rates 33 

Causes of Postfledging Mortality 35 

Age at Sexual Maturity 37 

Pesticides and Pollution 38 

Eggshell Thinning 39 

Residue Levels in Eggs - 40 

Polymorphism 41 

Annual Variation in Color Phase Ratios 43 

Sex, Age, and Color Phase 47 

Genetic Hj-potheses 49 

Brood Size and Color Phase 54 

Status of Population 55 

Index to Annual Abundance 55 

Life Equation Approach 58 

Summary of Status 59 

(Cont'd) 



CONTENTS (Cont'd) 

Summary 60 

Acknowledgments ■ : , . . 62 

Literature Cited 63 

Illustrations 

FIGURE 

Frontispiece, The cryptic coloration of the gray phase of the 

screech owl v 

1. A nest box is checked on the study area 1 

2. The four-county study area in northern Ohio 2 

3. An improved nest box which was used on the study area 

since the early 1960's 4 

4. A typical open field adjacent to a wooden creek bottom 

in the study area 6 

5. The directional preference of screech owl dispersal 

in the northeastern United States 23 

6. A banded screech owl being released at its nest box by 

the senior author 24 

7. Five downy screech owls 29 

8. Changes in the color phase ratios in the northern 

Ohio study area, 1944-73 43 



NOTE: For additional copies, write C. J. Henny, Building 16, Federal 
Center, Denver, Colorado 80225. 




Frontispiece— The cryptic coloration of the gray phase of the screech owl 
(photo courtesy of G. Ronald Austing). 



INTRODUCTION 

The screech owl (Otus asio) is native to North America and 
breeds throughout the United States and in portions of Canada and 
Mexico. It is a small owl, 20 cm (8 in) in length from the tip of the bill 
to the tip of the tail, with a wing span of 56 cm (22 in); it has yellow 
eyes and prominent ear tufts (see Frontispiece). Although the 
species is common throughout much of North America, it has not 
been studied intensively, particularly over a long period. The 
published literature is concerned mostly with food habits, color 
phase, taxonomy, and miscellaneous observations. Breeding 
biology and population dynamics have received little attention. 




Fig. 1. — A nest box is checked on the study area 
(photo courtesy of George Laycock). 



NORTH AMERICAN FAUNA 71 
OHIO 




Fig. 2.— The four-county study area in northern Ohio. 



This report presents the basic life history and population 
information about screech owls in northern Ohio over a 30-yr 
period. The owls studied were nesting in boxes (Fig. 1) established 
for wood ducks ( Aix sponsa) along rivers, creeks, and marshes in a 
four-county area (Ottawa, Sandusky, Wood, and Lucas Counties) 
near Lake Erie (Fig. 2). No special trapping techniques were 
required as the screech owls readily used these nesting boxes and 
could be easily captured while in them. More than 3,000 owls were 
captured and banded; 500 were recaptured after the initial banding, 
some 10 or 15 times. This process provided a large quanity of basic 
information for this report. 

The banding data provided useful information on dispersal, 
mortality, and age at sexual maturity; in several instances, 
however, the data were supplemented by banding data of other 
workers in the area north and east of Missouri. More than 500 
nesting attempts by the screech owls were recorded, and 53 
different species of birds were listed as food items in the nest boxes 
during the nesting season. Supplemental information on the 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 6 

seasonal food habits of the screech owl was obtained from food 
habit cards filed at the Patuxent Wildlife Research Center, Laurel, 
Maryland. 

The senior author initiated the study in 1944 and collected the 
field data through 1973. The junior author analyzed the data, wrote 
the report and coordinated the pesticide phase of the study with the 
Patuxent Wildlife Research Center in 1973. We have also brought 
together the relevant literature on the screech owl with emphasis 
on population ecology. This subject may be particularly timely 
since McLane and Hall (1972) showed that a low dietary dosage of 
DDE (a chlorinated hydrocarbon pesticide) causes eggshell 
thinning under laboratory conditions. Eggshell thinning has been 
associated with declines of many raptorial and fish-eating birds in 
both western Europe and North America (Ratcliffe 1967; Hickey 
and Anderson 1968; Lockie et al. 1969; Hickey 1969). Furthermore, 
the screech owl data have been subjected to a modeling process 
(Henny et al. 1970) to determine population parameters for the 
species in northern Ohio. 

The Study Population 

The screech owl population reported herein is one that nested in 
wood duck boxes in northern Ohio between 1944 and 1973. The fact 
that the birds were using artificial nesting structures may cause a 
bias in a portion of the findings, particularly in terms of population 
trends (indices) and annual recruitment rates. For example, trends 
in population numbers should not be based on the percentage of the 
nest boxes occupied each year because the nest boxes were always 
placed in suitable habitat. A population reduction from the loss of 
habitat would be undetected by such an approach. Our population 
index most likely represents the annual population nesting in 
suitable habitat. We believe, however, that most of the other 
findings presented in this paper are representative of the wild 
screech owl population in the study area, and, as such, were not 
biased by the type of nest site used. 

Screech owls began using the nest boxes in northern Ohio much 
earlier than wood ducks. In fact, we have found wood ducks nesting 
in boxes after young screech owls have fledged. It is our subjective 
assessment that screech owls keep starlings (Sturnus vulgaris), 
which are much more competitive than the owls, out of the nest 
boxes within the owls' feeding areas. Upon occasion, starlings will 
bury wood duck eggs under their nesting material and cause the 
hen to desert. We therefore contend that wood duck nesting success 
is enhanced by the presence of screech owl nesting. Adequate 
numbers of nest boxes should be provided for both species. 



NORTH AMERICAN FAUNA 71 




Fig. 3. — An improved nest box which was used on the study area since the early 
1960's. One hundred of this type of box were available annually during the last 
decade. 



Beginning early in the 1960's, most of the nest boxes were made 
with cylinders shaped like rockets (after the design of Bellrose 1955; 
Fig. 3). The owls used the boxes for nesting, roosting, and feeding. 

The Study Area 

Nearly all of the study area lies within the boundaries of what 
was called the "Black Swamp" during the early annals of North 
American history. The Black Swamp was drained late in the 
nineteenth century and is so transformed that an observer would 
have difficulty in estimating its former extent. Mayfield (1962) 
discussed changes in the natural history of the area since the 
coming of the white man, but indicated that agriculture had 
modified this region long before a competent naturalist had studied 
the plants and animals. 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO O 

From the beginning of settlement in the Black Swamp (circa 
1820), the entire agricultural system revolved around corn. Most of 
the conversion to agricultural land in northwestern Ohio took place 
after 1850 and continued until the turn of the century (Kaatz 1953). 
Within a few decades, the Swamp was transformed into one of the 
most productive agricultural regions in Ohio. By 1930, the Black 
Swamp was the most completely cropped area in the State 
(Sitterley and Falconer 1933). Today, like 60 or 70 yr ago, the Black 
Swamp lands are dominantly rural. Although there are urban and 
industrial areas around Toledo and other smaller cities, the rural 
scene prevails elsewhere. 

The Black Swamp, like most dense, unbroken forests, was not 
rich in wildlife (Kaatz 1953). Information on bird populations is 
scarce; however, it is almost certain that many of the species 
common at the time of settlement have been extirpated or greatly 
reduced in numbers. Conversely, some of the most common birds 
today were rare or nonexistant in the area when the first settlers 
arrived. Some species that are abundant today prefer nesting sites 
provided by the structures of modern man and are scarcely to be 
found in natural sites. Mayfield (1962) reviews the changed status 
of a number of bird species. It is unclear how or to what extent 
habitat changes have influenced the status of screech owls. In 
reference to central Ohio, Trautman (1940:275-276) reported, 
"Statements by hunters and farmers who were well acquainted 
with the screech owl indicate that before 1900 this species was 
decidedly uncommon in the area and that with the removal of most 
of the forests and the decrease in numbers, or extirpation, of barred 
and horned owls, the screech owl became increasingly numerous. 
These statements agree with my studies." Furthermore, Bent 
(1938:246) states, "The screech owl also has been known to nest 
frequently, even regularly, in cavities in trees close to houses in 
towns and cities, thus showing more confidence in human beings 
than most other owls show." 

An illustration of an open field adjacent to a wooded creek bottom 
where several screech owls nested annually is shown in Fig. 4. The 
loss of forested land in the four-county study area — 12% between 
1958 and 1967 (Table 1)— is continuing. Farmers are using more 
and more of their land each year for agriculture. The wooded creek 
bottom and woodlots are being cleared for mechanized agriculture. 
Only 7.6% of the study area was classified as forested land in 1967. 
This contrasts with about 90% of the area being forested in 1853 
(Diller 1962). In 1967, the four-county area consisted of 85.2% 
cropland, 1.9% pasture, 7.6% forested, and 5.3% other land (Anon. 
1971). The most important agricultural crops are corn and 
soybeans. 



NORTH AMERICAN FAUNA 71 




Fig. 4.— A typical open field adjacent to a wooded creek bottom in the study area. 



Table 1. The forest area in the four-county study area in northern Ohio, 1958 us. 
1967 (from Anonymous 1971). 

Forest area (acres) 



County 1958 1967 

Lucas - a 23,873 (19.5) b 

Ottawa 7,485 8,398 ( 5.9) 

Sandusky 21,000 15,932 ( 6.6) 

Wood 19,217 17,616 ( 4.9) 

Total 3 47,702 41,946 

The 1958 Lucas County figure was calculated with an incorrect standard (H. G. 
Smith, Soil Conservation Service, Columbus, Ohio, personal communication); 
therefore, the totals are for the three-county area with comparable data. 

In parentheses percent of county inventoried which was forested. 



Accumulation of Data 

A measure of the annual and seasonal effort to conduct this study 
is of importance. Unfortunately, the person-days spent in the field 
were not recorded. The number of nest boxes checked during the 
nesting season provides an index of annual effort during the 30-yr 
study. This measure is admittedly crude because the number of 
visits to each nest box was not recorded. For example, some of the 
boxes could not be visited more than once during several of the 
years (for additional details see results). Forty-five boxes were 
checked in 1944, the first year of the study. The average number of 
nest boxes checked for each 5-yr period during the study was as 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO / 

follows: 479, 727+, 985+, 730, 650, 678. The exact number of nest 
boxes checked from 1952 to 1955 is unknown; however, a high 
percentage of the boxes were checked only once at the time young 
were about to fledge. From 1956 to 1973, the annual effort was 
consistent. More than 4,249 boxes were checked during the 30-yr 
period. 

Breeding Season 

Pairs appeared in the nest boxes by the first week of February. 
During the initial years of the study, some nest desertions resulted 
from visiting the boxes in February and March. Therefore, in later 
years the nest boxes were not checked before mid-April. Most 
screech owls were incubating by mid-April, and, unless the 
incubating birds were handled, the nests were not deserted. Nest 
boxes were checked at this time and the males banded if they were 
in the nest box. Females were not handled until the young hatched. 
The nest boxes were generally checked three times between mid- 
April and early June. The females were banded during the first 
check in which young were in the nests. The young were not banded 
until the color phase could be determined. 

Postfledging Period 

Nest boxes were little used after the young fledged, but use began 
to increase in October when the deciduous trees lost their leaves. We 
visited nest boxes only rarely in late summer or early fall. 

Winter Period 

November, December, and January was a period of much nest 
box use by individual birds. The wood duck boxes were checked 
periodically during these months and a large number of screech 
owls was banded. Beginning in the early 1960's all boxes were 
visited during the annual maintenance check in January; not all 
boxes were visited during the winter in the earlier years. 

In addition to roosting, the screech owls used the boxes for 
feeding stations during winter. The trait of carrying prey to a 
cavity before feeding may have evolved: (1) to lower screech owl 
vulnerability to predation by larger owls, (2) to prevent other 
animals from eating the prey not consumed immediately by the 
owls, (3) as a food storage technique to carry the owls over periods 
of inclement weather or periods of low food availability, or (4) from 
the habit of the male feeding the female (Burton 1973). This food- 
carrying trait accounts for the large number of prey items recorded 
in the nest boxes. As mentioned above, the owls did not use the nest 
boxes in late summer or early fall when the leaves of deciduous 
trees are present. We believe the dense foliage protected the screech 



8 NORTH AMERICAN FAUNA 71 

owl from predation by larger owls during this period. Thus, the 
need for using a cavity as a feeding station was minimized in the 
late summer and early fall. 



TAXONOMY 

The screech owl is confined to North America and has a 
continuous breeding distribution from southeastern Alaska, 
southern British Columbia, southern Manitoba, southern Ontario, 
southern Quebec, and Maine south to the Cape district of Baja 
California, Jalisco, Hidalgo, southern Tamaulipas, and Florida 
(American Ornithologists' Union 1957). The American Or- 
nithologists' Union also recognizes the whiskered owl (O. trichop- 
sis) in the mountain areas from southeastern Arizona to Honduras 
and the flammulated owl (O. flammeolus) in southern British 
Columbia south through the mountains west of the Great Plains to 
the highlands of Mexico and Guatemala. Additionally, 18 
subspecies of the nonmigratory screech owl are recognized. 

Numerous other classifications were in effect before 1957 and 
agreement among taxonomists is still not complete. As with most 
species of birds, subspecies of the screech owl have been described 
on the basis of variation in the length of the wing, bill, tail, and 
tarsus; and in the color pattern of the plumage. In addition to 
geographical variation, the species is polymorphic in the eastern 
part of its range and has two distinct color forms: one with the 
plumage mainly bright rufous and the other with plumage mainly 
gray. An intermediate color phase also exists, but in low frequency. 
Elsewhere in the range, only gray birds occur. Owen (1963a) 
concluded that most of the subspecies had been separated by 
drawing arbitrary lines through clinal variations. He further 
indicated that new subspecies were still (in 1959) being described, 
sometimes based upon as few as five specimens, and thought that 
the situation should be remedied. Until such time as reproductively 
isolated sympatric populations are discovered in any part of the 
range, Owen believed that no subspecies should be recognized and 
the AOU Check-list entry should include geographical variation. 

Marshall (1967), on the other hand, stated that taxonomists have 
been so engrossed with subspecies that they have ignored a species 
problem existing in north and middle America. He proposed that 
seven species and four incipient species of Otus occupy north and 
middle America, where they overlap without interbreeding, in such 
combinations as their distributions and habitats permit. 
Marshall's conclusion were based on both morphology and song 
patterns. The Committee on Classification and Nomenclature of 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 

the American Ornithologists' Union has not commented on the 
screech owl since the 1957 Check-list was published. For the 
purposes of the present paper, no trinomial is used and the screech 
owl population is described on the basis of geography, i.e., the 
nonmigratory population resident in northern Ohio. 

FOOD HABITS 

The screech owl is one of the most nocturnal of North American 
owls. Allen (1924) made a series of observations on the feeding of a 
brood of young and concluded that the earliest time at which 
feeding began was 2025, and the latest was 2112; the earliest time at 
which feeding ceased was 0250, and the latest was 041 5. He further 
reported that both parent birds were engaged in caring for the 
young. These nighttime hunts yielded a variety of food items 
including many passerine birds that were apparently captured on 
their roosting sites. 

The food habits of most birds of prey were first studied in detail 
during the latter part of the 19th and early 20th centuries. Notable 
publications of this era were by Fisher ( 1 893) and Errington ( 1 932). 
In summary, they reported that the screech owl diet consisted of 
mice, shrews, rats, other mammals, small birds, lizards, 
amphibians, fish, crayfish, insects, spiders, and other in- 
vertebrates. Bent (1938) indicated that although birds do not form 
as large a portion of the food as mammals, the list of species is a 
long one. More recently, James and Martin (1950) and Stewart 
(1969) have added several species of birds to the list. 

McDowell and Luttringer (1948) estimated that the diet (percen- 
tage occurrence) of the screech owl consisted of 22.75% mice, 18% 
other mammals, 18% songbirds, with the remaining 41.25% 
reptiles, amphibians, fish, and insects, and other invertebrates. An 
analysis of 419 pellets collected in western Missouri from 1957 to 
1967 showed a preponderance of small rodents in screech owl diets 
(Korschgen and Stuart 1972). Mice and rats together made up 87% 
of all foods. Songbirds amounted to 8.4% in occurrence and only 
4.1% of the volume. Among birds in portions of the southwestern 
United States and western Mexico, Ross (1969) lists the common 
screech owl as being both carnivorous and insectivorous, the 
whiskered screech owl as being primarily insectivorous but on 
occasion supplementing its diet through carnivorous predation, 
and the flammulated screech owl, as being entirely insectivorous. 
Errington (1932) indicated that as a rule screech owls ate what was 
most convenient to catch and of a size easy to handle. He further 
noted that their preferred prey seemed to be mice if such were 



10 NORTH AMERICAN FAUNA 71 

available, but in the event of a mouse shortage they readily turned 
to birds. These studies suggest that the screech owl is an 
opportunistic predator whose diet includes nearly every class of 
animal life. 

During the Nesting Season 

Allen (1924) intensively studied a pair of nesting screech owls in 
New York for 45 days and found that birds represented an 
important portion of the diet. Allen provided a list of 77 birds of 18 
species brought to the young during the 45-day period. He 
concluded that because the feathers in the nest undoubtedly 
represent many more than one bird of each species, the grand total 
of birds required to feed the three young owls from the time of 
hatching until left by the old birds were certainly more than 100. 
Recently, Stewart (1969) also indicated that food items found at the 
nests of screech owls in Ohio and North Carolina consisted 
primarily of birds. However, Sherman (1911) reported 40 food items 
during the nesting season (apparently in Iowa), which included 3 
frogs, 1 shrew, 28 mice, and only 8 birds. 

During our nesting studies, 477 food items were recorded at wood 
duck nest boxes used by screech owls. Our observations were made 
between 26 March and 7 June, and nearly all food items found were 
intact. Most of the food was stored in the nests during the first 2 wk 
after the young hatched (late April and early May). Fifty-three 
species of birds were recorded, making up 64.8% of the items found 
during the nesting season (Table 2). An attempt was made to 
calculate biomass of the prey; however, sufficient information was 
not available regarding the age classes for several of the birds, 
mammals, and fishes which vary greatly in size. Preliminary 
calculations suggested that the percentage occurrence and percent 
biomass in the diet (with the exception of invertebrates which were 
probably eaten immediately and not adequately represented in the 
sample) were similar. 

Of the five most common species of birds found in the nests, three 
are transients (Campbell 1968): the yellow-rumped warbler 
(average arrival 19 April, average departure 20 May), white- 
throated sparrow (average arrival 14 April, average departure 22 
May), and the ruby-crowned kinglet (average arrival 11 April, 
average departure 17 May); one was a permanent resident (the 
house sparrow); and one was primarily a summer resident (the 
starling) nesting in northern Ohio in May. Furthermore, 49 of the 
53 species of birds found in the diet of the screech owl must be 
considered migratory. Therefore, the screech owl definitely takes 
advantage of the spring migration of small birds to feed its young. 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 11 

Table 2. Species stored as food by screech owls during the nesting season in 
northern Ohio a . 

Family Species n 

Birds 

Columbidae 

Mourning Dove (Zenaida macroura) 2 

Cuculidae 

Yellow-billed Cuckoo (Coccyzus americanus) 1 

Strigidae 

Screech Owl ( Otus asio) 1 

Picidae 

Common Flicker (Colaptes auratus) 1 

Downy Woodpecker (Dendrocopos pubescens) 1 

Tyrannidae 

Eastern Kingbird ( Tyrannus tyrannus) 1 

Unidentified Flycatcher 1 

Hirundinidae 

Purple Martin (Progne subis) 4 

Barn Swallow (Hirundo rustica) 4 

Tree Swallow (Iridoprocne bicolor) 2 

Bank Swallow (Riparia riparia) 1 

Cliff Swallow (Petrochelidon pyrrhonota) 1 

Corvidae 

Blue Jay (Cyanocitta cristiata) 4 

Certhiidae 

Brown Creeper (Certhia familiaris) 1 

Troglodytidae 

Winter Wren ( Troglodytes troglodytes) 1 

Mimidae 

Gray Catbird (Dumetella carolinensis) 5 

Brown Thrasher ( Toxostoma rufum) 3 

Turdidae 

American Robin ( Turdus migratorius) 10 

Hermit Thrush (Catharus guttatus) 8 

Swainson's Thrush (Catharus ustulatus) 7 

Wood Thrush (Hylocichla mustelina) 2 

Veery ( Catharus fuscescens) 2 

Unidentified Thrush 3 

Sylviidae 

Ruby-crowned Kinglet (Regulus calendula) 18 

Blue-gray Gnatcatcher (Polioptila caerulea) 4 

Golden-crowned Kinglet (Regulus satrapa) 2 

Unidentified Kinglet 1 

Sturnidae 

Starling (Sturnus vulgaris) 19 

Parulidae 

Yellow-rumped Warbler (Dendroica coronata) 40 

Palm Warbler (Dendroica palmarum) 5 

Common Yellowthroat (Geothlypis trichas) 3 

(Cont'd) 



12 NORTH AMERICAN FAUNA 71 

Table 2 (Cont'd) 

Cape May Warbler (Dendroica tigrina) 2 

Black-and-white Warbler (Mniotilta varia) 1 

Chestnut-sided Warbler (Dendroica pensylvanica) 1 

Tennessee Warbler ( Verminuora peregrina) 1 

Yellow Warbler (Dendroica petechia) 

Nashville Warbler ( Vermivora ruficapilla) - 1 

Canada Warbler ( Wilsonia canadensis) 1 

Unidentified Warbler 6 

Ploceidae 

House Sparrow (Passer domesticus) 25 

Icteridae 

Red-winged Blackbird (Agelaius phoeniceus) 5 

Common Grackle (Quiscalus quiscula) 5 

Brown-headed Cowbird (Molothrus ater) 1 

Thraupidae 

Scarlet Tanager (Piranga oliuacea) 1 

Fringillidae 

White-throated Sparrow (Zonotrichia albicollis) 37 

Song Sparrow (Melospiza melodia) 14 

Cardinal (Cardinalis cardinalis) 7 

American Goldfinch (Spinus tristis) 4 

Dark-eyed Junco (Junco hyemalis) 4 

Rose-breasted Grosbeak (Pheucticus ludovicianus) 3 

Chipping Sparrow (Spizella passerina) 2 

Field Sparrow (Spizella pusilla) 2 

Savannah Sparrow (Passerculus sandwichensis) 2 

Tree Sparrow (Spizella arborea) 1 

Vesper Sparrow (Pooecetes gramineus) 1 

Indigo Bunting (Passerina cyanea) 1 

Rufous-sided Towhee (Pipilo erythrophthalmus) 1 

Unidentified Sparrow 4 

Unidentified Family 

Small Bird 16 

Large Bird 1 

Subtotal (percent occurrence) 309 (64.8%) 

Mammals 

Soricidae 

Unidentified Shrew 4 

Chiroptera (Unknown Family) 

Unidentified Bat 2 

Sciuridae 

Gray Chipmonk ( Tamias striatus) 1 

Cricetidae 

Deer Mouse (Peromyscus sp.) 37 

Meadow Vole (Microtus sp.) 52 

Unidentified Mouse 3 

Muridae 

Norway Rat [young] (Rattus norvegicus) 6 

House Mouse (Mus musculus) 39 

(Cont'd) 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 13 

Table 2 (Cont'd) 

Leporidae 

Eastern Cottontail [leg] (Syluilagus floridanus) 1 

Subtotal (percent occurrence) 145 (30.4%) 

Amphibians 

Ranidae 

Leopard Frog (Rana pipiens) 5 

Subtotal (percent occurrence) 5 (1.0%) 

Fish 

Centrarchidae 

Green Sunfish (Lepomis cyanellus) ... 5 

Clupeidae 

Gizzard shad (Dorosoma cepedianum) 8 

Cyprinidae 

Unidentified Minnow 1 

Subtotal (percent occurrence) 14 (2.9%) 

Invertebrates 

Crayfish ( Cambarus sp.) 2 

Leech (Hirudinea) 2 

Subtotal (percent occurrence) 4 (0.8%) 

Grand Total 477 (99.9%) 

a Smaller food items were probably eaten immediately rather than being stored in 
the nest boxes and were undetected. 



A nest box containing four young screech owls at the Winous 
Point Shooting Club had 40 dead birds stored on 5 May 1972. These 
included 24 yellow-rumped warblers, 3 common yellowthroats, 1 
palm warbler, 2 white-throated sparrows, 1 Nashville warbler, 4 
blue-gray gnatchatchers, and 5 unidentified birds — 2 of which were 
sparrows but not house sparrows. When the nest was revisited on 
22 May 1972 to band the four young, no food was left in the box. 
Young flightless starlings were also taken from their nests on 
several occasions. Three young starlings and one adult were found 
in a nest box on 14 May 1955. Several trips were apparently made 
by the screech owl to the starling nest to fill its coffers. 

In Autumn and Winter 

Data collected by Wilson (1938) at Ann Arbor, Michigan, 
primarily in the autumn and winter, showed a preponderance of 



14 NORTH AMERICAN FAUNA 71 

mammals in the screech owl diet. Only four skulls of birds (0.3% 
frequency) were found in 1,408 pellets containing 1,549 skulls. 
Animals which were identified included: meadow mice (87%); deer 
mice (5%); Cooper lemming mice, Synaptomys cooperii (3%); short- 
tailed shrews, Blarina brevicauda (3%); the remainder (2%) 
included jumping mice, Zapus hudsonius; least shrews, Cryptotis 
parva; red squirrels, Sciurus hudsonicus; eastern moles, Scalopus 
aquaticus; Norway rats, birds, parts of crayfish, and insects. 
Wilson concluded by saying that the screech owl's diet showed a 
definite cycle. During the spring many owls fed upon insects and 
amphibians. Elsewhere in his paper Wilson (1938:193) notes, "The 
tendency for owls in this vicinity to ignore birds can possibly be 
explained by the abundance of mice . . . ." Similar results were 
reported from Michigan in a more recent study by Craighead and 
Craighead (1956) who reported that meadow mice and deer mice 
made up 95.3% of the diet in 1942 and 87.2% in 1948. The percentage 
of small birds (1.2 and 11.4%) showed a substantial increase in the 
latter winter. According to the Craigheads, this trend toward more 
small birds in the diet reflected the change in the relation of 
meadow mice to small bird densities of the latter year. 

Fall and winter food items found in the nest boxes during our 
study totaled 121. The information was collected between 15 
October and 23 February, with the majority being recorded in 
December. We estimated that the screech owl diet during the fall 
and winter in northern Ohio consisted of approximately 60.3% 
mammals, 26.4% birds, 5.8% fish, 5.0% frogs, and 2.5% crayfish 
(Table 3). The nonmigratory house sparrow and cardinal formed 
over half of the avian food items whereas the migratory bird 
species were most important during the nesting season. 

An observation made in January 1971 was of special note. A 
screech owl's talons had pierced the skull of a blue jay, but both 
birds were found dead entangled in a hawthorne tree {Crataegus 
sp.). A defective eye was noted in 13 owls captured during the study. 
Generally, the defective eyes were sealed shut. The defective eyes 
may have resulted from owls trying to capture prey in heavily 
wooded areas; however, the incidence of eye injuries was low (only 
13 in more than 3,000 birds handled). 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 15 

Table 3. Food items found at nest boxes occupied by screech owls in the fall and 
winter in northern Ohio a . 



Birds 



Mammals 



Fish 



Amphibians and 
crayfish 



House sparrow ... 9 Meadow Gizzard shad 5 Leopard frog . 6 

mouse .... 28 

Cardinal 8 Deer mouse . 20 Bullhead 1 Crayfish 3 

Mourning dove. . . 4 House mouse 10 Unidentified 

fish 1 

Blue jay 3 Norway rat . 9 

Tree sparrow 2 Shrew 5 

Starling 1 Unidentified 

mouse .... 1 
Common redpoll 

Screech owl 

Hermit thrush . . 
American robin . 
Common flicker . 



Total items 32 



73 



Percent 
(occurrence) . . 26.4 



60.3 



5.8 



7.5 



A few of the food items may have been taken earlier in the year. 



Fish in the Diet 

During the 1973 nesting season, a nest box on our study area was 
found with several dozen gizzard shad. Five green sunfish were 
also noted in a nest on 5 May 1972 and one minnow on 3 May 1973. 
Several other nest boxes smelled of fish when they were visited, 
although no fish were present. 

The presence of fish in the diet made us curious about 10 band 
recovery letters of screech owls that were reported to the Bird 
Banding Laboratory as drowned. It is doubtful that fishing 
activities by the owls led to their deaths. Six were found in water 
tanks (specifically, two in horse troughs and one in a windmill 
tank), two in rain barrels, one in a backyard swimming pool, and no 
details were given on the remaining recovery. Although it might be 
suspected that the drowned birds were all young and inexperienc- 
ed, only three were less than 2 mo old, whereas all of the others were 
more than 1 yr old. Kelso (1944) has included water as one of the 
chief considerations in the choice of territory, along with suitable 
roosting and nesting cavities, and the absence of larger species of 



16 NORTH AMERICAN FAUNA 71 

owls. Kelso indicated that screech owls will drink every day and 
bathe every 3 or 4 days if water is available. The birds that drowned 
may have been attempting to bathe or drink, or, more likely, 
attempting to catch insects or rodents in or near the water. Records 
of fishing appear in the literature (e.g., Treat 1889), and live fish 
have been found in tree cavities utilized by screech owls (Frazar 
1877). 

An incident in our study area on 8 December 1946, provides 
additional information concerning the relationship of screech owls 
to water. An oil slick was noticed on the Portage River and three 
screech owls from the area were captured that day with their legs 
and underparts covered with oil. The oil on one owl was still present 
on 14 January 1947 but it had been removed by 27 April 1947. We 
believe that if the owls were bathing in the water, oil would have 
been found throughout their plumage. These data lend support to 
the contention that screech owls take live prey from streams and 
ponds, and that aquatic food items found in nest boxes were not 
merely carcasses found along the shoreline. Based on our data, we 
cannot answer the question, "Do all screech owls take fish 
occasionally or do a few birds specialize in fish?" 

Seasonal Variation 

A seasonal pattern in the food habits of the screech owl became 
apparent during this study, although our sampling method was 
biased, i.e., most of the invertebrates taken by the screech owls were 
probably undetected in our study. Actually, the mammals and 
birds exchanged places of importance between the nesting season 
and the fall and winter (Table 4). Furthermore, the important bird 
species in the diet were nearly all migratory (Table 2) and not 
available during the other portions of the year. 

Table 4. The relative importance of birds to mammals in the seasonal diet of the 
screech owl (data obtained from nest boxes). 

Time No. No. Percent 

collected mammals birds birds 3 

Nesting season 145 309 68 

Fall and winter . . . . _J3 32 30 

The aggregate of birds plus mammals. 

Only rarely have authors discussed the food habits of the screech 
owl during all seasons of the year, although Baird et al. (187:57) 
stated, "The food is chiefly small quadrupeds, insects, and 
occasionally, when they have young, small birds." Fisher (1893) 
presented tabular data on mammals, birds, and other items found 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 17 

in the stomachs of birds collected during various months of the 
year, but did not analyze the data quantitatively for seasonal 
variation. Fisher (1893:166), however, did state, "It must be said 
that occasionally it is destructive to small birds, especially during 
the breeding season, when it has a number of hungry mouths to fill, 
and also in suburban districts where its favorite food is hard to 
procure." Fisher's data were collected primarily in the eastern 
United States. A reanalysis of his bird and mammal data 
suggested findings similar to ours reported in Table 4. 

The 479 food habit cards (approximately one-half of which were 
used by Fisher [1893]) filed at the Patuxent Wildlife Research 
Center, Laurel, Maryland, were reanalyzed for seasonal trends in 
the screech owl diet (Tables 5 and 6). Only data based on stomach 
contents were considered, and small sample sizes in June, July, 
August, and September made it necessary to pool 2-mo periods. The 
relative importance of arthropods in the diet throughout the year 
was evaluated on the basis of presence or absence in each stomach. 
This eliminated the problem of tremendous fluctuations in 
numbers present in individual stomachs and the problem of trying 
to obtain an exact count of the numbers present. The importance of 
arthropods (primarily insects) peaked in late summer and declined 
during the winter (Table 5). It must be emphasized that the 
arthropods were not detected in our study probably because they 
were eaten immediately instead of stored. The diet pattern followed 
availability, again indicating that the screech owl is an oppor- 
tunistic feeder. The relative importance of birds to mammals, based 
on the food habit cards (Table 6), was nearly identical to that found 
in our study (Table 4). 

Table 5. The relative importance of arthropods (primarily insects) in the seasonal 
diet of the screech owl (data obtained from 479 stomach content cards at 
Patuxent Wildlife Research Center). 

Month Stomachs No. samples with Percent with 

collected 3 checked arthropods arthropods 

January 56 6 11 

February 34 9 26 

March 38 9 24 

April 28 16 57 

May 46 34 74 

June-July 40 31 78 

August-September . 23 20 87 

October 40 24 60 

November 98 42 43 

December 76 12 16 



a 



Only samples collected in the Northeastern United States and Ontario. 



18 



NORTH AMERICAN FAUNA 71 



Table 6. The relative importance of birds to mammals in the seasonal diet of the 
screech owl (data obtained from 479 stomach content cards at Patuxent Wildlife 
Research Center). 

Month Stomachs No. No. Percent 

collected checked mammals birds birds 8 

January 56 33 "15 31 

February 34 15 5 25 

March 38 21 6 25 

April 28 8 11 58 

May 46 3 4 57 

June-July 40 3 6 67 

August-September 23 5 4 44 

October 40 21 8 28 

November 98 52 17 25 

December 76 45 14 24 

a The aggregate of birds plus mammals. April-September the percentage of birds 
was 57, whereas the percentage for October-March was 26. Other vertebrates in the 
stomachs include 7 toads, 7 frogs, 2 lizards, and 3 fish. 



The nesting season of the screech owl seems to be timed to take 
advantage of the spring migration of small birds. Lack (1968) 
concluded that breeding everywhere is timed in relation to the 
availability of food for the young. Eleanora's falcon (Falco 
eleanorae) of the bare rocky islands in the Mediterranean is 
another species whose nesting season is correlated with the 
migration of small birds (Vaughan 1961). This falcon, however, 
has young in September or October and feeds on the small birds 
migrating south at this season of the year. 

The food source for screech owls during the nesting season is 
possibly unique for owls. The species is not dependent upon a local 
rodent population, but is primarily dependent upon the spring 
migration of birds moving into its hunting territory from great 
distances away. A series of migrant species move through the 
nesting area on slightly different schedules in May and offers the 
screech owl a predictable prey source during the complete period 
when young are in the nest. 



MIGRATION AND DISPERSAL 

The 1957 Check-list of the American Ornithologists' Union 
states that the screech owl in the northeastern United States is 
mainly resident, but wanders in winter south to Alabama and 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 19 

Georgia. Owen (1963a), probably repeating the Check-list, also 
indicated that a few birds breeding in the North apparently move 
south in winter. Evidence of such movement between States, based 
on banding data, is nonexistent. Most interstate recoveries 
involved 16- to 32-km (10- to 20-mi) movements of birds banded near 
the State lines, although two definite exceptions were found. One 
bird was banded in Minnesota and reported 300 km (185 mi) 
southeast in Iowa and another was banded on our study area in 
northern Ohio and recovered 235 km (145 mi) northwest in 
Michigan. Both recoveries were reported during the first year of life 
from birds banded as nestlings. The first bird was banded on 25 
May 1932 and found dead on 20 December 1932, and the latter was 
banded on 14 May 1957 and found dead on 20 December 1957. Since 
the two recoveries were in opposite directions, they suggest a 
dispersal of young rather than migration. Our findings are in 
agreement with Fisher's (1893:167) statement, "This owl breeds 
throughout its range, and does not migrate or even wander far 
during the winter months." 

Dispersal Related to Age and Season 

Banding data show that young screech owls begin dispersing 
from their natal area in late summer and early fall (Table 7), and 
that by about 1 October, only one-fourth of the surviving young 
remain in the 10' block (approximately 23,500-hectare area 1 ) where 
they were hatched. Some young disperse more than 160 km (100 
mi). In contrast, about three-fourths of the adult birds remain 
sedentary and stay in the 10' block where banded (Table 7). In 
addition, 87% of the adults remained within 16 km (10 mi) of the 
banding site and no bird moved more than 64 km (40 mi) (Table 8). 
Furthermore, three of the four birds that moved beyond 32 km (20 
mi) were 6 yr old or older; they may have moved the additional 
distance to obtain a new mate after their original mate died (adult 
screech owls have a relatively high adult mortality rate). General- 
ly, there is little movement among adult screech owls. Hence, the 
major means by which a transfer of individuals occurs among 
populations of the nonmigratory screech owl is dispersal of young 
birds. The same findings were reported for the nonmigratory song 
sparrow by Johnston (1956). 



'An arbitrary geographical division of longitude and latitude used to report 
bandings of birds and band recoveries. 



20 



NORTH AMERICAN FAUNA 71 



Table 7. A comparison of the seasonal movement of young and adult screech owls 
banded in northern Ohio and in the remainder of the northeastern United 
States during the nesting season. 





NESTLINGS (Banded April-June) 




Recovery 


Time period of recovery" 




location 3 


Apr-June July-Sept Oct-Dec Jan-Mar 


Later years 


0-6 miles 


15 (94%) 20 (61%) 4 (24%) 2 (25%) 


8 (27%) 


More than 






6 miles 


1 (6%) 13 (39%) 13 (76%) 6 (75%) 


22 (73%) 


Total 


16 33 17 8 


30 




ADULTS (Banded March-June) 




Recovery 


Time period of recovery" 




location a 


Apr-June July-Sept Oct-Dec Jan-Mar 


Later years 


0-6 miles 


4 (100%) 4 (80%) 4 (80%) 8 (80%) 


23 (74%) 


More than 






6 miles 


( 0%) 1 (20%) 1 (20%) 2 (20%) 


8(26%) 


Total 


4 5 5 10 


31 



a Recoveries within same 10' block as banded were considered within 6 miles of the 
banding site. Owls recaptured by banders were not included, 
b Months refer to first year after banding. 



Dispersal Distance 

The mean distance moved by young birds (calculated after 1 
October) was 32 km (20 mi), although a majority of the birds were 
reported to have moved much shorter distances (Table 8). Several 
authors have attempted to determine if dispersal distance is 
random. Most of the tests have involved a comparison of the 
observed frequency distribution with the Poisson distribution 
(French et al. 1968). If the dispersal distance is random, then the 
number of animals in successive zones outward from the center 
should show a Poisson distribution. In evaluating this distribution 
the only data readily available on the place of banding and the 
place of recovery were the respective 10' blocks in which these 
events occurred. To change areas to locations for which distances 
can be calculated, we used the geographic centers of the 10' blocks 
involved. By using this approximation in Table 8, the hypothesis of 
random dispersal distance for young screech owls was rejected 
(P < 0.001). It appears that two different groups of young birds are 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 



21 



present in the population a group showing little dispersal (less than 
32 km [20 mi]), and a group wandering considerable distances (100 
to 300 km [60 to 185 mi]). Similar findings were presented for song 
sparrows by Johnston (1956) and for small mammals by Dice and 
Howard (1951), Howard (1960), and French et al. (1968). These 
authors all discussed the possibility that certain individuals within 
a population may desire to move long distances. Whether there is a 
genetic basis for his behavior, as proposed by Howard (1960), or 
whether all individuals are subject to this urge at a certain stage of 
their lives or season of the year, is uncertain. It should be noted, 
however, that about 6% of the screech owls moved beyond the 
expected range. The percentage is similar to the 10% reported for 
song sparrows in Ohio and California (Johnston 1956). Further- 
more, Henny and VanVelzen (1972) indicated that approximately 
11% of an osprey (Pandion haliaetus) population disperses from 
their natal area to breed. 



Table 8. Comparison of the observed dispersal distance of nestling screech owls 
with the Poisson distribution (includes recoveries from 1 October). The adult 
data are included in parentheses but are not statistically tested as little or no 
dispersal is indicated. 



Category 
(miles) 



Observed 
frequency 



Expected 
(Poisson) 



X 2 



0-10 

11-20 

21-30 

31-40 

41-50 

51-60 

61-70 

71-80 

81-90 

91-100 

101-110 

111-120 

121-130 

131-140 

141-150 

151-160 

161-170 

171-180 

181-190 

Total 



25 (48) a 4.5 




93.39 


21 (3) 11.2 


8.58 


3 (2) 14.0 


8.64 


2 (2) 11.6 


7.94 





7.2 









3.6 






1 


1.5 









0.5 









0.2 









T 









T 






1 


> 4 T 


> 13.0 


6.23 





T 









T 






1 


T 









T 









T 









T 






_1_ 


_T 






55 " 


54.3 




124.78 (P<0.001) 



a Number in parenthes refers to 55 recoveries of screech owls banded as adults in 
March through June. 



22 NORTH AMERICAN FAUNA 71 

The rigors of wandering during their first autumn and winter 
probably imposes considerable hardship on the young owls. Many 
have not yet located suitable roosting areas or other essential 
requirements, which may make them vulnerable to severe weather 
and predation from larger owls. However, it should be noted that 
theoretically, the apparent nonrandom distance of the dispersal 
has evolved because of its advantage to the species. Current ideas 
on nonrandom vertebrate dispersal can be conveniently divided 
into two viewpoints: (1) those that suggest an "inherent" or 
"genetic" difference between individuals causing them to be short- 
distance or long-distance dispersers (e.g., Johnston 1956, Howard 
1960, Lidicker 1962), and (2) those that suggest the only difference 
between individuals is their ability (relative dominance) to procure 
a residence (e.g., Murray 1967). 

The first group tends to think of innate dispersal in terms of 
benefiting the population (in spite of the high rate of mortality of 
long-distance dispersers). The advantages to the population from 
such behavioral characteristics are said to be: (1) the increase of 
gene flow to ensure a tendency toward greater panmixia, (2) the 
increase in ability to spread its range rapidly as favorable habitats 
are created and to quickly reinvade areas that may have been 
depopulated by catastrophes, and (3) the increase in ability to 
regulate population density below carrying capacity. Murray 
(1967) tends to think in terms of benefit to the individuals, i.e., the 
dominants that established a nearby residence would be at a 
distinct advantage. Furthermore, Murray (1967) believes the 
advantages cited by the first group are actually unselected 
consequences of selection for individuals that aggressively procure 
breeding sites but move away from dominant individuals. He 
concluded by saying there is no reason to believe that selection has 
occurred above the level of the individual. Although the 
mechanisms involved in dispersal are not fully understood, the 
many advantages of such a trait are readily apparent. 

Directional Pattern of Dispersal 

The directional pattern of screech owl dispersal in the 
northeastern United States was determined by using all birds 
recovered outside the 10' block where banded. Retrapped birds are 
not included here as they would be biased to the study area. As 
shown in Table 8, the majority of the birds were reported within 32 
km (20 mi) of the banding site. 

The apparent dispersal of more birds east and west is a result of 
the shape of the 10' blocks at the latitude where the data were 
collected (Fig. 5). The blocks are rectangular (approximately 21x11 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 23 



N W 

14 (1 7 71 



West 

35 (28.39) 



sw 

13 (17.71: 



North 

21 (1 518) 



7 Mi 



South 

15(15.1 8) 



NE 

14 (17.71) 



East 

30 (28.39) 



n= 158 R ECOV E R I ES 
X = 6.8* , 7 d. f 



S E 



16 (17.71) 



Fig. 5. — The directional preference of screech owl dispersal in the northeastern 
United States. 

km [13 x 7 mi]) and it is a shorter distance (by 53.8%) to an adjacent 
block if one moves east or west. The data in question are 
frequencies of recovery in eight recovery areas defined with 
reference to the 10' block of banding (Fig. 5). Again, we must 
assume that banding occurred at the center of the block. Because of 
the shape of the banding block and the recovery areas, these eight 
frequencies of recoveries would not have equal expected values, 
even if dispersal were random with respect to the direction of 
movement. The problem here is to ascertain if the data obtained are 
consistent with random direction of dispersal. 

We computed the distribution of such recovery frequencies 
conditional on the dispersal distance given a random direction of 
dispersal. 1 This computation then allowed a test of the null 
hypothesis that direction of dispersal is random. 

Given the length and width of the 10' block of banding, and a set 
of distances for a corresponding set of frequencies, the expected 



'For additional details, see Memorandum in files at Migratory Bird and Habitat 
Research Laboratory, "Distribution of band recovery frequencies for the screech owl 
data." dated 20 May 1974. 



24 



NORTH AMERICAN FAUNA 71 



frequencies can be computed under the hypothesis of random 
direction of dispersal. The formulae are not complex, except for 
requiring some inverse cosines. The theoretical frequencies of 
recovery by area are shown in parentheses in Fig. 5. The null 
hypothesis of random directional dispersal is not rejected (X 2 = 6.84, 
7 df, P = 0.50). Therefore, we conclude that the direction of dispersal 
of the screech owl is random. 

Records of Individual Birds 

Multi-recaptures on our study area in northern Ohio provided 
some additional information on movement or lack of movement in 
adult birds. Generally, after an adult bird established a nesting 
site, it was found only at that site in future years. For example, an 
adult female was brooding five young in a box on 12 May 1952. The 
bird was also found in the same box in 1954 (brooding five young), 
in 1957 (brooding three young), and 1959 (brooding four young). 
The bird was found dead in the immediate vicinity of the nest site 
on 31 March 1960. Records of this type were very common during 
the study. One of the longest records for an individual owl was a 
female that was banded in a box on 19 November 1945 and was 
retrapped eight times at the same nest box before it was killed by a 
car on 12 February 1958 (approximate age 13 yr) 400 m (0.25 mi) 
from the nest box. Figure 6 shows a banded screech owl being 
released at its nest box. 




Fig. 6.— A banded screech owl being released at its nest box by the senior author 
{photo courtesy of George Laycock). 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 25 

BREEDING BIOLOGY 

Breeding Season 

Carpenter (1883) discussed the breeding of a pair of screech owls 
that raised a brood of young in captivity in Massachusetts. He 
indicated that by 1 February 1883 the actions of the adult birds 
toward each other began to change and they became more attentive 
to each other. A clutch of three eggs was found on 15 April and 
incubation lasted 22 days. Kelso (1944) reported paired owls at 
Ithaca, New York, by 19 February, but egg-laying did not 
commence until 15 March. Sherman (1911) reported the first egg of 
a clutch laid on 27 March 1910 (apparently in Iowa) and that a 
complete clutch of four eggs was found on 4 April. The first two eggs 
hatched on 27 April and the remaining two on the next 2 days (26- 
day incubation period). She reported the birds leaving the nest 30, 
31, and 32 days after hatching. Kelso (1950) reported screech owls 
in New York leaving the nest in 31 to 35 days. Bent (1938) indicated 
that the incubation period is variously reported as lasting from 21 
to 30 days, but that the average is probably around 26 days as 
determined by Sherman (1911). Craighead and Craighead (1956) 
studied 28 nests in Superior Township, Michigan, in 1942 and 1948. 
They reported that the first selection of territories began in late 
February. The earliest egg-laying date was 18 April and the earliest 
hatching date was 12 May. The mean date for the broods leaving 
the nest was 1 June. 

During our study, pairs of owls were first seen together in the nest 
boxes as early as 1 and 3 February; egg-laying began considerably 
later. The chronology of the nesting season in northern Ohio may 
be determined from the data in Table 9. Small young (1 to 10 days 
old) were first seen in the nests during 11-20 April; however, the 
majority of small young (70%) were found during 21 April-10 May. 
Therefore, most of the eggs were hatched from 15 April-5 May. 
Assuming a 26-day incubation period, egg-laying began about 15 
March for the majority of the population with a few laying 5 to 10 
days earlier. A few small young were seen as late as the first week in 
June. These young were probably the results of renesting efforts. 
Thus, most young in the northern Ohio fledge during the last week 
of May or the first week of June. The young attain a bandable size 
with their color readily apparent at about 3 wk of age (10 days 
before fledging). 



26 



NORTH AMERICAN FAUNA 71 



Table 9. Percentages of three size categories of young screech owls in 10-day 
periods during the brooding season in northern Ohio. 

Small young Medium young Bandable young 

Date (^1-10 days old) (^11-20 days old (d21-30 days old) 



11-20 April 


6 








21-30 April 


33 


a 4 


5 


1-10 May 


37 


62 


17 


11-20 May 


18 


28 


43 


21-31 May 


5 


6 


30 


1-10 June 


2 





4 


11-20 June 








1 



Number of nests 
in sample 101 71 440 

a Longitudinal line refers to peak period for young in each size category, which in 
turn documents the progression of the breeding season. 



Clutch Size 

Bent (1938) indicated that screech owls lay three to seven eggs, 
but usually four or five, with the average in favor of five. He further 
indicated that the extremely large or small sets are rare, and the 
reports of eight or nine seem doubtful. The average clutch in our 
study was 4.43 (Table 10). 

Among egg sets from 12 museums in the eastern half of the 
United States, the largest clutches were found in the north-central 
part of the region and the smallest in the Southeast (Table 10). This 
suggests a clinal pattern in which clutch size, in general, increased 
from east to west and from south to north. One unusual clutch of 10 
eggs was found during our study. The bird was incubating on 2 
May 1967 and three medium-sized young were found in the nest on 
18 May. Two birds probably laid the eggs. We banded seven young 
in another nest on 7 May 1954. Since this was our only record of 
seven young in a nest and there were no infertile eggs found, it is 
probably safe to assume that the clutch size was seven. The record 
of this nest is not included in the table, however, because it was not 
visited when eggs were in the nest. Little effort was made to obtain 
information on clutch size during our study because disturbance 
during the incubation period seemed to cause nest abandonment. 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 27 

Table 10. Clutch sizes of screech owls in northern Ohio compared with egg sets 
in 12 museums (nearly all museum sets were collected before 1900). 

Clutch size 

Location 12 3 4 5678n Mean 

Northeast 3 3 17 36 28 8 1 93 4.27 

Northern Ohiob 1 10 37 35 8 91 4.43 

Northcentral c 2 7 6300 18 4.56 

Midwest d 1 6 5 3000 15 3.67 

Arkansas-Oklahoma- 
Texas 7 9 0000 16 3.56 

Georgia-South 

Carolina-Tennessee... 2 5 3 10 3.10 

Florida 1 13 32 7 4 57 3.00 

a Massachusetts, Rhode Island, New York, Pennsylvania, New Jersey, 
Maryland, and Washington, D.C. 
bDoes not include one clutch of 10 eggs. 
c Ohio, Indiana, Illinois, and Wisconsin, 
d Missouri, Iowa, Kansas, and Nebraska. 



Nesting Success and Fledging Rates 

Craighead and Craighead (1956) provide the only estimate of the 
number of young fledged per nesting attempt; however, their 
estimate was based on several assumptions because the informa- 
tion from most nests was incomplete. They concluded that between 
2.6 and 3.0 young were fledged per pair of adults in Michigan and 
Wyoming. 

Much of our information from nests was also incomplete. Many 
of the nests were not visited early, i.e., during the incubation period, 
and some unsuccessful nests were not detected. The number of 
active nests in 1944-73 (Table 11) partially reflects the authors' 
effort rather than abundance. From these 440 successful nestings, 
1,673 young were believed to have been fledged (3.80 young per 
successful nesting). The annual variation in numbers fledged per 
successful nesting was small (coefficient of variation [* 100] of 8.2). 



28 



NORTH AMERICAN FAUNA 71 



Table 11. Success of screech owl nests in northern Ohio, 1944-73. 













Young 








Percent 




fledged per 




No. active 


No. nests 


of nests 


No. young 


successful 


Year 


nests 


successful 8 


successful 


fledged 


nest 


1944 


4 


4 


100.0 


-17 


4.25 


1945 c 


5 


4 


80.0 


16 


4.00 


1946 c 


9 


6 


66.7 


22 


3.67 


1947 c 


19 


14 


73.7 


56 


4.00 


1948 c 


20 


11 


55.0 


37 


3.36 


1949 c 


19 


15 


78.9 


61 


4.07 


1950 


26 


24 


92.3 


87 


3.63 


1951 


11 


10 


90.9 


39 


3.90 


1952 


25 


23 


92.0 


85 


3.70 


1953 


34 


31 


91.2 


116 


3.74 


1954 


49 


44 


89.8 


197 


4.48 


1955 


42 


42 


100.0 


161 


3.83 


1956 


22 


20 


90.9 


81 


4.05 


1957 


16 


14 


87.5 


54 


3.86 


1958 


17 


14 


82.4 


50 


3.57 


1959 


15 


12 


80.0 


49 


4.08 


1960 c 


17 


11 


64.7 


40 


3.64 


1961 c 


12 


9 


75.0 


32 


3.56 


1962 


20 


18 


90.0 


70 


3.89 


1963 c 


16 


11 


68.8 


40 


3.64 


1964 


9 


9 


100.0 


.34 


3.78 


1965 


8 


7 


87.5 


29 


4.14 


1966 


9 


9 


100.0 


30 


3.33 


1967 c 


18 


16 


88.9 


54 


3.38 


1968 


13 


13 


100.0 


49 


3.77 


1969 c 


12 


10 


83.3 


35 


3.50 


1970 


13 


12 


92.3 


45 


3.75 


1971 c 


13 


11 


84.6 


34 


3.09 


1972 


18 


16 


88.9 


53 


3.31 


1973 d 


— 


— 


— 


— 


— 


Total 


511 


440 


86.1 


1,673 


3.80 



a Fledged at least one young. 

bThis percentage is biased high because many nests were visited only once when 
the young attained bandable size. 

c Years when 75 percent of nests were visited on two or more occasions. 

dData excluded because one egg from each clutch was collected for pesticide 
study. 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 



29 



The banding date disclosed variation in the number of young 
reported per successful nesting (Table 12). The earliest nesting 
attempts produced slightly more young than later attempts. This 
may be partially due to the older, more experienced birds nesting 
first, whereas the late nests may include more first-year breeders. 
Also, a few renests with small clutch sizes may have been included 
in the last category. A brood of five young is shown in Fig. 7. 




Fig. 7. — Five downy screech owls {photo courtesy of G. Ronald Austing). 



Table 12. Number of young screech owls per successful nest during five banding 
periods (from early to late nesters) in northern Ohio. 





Successful 


Number 


Mean 


Banding date 3 


nests 


banded 


per nest 


22 April — 9 May 


88 


367 


4.17 


10 May — 15 May 


88 


336 


3.82 


16 May — 19 May 


88 


335 


3.81 


20 May — 23 May 


88 


334 


3.80 


24 May — 20 June 


88 


301 


3.42 


Total 


440 


1,673 


3.80 



a The banding dates were adjusted so that an equal number of successful nests 
were reported in each banding period. 



30 NORTH AMERICAN FAUNA 71 

Because the data presented in Table 11 include a number of nests 
that were visited only once, i.e., at the time of banding, a rationale 
was needed to obtain an estimate of the number of young fledged 
per breeding pair. The data are obviously an overestimate of the 
percentage of nesting attempts that were successful. Two methods 
were used to estimate the number of young fledged per breeding 
pair: (1) use of only nests with the most complete set of 
observations, i.e., nests visited initially during incubation, and (2) 
the Mayfield (1961) Exposure-day Method (and the data which met 
his criteria). 

Nests Visited Initially During Incubation 

This method was restrictive and reduced the number of usable 
nesting records from the total of 511 to 165, which were the most 
complete ones (Table 13). Because egg-laying in northern Ohio 
begins about 15 March for most of the population, the nests visited 
between 18 March and 10 April best represent the fledging rate per 
nesting attempt (Table 13). The fledging rate estimates for 11-30 
April were believed to be inflated because of undetected losses 
before the first visit to the nest. Thus, we believe the best estimate 
by this rationale is 2.55 young fledged per pair. 

Table 13. Screech owl nesting success and fledging rates as related to the date the 
nest was first visited. Only nests with eggs present at first visit were included. 









Young fledged 




No. of 


No. of young 


per nesting 


Date first visited 8 


nests 


fledged 


attempt 


18 March — 10 April 


20 


51 


2.55 


11 April — 20 April 


64 


178 


2.78 


21 April — 30 April 


81 


238 


2.94 



All years combined. 



Mayfield Exposure-day Method 

Other field workers have often been faced with interpreting 
nesting success with data from nests that were not observed from 
the start of incubation. Further, the sample may include some nests 
for which success of nesting was unknown. If such partially 
complete records are included (or excluded) from his calculations, 
there is a danger of distorting the conclusions. Mayfield (1961) 
presented a way of dealing with this problem by reducing the data 
to units of exposure, which reflects not only the number of nests but 
also the length of time each was under observation. A convenient 
measure of exposure is the nest-day (equivalent to one nest for 1 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 31 

day). With this method, all single or multiple observations of 
individual nests can be incorporated into the sample, even though 
some of these do not go back to the very beginning or do not carry 
through to the end. Mayfield (1961) points out that after mortality 
and survival rates are expressed per nest-day, the probability of 
survival of eggs and young may be calculated for all or any part of 
this nesting period. If the survival rate per nest-day is S, the 
probability of survival of a nest for d days is S d . Mayfield further 
pointed out that the survival rate during incubation is different 
from the rate during the nestling period, so these two stages must 
be treated separately. Also, egg survival must be considered 
separately from nest survival because of the loss of individual eggs, 
particularly at hatching time. The probabilities of survival in 
different stages of nesting may be combined through the 
mathematical principle that the probability of a succession of 
events is the product of their separate probabilities. 

We found that only 13 young from successful nestings (nests 
from which at least 1 young is fledged) were lost in 1,090 exposure 
days between 1944 and 1973 (loss per exposure-day of 0.012). 
Furthermore, the loss of young per exposure-day was quite 
consistent throughout the brood rearing period in successful 
nestings (i.e., 22 April-9 May— 0.010, 10-20 May— 0.012, and 21 
May-20 June — 0.015). Based on a 30-day period for the young in the 
nest, an estimated 0.36 young was lost per successful nesting from 
the period of hatching to fledging. Thus, we worked backwards 
from the number of young fledged per successful nesting (3.80 from 
Table 11) to determine the number of young hatched per successful 
nest. This approach was a necessity because of our weak data for 
early in the incubation period. Early visits had the potential to 
cause nest abandonment and thus, it was not possible to estimate 
egg loss on an exposure-day basis. Egg loss in successful nests was 
estimated by difference. With an estimated 3.80 young fledged per 
successful nesting, we estimate that approximately 4.16 young 
hatched per successful nesting from the 4.43 eggs laid (Table 10). 
Thus, by difference, an estimated 94% of the eggs in successful 
nests hatched. The loss of young after hatching from broods that 
were not completely destroyed appeared to be low. 

As mentioned earlier, the percentage of successful nesting was 
overestimated in Table 11, particularly during years when less 
than 75% of the nests were visited two or more times (Table 11, 
footnote c). It is most important to obtain an estimate of the 
percentage of nestings that were successful. Mayfield's (1961) 
method is again believed applicable. Since few nests were visited 
more than twice, a problem exists regarding the timing of nest loss. 



32 NORTH AMERICAN FAUNA 71 

Therefore, we must of necessity assume that the loss rate of 
complete nests was constant through time. This assumption may 
or may not be true; therefore, our recruitment estimates based on 
this rationale must be regarded as tentative. Our pooled data 
indicate that during 5,189 exposure days, 34 nests were lost or 
believed lost (young in 2 nests may have fledged before being 
revisited; however, we assumed that one nest was successful). An 
estimated 0.006552 nest was lost per nest-day. With an incubation 
period of 26 days and a nestling period of 30 days, each nest was 
susceptible to loss for 56 days. By applying Mayfield's formula, 
(0.993448 56 ) we estimated that 69.2% of the screech owl nesting 
attempts in northern Ohio were successful during the study. With 
3.80 young fledged per successful nest, an estimated 2.63 young 
were fledged per nesting attempt based on the Mayfield Exposure- 
day Approach. This estimate is in close agreement with that of 2.55 
fledged young based on a much smaller sample of nests observed 
throughout the breeding season. In summarizing our data on 
recruitment, we believe that the best estimate of the number of 
young fledged in northern Ohio during the 30-yr study was about 
2.5 to 2.6 young per nesting attempt. 

Causes of Nest Loss 

Sixty-six unsuccessful nests were found during our study. The 
actual cause of nest failure was determined for a few of the nests 
listed in Table 14. Most of the nests (41) were either deserted or 
destroyed. Raccoons (Procyon lotor) were implicated in the loss of 
15 nests, and the eggs failed to hatch in 7 nests. Craighead and 
Craighead (1956) noted that fox squirrels (Sciurus niger) destroyed 
the eggs of two pairs of screech owls in Michigan and appropriated 
the nest hollows. They also pointed out that screech owl nests and 
ranges are commonly within the nesting range of the great horned 
owl (Bubo virginianus) and that young suffer their heaviest 
mortality soon after leaving the protection of the nest hollow. 

Table 14. Causes for screech owl nests being unsuccessful in northern Ohio, 
1944-72. 

Cause of loss Number of nests 

Deserted or destroyed (reason unknown) 41 

Raccoon predation 15 

Eggs failed to hatch 7 

Young dead in nest 2 

People (children) took young 1 

Total 66 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 33 

POPULATION DYNAMICS 

Sex Ratio in Population 

Behavioral differences between sexes are to be expected in most 
species. Bellrose et al. (1961) described biases involved in sampling 
waterfowl populations to obtain sex ratios. For waterfowl and 
many other groups of birds, a bias may result from (1) a differential 
migration of the sexes (time of year sample collected), (2) the type of 
area where collection was made (characteristics of the habitat), or 
(3) a differential vulnerability of the sexes to the method of 
collection (e.g., shot, trapped, etc.). 

The information on screech owls concerning sex ratios may also 
be biased, although the nonmigratory characteristic of the species 
tends to minimize several potential biases. The sex data were 
obtained from the stomach content records filed at the Patuxent 
Wildlife Research Center. Not all records contained information on 
sex. To eliminate one potential bias, the information from March 
through June (the nesting season when the female was probably on 
the nest) was eliminated from the sample. Thus, for July to 
February, of 234 screech owls collected in the northeastern United 
States, 113 (48.3%) were males and 121 (51.7%) were females. The 
sample did not deviate significantly from the expected 50:50 sex 
ratio (chi-square = 0.13, 1 df). 

Data obtained in our study area could not be analyzed for sex 
ratio information because of the potential bias associated with the 
sampling scheme (nearly all birds captured at nest boxes were 
paired). 

Mortality Rates 

Mortality rates for the screech owl have not been estimated 
previously. In this study, life tables were made from band recovery 
data in northeastern United States and Ontario; a large percentage 
of the data came from our study area. Recoveries were not included 
if information was insufficient to tell if the bird was dead or alive 
and released. Also, all retrapped birds, sight records, and skeletal 
remains were omitted. Nestlings were generally banded about 10 
days before fledging and, therefore, recoveries during the first 10 
days after banding were not included. An initial date (the date after 
which recoveries would be used in the life tables) of the first 15 May 
after banding was used for the adults, as opposed to 1 January, 
because we believe that the nonmigratory screech owl does not 
attain an adult mortality schedule by 1 January. Henny (1972) 
indicated that most migratory passerines achieve a schedule of 



34 NORTH AMERICAN FAUNA 71 

adult mortality by the first 1 January after hatching; however, the 
more sedentary species (e.g., cardinal, black-capped chickadee 
[Parus atricapillus] blue jay) in the northern portion of the United 
States do not attain their schedule of adult mortality rates until 
later. This phenomenon was first reported for the sedentary 
California quail (Lophortyx californicus) by Emlen (1940). 

Nestlings and adults banded through 1964 and recoveries 
processed through August 1974 were included in the analysis. It is 
believed that the life span of the banded cohort was nearly 
completed by 1974 as only two recoveries greater than 9 yr of age 
have been reported during the last 50 yr. However, the possibility 
still exists that the mortality rate estimates from the composite 
dynamic life table (Hickey 1952) may be biased slightly on the high 
side. 

Mortality rate estimates shown in Table 15 suggest that 
approximately 32.8 to 39.0% of the adult population dies annually. 
The range in the estimates of adult mortality rate is undoubtedly 
due to small sample sizes, particularly for the cohort banded as 
nestlings. Probably the best estimate of adult mortality rate would 
be based on the pooled recoveries from bandings of both nestlings 
(second year and later) and adults. The combined data yielded an 
annual adult mortality rate estimate of 33.9%. The first-year 
mortality rate for young birds is much higher, about 69.5%. 

Table 15. Estimates of mortality rates for screech owls banded as nestlings and 
adults in the northeastern United States and Ontario during 1915-64. 

A Adults 3 Nestlings 

interval Number of Alive at Mortality Number of Alive at Mortality 
years recoveries beginning rate recoveries beginning rate 

0.328 b 



0-1 


55 


134 


1-2 


25 


79 


2-3 


13 


54 


3-4 


9 


41 


4-5 


7 


32 


5-6 


8 


25 


6-7 


6 


17 


7-8 


3 


11 


8-9 


5 


8 


9-10 


1 


3 


10-11 


1 


2 


11-12 





1 


12-13 


1 


1 



73 


105 


0.695^ 
0.390 


13 


32 


7 


19 




4 


12 




2 


8 




2 


6 




3 


4 




1 


1 













































a An initial date of the first 15 May after banding was used for all birds not banded 
as nestlings. On this date, the birds were arbitrarily classified as adults. 



"Annual adult mortality rate estimate. 
c First-year mortality rate estimate. 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 35 

Causes of Postfledging Mortality 

Band recoveries provide some information on causes of mortali- 
ty, although dead owls found by humans are not a random sample 
of the mortality occurring in a population. For example, predation 
by other raptors would most likely be undetected because the 
carcasses would disappear before being found. Actually, most 
bands reported to the Bird Banding Laboratory are from birds that 
died as a result of man's activities (his vehicles, pets, etc.). 
Furthermore, the majority of the bands are reported as found on 
dead birds with no cause of death mentioned. 

Screech owls hit by motor vehicles and found along roads appear 
frequently in the sample reported to the Bird Banding Laboratory 
(Table 16). Sutton (1927) reported 113 requests for permits to retain 
specimens of the screech owls which had been found dead or in 
weakened or wounded condition in Pennsylvania. The causes of 
death included 6 which may have died of starvation or illness, 2 
which had flown into window panes, 7 which were caught by steel 
traps, 1 which had been killed by the felling of a tree, 13 which had 
been shot, 2 which were alive but in poor condition, and 82 which 
evidently were killed by flying into automobiles. Schorger (1954) 
recorded 235 road-killed screech owls during a period of 18 yr 
between Madison, Wisconsin, and Freeport, Illinois; Stupka(1953) 
examined 41 screech owls along the highways in or near Great 
Smoky Mountains National Park. Both Sutton (1927) and Stupka 
(1953) believed that in most instances these birds were struck and 
killed while feeding upon prey, primarily insects, along highways. 

The recovery data submitted to the Bird Banding Laboratory, as 
anticipated, suggested that predation on the screech owl by other 
raptors was of minor importance. Craighead and Craighead (1956), 
however, indicated that the screech owl was fairly vulnerable to the 
great horned owl because both often hunt the same habitat. 

The importance of the various mortality factors affecting a 
screech owl population cannot now or perhaps ever be determined. 
However, it is important to recognize that man, his machinery, his 
vehicles, his pets, and his buildings and other structures do take 
their toll; at the same time, it should be understood that many of 
man's alterations of the habitat have benefited the screech owl. 
Numerous authors have stated that screech owls seem to take 
kindly to man's occupation of the land and breed and thrive in the 
vicinity of his dwellings, in spite of the injudicious warfare which is 
sometimes waged against them. 



36 



NORTH AMERICAN FAUNA 71 






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THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 37 

Age at Sexual Maturity 

The age at sexual maturity is one of the most important and 
difficult parameters to determine for a wild population of birds, and 
published information on the subject for the screech owl was not 
available. Screech owls were recorded breeding as 1-yr-olds on 10 
occasions (7 females and 3 males) during our study, and 3 other 1- 
yr-olds were found in nest boxes in April but with no sign of eggs or 
a mate. These birds were all banded as nestlings and found the 
following spring. Of particular interest was a 1-yr-old male that 
mated with its mother in 1971 and produced one young. Nestings 
where a 1-yr-old was involved fledged 3.55 young per successful 
attempt (slightly lower than the 3.80 average for the population, 
Table 11). Ten is a small number to have of definite records of 1-yr- 
olds breeding considering the amount of time spent in the field 
during the 30-yr study; however, it must be remembered that the 
young disperse during the late summer and early fall (Table 7). 
Thus, the chance of recapturing a banded 1-yr-old screech owl in 
another nest box is remote. 

As well as establishing the capability of 1-yr-olds to breed, we 
need to know the percentage that do breed at this age. Of 13 1-yr- 
olds found during the nesting season, 10 (77%) were nesting. This 
estimate may be biased because other nonbreeding 1-yr-olds may 
not associate with nest boxes and therefore be undetected. Another 
approach is to compare the number of breeding 2-yr-olds found 
nesting during the study with the number of 1-yr-olds found 
nesting. Then, given the average annual adult mortality rate of 
approximately 33.9% (from previous section), and given that eight 
2-yr-olds were found nesting, we estimated that there should be a 
total of 12.1 breeding 1-yr-olds located if all 1-yr-olds and 2-yr-olds 
attempted to nest. Of course, only 10 were found, which provides an 
estimate of 83% (10/12.1) of the 1-yr-olds attempting to nest 
annually. 

Conclusions regarding the percentage of the 1-yr-old screech 
owls that attempt to nest annually must be regarded as tentative at 
this time. Furthermore, the percentage may fluctuate from year to 
year. Our two estimates (based on very small sample sizes) suggest 
that an average of possibly 77 to 83% of the 1-yr-olds attempt to nest 
annually, but the percentage may be lower. 

In comparison, Henny (1972) summarized information from 
several published and unpublished sources on age at sexual 
maturity of great horned owls. He concluded that most individuals 
do not breed until they are 2-yr-olds, although an average of about 
one-fourth of the population nests as 1-yr-olds, with the percentage 



38 NORTH AMERICAN FAUNA 71 

varying from year to year depending upon local conditions. Similar 
but more dramatic findings have been reported for other owls that 
are primarily dependent upon one food source. Barn owls (Tyto 
alba) were reported to nest at the end of their first year of life 
(Schneider 1937, Stewart 1952); however, in general, barn owls 
seem to breed irregularly. Wallace (1963:209) stated, "The barn owl 
may breed almost continuously during peak years in the Microtus 
(meadow mouse) cycle, but slow down or skip a breeding season 
when its staple prey is scarce." Similar observations were recorded 
for the tawny owl (Strix aluco) in England. Lack (1966) reported 
that 59% of the population was breeding pairs, but the percentage 
that nested in specific years ranged from to 90%. Increases in 
clutch size also were reported for some species of owls in years when 
rodents were abundant. Lack (1968) reported that the short-eared 
owl (Asio flammeus) may lay nine eggs during a vole plague, but 
only about half that many during other years. 

The number of young fledged per successful screech owl pair was 
very consistent from year to year (Table 11). This is probably the 
result of the species being an opportunistic feeder. We do not believe 
that large numbers of breeding-age screech owls fail to produce in 
years when the local rodent cycle is low, although rodent cycles 
may have some influence on 1-yr-olds. Unlike the barn owl, tawny 
owl, and short-eared owl, the screech owl is not totally dependent 
upon local rodents (or one genus of mouse) for its food supply. In 
fact, the young are fed a diet which consists largely of small birds of 
many species which migrate through the area (Table 2). Thus, the 
varied food supply for the young is not produced locally; it is highly 
dependable and actually migrates to the owl's hunting areas 
annually from numerous places, some being considerable dis- 
tances away. The opportunistic feeding habits of the screech owl 
apparently tend to maintain a very uniform reproduction annually 
and, to our knowledge, nonbreeding is limited to a small percentage 
of the 1-yr-olds. We must caution, however, that more research is 
needed on sexual maturity. 



PESTICIDES AND POLLUTION 

Eggshell thinning has been found to occur widely and is well 
documented, especially in the United States and England. In the 
United States, natural populations of at least 22 species represen- 
ting seven orders have been affected. In seven of eight species 
where shell thinning exceeded 20%, there was an associated 
population decline (Anderson and Hickey 1972). The thinning 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 39 

began to occur in the United States and England in the mid-forties 
at the time DDT and other organochlorine insecticides were first 
available for widespread use (Ratcliffe 1967, Hickey and Anderson 
1968). The possible relationship of shell thinning to the use of DDT 
led to a series of studies to test the effects of DDT and its 
metabolites on shell thinning and reproduction of birds. 

Initial laboratory studies were conducted on mallards (Anas 
platyrhynchos), black ducks (Anas rubripes), and American 
kestrels (Falco sparverisu) and showed a direct causal relationship 
between DDE and eggshell thinning (Heath et al. 1969; Longcore et 
al. 1971; Wiemeyer and Porter 1970). The next species to be studied 
in a similar manner was the screech owl (McLane and Hall 1972). 
The owls were given a dietary dosage of 2.8 ppm of DDE (equivalent 
to 10 ppm dry weight). Birds fed untreated food in 1970 and DDE 
dosage in 1971 laid eggs with shells that were 12% thinner than in 
1970. McLane and Hall (1972) concluded that screech owls seem to 
show greater shell thinning than American kestrels which showed 
10% thinning on a similar diet (Wiemeyer and Porter 1970). 

Eggshell Thinning 

The work by McLane and Hall (1972) indicated that screech owls 
were susceptible to the eggshell thinning phenomenon. In 
cooperation with the Patuxent Wildlife Research Center, one egg 
was collected from each of the 21 clutches on our study area in 1973. 
The eggs were measured for eggshell thickness and analyzed for 
chlorinated hydrocarbon residues. The remainder of the clutch was 
observed for hatchability and fledging rates. Klaas and Swineford 
(In preparation, Patuxent Wildlife Research Center) compared 
shell thickness in these 1973 eggs with shell thickness in eggs 
collected before 1947 in Ohio (Table 17). No significant decrease in 
eggshell thickness in the 1973 population was detected. 

Eggshells collected from the field in 1973 were about 8% thicker 
than the controls in the laboratory experiment (Table 17). This is of 
interest because many of the owls in the laboratory experiment 
were obtained from our study area in northern Ohio in 1967. The 
reason for the thinner control eggs in the laboratory study is 
unknown, but it may be due to a number of factors (e.g., age of birds, 
diet, etc.). 



40 NORTH AMERICAN FAUNA 71 

Table 17. Shell thickness of screech owl eggs. 







No. of 










No. of 


eggshells 


Shell thickness (mm) 


Location 


females 


measured 


Mean 


(S.E.) 


Range 


Laboratory experiment 


a 










Controls 


14 


66 


.218 




.180-.253 


Experimental (DDE) 


6 


28 


.189b 




.170-.205 


Field study c 












Pennsylvania 












pre-1946 d 


37 


37 


.241 


.003 


.197-.277 


Ohio pre-1946 d 


12 


12 


.244 


.004 


.230-.287 


Ohio 1973 












(random, early) d 


19 


19 


.234 f 


.004 


.197-.260 


Ohio 1973 (addled) e 


8 


16 


.243 f 


.007 


.157-.270 



a Patuxent Wildlife Research Center (McLane and Hall, 1972). 

D Significantly different from shell thickness of controls (P<0.05). 

c Klaas and Swineford (In preparation, Patuxent Wildlife Research Center). 

dOne egg selected at random from each clutch in early stages of incubation, but 
after clutch was completed. 

e Unhatched eggs removed from nest after incubation was complete. Includes 11 
eggs from three clutches in which no eggs hatched. 

f No statistical differences were detected in the means for the four groups of eggs. 
t-test(PX).l). 



Residue Levels in Eggs 

The 19 fresh eggs collected early in the nesting season and an 
additional 16 addled eggs found in the same nests at the end of the 
1973 nesting season were analyzed for chlorinated hydrocarbons 
(Klaas and Swineford, in preparation, Patuxent Wildlife Research 
Center). DDE and polychlorinated biphenyls (PCB's) were found in 
all 35 samples (Table 18), although the levels were generally low, as 
expected in view of the lack of eggshell thinning in the population 
(Table 17). Eight eggs in the sample (all from separate clutches) 
contained dieldrin (range 0.10-0.24 ppm). 

Table 18. Residue levels of DDE and PCB's in 35 screech owl eggs collected in 
northern Ohio, 1973 (from Klaas and Swineford in preparation, fatuxent 
Wildlife Research Center). 



Category 


Arithmetic mean 3 
(ppm) 


Range 
(ppm) 


DDE 
PCB 


1.29 
1.32 


0.33-2.8 
0.26-3.4 


a Wet weight. 







THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 41 

Numerous authors have stated that bird-eating and fish-eating 
raptors usually build up higher residues and have greater shell 
thinning than do rodent-eaters (review by Stickel 1973). Why did 
the screech owls, whose diets contained large quantities of migrant 
birds, not display these characteristics? We can only speculate that 
the lack of shell thinning and high egg residues in the Ohio 
population was because the eggs were laid before the predominance 
of avian prey became available to the owls. The owls just prior to 
and during the egg-laying period were feeding primarily on 
rodents. Thus, the seasonal pattern in the food habits of the screech 
owl may be offering the species some protection against the 
accumulation of high pesticide loads. 

Eggs in three clutches contained the organochlorine pesticide 
mirex (three eggs in one clutch, mean 0.10 ppm, one egg in another 
clutch, 0.12 ppm, and one in another clutch 0.16 ppm). Mirex has 
stimulated considerable controversy during the past several years 
because of its actual and proposed widespread aerial application 
throughout the southeastern United States to control the imported 
fire ant (Solenopsis saeuissima richteri). Recent laboratory studies 
by Heath and Spann (1973), however, showed no perceptible 
reproductive effects on bobwhite quail (Colinus virginianus) or 
mallards. Since mirex spray activities have been confined to the 
southeastern United States, we believe the screech owls in northern 
Ohio obtained their mirex from migratory passerine birds that 
wintered in the Southeast. The diet of screech owls during certain 
periods of the year consists mainly of migratory passerines 
(Table 2). 

POLYMORPHISM 

The existence of bimodal or polymodal variation within a species 
is referred to as polymorphism, in which intermediate forms occur 
at low frequency or are even absent (Owen 1963b). Polymorphism 
may be defined as the occurrence together in the same habitat of 
two or more distinct genetic forms of a species of animal or plant in 
such proportions that the rarest of them cannot be maintained by 
recurrent mutation (Ford 1940). Differences between the sexes, 
between young and older individuals, and among seasons are 
excluded from this definition. Hence, an extremely variable species 
may not necessarily be polymorphic. Huxley (1955) provided an 
outline for the analysis of polymorphism which included among 
other items: (1) the ecogeographical mapping of morph-frequency 
and determination of ratio-clines (if present, this should be 
repeated at regular intervals), (2) experimental and statistical 



42 NORTH AMERICAN FAUNA 71 

evaluation of viability and other intrinsic or extrinsic selective 
advantages of the morphs concerned, and (3) genetical analysis of 
the underlying balance and stability mechanisms. In evaluating 
polymorphism in the screech owl, we attempt to follow the outline 
presented by Huxley. 

The existence of two distinct color forms of the screech owl has 
been known since about 1874 when Ridgway (Baird et al. 1874) 
realized that rufous and gray birds were of the same species and 
that the forms were independent of age, sex, or season. Earlier, 
rufous and gray birds had been variously interpreted as belonging 
to different sexes or to different age classes (review by Hasbrouck 
1893). Ridgway later published in many natural history and 
scientific journals an appeal for information on the frequency of 
rufous and gray birds, nesting information, and any other data 
that might help to solve the problem. The information was passed 
over to E. M. Hasbrouck who published an account of his findings 
(Hasbrouck 1893). A number of inaccuracies in Hasbrouck's paper 
were pointed out at the time in a critical review (Allen 1893). It was 
70 yr until another full survey of the geography of color forms in the 
screech owl was attempted (Owen 1963b), although several authors 
discussed small geographic areas (Martin 1950, Schorger 1954, 
Hrubant 1955). 

Owen (1963b:189) summarized his description of geographical 
trends in the occurrence of color forms of the screech owl as follows: 
"Throughout most of North America east of about 104° 
there are two forms of the screech owl: one with the 
plumage mainly bright rufous and the other with the 
plumage mainly gray. Birds of intermediate coloration 
also exist, but in most areas they are rare. The relative 
frequency of rufous birds varies geographically in the form 
of a cline from north to south; about a quarter or less of the 
northern population is rufous, while in the south (the Gulf 
coast and Florida excepted) up to three quarters of the 
population may be rufous. . . . The geography of 
polymorphism in the screech owl appears to be an unusual 
pattern of variation within an animal species." 
Owen (1963b) indicated that the presence of polymorphism in the 
screech owl probably represents balanced adaptation of the forms 
to varying environmental conditions. After noting that the 
adaptive significance of the polymorphic forms was not un- 
derstood, Owen indicated that a cline in the relative frequency of 
the forms as opposed to random or irregular distribution supports 
the view that polymorphism in the species is maintained by 
selection operating along environmental gradients. Fisher (1930) 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 43 

indicated that it is extremely unlikely that two or more very 
different phenotypes would be equally adapted to the environment 
in which they live; a balance of selective forces must be involved, 
for if not, one form would rapidly replace the other and there would 
be no polymorphism. 

Annual Variation in Color Phase Ratios 

Hasbrouck (1893) and Owen (1963b) studied the general patterns 
in the geographical occurrence of the two color phases of the 
screech owl. However, to explore the factors responsible for the 
patterns observed, we believed it would be fruitful to study yearly 
fluctuations in a given population, preferably one near the edge of 
its range. Although the screech owl's range extends into southern 
Ontario (Godfrey 1966), our study area in northern Ohio was 
sufficiently close to the northern edge of the range that yearly 
fluctuations were probable. During our 30-yr study, more than 
4,000 observations of screech owls allowed us to assess fluctuations 
in the composition of the two morphs within the population. 



Q 


30 


UJ 




on 




t- 


25 


z 




UJ 


20 


U 





flj 



fl u B 



•o -© 



YEARS 



Fig. 8. — Changes in the color phase ratios of screech owls in the northern Ohio 
study area, 1944-73 (mean with 95% CI). 



44 NORTH AMERICAN FAUNA 71 

The population during the first 8 yr of the study (1944-51) 
consisted of about 25% red-phase birds. During December 1951, the 
percentage of reds decreased significantly (Table 19, Fig. 8). No 
significant annual change in the color phase ratio occurred during 
the remainder of the 30-yr period, although the red phase seemed to 
increase slightly during the early and mid-1960's. Owen's (1963b) 
suggestion that environmental gradients were probably responsi- 
ble for the clinal pattern observed in North America led us to review 
weather data. The Toledo weather station (Toledo Express Airport) 
is within the study area and monthly summaries for the 30 winters 
(November- April) were obtained for that station from the National 
Weather Records Center (Asheville, N.C.). The deepest snowfall 
during the 30-yr study occurred in December 1951 as a snowcover of 
10.16 cm (4 in) or more lasted for 17 days, averaging 21.34 cm (8.4 in) 
(25.40 to 33.02 cm [10-13 in] on the ground for 6 days). The snow was 
accompanied by below-normal temperatures; the lows during the 
first 15 days of the snowcover averaged -15C and for 4 days the 
temperature was below -18C. 



Table 19. A summary of the color phase percentages of screech owls by 2-year 
periods in the Ohio study area. (Includes retrapped birds banded in previous 
years.) 





Percent 


Percent 


Number 


Standard 


Percent red 


Years 


red 


gray 


observations 


deviation 


95% C.I. 


1944-45 


26.8 


73.2 


149 


3.63 


19.7 to 33.9 


1946-47 


25.5 


74.5 


267 


2.67 


20.3 to 30.7 


1948-49 


26.2 


73.8 


267 


2.69 


20.9 to 31.5 


1950-51 


23.3 


76.7 


275 


2.55 


18.3 to 28.3 


1952-53 


14.7 


85.3 


537 


1.53 


11.7 to 17.7 


1954-55 


9.8 


90.2 


695 


1.13 


7.6 to 12.0 


1956-57 


13.0 


87.0 


300 


1.94 


9.2 to 16.8 


1958-59 


9.3 


90.7 


323 


1.62 


6.1 to 12.5 


1960-61 


16.5 


83.5 


176 


2.80 


11.0 to 22.0 


1962-63 


13.6 


86.4 


206 


2.39 


8.9 to 18.3 


1964-65 


16.7 


83.3 


120 


3.40 


10.0 to 23.4 


1966-67 


19.2 


80.8 


156 


3.15 


13.0 to 25.4 


1968-69 


12.3 


87.7 


179 


2.46 


7.5 to 17.1 


1970-71 


9.8 


90.2 


174 


2.25 


5.4 to 14.2 


1972-73 


11.9 


88.1 


218 


2.19 


7.6 to 16.2 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 45 

The combination of heaviest snowfall during the study and 
extremely cold weather seemed to be correlated with the only 
significant change in color phase ratios during the 3 decades. Thus, 
we hypothesize that heavy snowfall in combination with low 
temperatures plays a direct or indirect role in the relative 
abundance of the two morphs in the northern portion of the range. 
The gray phase, which is more abundant in the northern latitudes 
where winters are more extreme, seemed to survive the critical 
period much better than the red phase. 

Retrap records of adult screech owls banded on the study area 
before the extreme winter provided information about relative 
survival of the two color phases during December 1951. Adult birds 
banded during 1947-50 were used in the analysis, and retrap 
records during 1952 or later were considered envidence of possible 
differential survival of the two morphs during the critical period of 
December 1951 (Table 20). This admittedly small sample suggests 
that 44% (10.8-6.0/10.8 = 44%) more red-phased birds than grays 
were lost during the interval which corresponds quite well with the 
drop in the ratio of reds from 1950-51 to 1952-53 (change from 23.3 
to 14.7, a drop of 37% [23.3-14.7/23.3=37%]). 



Table 20. A comparison of adult screech owls banded in 1947-50 and retrapped in 
1952 or latter a- 

Category Red Gray 

Number banded 50 157 

Number retrapped 3 17 

Percentage retrapped 6.0 10.8 

a Only banded birds retrapped at least once after banding were included (they 
may have been retrapped initially at any time). 



A comparison between the two phases was then made for adult 
birds banded between 1953 and 1970 to determine if the gray phase 
birds were always retrapped at a higher rate than the red phase. 
The percentage of birds retrapped 2 or more yr after banding was 
nearly identical for both color phases during the 18-yr period (Table 
21). This lends further support for our differential survival 
hypothesis for December 1951. Differential survival, of course, 
would lead to a change in the ratio of the two morphs. No 
significant change in the ratio of the two morphs was observed 
between 1952-53 and 1972-73 when each morph was retrapped in 
nearly identical percentages. 



46 NORTH AMERICAN FAUNA 71 

Table 21. A comparison of adult screech owls banded in 1953-70 and retrapped 2 or 
more years after banding °. 

Category Red Gray 

Number banded 120 831 
Number retrapped 9 65 
Percentage retrapped 7.5 7^8 

a Only banded birds retrapped at least once after banding were included (they 
may have been retrapped initially at any time). 



It is tempting to speculate about the population composition over 
the last 40 or 50 yr in light of snowfall patterns. Based on records 
for the study area during the last 85 yr, snowfall was less than 
normal during 1920-50 and more than normal during the last 20 yr. 
The less than normal snowfall during the 25 yr before the study 
began may have allowed the red phase to increase in abundance; 
then, the severe weather in December 1951 directly or indirectly 
seems to have caused a decrease in the red-phased segment of the 
population. The more than average snowfall during the next 20 yr 
may be responsible for maintaining the population composition at 
the lower ratio of red to gray phases, although a hint was present 
that the red phase attempted to increase in abundance at times. 

The ruffed grouse (Bonasa umbellus), another polymorphic 
species in North America, has a somewhat similar clinal pattern in 
color phase ratios (red and gray) within its range and some 
knowledge is available for this species. Gordon W. Gullion has 
worked for many years with survival rates of grouse, including 
survival related to color phase. Gullion and Marshall (1968:157) 
stated, "As a generalization, the prevalence of red-phase grouse in 
the population increases as the climate grades from the Arctic 
south. This suggests that the difference in mortality may be related 
to the severity of the winter; or more specifically, that there may be 
a color-linked susceptibility to cold or other climatic conditions, 
which selects against the red-phase grouse." Gullion (1966) 
indicated that during winters which lacked sufficient snow for 
burrowing, mortality among red-phase grouse was greater than 
among gray birds, but with better snow conditions the red-phase 
birds outlived the gray. 

Gullion and Marshall (1968:157) indicated, "Earlier we believed 
that this differential survival represented periodically increased 
vulnerability to predation among red-phase birds (primarily as a 
result of insufficient burrowing snow), and we still cannot discount 
this idea entirely. . . . We have suggested that the red-phase 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 47 

grouse may be more conspicuous to raptors hunting overhead than 
are the gray-phase birds." Thus, in summary, the increased 
mortality of the red-phase grouse may be due to a color-linked 
susceptibility to severe weather or a predation problem associated 
with color-linked vulnerability. The same conclusions seem to be 
applicable to the screech owl; in years of heavy snowcover, the red 
phase screech owls appear to be more prone to mortality because 
they have not developed the burrowing behavior of the ruffed 
grouse. The most important natural predator of the screech owl 
would probably be the great horned owl (Craighead and Craighead 
1956), but we are doubtful that predators over a very short term 
could be responsible for the major change in color phase ratios in 
1951. Lustick (1969) studied the effects of artificial radiation on bird 
energetics and concluded that radiant energy reduces oxygen 
consumption in birds and that the reduction is correlated with 
feather color, being greater in dark birds. Mosher and Henny (in 
press) recently conducted metabolic experiments with screech owls 
to attempt to ascertain the mechanism responsible for the change 
in color phase ratios in the population in 1951 by exposing birds of 
the two color phases to various temperature regimes. These data 
demonstrated a significant difference in oxygen uptake between 
color phases at -10 and -5C. This supports the hypothesis that red- 
phase screech owls are restricted in their northern distribution by 
color related metabolic differences from the gray-phase birds. It 
also suggests that the extremely cold temperatures for a prolonged 
period of time in December 1951 were responsible for excessive 
mortality of red-phase birds. 

Sex, Age, and Color Phase 

Baird et al. (1874:51) first concluded that plumage color was 
independent of sex and age when they stated, "These two very 
different plumages are entirely independent of age, sex, or season, 
and that they are purely individual there can be no doubt, since in 
one nest there may often be found both red and gray young ones, 
while their parents may be either both red or both gray, the male 
red and the female gray, or vice versa." To our knowledge a large 
sample of birds from a given area has not been available to 
determine if the color is independent of sex. Hasbrouck (1893) 
presented data showing the relation of color to sex based on 646 
birds from throughout the breeding range, but since it is doubtful 
that each sex was sampled equally throughout the species range, 
valid comparisons could not be made. Martin (1950) compared 75 
study skins from Ontario and found no significant relationship 



48 NORTH AMERICAN FAUNA 71 

between sexes and color phases. Similarly, we detected no signifi- 
cant relationship between sex and color phase in our sample of 760 
nesting screech owls (Table 22). Fewer males were in the sample 
because they were often away from the nest at the time it was 
visited. 



Table 22. The color phase composition of adult nesting screech owls, 1944-73 
(includes pairs in which the male was unknown). 

Gray Red Percent red a 

Males 220 40 15.4 

Females 403 97 19.4 

No significant difference in color phase composition of nesting males and 
females (chi-square 1.87, 1 df NS). The intermediates were not included here. 



The red and gray birds were found in almost identical ratios in 
both breeding populations and the young birds produced (Table 
23). The intermediates were also consistent in both age categories 
(2.6 and 2.3%). 

Table 23. Classification of screech owl color morphs from northern Ohio. The data 
include only pairs in which the color phase of both parents was known. 
Percentages are in parentheses. 



Category 


Red 


Intermediate 


Gray 


Total 




1944-73 






Breeding population 


81 (16.1) 


13 (2.6) 


408 (81.3) 


502 (100.0) 


Offspring 


126 (15.3) 


19 (2.3) 


681 (82.4) 


826 (100.0) 


Totals 










Including 










intermediates 


207 (15.6) 


32 (2.4) 


1,089 (82.0) 


1,328 (100.0) 


Excluding 










intermediates 


207 (16.0) 


— 


1,089 (84.0) 


1,296 (100.0) 




1952-73 






Breeding population 


62 (15.4) 


12 (3.0) 


328 (81.6) 


402 (100.0) 


Offspring 


91 (14.1) 


17 (2.6) 


536 (83.2) 


644 ( 99.9) 


Totals 










Including 










intermediates 


153 (14.6) 


29 (2.8) 


864 (82.6) 


1,046 (100.0) 


Excluding 










intermediates 


153 (15.0) 


— 


864 (85.0) 


1,017 (100.0) 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 49 

Genetic Hypotheses 

Hrubant (1955) analyzed screech owl data by the gene frequency 
method to test whether the matings occurred at random, and 
whether the offspring were produced in agreement with any of 
several genetic hypotheses. The genetic hypotheses tested 
were: (1) one pair of genes with dominance, (2) one pair of genes 
without dominance, (3) multiple alleles, and (4) sex associated 
inheritance. He concluded that lack of dominance may be 
dismissed as an acceptable mode of inheritance since neither of the 
ways in which this mode could be applied could account for the 
distribution of colors of the young produced and that none of the 
sex-associated modes are acceptable since the two attributes, color 
and sex, are independent of each other. This left Hrubant (1955) 
with but two modes of inheritance as possible explanations of the 
observed data. In testing the assumption of simple dominance, it 
was noted that red by red produce only red. Some grays were 
expected, but the lack of grays may be attributed to the small 
sample analyzed (only 10 offspring). Under the hypothesis of 
multiple alleles, the observed value of the intermediate class from 
red by gray matings deviated from the hypothetical value by a 
significant amount; however, small samples (only 10 offspring 
from red by red and 55 offspring from red by gray) made Hrubant's 
findings somewhat inconclusive. 

One Pair of Genes with Dominance 

Using the same approach as Hrubant, but with considerably 
larger sample sizes, we again tested the hypothesis of simple 
dominance. The method requires the assumption that the popula- 
tion is in equilibrium with respect to mating type, phenotype, and 
genotype. The significant change in color phase ratios between 
1951 and 1952 suggested that the data collected during the first 8 yr 
of the study (1944-51) should not be pooled with the latter data. 
Therefore, genetic hypotheses were tested with information 
collected during the last 22 yr of the study (1952-73). To test this 
hypothesis the owls may be classified merely as red or gray, and the 
difference assumed to be caused by one pair of genes with the gene 
for red exhibiting dominance over the gene for gray. The 
intermediate owls may be regarded as misclassified gradations of 
the red and gray. Removal of the intermediates from the sample 
total in Table 23 leaves 864 gray and 153 red owls. Since an 
overwhelming number of the matings were gray by gray (Table 24), 
and since these matings produced only gray offspring, the gray 
phenotype may be assumed to be due to the recessive gene, and red 
due to its dominant allele. 



50 NORTH AMERICAN FAUNA 71 

Table 24. Mating types of progeny of screech owls in northern Ohio with respect to 
color, 1952-73. 





Mating type 




adult 


1. 


Red x red 


2. 


Red x gray 


3. 


Red x Inter. 


4. 


Inter, * Inter. 


5. 


Inter. * gray 


6. 


Gray * gray 



Number of 
successful 




Progeny 




matings 


Red 


Inter. 


Gray 


8 


23 





5 


46 


68 


1 


63 


























12 





16 


29 


135 








439 



Totals 201 91 17 536 



Let R stand for the dominant gene for red pigment and r stand for 
its recessive allele for gray pigment. Let p be the frequency of r in 
the sample, such that p + q = 1. Values of q and p may be obtained 
from the sample so that they are the maximum likelihood estimates 
of 9 and 1-9, the true frequencies of r and R in the population. 

Following the discussion by Hrubant (1955), the estimates may 
be based upon the parents only, the offspring only, or upon the 
entire sample of 1,017 owls, omitting the intermediates. The entire 
sample of 1,017 appears to be the best choice since there is no 
marked evidences of heterogeneity within it and since the parents 
and offspring contain about the same frequencies of red and gray 
owls. Of 1,017 owls in the entire sample (omit ting interm ediates), 
864 were gray. The frequency of r thus q = /864/1.017 = 0.9217 
and of R is p = 0.0783. 

The theoretical mating type frequency, determined by the 
expansion of the binomial ((1-9) + 9) 4 using q = 0.9217 as an 
estimate of 9, and the observed mating frequencies are compared in 
Table 25. The data in this table show that the various kinds of 
matings occurred with frequencies in agreement with those 
expected if mating is at random. The computed chi-square is 
smaller (3.30) than the critical (5.99) at the 5% level of significance 
of 2 degrees of freedom. 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 51 

Table 25. Testing specific mating types of screech owls under hypothesis of one 
pair of genes with dominance. 





Mating 


Theoretical 


Expected 


Expected 


Observed 




type 


frequency 


proportions 


frequency 


frequency 


1. 


Red x red 

RR x RR 


(1-9) 4 


0.00004~ 








2. 


Red x red 
RR x R r 


40(1-9)3 


9.09177 


> 9.92264 


4.3 


8 


3. 


Red x red 


4(9)2 (1-9)2 


9.92983_ 









Rr x R r 

4. Red x gray 2(9) 2 (1-9) 2 9.91942" 

RR x rr > 9.25566 48.3 46 

5. Red x gray 4(9 ) 3 (1-9) 9.24524_ 
Rr x rr 

6. Gray x gray (9) 4 9.72179 9.72179 136.4 135 
rr x rr 



1.99999 189.9 189 

X 2 = 3.39, 2 d.f. 



Table 26 contains the test of the observed distribution of young 
produced by individual mating types compared to their expected 
distribution. The expected numbers of offspring are found from the 
relative frequencies of the matings given in Table 25. The data 
show that in all three mating combinations the observed numbers 
of offspring occur with frequencies in agreement with those 
calculated if the difference betwen red and gray is due to only one 
pair of genes with red dominant to gray. In fact, the observed and 
expected frequencies for red by gray and gray by gray are in 
complete agreement, whereas the red by red pairings are in nearly 
complete agreement (chi-square 0.22). The critical level for rejection 
of the hypothesis is 3.84 at the 5% level of significance for 1 degree 
of freedom. The observed data are nearly a perfect fit. 



52 NORTH AMERICAN FAUNA 71 

Table 26. Distribution of young screech owls produced by mating phenotypes 
under hypothesis of one pair of genes with dominance. 





Observed 


number 


Expected 


number 




offspi 


ring 


offspring 


Mating 


Red 


Gray 


Red 


Gray X 2 


1-3 Red x red 


23 


5 


21.6 


6.4 0.22 


4-5 Red x gray 


68 


63 


68.2 


62.8 - 


6 Gray * gray 





439 





439.0 — 



Multiple Alleles 

The other possible mode of inheritance (Hrubant 1955) is a 
multiple allelic system. Assume the red, intermediate, and gray 
phenotypes are inherited in this manner with a graded order of 
dominance of Y (red) dominant to y' (intermediate) dominant to y 
(gray). Table 27 illustrates the observed and expected sample 
frequencies and theoretical population frequencies used in deriving 
the values for the statistics r = 0.0760, s = 0.0152, and t = 0.9088, 
as estimates of the true frequencies of genes. 



Table 27. Sample frequencies and estimates of population gene frequencies of 
screech owls under hypothesis of a system of multiple alleles. 





Theoretical 


Observed 


Expected 


Genotype 


frequency 


frequency 


frequency 


YY 


P 2 




0.0058 


Yy' Red 


2pa 


0.146 


0.0023 


Yy 


2p7 




0.1381 


y'y' 


02 




0.0002 


Inter. 




0.028 




y'y 


2 ay 




0.0276 


yy Gray 


y 2 


0.826 


0.8259 



t= v/864/1046 =0.9088, frequency of y(y) 

s= 7864+29/1046-0.9088=0.0152, frequency of y'(o) 
r= l-(t+s) =0.0760, frequency of Y(p) 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 53 

The theoretical mating type frequency determined by the 
expansion of the trinomial (p + ° + ? ) 4 and the observed mating 
type frequencies are compared in Table 28. The data in this table 
show that the various kinds of matings occurred with frequencies 
in agreement with those expected if mating is at random. The 
computed chi-square (5.99) is smaller than the critical (11.07) at the 
5% level of significance for 5 degrees of freedom; therefore, we 
concluded that the matings were at random. 



Table 28. Testing specific mating types of screech owls under hypothesis of a 
system of multiple alleles. 



Mating type 


Theoretical 


Expected 


Expected 


Observed 


by phenotypes 


frequency 


proportions 


frequency 


frequency 


1. Red x red 


p +4p o+4p < y+4p a + 










8p ay+4p y 


0.0213 


4.3 


8 


2. Red x gray 


2p 2 7 2 +4pa7 +4py 3 


0.2415 


48.5 


46 


3. Red x inter. 


2pV+4pa 3 +12pa 2 7+ 










4p ay+8pay 2 


0.0081 


1.6 





4. Inter. * inter. 


a +4a y+4a 2 y 2 


0.0008 


0.2 





5. Inter. * gray 


2o*y"+4oy } 


0.0460 


9.2 


12 


6. Gray * gray 


4 

7 


0.6821 


137.1 


135 






0.9998 


200.9 


201 








X 2 - 5.99, 5 d.f. 



The observed distribution of young produced by the specific 
phenotypic mating types as compared to their expected distribu- 
tion is shown in Table 29. The data show agreement in all cases 
between the frequencies of the observed numbers of offspring and 
those calculated if the difference between red, intermediate, and 
gray were due to a multiple allelic genetic system. This system 
involves a graded order of dominance of red over intermediate over 
gray. 



54 NORTH AMERICAN FAUNA 71 

Table 29. Distribution of young screech owls produced by mating phenotypes 
under hypothesis of a system of multiple alleles. 







Observed num 


bers 


Expected num 


bers 






Mating 




offspring 






offspring 








Red 


Inter. 


Gray 


Red 


Inter. 


Gray 


X 2 


1. 


Red x red 


23 





5 


21.3 


0.1 


6.6 


0.63 


2. 


Red x gray 


68 


1 


63 


68.6 


1.1 


62.3 


0.02 


3. 


Red x inter. 


— 


— 


— 


— 


— 


— 


— 


4. 


Inter. x inter. 


— 


— 


— 


— 


— 


— 


— 


5. 


Inter, x gray. 





16 


29 





22.3 


22.7 


3.53 


6. 


Gray * gray 








439 








439.0 


— 



Unfortunately then, neither of the two hypotheses of gene action 
(one pair of genes with dominance and multiple alleles) that 
appeared satisfactory to Hrubant (1955) can be eliminated on the 
basis of breeding data presented here. In fact, the evidence for the 
two hypotheses was strengthened by the new data as the expected 
values were in nearly perfect agreement with the observed breeding 
data. The fact that red by red produced approximately 25% grays 
was a most important finding because Hrubant noted that red by 
red produced only reds in his small sample. Therefore, the 
following hypotheses remain for the gene action: (1) If the 
intermediate phenotype is assumed to be due to genetic modifiers, 
the difference between red and gray is due to one pair of alleles, the 
gene for red exhibiting dominance over its allele for gray; (2) if all 
three colors are considered, the colors are inherited on the basis of a 
series of three alleles with a graded order of dominance of 
red -*■ intermediate -*gray. 

Brood Size and Color Phase 

Data were insufficient to provide a fledging rate estimate (per 
nesting attempt) for each color phase; however, a pooled estimate of 
the number of young fledged per successful nest was obtained. The 
color phase of both parents was documented for 211 successful 
nesting attempts during 1944-72 (Table 30). The data for 1973 were 
not included because one egg was taken from each nest for pesticide 
analysis. No significant difference in the number of young fledged 
for the various breeding combinations could be detected. 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 55 

Table 30. A summary of the estimated number of young screech owls fledged per 
successful nest for the various combinations of color phase pairs, 1944-72. 



















Young fledged 








Young fledged 




Number 
nests 


per 


successful 
nest 




Category 


1 


2 


3 


4 


5 


6 


95% C.I. 


Gray * gray 


18 


7 


35 


49 


30 


8 


147 




3.61 


3.39-3.83 


Gray * red 


1 


5 


8 


15 


13 


1 


43 




3.86 


3.52-4.20 


Red x red 


1 





1 


4 


3 





9 




3.89 


3.06-4.72 


Inter. * gray 


1 





1 


5 


3 


1 


11 




4.09 


3.32-4.86 


Inter. * red 











1 








1 




4.00 


— 


Total 


21 


12 


45 


74 


49 


10 


211 




3.70 





STATUS OF POPULATION 

The status of any wildlife population may be evaluated in three 
general ways: (1) a complete count or direct enumeration on an 
annual basis, (2) an index to abundance at some time period during 
the life cycle, or (3) a life-equation approach which provides an 
indirect evaluation of the population's condition. These ap- 
proaches are not new and fall within the general outline presented 
by Leopold (1933:139): "(1) Census (Measuring the Stock on 
Hand), and (2) Measuring the Productivity of the stock and 
comparing it with a standard." Furthermore, it is always useful to 
compare results obtained from several approaches. The complete 
count or direct enumeration of screech owls is impossible due to 
their habitat preferences. Therefore, we will rely upon an index to 
annual abundance and the life-equation approach to evaluate the 
status of the screech owl population in northern Ohio. 

Index to Annual Abundance 

The occupancy rate of wood duck boxes by screech owls during 
the nesting season may provide some insight into the relative 
annual abundance of the species on the study area. However, since 
the nest boxes were always placed in suitable habitat and loss of 
habitat has occurred on the study area (Table 1 ), the occupancy rate 
for nest boxes would provide only an index to abundance in 
suitable habitat. Thus, the possibility of an undetected decline due 
to loss of habitat exists. To further complicate the matter, we have 
been forced to use an index to annual abundance based on 
successful nests. This modification was necessary because during 



56 



NORTH AMERICAN FAUNA 71 



some years the boxes were visited early in the nesting season (a 
chance to observe nests that were later abandoned and un- 
successful), whereas in other years nests were visited only at 
banding time (successful nests only recorded). Therefore, because 
of the fluctuations in annual effort we believe that a more realistic 
annual comparison of abundance would be obtained from 
successful nests only. 

Although indices based on wood duck box usage have many 
potential biases, two factors present during this study tend to 
minimize these biases: (1) the same observer checked the boxes 
annually, and (2) approximately the same number of boxes were 
available each year. 

The information collected during the 30-yr period suggested no 
long-term trend in abundance in suitable habitat, as measured by 
numbers of successful nests (Table 31). The 4 yr (1952-55) when 
students assisted in the study could not be evaluated because the 
number of nests checked was unknown. Excluding those 4 yr, the 
10-yr indices to abundance were as follows: 1944-51, 10.1; 
1956-63, 8.4; and 1964-73, 9.0. Successfully nesting screech owls 
were found annually in approximately 8.9% of the nest boxes 
during the 30-yr study. Assuming that 69.2% of the screech owl 
nesting attempts were successful (see section on BREEDING 
BIOLOGY), another 4.0% of the nest boxes were used annually, but 
without success, making a total of about 12.9% of the boxes utilized 
annually during the study. 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 57 

Table 31. An index to the abundance of screech owls in suitable habitat in 
northern Ohio, 1944-73. 



Year ! 



No. of 
boxes checked 



No. of 
successful nests 



Percent with 
successful nests 



1944 
1945 
1946 
1947 
1948 
1949 
1950 
1951 

Total 1944-1951 



45 
78 
95 
128 
133 
139 
140 
145 

903 



4 
4 
6 
14 
11 
15 
24 
10 

88 



9 
5 
6 

11 

8 
11 

17 

7 

10.1 b 



1956 


210 


1957 


208 


1958 


190 


1959 


180 


1960 


155 


1961 


140 


1962 


130 


1963 


125 


Total 1956-63 


1,338 


1964 


125 


1965 


135 


1966 


130 


1967 


130 


1968 


130 


1969 


130 


1970 


130 


1971 


130 


1972 


135 


1973 


153 



Total 1964-73 



1,328 



20 
14 
14 
12 
11 
9 
18 
11 

109 

9 
7 
9 
16 
13 
10 
12 
11 
16 
19 

122 



10 

7 

7 

7 

7 

6 
14 

9 

8.4b 

7 

5 

7 
12 
10 

8 

9 

8 
12 
12 

9.0 b 



Total 1944-73 



3,569 



319 



8.9 1 



a Numbers of boxes checked in 1952-55 are unknown. 
bEach year given equal weight. 



58 NORTH AMERICAN FAUNA 71 

Life Equation Approach 

A mathematical model showing the relations between popula- 
tion parameters that yield stable populations was developed by 
Henny et al. (1970). Information needed for the model includes (1) 
mortality rate schedules (obtained from recoveries of banded 
birds), (2) recruitment rates, and (3) age at sexual maturity. 
Estimates of these parameters have been presented in this paper 
for the screech owl. 

We know that screech owls are capable of breeding at the end of 
their first year of life (as 1-yr-olds), although the percentage in this 
age class that breeds is not known with certainty. However, we still 
believe that the excercise in constructing a life-equation model is 
worthwhile, because it will point out future research needs and act 
as a check on the internal consistency of the input data. Adult 
mortality rate estimates for the screech owl range from 32.8 to 
39.0% (Table 15), with a pooled estimate of 33.9%. The mortality rate 
estimate for the first year of life was 69.5% (Table 15). The observed 
recruitment rate per breeding pair was estimated at between 2.55 
(Table 13) and 2.63 based on the exposure-day method. 

Initially, let us assume that all screech owls breed as 1-yr-olds, 
although we have some evidence that a portion of the 1-yr-olds do 
not nest. The estimated number of young that must be fledged per 
breeding-age pair to maintain a stable population may be 
estimated by the following formula (from Henny 1972): 



_ 1 - s 

m = 



s (1 - S + Si) 

where 

m = the average number of female fledglings produced per 
breeding- age female (2fn = the total number of young 
produced per breeding female assuming an equal sex ratio of 
fledglings) 

s = third year and later survival rate 
s = first year survival rate 
Si - second year survival rate 
By using the pooled estimate for adult mortality and assuming that 
all 1-yr-olds attempt to nest, it is estimated that only 2.22 young are 
required per breeding pair to maintain a stable population. This is 
slightly less than the observed recruitment (2.55 to 2.63), and, 
although the mortality rates may not be precise, the modeling 
approach suggests that not all 1-yr-old screech owls need to attempt 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 59 

to nest annually in order to maintain a stable population. To pursue 
this matter a little further, let us assume for the moment that the 
recruitment rates and the mortality rates are correct and, through 
the modeling process, let us calculate the percentage of 1-yr-olds 
(solve for p x ) that must attempt to nest annually to balance the life 
equation (see Equation 1 in Henny 1972:6). Given the above 
constraints and assuming that 2-yr-olds and older attempt to nest 
each year, the population could remain stable if at least an average 
of 60% of the 1-yr-olds nested annually. This estimate is in farily 
close agreement with the 77 to 83% estimate based on a small 
sample of field data which may be biased upward. 

Summary of Status 

In summary, we believe that the screech owl population in 
northern Ohio has fluctuated in abundance during the last 30 yr, 
with no long-term trend being apparent in suitable habitat (Table 
31). In addition, the recruitment rate observed during the long-term 
study seemed to be adequate to maintain the population numbers. 
We have indirect evidence that all 1-yr-olds are not required to nest 
(from modeling approach) and field evidence suggesting that a 
sufficient percentage of 1-yr-olds are nesting. Furthermore, we 
have no evidence of eggshell thinning in the population (Table 17), 
although laboratory tests have shown that the species is suscepti- 
ble to the eggshell thinning phenomenon. Residue levels of DDT 
and its metabolites along with PCB's were low (Table 18). It should 
be cautioned, however, that even though screech owls seem to be 
adaptable to people and their structures, continued habitat loss 
may gradually reduce the population numbers in northern Ohio as 
the woodlots and creek bottoms become cleared and farmed more 
intensively. The forested lands in the four-county study decreased 
12% between 1958 and 1967. 



60 NORTH AMERICAN FAUNA 71 



SUMMARY 

1. The life history and population ecology of the screech owl was 
studied in northern Ohio between 1944 and 1973. The owls 
nested in boxes established for wood ducks. The birds were 
banded and periodically recaptured. Food habits, productivity 
information, and color phase of the parent birds and offspring 
were recorded. 

2. The screech owl is an opportunistic feeder. The diet changes 
with the seasons of the year, i.e., during the nesting season 
migrant birds replace mammals in importance, and during the 
late summer insects become important. 

3. There is no evidence from banding data to suggest that screech 
owls in the northeastern United States migrate. 

4. Young screech owls begin dispersing from their natal areas in 
late summer or early fall, with only about one-fourth of the 
young birds remaining within 10 km (6 mi) of the banding site. 
On the contrary, adult birds remain close to the area where they 
previously nested. 

5. The hypothesis of random dispersal distance in young screech 
owls was rejected. There appear to be two groups in the 
population: (1) a group showing little dispersal, and (2) a 
group wandering considerable distances. 

6. The directional pattern of dispersal was random. 

7. Pairs of screech owls were noted in the nest boxes in early 
February, but egg laying did not peak until about 15 March. 
Hatching takes place in mid-April to early May and most of the 
young leave the nest the last week of May or the first week in 
June. 

8. The mean clutch size was 4.43 with a mean of 3.80 young 
fledged per successful nest. An estimated 69.2% of the nesting 
attempts were successful. Including unsuccessful nesting 
attempts, an estimated 2.55 to 2.63 young were fledged per 
breeding pair. 

9. Annual variation in the number of young fledged per 
successful nest was small. The lack of variability was probably 
due to the predictable food source of passerine birds that 
migrated through the area each spring when young were in the 
nest. This contrasts with the highly variable productivity 
reported for owls dependent upon local cyclic rodent pop- 
ulations. 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 61 

10. Raccoon predation was implicated in a number of unsuccessful 
nesting attempts, but the overall effect of the raccoon on the 
screech owl population was not fully assessed. 

11. The sex ratio in the population did not differ significantly from 
a 50:50 ratio. 

12. The first year mortality rate of the screech owl was estimated at 
69.5%; adult mortality was estimated at 33.9%. 

13. Male and female screech owls banded as nestlings were known 
to nest successfully at the end of their first year of life (as 1-yr- 
olds). Two estimates of the average proportion of 1-yr-olds 
nesting ranged from 77 to 83%, but the percentage may be 
lower. 

14. No significant eggshell thinning was detected in the popula- 
tion in 1973, and residue levels of DDE and PCB's were low. 

15. In northern Ohio, the screech owl population consists mainly 
of a gray- and red-phased birds, but about 2 to 3% of the birds 
are intermediate in color. 

16. A significant change in the ratio of red- to gray-phased birds 
occurred in December 1951. This change coincided with the 
lowest temperatures during the 30-yr study and the heaviest 
snowfall. The red-phased birds decreased from 23.3% of the 
population to 14.7%. Additional evidence from retrap informa- 
tion suggests that the grays survived much better than the reds 
during the stressful period. The ratio of reds in the population 
failed to increase to pre-1951 levels during the next 20 yr. 

17. No significant relationship existed between sex and color 
phase in our sample of 760 nesting screech owls. 

18. The following hypotheses of gene action remain: (1) If the 
intermediate phenotype is assumed to be due to genetic 
modifiers, the difference between red and gray is due to one pair 
of alleles, the gene for red exhibiting dominance over its allele 
for gray; or (2) if all three colors are considered, the colors are 
inherited on the basis of a series of three alleles with a graded 
order of dominance of red -^-intermediate -^.gray. 

19. No significant difference in brood size and color phase of the 
parents could be detected. 

20. An index to annual abundance suggests that the screech owl 
population fluctuated in northern Ohio during the last 30 yr, 
but with no long-term trend apparent in available habitat. The 
life-equation approach also suggests that, over the long term, 
productivity probably balanced mortality. 



62 NORTH AMERICAN FAUNA 71 



ACKNOWLEDGMENTS 

The assistance of Michael Smith, Larry Shafer, Lee Garling, 
Denny Genzman, and Gene Walston in the field aspects of the 
study during the mid-1950's is appreciated. Informative dis- 
cussions with Milton B. Trautman and Ronald L. Stuckey of the 
Ohio State University, and Theodore A. Bookhout of the Ohio 
Cooperative Wildlife Research Unit, led to the location of some 
interesting references concerning changes in the flora and fauna of 
the region. Kenneth P. Burnham of the Migratory Bird and Habitat 
Research Laboratory assisted in the analysis of directional dis- 
persal, and Erwin E. Klaas of the Patuxent Wildlife Research 
Center conducted the pesticide analysis. William H. Stickel of the 
Patuxent Widlife Research Center brought to our attention the vast 
food habit files that are stored at the Center. Richard D. Porter of 
the Denver Wildlife Research Center kindly reviewed the 
manuscript and Richard Wilmot reviewed the section concerning 
polymorphism. G. Ronald Austing and George Laycock provided 
several photographs to illustrate the publication. Our sincere 
thanks is expressed to all who assisted. 



THE SCREECH OWL: LIFE HISTORY AND POPULATION ECOLOGY IN NORTHERN OHIO 63 



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