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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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