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RAPTOR RESEARCH

Voliune M

Niiml>er I

Spring J

Rnptui' Rese:irdi Fniiud^iinn, hu%

Pnivii, Uti^lK U.S,A.

Spring 1980

RAPTOR RESEARCH

Volume 14, Number 1, Pages 1-32

CONTENTS

SCIENTIFIC PAPERS

Nest Site Selection and Productivity
of Great Homed Owls in Central

Minnesota— Robert T. Rohm 1

Bacterial Isolates from the Pharynx
and Cloaca of the Peregrine Falcon
{Falco peregrinus) and Gyrfalcon
(F. rusticolus) (Bacteria from

Falcons)—]. E. Cooper et al 6

Some Considerations for Future Raptor
Rehabilitation— Jerry Olsen and

Penny Olsen 10

Spring Hawk Migration in Eastern

Mexico— J. M. Thiollay 13

Comments on the Recognition of Off-
spring by Raptors— Helmut C. Mueller 20

Nesting Populations of Red-tailed Hawks
and Great Horned Owls in Central

Ohio— John S. Kirkley and Mark A. Springer 22

Pre-nesting Behavior of the Swallow-
tailed Kite {Elanoides forficatus).

Including Interference by an Unmated
Male with a Breeding Pair—

Lawrence Kilham 29

Juvenile Prairie Chicken Predation

by Marsh Hawk— W. Daniel Svedarsky 31

ANNOUNCEMENTS 9, 32

RAPTOR RESEARCH

Published Quarterly by the Raptor Research Foundation, Inc.

Editor Dr. Clayton M. White, Dept, of Zoology, 161 WIDE, Brigham Young
University, Provo, Utah 84602

Editorial Staff Dr. Frederick N. Hamerstrom, Jr. (Principal Referee)

Dr. Byron E. Harrell (Editor of Special Publications)

The Raptor Research Foundation, Inc., welcomes original articles and short
notes concerning both diurnal and nocturnal birds of prey. Send all papers
and notes for publication and all books for review to the Editor. Most longer
articles (20 or more typeset pages) will be considered for publication in Rap-
tor Research Reports, a special series for lengthy and significant contributions
containing new knowledge about birds or new interpretations of existing
knowledge (e.g., review articles). However, authors who pay page costs (cur-
rently $20.00 per page) will expedite publication of their papers, including
lengthy articles, by ensuring their inclusion in the earliest possible issue of
Raptor Research. Such papers will be in addition to the usual, planned size
of Raptor Research whenever feasible.

SUGGESTIONS TO CONTRIBUTORS: Submit all manuscripts in duplicate,
typewritten, double spaced (all parts), on one side of 8V2 X 11 inch paper,
with at least 1 inch margins all around. Drawings should be done in India
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plicate material in either the text or graphs. For advice concerning format
refer to the Council of Biological Editors’ Style Manual for Biological Jour-
nals or to previous issues of Raptor Research. Proofs will be sent to senior au-
thors only. Major changes in proofs will be charged to the authors. Reprints
should be ordered when proofs are returned.

NEST SITE SELECTION AND PRODUCTIVITY OF GREAT HORNED
OWLS IN CENTRAL MINNESOTA

Robert T. Rohm
520 7th Ave. North
Sauk Rapids, MN 56379

Abstract

Thirty-three active nests (16 artificial, 17 natural) of the Great Horned Owl (Bubo
virginianus) were found in central Minnesota in 1977. Sixteen of the 17 natural nests
were originally built by the Red-tailed Hawk (Buteo iamaicensis). Average nest height
was 14 m; 42% of the nests were in northern pin oak (Quercus ellipsoidalis); 79 percent
in edge areas, 21 percent in woodlot interiors. Nests averaged 0.59 km from the closest
human dwelling and 0.58 km from the closest graded road. Nest density in a thoroughly
searched area was 0.21 per km^ Utilization of artificial nest platforms was 52 percent.

The first incubating owl was seen on 20 February; the latest clutch was laid during
the first week in April. Five of 7 clutches contained two eggs; 10 of 19 successful nests
contained two nestlings. Forty-two percent of initial nesting attempts failed. Successful
nests produced 1.8 young per nest. Artificial nests were less successful than natural nests,
probably because of more human activity at artificial nest sites. An unusual distraction
display involving an adult owl is described.

Introduction

The Great Horned Owl {Bubo virginianus) has adapted to a variety of habitats
throughout most of North, Central, and South America (Bent 1938). In the rather exten-
sively farmed rural areas of central Minnesota, it is a common inhabitant of woodlots
and forested riverbottoms, often sharing these areas with another common raptor, the
Red-tailed Hawk {Buteo jamaicensis). Because of their close association, I had an excel-
lent opportunity, in conjunction with a study of the Red-tailed Hawk, to collect data on
Great Horned Owls.

Methods and Materials

Nests were located in February, March, April, and May 1977 by systematically
searching wooded areas for Red-tailed Hawk nests and when rechecking raptor nests
located in previous years. A nest was classified as active when an owl was observed sit-
ting atop the nest, apparently incubating, on at least two occasions during the nesting
cycle. To create as little disturbance as possible at nests, observations were made, when
possible, from a distance. To avoid the chilling of eggs and/or young, nest trees were
generally not climbed during February, March, and early April. Nest heights were de-
termined with an optical measuring device called a Relaskop. Nestlings were banded
prior to fledging. Observations were by 10 by 50x binoculars and a 15-60x spotting
scope. Nest locations were plotted on aerial photo maps (1:24,000), from which dis-
tances were calculated.

The Study Area

Thirty-three Great Horned Owl nests were found in central Minnesota in 1977: 15, 8,

Raptor Research 14(1): 1-6

RAPTOR RESEARCH

Vol. 14 No. 1

5, and 5 nests, respectively, in Benton, Morrison, Sherburne, and Stearns counties. Habi-
tat varied within the study area, but woodlots are typically dominated by pin oak
(Quercus ellipsoidalis)/ red oak {Quercus borealis), trembling aspen {Populus tremu-
loides), or sugar maple [Acer sacc/iamm) /basswood {Tilia americana). Tree species in
low-lying areas and riverbottoms include tamarack (Larix laricina), American elm
{Ulmus americana), green ash (Fraxinus pennsylvanica), black willow {Salix nigra), and
others. Some nests in Morrison County were in stands of jack pine {Finns Banksiana)
and large-toothed aspen {Populus grandidentata). Terrain in all counties is generally flat
to moderately rolling. Although the study area is a transition zone containing sugar
maple /basswood climax forest, oak savanna, and tall grass prairie, agricultural and lum-
bering practices have greatly changed the original appearance of the area.

Results and Discussion

Nest Site Selection. Sixteen of the 33 nests utilized by Great Horned Owls were man-
made nest platforms. I erected 14 of them in 1976. Two were erected by other individ-
uals prior to 1976. Of the 17 natural nests, one was a small leaf-and-twig structure prob-
ably built by squirrels {Sciurus spp.), and 16 were old Red-tailed Hawk nests. In 1976, 12
of the 16 red-tail-built nests were used by redtails, one was used by Great Horned Owls,
and one was inactive. The remaining two nests were probably also used by redtails as
they were still in excellent repair when 1 discovered them in 1977.

Fourteen nests were found in pin oak, 4 in bur oak {Quercus macrocarpa), 2 each in
jack pine, white pine {Finns Strobus), American elm, and trembling aspen, and one in
red oak, basswood, tamarack, green ash, large-toothed aspen, cottonwood {Populus del-
toides), and black willow.

Nest heights averaged 14.0 m and ranged from 5.5 m (the squirrel nest, in a pin oak)
to 22.6 m (an artificial nest in a whiteXpine). Artificial nests averaged 14.1 m in height
(range: 10.1 m to 22.6 m). Natural nests averaged 13.9 m (range: 5.5 m to 19.2 m).

Twenty-six (79%) nests were in woodlot edges (arbitrarily defined as within 15 m of
the outer boundary) or in scattered trees ih\open locations, such as in fencerows or pas-
tures; 7 nests (21%) were in the interior of Wooded areas. Of the natural nests, 82 per-
cent (14) were classified as edge nests, and 18 percent (3) were classified as interior
nests. Of the artificial nests, 75 percent (12) were edge nests, and 25 percent (4) were
interior nests.

Nests were often quite close to areas of human activity. Active nests averaged 0.59
km from the nearest occupied human dwelling (range: 0.16 km to 2.4 km), and 0.58 km
from the closest improved (at least periodically graded) road (range: 0.16 km to 2.6 km).

Placement and Success of Artificial Nests. Fifty artificial nests were erected in the
study area in 1976: 25 in January, February, and March, and 25 in July and August. The
procedure that 1 used to construct them is outlined in an earlier paper (Bohm 1977). My
primary objective was to determine how readily they would be accepted by raptors,
redtails in particular. In 1976, redtails nested on two of the platforms that were avail-
able to them (the first 25); none was used by owls. According to Baumgartner (1938),
nest selection by Great Horned Owls occurs in late fall, several months prior to actual
nesting. The fact that owls used none of the nests that were erected in January, Febru-
ary, and March seems to substantiate this.

In the 1977 nesting season, there were 50 artificial nests available. Because nests were
often placed relatively close to each other, in clusters or groups, 1 estimated that the 50

Spring 1980

Bohm— Great Horned Owls

nests represented 28 probable territories, i.e., areas that would likely support only one
breeding pair of raptors of the same species. One of these territories was eliminated
when a woodlot was logged in the fall of 1976. Nests in 14 (52%) of the remaining 27
areas were used in 1977 by Great Horned Owls. The only nest used by redtails in 1977
was one of these same 14. I believe a significantly higher utilization rate could have
been attained if only one or two nests had been placed in a potential territory. Several
nests were also purposely placed in areas that did not seem to be particularly favorable
locations for raptor nests; none of these was utilized. Several artificial nests were se-
lected when placed in woodlots that seemed to be suitable for raptors but that did not
contain any natural nests.

Nest Density. Nests were often widely scattered throughout the study area. However,
in a 28.5 km^ area that was thoroughly searched, I believe all active nests were located.
Six were found, representing 0.21 nests per km^ I found no redtail nests. In the previous
nesting season, 1976, I found 3 active Great Horned Owl nests and 3 redtail nests within
the same area. In the more heavily wooded areas of central Minnesota, the density of
nesting owls is perhaps higher. In Wisconsin, Orians and Kuhlman (1956) found that the
Great Horned Owl population ranged from 0.05 to 0.08 pairs per km^ (1953-1955). Ha-
gar (1957), in New York, found 0.09 pairs per km^ and Smith (1969), in Utah, found 0.14
pairs per km^ On the basis of hooting censuses, Baumgartner (1939) estimated 0.39 to
1.16 pairs per km^ near Lawrence, Kansas.

Productivity. I considered a nest to be successful if at least one nestling survived to
approximately four weeks of age. At this age, most young Great Horned Owls have a
fair chance of survival if forced from the nest. Premature departure may in fact be a
fairly common phenomenon, for many of the nests used by owls are already in poor
condition at the beginning of the nesting season. In a Wisconsin study, Orians and Kuhl-
man (1956) found that all the owlets that prematurely left their nests eventually sur-
vived. Similar results were found by Errington (1932). I found this to be true also. In
two instances I found owlets that had prematurely left their nests. On one occasion, af-
ter an extremely windy night, I discovered two owlets on the ground near the base of
their nest tree. They both seemed to be in good condition, even after a 16 m tumble.
They were apparently being fed by an adult, as several small chunks of flesh were on
the ground between them. In another area, about a week later, I found an owlet at the
base of a large tree, approximately 150 m from the original nest tree. Again, it was ap-
parently being fed by at least one adult, as it was surrounded by parts from several Red-
wings {Agelaius phoeniceus). In both of these instances the owlets appeared to be 3 to 4
weeks old when I found them; I located them again approximately two weeks later, and
they appeared to be doing well.

Nesting Success. Forty-two percent (14 of 33) of the initial nesting attempts were un-
successful. Failure rates in other studies have been 36 percent (4 of 11) in Montana (Sei-
densticker and Reynolds 1971), and 31 percent (4 of 13), 5 percent (1 of 17), and 27
percent (3 of 11) in Wisconsin in 1953, 1954, and 1955, respectively (Orians and Kuhl-
man 1956). The 19 successful nests (58%) produced 35 young, or 1.8 per nest. Successful
nests in other investigations produced 1.7 owls (Hagar 1957), 1.8 (Seidensticker and Rey-
nolds 1971), and 1.6, 2.0, and 1.8, in 1953, 1954, and 1955, respectively (Orians and
Kuhlman 1956).

Fifty percent of the artificial platforms that were utilized were successful; 65% of the
natural nests were successful (table 1). The high rate of failure of artificial platforms

RAPTOR RESEARCH

Vol. 14 No. 1

Table 1. Nesting Success in Artificial and Natural Nests.

Nest

type

Number

Successful

Unsuccessful

Total

young

Young per
nesting
attempt

Young per
successful
nest

Artificial

0.9

1,8

Natural

1.2

1.9

may be explained in part because they were often erected in locations where they were
quite accessible. My activity around the nests may have attracted additional human at-
tention. Also, two of three late nesting attempts were on artificial platforms. These at-
tempts, perhaps renesting attempts, were particularly unsuccessful; all failed. I checked
the latest of these on 10 May 1977 and found two owlets, both with their eyes still
closed. Assuming them to be no older than one week, and using a 28-day incubation
period (Bent 1938), I estimated that the clutch was laid during the first week of April. I
found that most owls began incubating by the end of the first week in March; the
earliest incubating owl that I saw was on 20 February.

The success rates of edge nests (58%) and interior nests (57%) were similar (table 2).
This finding surprised me somewhat since I had thought that owls using the more con-
spicuous edge nests would be more vulnerable to human-related disturbances. (I believe
that this aspect of Great Horned Owl productivity, comparing nest success to nest loca-
tion, warrants further investigation.)

Nest Failures. Causes of nest failures were impossible to determine in most cases. It
appeared, however, that at least three were caused by human interference. A fourth
nest, which was being used by owls in February was being used by redtails in April. It
was not known whether interaction occurred between the hawks and owls, or whether
the hawks appropriated the nest after the attempt by the owls had already failed. A
similar situation occurred in Montana (Seidensticker and Reynolds 1971). Raccoons {Pro-
cyon lotor) may have been responsible for some nest failures. On several occasions I saw
them sleeping on leafy nests high in the treetops, most often on sunny days in April and
early May. The crow (Corvus brachyrhynchos) may also be responsible for some nest
failures. At one nest I saw nearly a dozen of them mob an incubating owl and chase it
from its nest. This particular nest, however, eventually proved to be successful.

Clutch Size. The clutch size was known in seven nests in 1977; five nests contained
two eggs, and two nests contained single eggs. Although I found 10 owl nests in 1976, I
did not know their clutch sizes. However, I did know the number of nestlings per suc-
cessful nest for both years (table 3). In 1977, 10 of 19 successful nests contained two
owlets. The frequency of three-owlet nests was noticeably lower than in the 1976, when
half the nests that I examined contained three nestlings. It would be interesting to know
how weather conditions affect productivity. The winter of 1976-1977 was particularly
severe in central Minnesota. Daily temperatures averaged 4.7 degrees C below normal
(mean -13.7 degrees C, range -36.6 degrees C to 5.6 degrees C) in December and 6.2
degrees C below normal in January (mean -16.5 degrees C, range -41.7 degrees C to 1.1
degrees C) (U.S. Weather Bureau, St. Cloud, MN). Extreme conditions of this sort cer-
tainly place an increased energy demand upon organisms and are perhaps reflected in
reproductive productivity. Food availability during the nesting cycle would, of course,
also affect productivity. Hagar (1957) and Smith (1969) speculated that winter weather

Spring 1980

Bohm— Great Horned Owls

conditions may have affected productivity changes in Great Horned Owl populations in
New York and Utah, respectively.

Behavior. While we were banding nestlings, we observed some rather unusual behav-
ior. At one nest, just as I was preparing to climb to the nest, another individual and I
were surprised to see an adult Great Horned Owl land on the ground perhaps 30 m
from us. The owl proceeded to shuffle about among the dry leaves, holding its wings out
and away from its body, with the underside toward us, in a manner similar to the threat
display posture used by young redtails. At no time did it turn the backs of its wings
toward us, as nestling Great Horned Owls characteristically do when threatened. This

Table 2. Nesting Success in Edge and Interior Locations.

Nest

type

Natural

Artificial

Young

Young per
nesling

Young per
successful

Number

suc/unsuc

produced

attempt

nest

Edge 26 9 5 6

Interior 7 2 1 2

28 1.1 2.0

7 1.0 1.8

Table 3. Numbers of Nestlings per Successful Nest.

Year 1 2

1976 2 3

1977 6 10

Nestlings per .successful nest
3

display lasted nearly a minute and was accompanied by bill snapping. The owl then
flew into the woods, only to return and land on the ground again. This time the display
lasted only 10-15 seconds before the owl flew back into the woods. When I climbed to
the nest, both adults flew from tree to tree and hooted intermittently but came no closer
than about 40 m. Errington (1932) saw a similar display by an adult owl in which the
owl flapped about on the ground as though it were injured.

Adult behavior at any nest is unpredictable. On a large artificial platform containing
a pair of three-week-old young, the adult sat tight until the climber thumped on the
bottom of the nest with his fist. Other adults flushed when a climber was partially up
the tree. No climber was actually struck by an owl. Behavior of this sort was perhaps
discouraged in many cases by the presence of one or two other persons at the nest site.
However, even with several people in the vicinities of nests, adult owls often became
quite excited as the climber neared the nest. Several uttered an amazing variety of calls,
perhaps best described as a mixture of hoots, barks, and whistles.

Acknowledgments

I would like to thank Douglas H. Hedtke, David C. Pauly, and Richard W. Peifer for
helping me band nestlings; Dr. Alfred H. Grewe Jr., for criticisms of this manuscript;
and the Department of Biological Sciences at St. Cloud State University for providing
equipment.

Literature Cited

Baumgartner, F. M. 1938. Courtship and nesting of the Great Horned Owls. Wilson
Bull. 50:274-285.

1939. Territory and population in the Great Horned Owl. Auk 56:274-282.

Bent, A. C. 1961. Life histories of North American birds of prey. Vol. 2. Dover, New
York.

Bohm, R. T. 1977. Artificial nest platforms for raptors. Raptor Res. ll(4):97-99.

Errington, P. L. 1932. Studies on the behavior of the Great Horned Owl. Wilson Bull.
44:212-220.

Hagar, D. C., Jr. 1957. Nesting populations of Red-tailed Hawks and Horned Owls in
central New York State. Wilson Bull. 69:263-272.

Orians, G., and F. Kuhlman. 1956. Red-tailed Hawk and Horned Owl populations in
Wisconsin. Condor 58:371-385.

Seidensticker, J. C. IV and H. V. Reynolds III. 1971. The nesting, reproductive perform-
ance, and chlorinated hydrocarbon residues in the Red-tailed Hawk and Great
Homed Owl in south-central Montana. Wilson Bull. 83:408-418.

Smith, D. G. 1969. Nesting ecology of the Great Horned Owl, Bubo virginianus. Brig-
ham Young Univ. Sci. Bull. Biol. Ser. 10:16-25.

BACTERIAL ISOLATES FROM THE PHARYNX AND CLOACA OF THE
PEREGRINE FALCON {FALCO PEREGRINUS) AND GYRFALCON
(F. RUSTICOLUS) (BACTERIA FROM FALCONS)

J. E. Cooper*

Clinical Research Centre
Watford Road, Harro^v
Middlesex, HAI 3UJ, England

P. T. Redig

Dept, of Veterinary Biology
College of Veterinary Medicine
St. Paul, Minnesota 55101, USA

W. Burnham

Peregrine West, 1424 N.E.

Frontage Road

Fort Collins, Colorado 80521, USA

Abstract

Swabs taken from the pharynx and cloaca of Peregrine Falcons {Falco peregrinus) and
Gyrfalcons {Falco rusticolus) yielded many species of bacteria, including E. coli, Proteus
sp., Staphylococcus aureus, Pasteurella anatipestifer, and Pseudomonas aeruginosa. Some
of these organisms may be significant in the context of raptor disease.

’Present address: Royal College of Surgeons of England
35-43, Lincoln’s Inn Fields
London WC2A 3PN
England

RAPTOR RESEARCH 14(l):6-9

Spring 1980

Cooper, et al.— Bacterial Isolates

Introduction

The determination of normal bacterial floral populations in the gastrointestinal tracts
of wild raptors has received little attention. Such knowledge would help in the recogni-
tion of potential pathogens in these birds. Field studies involving the Peregrine Falcon
and the Gyrfalcon by one of the authors (WB) presented an opportunity to sample the
bacterial flora of these threatened falcons, since nestling and adult birds were being han-
dled for banding purposes.

During the fall of 1972 pharyngeal swabs were taken from seven wild Peregrine Fal-
cons trapped on their southward migration. For comparative purposes, pharyngeal
swabs were also taken from seven captive peregrines. In the spring of 1973 pharyngeal
and cloacal swabs were taken from 25 nestling Peregrine Falcons and 13 young Gyrfal-
cons from Greenland eyries.

Samples were transported from the field to the laboratory in thioglycolate medium
(1972) and Stuart’s improved transport medium (1973) which were packed in ice. Sub-
sequent laboratory culture, isolation, and identification followed standard micro-
biological techniques for aerobic bacteria.

Discussion

A large number of organisms, of many different species, was isolated from the pha-
rynx and cloaca of both F. peregrinus and F. rusticolus. Since this study was intended
primarily as a survey, it would be wrong to attempt too detailed an interpretation of the
findings, especially on a quantitative basis, but certain points should be made.

Many of the organisms isolated are well recognized as part of the bacterial flora of
raptors. Examples are E. coli and Proteus spp., both of which have been reported in pre-
vious surveys (Gooper 1973). Staphylococcus aureus is a potential pathogen of birds of
prey, where it often results in infections of the feet (“bumblefoot”). Its isolation from
the pharynx of captive peregrines but not from free-living birds of either species is of
considerable interest. In a previous paper Gooper and Needham (1976) postulated that
captive raptors might acquire their staphylococci from human sources, and the findings
in the present study could support this postulation. S. epidermidis, however, was isolated
from both free-living and captive birds.

One of the most significant isolates in this survey was probably Pasteurella anatipesti-
fer, which is a known pathogen of waterfowl. As may be seen in Table 1, four out of
seven free-living peregrines yielded this organism. All seven were trapped on the east-
ern coast of the United States while on passage south in October, and it is possible that
the P. anatipestifer was acquired from diseased waterfowl.

Table 1. Isolate.s from Wild Trapped Peregrines 1972

Organisms Number of birds

(total of 7)

Staphylococcus epidermidis 5

Klebsiella pneumoniae 4

Escherichia coli 4

Pasteurella anatipestifer 4

Streptococcus sp. 3

Enterobacter sp. 3

Pseudomonas aeruginosa 2

Bacillus sp. 1

RAPTOR RESEARCH

Vol. 14 No. 1

Table 2. Isolates from Captive Peregrines 1972
Organisms

Number of birds

(total of 7)

Streptococcus sp.

Bacillus sp.

Escherichia coli

Enterobacter sp.

Klebsiella pneumoniae

Staphylococcus aureus

Staphylococcus epidermidis

Pseudomonas alcaligenes

Achromobacter anitratus

Neisseria pharyngitis

Neisseria catarrhalis

Proteus mirabilis

Table 3. Isolates from Nestling Peregrines 1973

Number of pharyngeal

Number of cloacal

isolates

Organisms

(total of 25)

Streptococcus sp.

Escherichia coli

Staphylococcus epidermidis

Enterobacter cloacae

Proteus mirabilis

Corynebacterium xerosis

Proteus rettgeri

Table 4. Isolates from Gyrfalcons 1973
Organisms

Number of pharyngeal
isolates
(total of 13)

Number of cloacal
isolates
(total of 13)

Escherichia coli

Steptococcus sp.

Staphylococcus epidermidis

Haemophilus aphrophilus

Proteus mirabilis

Proteus vulgaris

Actinobacillus sp.

Pseudomonas aeruginosa is ubiquitous in nature and can occur as part of the intestinal
flora of both mammals and birds (Bailey and Scott 1970). However, it can also cause
disease in raptors, especially if wounds become infected, and it is noteworthy that thera-
py of such cases is frequently difficult.

spring 1980

Cooper, et al.— Bacterial Isolates

The role and significance of many of the bacteria isolated remain uncertain. It is
probable that birds of prey have a degree of resistance to some bacteria and that the
isolation of the organism is not, per se, indicative of pathogenicity. It is also likely that
contaminated prey could be the source of the infection and that the bacterium would
not, under normal circumstances, persist for any length of time in the raptor host. De-
spite great advances in our understanding of raptor pathology in the past ten years
(Cooper 1978, Keymer 1972, Trainer 1969), much remains to be learned of the impor-
tance of many organisms, among them bacteria. Such information could prove useful in
studies on free-living raptor populations where predator/prey pathogen relationships
are as yet little understood.

A cknowledgments

We are grateful to Mr. Roger Scammell and Miss Sue Hudson for assisting with the
collation of data and to Mr. Jay Renicker for carrying out part of the laboratory exam-
ination.

Literature Cited

Bailey, R., and Scott, E. 1970. Diagnostic microbiology. 3rd edition. Mosby, Saint Louis.
Cooper, J. E. 1973. Post-mortem findings in East African birds of prey. Journal of Wild-
life Diseases 9:368-375.

1978. Veterinary aspects of captive birds of prey. The Standfast Press, Glouces-
tershire, England.

Cooper, J. E., and Needham, J. R. 1976. An investigation into the prevalence of S.

aureus on avian feet. Veterinary Record 98:172-174.

Keymer, I. F. 1972. Diseases of birds of prey. Veterinary Record 90:579-594.

Trainer, D. O. 1969. Diseases in raptors: A review of the literature. In J. J. Hickey (ed.),
Peregrine falcon populations; Their biology and decline. University of Wisconsin
Press, Madison.

ANNOUNCEMENT

NOTICE OF SYMPOSIUM ON THE BALD EAGLE IN WASHINGTON

A symposium to review past research on the American Bald Eagle in Washington and to
stimulate further investigation of this threatened species will be held June 14-15, 1980,
at the City of Seattle Aquarium, co-sponsored by the National Wildlife Federation,
Seattle Aquarium, Seattle Audubon Society, Seattle City Light, Seattle Woodland Park
Zoological Gardens, The Nature Conservancy, U.S. Forest Service, U.S. Fish and Wild-
life Service, and Washington Department of Game. Preregistration is recommended as
limited space is available. Interested persons should forward $5 to cover registration fees
to Washington Bald Eagle Symposium, c/o Jeremy Robertson, 2357 N.W. 70th, Seattle,
WA 98117; phone (206) 789-6056; by May 31. A published copy of the symposium pro-
ceedings is included in registration fees.

SOME CONSIDERATIONS FOR FUTURE RAPTOR REHABILITATION

Jerry Olsen and Penny Olsen
Goswood, Sutton, NSW, Australia 2620

Abstract

A female Wedge-tailed Eagle {Aquila audax) was released in a national park where
myxomatosis had been introduced into the rabbit population. The release eventually
failed because of the eagle’s aggressiveness towards humans. The use of national parks
and the need for conditioning rehabilitated raptors to fear humans are discussed.

Introduction

Techniques for rehabilitating raptors have been well documented; however, we know
of no failures that have been discussed even though considerable information can be
gained from their analysis. Many sources (e.g., Hamerstrom 1970) discuss release pro-
cedures, but, because the outcome of these releases are unknown, the methods used can-
not be accurately assessed.

Discussed below are procedures used on a female Wedge-tailed Eagle {Aquila audax).
Though they were not completely successful, they do suggest a number of consid-
erations for future rehabilitation work.

Release Procedure

The eagle came from Melbourne Zoo; nothing was known about her previous history.
In planning her release we decided to avoid any techniques related to falconry, if pos-
sible, because of their obvious drawbacks (tameness, aggression, dependence on man, ex-
pensiveness in terms of time and labour, etc.). A 5,500-hectare nature reserve near Can-
berra was chosen because previous experience with a number of species, including
Wedge-tailed Eagles, had shown that releases in national parks and reserves, where
progress could be monitored by park personnel, provided valuable data on the effective-
ness of release procedures. After conferring with authorities we decided to release her
immediately after myxomatosis was introduced into the park’s plentiful rabbit popu-
lation, This disease is harmless to eagles (Bull and Dickinson 1937), and we hoped that
.she would learn to hunt the many sick rabbits and gradually improve her skills as the
population was reduced. It was fairly certain that adequate numbers of rabbits would
remain even after the myxomatosis had run its course (e.g., Fullagar 1977). A resident
ranger left her dead laboratory rats, to which she was accustomed, every second day.

Results

Two weeks after her release .she was hunting rabbits, ignored all food put out for her,
and appeared to be totally independent. Curiously, she settled within a large (4-ha) wa-
terfowl enclosure surrounded by a 3-m-high fence.

Before her release this eagle was fearful of any approaches by man, especially strang-
ers, and she would never eat in our presence. However, two months after her release she
began to harry rangers who ventured into her hunting area. These attacks were low-

'‘Division of Wildlife Research, CSIRO, P.O. Box 84, Lyneham, ACT, Australia 2602.

Raptor Re.search 14(1): 10-12

Spring 1980

RAPTOR RESEARCH

angled swoops made at high speed with opened feet. Most people were able to drop to
the ground quickly enough to avoid being struck, but one ranger was cut on the arm. As
she would not come to food, she was trapped with the use of a noose carpet tied to a
limb used as a perch. When returned to her pen, she again became fearful of humans.

Discussion

Use of National Parks. The release of the eagle where myxomatosis had been in-
troduced was effective. From the aspect of an eagle’s welfare, large national parks have
several advantages over release sites in more remote areas. The bird’s progress can be
monitored by park personnel, and if something does go wrong, the no-shooting laws and
sympathetic personnel in these parks give some guarantee that no harm will come to a
released raptor. There are often large food supplies in these parks which can keep a
released raptor resident. The disadvantages of some remote areas are that more per-
secution (e.g., shooting or poisoning) of raptors often takes place, and, if a bird does not
fear humans, it may drift away from a remote area toward settlement particularly if
food supplies are inadequate. This eagle chose a fenced compound to settle in. We have
had a number of rehabilitated raptors, apparently accustomed to man-made structures,
travel distances up to 800 km from their release points to built-up areas.

Loss of fear of humans. The degree of fear of humans in captivity is often an irrele-
vant and totally inadequate criterion for predicting how fearful of humans a raptor will
be after release. Restraining a raptor can compound or amplify a fearful or stressful situ-
ation (McElroy 1972, Stevens n.d.) and semi-wild raptors being tame hacked can very
abruptly lose their fear of man after they are first flown free. This eagle’s aggressive
behavior superficially resembled that of an “imprinted” raptor (e.g., McElroy 1972);
however, some wild-caught raptors that have been trained for falconry or kept in zoos
for long periods before their release exhibit similar behavior (pers. obs.).

There are probably a number of released raptors that die each year because they have
lost their fear of man. A Whistling Kite {Haliastur sphenurus) was shot in Melbourne by
wildlife authorities because it had attacked a number of people, injuring one. A number
of raptors, apparently escaped captives, have been sent to us after being secured while
diving at people or after landing on urban roofs.

Instilling a Fear of Man. Raptor ethologists, rehabilitators, and falconers could ex-
plore behavioral mechanisms, in particular those involved in imprinting, adult-fledgling
relationships, “play,” territoriality, social interactions, and learning, which may reveal
methods of eliminating aggression toward humans in raptors before their release. A
more fruitful approach might involve the exploration of methods to systematically teach
tame or hand-raised raptors to fear humans, on the assumption that aggressive behaviors
will disappear as a result. This could also help overcome the main drawback that results
from all tame hack methods— the tameness of the raptor.

Much has been written in falconry works about methods of systematically eliminating
innate fear in raptors, but very little has been written on how to instill fear. Most rap-
tors probably have a basic innate fear of humans (Brown 1955), which is amplified by
the birds’ learning and experience. Methods of taming or “manning” (Stevens n.d.) in-
volve learning not to fear man, and these methods (perhaps coincidentally) very closely
resemble some Cx the techniques that behavior therapists use to cure phobias in humans,
e.g., flooding and systematic desensitization (Eysenk 1977). The use of similar forms of
operant or classical conditioning, as outlined by Ferster et al. (1975), should provide

RAPTOR RESEARCH

Vol. 14 No. 1

means to systematically teach raptors to fear people. When we were trying to trap the
female eagle, she ceased launching attacks at us. Consequently, we began walking to-
ward her to flush her to the tree containing the noose carpet. This offensive behavior on
our part, contrasted with the defensive or fleeing behavior she elicited from most hu-
mans she attacked, appeared to make her fearful of us, and by the second day she would
fly if we approached any closer than 150 m. However, these fears did not generalize,
and she continued her attacks on other park personnel who ventured into her territory
during the two days we were trying to trap her. Perhaps a variety of individuals chasing
her and a consistent, offensive response from all humans would have produced more
generalization and reinforcement of any fear of humans. The systematic use of pyro-
technics, air rifles, or firearms could aid in this type of conditioning if used after the bird
has settled into an area as this eagle had.

Summary

Careful consideration is necessary before any raptors are released, particularly if they
have been hand raised or are of unknown origin. “Ability to hunt” should not be the sole
criterion for release. There may be a risk to humans and, even if the release site is re-
mote, to the birds themselves as well as potential repercussions for future rehabilitation
work.

Though it is preferable to prevent imprinting, rehabilitators often receive imprinted
raptors. We release no eagles known to be imprinted, even into remote areas.

The teaching of raptors, and some other animals, to fear man would seem to be an
important area of research that could also be applied to wild endangered raptors at risk
from persecution. It is desirable for a raptor to fly when humans approach to within
shooting range; very few rehabilitated, hand-raised raptors are fearful enough to do so.

Acknow ledgments

We are particularly grateful to Terry Dennis of National Parks and Wildlife Service,
South Australia, and Peter Hann of Conservation and Agriculture, Australian Capital
Territory, for their invaluable assistance in the release of various raptors. We are also
indebted to R. Wayne Nelson, Nick Mooney, Frances Hamerstrom, Alan Kemp, D. Gar-
celon, and Jeff Lincer for their helpful comments on the manuscript.

Literature Cited

Brown, L. 1955. Eagles. Michael Joseph, London.

Bull, L. B. and C. G. Dickinson 1937. The specificity of the vims of rabbit myxomatosis.

/. Coun. Sci. Industr. Res. Aust. 10:291.

Eysenk, H. J. 1977. You and neurosis. Billing and Sons, London.

Ferster, C. B., S. Culbertson, and M. C. P. Boren. 1975. Behaviour principles. Prentice-
Hall, Englewood Cliffs, N.J.

Fullagar, P. J. 1977. Observations on myxomatosis in a rabbit population with immune
adults. Aust. Wildl. Res. 4(3):263-280.

Hamerstrom, F. 1970. An eagle to the sky. Iowa State Univ. Press, Ames.

McElroy, H. 1972. Desert hawking. Cactus Press, Yuma, Arizona.

Stevens, R. n.d. Observations on modern falconry. Privately printed.

SPMNG HAWK MIGRATION IN EASTERN MEXICO

J. M. Thiollay

Institute de Ecologia, A.C., Mexico 18, D.F.

Laboratoire de Zoologie, ENS
46 rue d’Ulm, 75230 Paris cedex 05

Abstract

Hawk migration was witnessed along the eastern coast of Mexico for 23 days from 6
April to 6 May 1978 north of Veracruz. More than 262,000 hawks of 17 species were
recorded, mainly Buteo platypterus (77%), Buteo swainsoni (6.4%), Cathartes aura
(9.8%), and Ictinia misisippiensis (4.7%). A large proportion of the migrants may have
passed in March and early April and a few after 6 May. Daily numbers of most species
quickly decreased after 27 April. The importance of this flyway was evident from the
13,600 Pelecanus erythrorhinchos counted and from the several million other migrant
birds seen.

Some behavioral features of migrants are described. The need for further survey work
is emphasized.

Introduction

1 know of no accurate and extensive records of migrating North American birds of
prey south of the United States border." A huge migration was noted in early autumn
1976 about 15 km north-northwest of Veracruz (Thiollay 1977). This migration was
studied in spring 1978, but most of the time was devoted to the ecology of resident rap-
tors with observation of migrants as a sideline.

Methods

1 tried to quantify the hawk migration from 6 April to 6 May 1978 along 20 km of the
road leading east-west at a right angle from the coast, from just north of Palma Sola to
the valley of Plan de Las Hayas. This unpaved road crossed first the narrow coastal
plain, then hills and plateaus to an altitude of about 800 m on the eastern edge of the
Sierra Madre Oriental, which rises well over 1,000 m (the Orizaba Peak, just southwest
of our area, is 5,563 m above sea level). Steep slopes and canyons in this ridge provide a
variety of thermals and updrafts readily used by hawks.

The weather was calm, hot, and clear during 23 of the 31 days spent there although it
was sometimes cloudy on the mountains in late afternoon, with light to moderate south
wind (optimal conditions for migration). By contrast, the remaining 8 days were cold,
with strong north winds and low, dark clouds, but little rain. However, hawks did not
stop their migratory movements for more than a few hours.

Three well-trained persons with 10 x 40 binoculars were always involved in searching
carefully for migrants. According to the behavior of the migrants (see below), the same
procedure was followed every day. During the morning, we waited along the road on
the slopes above the coastal plain until the first kettles were detected. We then tried to
remain all day in close contact with the flight line, often moving up and down the road
within the 20-km section suitable for observation. Thus, the three persons were always
together at a place believed to be under the main migrant flow. This was necessary for

Raptor Research 14(1): 13-20

RAPTOR RESEARCH

Vol. 14 No. 1

accurate counts of large multispecies flocks, but we never knew whether other birds
were passing elsewhere, i.e., more than 2 km on either side of our post. The possibility
of undetected migrants elsewhere is why our results represent only minimum figures.

Most of the groups were detected at a great distance before they crossed the road,
and thus they could be followed for a long time. Their specific composition was first
assessed as they circled. Then the total number of individuals was counted when the
flock was gliding on a long line. Lastly, the birds were again counted and identified
when passing overhead. Only very few distant groups, which we could not reach in
time, were estimated. More than 90 percent of the birds seen were actually counted to
the nearest 5 or 10 individuals, rarely to the nearest 100 (in the case of huge, dense
flocks).

As the temperature increased, most birds flew over the mountain west of the valley
Plan de Las Hayas, where the road did not cross. They were then impossible to see. This
happened sometimes as early as 2:00 or 3:00 p.m., so a large part of the daily migration
was out of sight. On the 8 days when migratory conditions seemed to be best, watching
was imsatisfactory because the flight could not be seen. Hence, no attempt was made to
study the migration.

Resident raptors in the open habitats of the Palma Sola area included the Turkey Vul-
ture {Cathartes aura). Black Vulture {Coragyps atratus), White-tailed Kite {Elanus leuc-
urus), Common Black Hawk {Buteogallus anthracinus). Gray Hawk {Buteo nitidus).
Roadside Hawk (Buteo magnirostris). Short-tailed Hawk (Buteo brachyurus), White-
tailed Hawk (Buteo albicaudatus). Laughing Falcon (Herpetotheres cachinnans). Bat Fal-
con (Falco rufigularis), and Aplomado Falcon (Falco femoralis). Since local Turkey Vul-
tures were nearly always low-flying, lone birds, they were easily differentiated from the
high, northward-flying, migrant flocks. However, local Black Vultures often flew high in
groups, even joining migrant raptors for some distance. They were identified as migrants
only after we followed them a long time, and many of them, although behaving like
migrants, were uncoimted if we had doubts about them.

The few local wintering birds which remained at that time were distinguished from
migrants by their behavior. They were mainly the American Kestrel (Falco sparverius)
although the Marsh Hawk (Circus cyaneus) and the Sharp-shinned Hawk (Accipiter
striatus) were also seen. Birds of migrant species seen hunting, roosting, or flying south-
ward were not counted.

Results

The number of birds actually recorded is given in table 1. If we attempt a rough esti-
mate of the migration which probably occurred during each of the 8 unstudied days
(taking the mean number of birds counted on the days just before and just after), we
reach a monthly total of about 350,000 hawks, not including the Unknown number of
birds possibly missed on other days.

The period covered was only a part of the spring hawk migration, which may last
from February to the end of May. From the curve of daily numbers recorded, no single
species migration period has been completely covered.

Turkey Vulture (Cathartes aura). The migration of Turkey Vultures seemed to reach
its peak during April, with a daily mean of more than 1,500 birds, although the number
decreased rapidly. Flights may begin early in the season since a group of 140 migrants
was recorded in Texas one year on 26 January (Schumacher Donohue 1978).

Spring 1980

Thiollay— Spring Hawk Migration

Black Vulture {Coragyps atratus). The small number of Black Vultures identified as
migrants (with a mean of 22 per day during the first three weeks) declined sharply at
the end of April. Similar movements have been recorded in Panama (Heintzelman 1975)
and Texas as early as 3 March (Schumacher Donohue 1978).

Osprey (Pandion haliaetus). Four hundred Ospreys were recorded, an encouraging
number in light of the slowly reeovering North American population, especially since
this may be a small part of the latter. Ospreys migrate over a wide front in Mexico
(Friedman et al. 1950), some winter north of Veracruz (Bent 1961), and the first mi-
grants reach Texas at the end of February. Osprey populations, however, are evidently
sparse in western North America (Snow 1974).

Swallow-tailed Kite {Etanoides forficatus). The absence of the Swallow-tailed Kite
(only two recorded in spring and one in autumn) is surprising because, from its south-
eastern distribution, one would expect the U.S. population to migrate along the Mexican
coast.

Everglade Kite (Rostrhamus sociabilis). Although apparently not true migrants, some
individual Everglade Kites may wander long distances along the coastal lagoons. Wet-
more (1943) also noticed a migrating (?) bird on 1 April in Veracruz.

Mississippi Kite {Ictinia misisippiensis). Flocks of the Mississippi Kite were probably
missed because they often traveled outside the main flow of migrants. The migration
began in the first days of April, reached a maximum in the second half of that month,
and continued past 6 May. This corresponds to the extreme dates (28 March and 24
May) given for Texas in 1977 (Schumacher Donohue 1978). The peak recorded in 1978
was far below the 5,130 individuals counted on 5 September 1976 (Thiollay 1977a).

Marsh Hawk (Circus cyaneus). A fair proportion of migrant Marsh Hawks may have
passed during March (first ones on 7 March in Texas according to Schumacher Donohue
1977), with their number declining in April. None was seen in May.

Sharp-shinned Hawk (Accipiter striatus). The daily mean numbers of the migrant
Sharp-shinned Hawks were 119 in the first half of April, 33 in the second half, and 5 in
the first week of May. In 1977, the true migration began on 25 March in Texas where
220 birds were still counted on 24 and 25 April.

Cooper’s Hawk (Accipiter cooperii) and Red-shouldered Hawk (Buteo lineatus). It is
interesting to point out the few Cooper’s Hawks and Red-shouldered Hawks that were
identified since they belong to rather rare wintering species south of Veracruz.

Broad- winged Hawk (Buteo platypterus). The Broad- winged Hawk is by far the most
abundant migratory hawk in eastern Mexico. The 202,000 birds counted are but a part
of the total North American population, which may amount to more than one million
(see huge autumn 1977 migration in Texas [Schumacher Donohue 1978]). The flights
gradually decreased throughout the period studied and probably peaked before our ar-
rival. Broad-winged Hawks have been reported on 6 and 9 arch (1975, 1976) with the
peak numbers (up to 100,000 a day) from 26 March to 2 April (Kennedy 1977, Schuma-
cher Donohue 1978), Some flocks of migrants have also been recorded along the west
coast of Mexico (Friedman et al. (1950) and at the southern tip of Florida (Heintzelman
1975), so this may not be the only migration route followed by the North American
Broad-wing population.

Swainson’s Hawk (Buteo swainsoni). The eastern coast of Mexico is likely to be on the
edge of the regular flyway of Swainson’s Hawk whose breeding distribution is more
western. Flights remained about the same through April, decidedly decreasing from 28

RAPTOR RESEARCH

Vol. 14 No. 1

April onward. Kennedy (1977) reported 100 migrants north of the Mexican border as
early as 27 March. After 6 May, few additional birds are to be expected.

Red-tailed Hawk {Buteo jamaicensis). Surprisingly few Red-tailed Hawks were seen
although the North American population is supposed to winter occasionally as far south
as Guatemala (Land 1970).

American Kestrel {Falco spawerius). The small American Kestrel migrates over a
much wider front than the soaring birds and is thus proportionately underrepresented in
the counts. It decreased quickly during April (respectively 617, 23 and 11 per day dur-
ing the first, second, and third decades), and none was seen in May.

Pigeon Hawk {Falco columbarius). Migrants are possibly more numerous in March.
The latest was recorded on 5 May 1977 in Texas (Schumacher Donohue 1978).

Prairie Falcon {Falco mexicanus). The only individual identified (April 7) indicates
that this species rarely winters south of Veracruz (Friedman et al. 1950).

Peregrine Falcon {Falco peregrinus). The daily mean number of migrant Peregrine
Falcons steadily remained around 2 all during the observations. Canadian populations
are probably involved. In Texas, Schumacher Donohue (1978) records similar late arriv-
als.

Table 1. Migrants counted in spring

APRIL

Cathartea

aura

2,928

2,079

158

2,543

3,192

1,008

1,810

228

592

134

272

doragypa

atratus

Pandion

haliaetUB

Elanoidea

forficatua

Roatrhamua

Bociabilie

Ictinia

mtotgippygnaifl

131

550

791

197

322

129

968

250

665

Ciroua

cyaneua

Aacipiter

atriatue

333

104

263

Aacipiter

cooperi

tsuteo

lineatua

auteo

platyp terua

42,988

28.100

20,320

4,765

8,176

11,630

6,858

32,850

16,830

5,847

1,685 i

tJuteo

awainaoni

3,762

1,628

162

120

862

1,402

3,554

570

576

136

174 ■

duteo

joTnaiaenaia

Falao

aparveriua

1,402

313

137

143

167

27 ■

Fat CO

aolumbariua

Falco

pp.riarinua

TOTAL

51,642

32,246

21,106

8,209

13,210

14,518

12,708

33,980

19,431

6,504

2,903 1

Spring 1980

Thiollay— Spring Hawk Migration

Non-raptor soaring species

Mixed with hawks were 762 Anhinga, 50 Phalacrocorax, 15 Plegadis, and 10 Mycteria.
They were much less numerous than in September 1976 (Thiollay 1977a). The White
Pelican {Pelecanus erythrorhynchos) deserves a special mention: There were 13,588
counted in 23 days (19,000 estimated for 31 days). This figure is only a part of the whole
population since the species winters in the southern United States and along both coasts
of Mexico (AOU checklist 1957, Peterson and Chalif 1973), and neither the beginning
nor the end of the passage has been seen.

Behavior of migrants

Since this work was intended to determine the magnitude of the migration, all my
time was devoted to a thorough search and census of the migrants, and no detailed study
of their behavior could be done.

Migration began rather late in the morning. Large flocks rarely appeared before
12:00 noon, never before 11:00 a.m., and sometimes after 1:00 p.m. Only some groups of

west of Palma Sola, northern Veracruz (Mexico).

TOTAL

Count6d during
23 observation days

385

3,418

1,067

3,642

681

341

292

325

25,820

333

■ 21

380

1,238

1,547

2,371

442

219

1,853

142

12,432

170

1,396

1,618

1,970

302

4,984

1,230

3,670

898

129

1,429

450

202,147

1,135

161

1,779

16,684

2,597

2,159

7,835

3,105

12,881

2,514

4,328

1,055

152

564

3,702

671

262,110

RAPTOR RESEARCH

Vol. 14 No. 1

Vultures or Buteo, which had spent the night in the surrounding hills, started between
8:00 and 10:00 a.m. Flights were dependent on weather conditions and usually lasted 2
to 6 hours. In the beginning of the afternoon Buteo formed the bulk of the migrants, but
in the last two hours small hawks {Falco, Accipiter) increased markedly, often along with
Ictinia and Cathartes whose large flocks frequently appeared very late (around 5:00
p.m.).

Birds showed their usual migration pattern, soaring to get height, then gliding on long
distances and soaring again. Their flyway, however, shifted all day between the coast
and the mountains, even without detectable change in local weather conditions. Thus
with sunny weather and a south wind the early migrants were seen above the eastern
hills. Then they crossed the Palma Sola-Plan de las Hayas road more and more west-
ward, and after 3:00 p.m. they sometimes flew well above the distant mountains. Fortu-
nately this tendency was reversed with north wind and clouds on the high range, and in
the afternoon birds drifted toward the coastal plain above which they migrated even
when fog covered the hills.

A commonly observed behavior was a reluctance to fly under the dark clouds even if
conditions remained favorable. When clouds began to build up over the mountains, the
birds tended to fly around them.

Buteo and Cathartes are strongly gregarious, associating with any other species. Ic-
tinia shows intra- but less interspecific attraction and often forms dense monospecific
flocks. Pandion, Circus, Accipiter, and Falco travel individually even when they are
within a short distance of one another or temporarily join other species. Flocks rising
quickly attract other birds, especially those flying low, which often turn up to 90° from
their primary direction to join them and take advantage of the best thermals.

Each species has its own flight characteristics: Only soaring and gliding for Cathartes,
occasional flapping for Buteo, strong wing beats and long glides for Ictinia, mainly rap-
id, flapping flight for Falco and Accipiter, etc. As a result, their speeds are different.
Mississippi Kites are the fastest of all, flying so quickly that some of their flocks can be
overlooked if one is not searching constantly.

Roosting behavior was also very different from one species to another. At the end of
the afternoon kestrels went down anywhere, perching on isolated trees. Broad-winged
and Swainson’s Hawks concentrated over remote wooded slopes, soaring for a while be-
fore going down and scattering themselves over a large area. Turkey Vultures appeared
shortly before sunset in long lines of several hundred birds gliding slowly, lower and
lower, to roost in deep valleys (barrancas), sometimes more than 20 birds on the same
tree. The flocks of Mississippi Kites often went down very late, perching immediately
on 2 or 3 large trees.

As usual, very few of these birds were ever seen hunting, even before departing. In
the morning most of them stayed on their roosting sites until 9:00 or 10:00 a.m. If up-
drafts were not good enough, they began to fly low over the slopes and valleys to search
for a suitable updraft, sometimes many kilometers from their roost, and then they went
up quickly and disappeared.

Discussion

Three to four hundred thousand hawks (77% Broad-winged) migrated in April along
the northeastern coast of Mexico. It is five times what can be seen in a whole autumn in
the best lookouts of North America (Heintzelman 1975, W. S. Clark, pers. comm.) ex-

spring 1980

Thiollay— Spring Hawk Migration

cept in Texas, and more than anywhere in Europe (Thiollay 1977b). The importance of
this flyway is emphasized by the huge and continuous migration of passerines and oth-
ers, involving often half a million birds a day.

Since the central part of Mexico is a high plateau with elevations of around 2,000 m
edged on both sides by high ranges, it is easy to understand why soaring birds concen-
trate on the coastal slopes of this relief, to enter the United States in Texas. But we have
seen that the flight lines could change over a wide front according to the weather and
are difficult to fully control because of the few roads crossing these mountains. More-
over, we do not know the magnitude of possible central or western flyways.

Counting migrants is the easiest way to evaluate breeding populations of very large
areas. Most if not all the world populations of species such as Buteo platypterus, B.
swainsoni or Ictinia misisippiensis cross the Isthmus of Tehuantepec twice yearly. This
bridge is the only locality north of Panama suitable to initiate a complete migratory
survey. However, a numerous and well-trained team is necessary to control during 3
months some 100 km of hilly country over which hawks are often flying very high.

A cknowledgments

This work was a part of the ecological survey of the Laguna Verde atomic plant area,
supported by the Mexican Federal Commission of Electricity and the Institute of Ecolo-
gy of Mexico. I gratefully acknowledge Drs. G. Halffter and P. Reyes of the institute for
their invaluable help, J. Nocedal (experienced Mexican ornithologist), and my keen-eyed
wife Fran 9 oise for their tireless assistance in the field, together with C. M. White and
W. S. Clark for comments on an earlier draft and improvement of my English,

Literature Cited

American Ornithologists’ Union Committee. 1957. Checklist of North American Birds.
A.O.U.

Bent, A. C. 1961. Life histories of North American birds of prey. 2 vols. Dover, New
York.

Friedman, H., L. Criscom, and R. T. Moore. 1950. Distributional checklist of the birds
of Mexico, part 1. Pacific Coast Avifauna Ser. n. 29. Cooper Orn. Soc., Berkeley.
Heintzelman, D. S. 1975. Autumn hawk flights. Rutgers Univ. Press, New Brunswick.
Kennedy, R. S. 1977, Spring records 1976. Report for the South. Newsletter Hawk Migr.
Assn. N. Amer. 2(l):5-6.

Land, H. C. 1970. Birds of Guatemala. Livingstone, Wynnewood, Penn.sylvania.

Peterson, R. T., and E. L. Chalif. 1973. A field guide to Mexican birds. Houghton Mif-
flin, Boston.

Schumacher Donohue, G, 1978. Spring and autumn records 1977. Reports for South
central. Newsletter Hawk Migr. Assn. N. Amer. 3(l):7-8, 3(2): 16-18.

Snow, C. 1974. Habitat management series for unique or endangered species. Report
No. 12. Osprey. USBLM Tech. Note T-N-254.

Thiollay, J. M. 1977a. La migration d’automne sur la cote orientale du Mexique. Alauda
45:344-346.

1977b. Importance des populations de rapaces migrateurs en Mediterranee oc-

cidentale. Alauda 45:115-121.

Wetmore, A. 1943. The birds of .southern Veracruz, Mexico. Proc. U.S. Nat. Mus.
93:215-340.

“Ed. Note; N. G. Smith has been working on raptor migration in Panama for some years. See Smithsonian Institute Report 7, Winter 1974.

•Oaxaca

COMMENTS ON THE RECOGNITION OF OFFSPRING BY RAPTORS
by

Helmut C. Mueller

Department of Zoology and Curriculum in Ecology
University of North Carolina
Chapel Hill, North Carolina 27514

Stinson (1976) presents anecdotes which suggest that Ospreys {Pandion haliaetus) may
be able to recognize their own fledged young, and he contrasts his observations with
those of Postulpalsky and Holt (1975), which suggest that Bald Eagles {Haliaeetus leu-
cocephalus) may not recognize their own unfledged young. Although it is possible to
argue that recognition, or lack thereof, has not been clearly demonstrated in either pa-
per, let us assume that it has.

Raptor Research 14(1):20-21

Spring 1980

Mueller— Recognition of Offspring

There is no adaptive rationale for the recognition of unfledged young in altricial spe-
cies in which there is any distance between nests, because there is no natural way for a
strange young to get from one nest to another. Once the young are on the wing, how-
ever, it would be adaptive for parents to recognize their own young, particularly in Os-
preys, which often nest relatively near to other pairs and which exhibit little territorial
behavior.

Territorial defense, nest-building, courtship, egg production, incubation, and the rear-
ing of young constitute a considerable energy drain on the parents, increase their sus-
ceptibility to injury and disease and, by decreasing chances for individual survival, con-
stitute a loss in genetic fitness to the adult. This loss in fitness is compensated for by the
genes of the adult which have been passed to its young, resulting usually in a net gain. If
adults feed unrelated young, there is a loss in fitness, so adults should recognize their
own offspring. As long as the young are in the nest, recognition of the nest is sufficient.
Once the young have fledged, individual recognition of the young increases the fitness
of the adult if there is any chance that unrelated young will be fed. In species which
defend large territories, and in which the young remain well within the territory, indi-
vidual recognition of the young may be unnecessary.

Davies and Garrick (1962) were probably the first to suggest that individual recogni-
tion of young develops as the young become mobile. Extremes in the onset of individual
recognition can be seen in (1) the Common Murre (Uria aalge) which lays its egg on a
rock edge in extremely close proximity to those of neighbors, the eggs can roll a bit on
slanting ledges, and the parents recognize their own eggs (Tschanz 1959); and (2) Barn
Swallows {Hirundo rustica) and Tree Swallows [Iridoprocne bicolor), which do not recog-
nize their own young until about the time of fledging (Burtt 1977).

It would be interesting to determine, experimentally, the age at which Ospreys and
other raptors begin to recognize their own young, but I would argue against such a
study. There have been more than a few studies on parental recognition of young in a
variety of birds, and there is every reason to expect that the phenomenon occurs in al-
most all species. Further studies of the phenomenon should not be performed on species
that are suffering from low or declining populations, which includes too many of our
raptors.

Literature Cited

Burtt, E. H., Jr. 1977. Some factors in the timing of parent-chick recognition in swal-
lows. Anim. Behav. 25:231-239.

Davies, S. J. J. F., and F. Garrick. 1962. On the ability of Crested Terns (Sterna bergii) to
recognize their own chicks. Aust. J. Zool. 10:171-177.

Postulpalsky, S., and J. B. Holt, Jr. 1975. Adoption of nestlings by breeding Bald Eagles.
Raptor Res. 9:18-20.

Stinson, C. H. 1976. On the recognition of offspring by raptors. Raptor Res. 10:30-32.
Tschanz, B. 1959. Zur Bmtbiologie der Trottellumme lUria aalse aalpe Pont.). Behav-
iour 14:1-100.

NESTING POPULATIONS OF RED-TAILED HAWKS AND
GREAT HORNED OWLS IN CENTRAL OHIO

by John S. Kirkley‘ and
Mark A. Springei^

Ohio Wesleyan University

Abstract

Nesting populations of Red-tailed Hawks {Buteo jamaicensis) and Great Horned Owls
{Bubo virginianus) were studied from 1974 through 1976 on a 510-km^ (197-sq.-mi.) area
located in Delaware County, Ohio.

In 1976, 59 nesting pairs of Great Horned Owls, 6 non-nesting pairs, and 9 unpaired
individuals were located, while 78 nesting pairs of Red-tailed Hawks, 7 non-nesting
pairs, and 7 unpaired individuals were located. Density of Great Horned Owls was one
nesting pair per 8.2 km^ (3.16 sq. mi.) of usable habitat, and density of Red-tailed Hawks
was one nesting pair per 6.2 km^ (2.39 sq. mi.). The rate of nonbreeding for Great Horn-
ed Owls and Red-tailed Hawks was 15 percent and 12 percent, respectively. Productiv-
ity of Great Homed Owls averaged 2.0 eggs per observed clutch, 1.9 nestlings per
hatched clutch, 1.7 fledglings per successful nest, and 1.2 young fledged per nesting at-
tempt. Red-tailed Hawk productivity averaged 2.0 eggs per observed clutch, 2.12 nest-
lings per hatched clutch, 1.96 fledglings per successful nest, and 1.29 young fledged per
nesting attempt. Nest failure rate for Great Horned Owls was 25 percent. Red-tailed
Hawk nest failure rate was 34 percent, and nestling mortality was 19.7 percent. Causes
of Red-tailed Hawk nest failures were largely undetermined although predation by
Great Horned Owls was often suspected.

Introduction

Recent studies of Red-tailed Hawks {Buteo iamaicensis) and Great Horned Owls
{Bubo virginianus) suggested that nesting densities of these raptors in Ohio were
markedly lower than densities reported elsewhere (Shelton 1971, Cornman 1973, Mis-
ztal 1974). This study ranged from 1974 through 1976. Most of the information on pop-
ulation density and productivity of these birds, however, was obtained during the last
year. Therefore, this report will focus on the results of that year.

Study Area and Methods

A 510-km^ area was selected in Delaware County, Ohio (approximate coordinates,
north latitude 40° 08' and 40° 20'; west longitude 83° 10' and 82° 96'; see figure 1).
Topography in Delaware County is flat to gently rolling, and land is devoted largely to
agriculture (67%) with small percentages of pasture (16%) and woodlots (9%). Three ma-
jor drainages traverse the study area, two of which have reservoirs comprising a com-
bined area of 17 km^ Urban and residential development occupies approximately 8 per-
cent of the county, where the city of Delaware covers approximately 26.4 km^ of this
land.

'Department of Biology and the Ecology Center, Utah State University, UMC 5.3, Logan, Utah 84322.
“Department of Anatomy, School of Medicine, Wright State University, Dayton, Ohio 454.35,

Raptor Research 14(l):22-28

Spring 1980

Kirkley & Springer— Nesting Populations

Nesting Red-tailed Hawks (RTH) and Great Horned Owls (GHO) were located from
foot surveys and road surveys during a period from early February through April. We
considered a nest to be active if an adult raptor was observed on the nest or if signs of
recent occupation were evident (i.e., eggs, eggshells, fresh nest lining, abundant white-
wash, and fledglings). Individual or paired birds were listed as nonbreeding when re-
peated searches failed to reveal an active nest. Inasmuch as other investigators have re-
ported high rates of abandonment of incubating raptors disturbed by climbers (Fitch et
al. 1946, Luttich et al. 1971), we obtained clutch-size data without climbing by using a
mirror and pole device (Parker 1972). It was necessary to climb only once to most nests,
to determine hatching success and to weigh nestlings; this was usually done within three
weeks after hatching. Age of nestlings was determined by comparing body weights of
each nestling to known growth rates of five nestling RTHs and two GHOs measured on
the study area in 1975 and 1976. Beyond this time, age was determined by the linear
measurement of the 7th primary, and/or overall size and plumage development. Final
reproductive outcome was determined by counting the four- to six-week-old nestlings
from the ground with the aid of a mirror and pole device and binoculars.

Results and Discussion

Breeding chronology. Great Horned Owls: Of 53 nests occupied by GHOs in 1976, 31
(59%) were former RTH nests, 17 (32%) were tree hollows, and 5 (9%) were arboreal
squirrel nests.

We visited 10 nests_while hatching was in progress. Dates of first hatch ranged from 7
March to 21 March (X: 13 March, 10 nests). In 16 other nests where nestling were aged
by weight, hatch dates ranged from 15 February to 26 March (X: 10 March). We used
34 days as an estimated incubation period (Austing_and Holt 1966) and estimated laying
occurred between 12 January and 21 February (X; 5 February, 26 nests). No attempt
was made to determine actual fledging dates, but newly fledged young were observed in
the last week of March. Assuming the variable nestling period to average about 45 days
(Hoffmeister and Setzer 1947), fledging dates ranged from the last week of March to the
first week of May.

Red-tailed Hawks: Nest-building activities were observed on 27 January 1975 and on
4 February 1976. Both instances occurred on unseasonably warm, sunny days. Of 41
RTH territories active in 1975, 34 (83%) were reoccupied by RTHs in 1976. Pairs in the
34 reoccupied territories built new nests in 22 (65%) and reused former nest sites in 12
(35%) territories. Of the 29 nest sites in 1975 not reused by RTHs in 1976, 10 (34%) were
occupied by GHOs, 9 (13%) were unused, and 10 (34%) had been destroyed, presumably
by weather.

In 11 nests visited while _hatching was in progress, date of first hatch in 1976 ranged
from 11 April to 2 May (X: 19 April). In 44 other nesj^ where young were aged by
weight, the hatch dates ranged from 9 April to 10 May (X: 19 April). Our record of the
first egg in a clutch hatching 34 days after it was first observed as a single egg agrees
with the length of the incubation period observed by Hardy (1939). Assuming the in-
cubation period to ^yerage 34 days, estimated laying dates at 55 nests ranged from 6
March to 6 April (X: 16 March). Actual dates of fledging were generally not deter-
mined, but the nestling period averaged 42 days for nestlings weighed on a daily basis. If
the nestling period is assumed to average 42 days, estimated fledging dates ranged from
the last week of May through the last week of June.

Nesting Season Density. Great Horned Owl: In the study area in 1976, Great Horned

RAPTOR RESEARCH

Vol. 14 No. 1

Owls numbered 59 nesting pairs, 6 non-nesting pairs, and 9 unpaired individuals, thus a
total of 74 activity centers (figure 1). An owl density of one nesting pair per 8.2 km^ was
calculated for the 483.6 km^ of usable habitat in the study area, excluding the land occu-
pied by the city of Delaware. Great Horned Owls in our study area were distributed in
considerably higher densities than those previously reported in Ohio (i.e., 82.9 and 171.7
km^ per nesting pair reported by Cornman 1973 and Misztal 1974, respectively). Dis-
tances between the nearest adjacent owl nests average 2.0 km, with a minimum distance
between active owl nests of 0.9 km. Nonbreeding in GHOs in the study area in 1976 was
approximately 15 percent.

Red-tailed Hawk: The RTH population in 1976 consisted of 78 nesting pairs, 7 non-
nesting pairs, and 7 unpaired individuals, yielding a total of 92 activity centers (figure
1). Red-tailed Hawk density on the usable habitat of the study area averaged one nesting
pair per 6.2 km^ Densities on selected 52-km^ plots ranged from 10.4 km^ per nesting
pair in the sparsely wooded uplands to 3.4 km^ per nesting pair along the heavily wood-
ed river drainages. The average distribution of RTHs in the study area in 1976 was high-
er than those reported elsewhere in Ohio (i.e., 24.9, 43.0, and 50.0 km^ per nesting pair
reported by Cornman 1973, Misztal 1974, and Shelton 1971, respectively). Adjacent
RTH nests were spaced an average distance of 1.5 km, with a minimum distance be-
tween active nests of 0.6 km. Nonbreeding in RTHs in the study area in 1976 was ap-
proximately 12 percent.

Productivity and Mortality. Great Horned Owl: Clutches in 19 Great Horned Owl
nests ranged from 1 to 4 with a mean of 2.0, and broods in 32 nests ranged from 1 to 3
with a mean of 1.9 (table 1). We found that 8.8 percent of the eggs in 17 nests failed to
hatch. Nestling mortality in 32 nests was approximately 13 percent. Great Horned Owl
productivity in 42 nests where young were successfully fledged averaged 1.69 young per
successful nest; but, when unsuccessful nesting attempts are included in the average, the
net productivity is 1.27 young fledged per nesting attempt.

Of 56 Great Horned Owl nesting attempts with known outcome in 1976, 14 (25%)
failed to fledge young. Many nest failures were believed to have occurred during in-
cubation or shortly after the eggs hatched, judging from the lack of an accumulation of
whitewash at the nest. Human disturbance was the probable cause of three nest failures,
and predation by raccoons and wind damage each caused two nest failures.

Red-Tailed Hawk: The clutch and brood sizes of RTHs ranged from 1 to 3 with a
mean clutch size in 38 nests of 2.0, and a mean brood size in 57 nests of 2.12 (table 1).
(Average brood size is larger than average clutch size because of the low hatching suc-
cess (16.7%) of single-egg clutches). In 26 nests for which clutch size and successful
hatching are known, 4 had one addled egg each, a hatching failure rate of 7.3 percent.
Nesting mortality rate in 56 nests was 19.7 percent, with loss of the entire brood ac-
counting for 13 mortalities in 6 unsuccessful nests, and brood reductions accounting for
11 mortalities in 10 successful nests. The productivity in 50 successful nests averaged
1.96 young fledged per successful nest, but, when all nesting attempts are included, the
net productivity averaged 1.29 per nesting attempt.

Because of low hatching success, single-egg clutches had a nesting success rate (16.7%)
which was significantly (X^ P < 0.0001) lower than that of 2-egg clutches (87.5%) or 3-
egg clutches (86.7%). The final productivity of 2-egg and 3-egg clutches averaged 1.8
and 2.4 young fledged per successful nest, respectively. Brood reductions, occurring 10
times as frequently in broods of three (62%) as in broods of two (6%), suggest a higher

Spring 1980

Kirkley & Springer— Nesting Populations

Table 1. Population and productivity statistics for Red-tailed HaAvks and Great Horned Owls.

Red-tailed Hawk Studies

Average Average

Researcher

density:

km-

per

nesting

pair

Average

non-

breeding

rate

Average

clutch

size

Average

brood

size

no, young
fledged
per

nesting

attempt

Average

nest

failure

rate

Hawk

owl

nesting

ratio

Kirkley and
Springer 1980 (Ohio)

6.2

2.0

2.12

1.29

1.3:1

Cornman 1973 (Ohio)

24.9

8.4

1.5

5:1

Craighead and (Michigan)

19.2

2,0

0.9

Craighead 1956 (Wyoming)

2.6

2.3

—

1.4

3:1

Fitch et al. 1946 (California)

1.3

—

2.0

2.2

0.9

—

Gates 1972 (Wisconsin)

10.6

—

1.1

10:1

Hagar 1957 (New York)

8.0

—

1.9

1.1

1.5:1

Johnson 1975 (Montana)

8.0

2.53

1.57

—

Luttich et al. 1971 (New York)

7.0

2.0

1.9

—

Mclnvaille and
Keith 1974 (Canada)

7.5

2.1

2.0

0.92

1.3:1

Misztal 1974 (Ohio)

43.0

—

Orians and

Kuhlman 1956 (Wisconsin)

7.3

1.9

1.4

2.3:1

Seidensticker and
Reynolds 1971 (Montana)

2,0

2.6

0.9

3:1

Shelton 1971 (Ohio)

50.0

—

Wiley 1975 (California)

3.2

4.5

2.53

2,19

1,64

26.4

Great Horned Owl Studies

Kirkley and
Springer 1980 (Ohio)

8.2

2.0

1.9

1.27

Baumgartner 1939 (Kansas)

1.3

—

Boswell 1974 (Ohio)

—

2.0

1.45

1.18

(Colorado)

—

2.66

1,85

1.69

Cornman 1973 (Ohio)

82.9

—

Craighead and (Michigan)

19.2

1.9

1.1

Craighead 1956 (Wyoming)

7.8

2.2

—

2.0

—

Hagar 1957 (New York)

12.2

1,8

1.5

Houston 1975 (Canada)

5.2

(3-56)

—

6.5

Mclnvaille and
Keith 1974 (Canada)

22.0

(0-80)

2.37

2.2

1.8

Misztal 1974 (Ohio)

171.7

—

Orians and

Kuhlman 1956 (Wisconsin)

14.5

1.82

1.46

Seidensticker and
Reynolds 1971 (Montana)

—

2.2

1.2

degree of nestling competition in the larger broods. Fratricide may account for some of
the brood reductions. In 1974, we observed from a blind a sibling fratricide sequence in
which the largest of three RTH nestlings pecked both its nestmates to death. At one nest
in 1976, we found evidence of sibling aggression where the smaller of two nestlings had
parts of its head scarred and bare.

RAPTOR RESEARCH

Vol. 14 No. 1

In 76 RTH nests with known outcome, 26 (34%) failed to fledge young. The causes of
RTH nesting failures were often difficult to determine since the majority of the failures
occurred during the incubation or early brood stages. Of the known causes of nest fail-
ure in 1976, human disturbance caused the desertion of three nests, wind and lightning
each destroyed one nest, and predation by raccoons, crows, and GHOs (Springer and
Kirkley 1978) accounted for the failures of one, two, and three nests, respectively.

Summary

Nesting populations of Red-tailed Hawks and Great Horned Owls were studied from
1974 to 1976 on a 510-km^ area in Delaware County, Ohio. Population density and pro-
ductivity of both raptors were similar to those values reported elsewhere in North
America, but densities of these two raptors were considerably higher than those report-
ed previously in Ohio.

In 1976, Great Horned Owls number 59 nesting pairs, 6 non-nesting pairs and 9 un-
paired individuals, yielding an average density of 1 nesting pair per 8.2 km^ of msable
habitat and a nonbreeding rate of 15 percent. The estimated mean hatching date in
1976 was 10 March. Great Horned Owl productivity averaged 2.0 eggs per observed
clutch, 1.9 nestlings per hatched clutch, 1.7 fledglings per successful nest, and 1.2 fledg-
lings per nesting attempt. A hatching failure rate of 8.8 percent, a nestling mortality
rate of 13 percent, and a nest failure rate of 25 percent were recorded for Great Horned
Owls in 1976.

In 1976, Red-tailed Hawks numbered 78 nesting pairs, 7 non-nesting pairs, and 7 un-
paired individuals, yielding an average density of one nesting pair per 6.2 km^ and a
nonbreeding rate of 12%. In 1976 the estimated mean hatching date was 19 April. Red-
tailed Hawks produced an average of 2.0 eggs per observed clutch, 2.12 nestlings per
hatched clutch, 1.96 fledglings per successful nest, and 1.29 fledglings per nesting at-
tempt. A hatching failure rate of 7.3 percent, a nestling mortality rate of 19.7 percent,
and a nest failure rate of 34 percent were also recorded for Red-tailed Hawks in 1976.

A cknow ledgments

We would like to thank Drs. Dennis C. Radabaugh, William D. Stull, Frank L.
Damm, James W. Parker, and Frank C. Craighead, Jr. We are deeply indebted to Rita
Apanius, Gael Bissel, Janet Johnson, Margaret Macivor, and Richard Tuttle for their in-
valuable contribution in the field. This study was financed in part by the Ohio Biologi-
cal Survey, and field equipment was provided by Ohio Wesleyan University, the USD A
Forestry Laboratory in Delaware, and the Ohio Bell Telephone Company. We would
also like to thank Kate Tremblay for her support in typing this manuscript.

Literature Cited

Austing, G. R., and J. B. Holt. 1966. The world of the Great Horned Owl. Lippincott,
Philadelphia and New York.

Baumgartner, F. M. 1939. Territory and population in the Great Horned Owl. Auk
56:274-282.

Boswell, R. S. 1974. A comparative study of the population biology of two subspecies of
the Great Horned Owl {Bubo virginianus occidentalism Bubo virginianus virgi-
nianus). M.S. thesis. Bowling Green State University, Bowling Green, Ohio, 57 pp.

Spring 1980

Kirkley & Springer— Nesting Populations

Cornman, D. D. 1973. Red-tailed Hawk population ecology in Wood County, Ohio.
M.S. thesis, Bowling Green State University, Bowling Green, Ohio.

Craighead, J. J., and F. C. Craighead, Jr. 1956. Hawks, owls, and wildlife. Stackpole,
Harriburg, Pennsylvania.

Fitch, H. F., F. Swenson, and D. F. Tillotson. 1946. Behavior and food habits of the Red-
tailed Hawk. Condor 48:205-237.

Gates, J. M. 1972. Red-tailed Hawk populations and ecology in east-central Wisconsin.
Wilson Bull 84:421-433.

Hagar, D. C., Jr. 1957. Nesting populations of Red-tailed Hawks and Horned Owls in
central New York State. Wilson Bull 69:263-272.

Hardy, R. 1939. Nesting habits of the western Red-tailed Hawk. Condor 41:79-80.

Hoffmeister, D. F., and H. W. Setzer. 1947. The postnatal development of two broods
of Great Horned Owls {Bubo virginianus). Univ. of Kansas Pubis., Mus. Nat. Hist.
1:157-173.

Houston, G. S. 1975. Reproductive performance of Great Horned Owls in Sasketche-
wan. Bird Banding 46:302-304.

Johnson, S. J. 1975. Productivity of the Red-tailed Hawk in southwestern Montana. Auk
92:732-736.

Luttich, S. M., L. B. Keith, and J. D. Stephenson. 1971. Population dynamics of the Red-
tailed Hawk {Buteo jamaicensis) at Rochester, Alberta. Auk 88:75-87.

Mclnvaille, W. B., Jr., and L. B. Keith. 1974. Predator-prey relations and breeding biol-
ogy of the Great. Horned Owl and Red-tailed Hawk in central Alberta. Can. Field-
Nat. 88:1-20.

Misztal, A. 1974. The population and nesting of Red-tailed Hawks (Buteo jamaicensis)
and Great Horned Owls (Bubo virginianus) on the Wisconsin till plains of western
Ohio. M.S. thesis, Ohio State Univ., Columbus, Ohio.

Orians, G., and F. Kuhlman. 1956. Red-tailed Hawk and Horned Owl populations in
Wisconsin. Condor 58:371-385.

Parker, J. W. 1972. A mirror and pole device for examining high nests. Bird Banding
43:216-218.

Seidensticker, J. C. IV, and H. V. Reynolds III. 1971. The nesting, reproductive per-
formance, and chlorinated hydrocarbon residues in the Red-tailed Hawk and Great
Horned Owl in south-central Montana. Wilson Bull. 83:408-418.

Shelton, A. D. 1971. A population study of the breeding hawks of the genus Buteo in
Marion County, Ohio. M.S. thesis, Ohio State Univ., Columbus, Ohio.

Springer, M. A., and J. S. Kirkley. 1978. Inter- and intraspecific interactions between
Red-tailed Hawks and Great Horned Owls in central Ohio. Ohio J. Sci.
78:323-328.

Wiley, J. W. 1975. The nesting and reproductive success of Red-tailed Hawks and Red-
shouldered Hawks in Orange County, California, 1973. Condor 77:133-139.

28 RAPTOR RESEARCH Vol. 14 No. 1

RAPTOR ACTIVITY CENTERS

water

1-talled Hawk

Great Horned Owl

urban, residential

nesting

▼ nesting

1 mile

non-nesting

V non-nesting

2 kilometers

Figure 1. Distribution of Red-tailed Hawks and Great Horned Owls on the study area in 1976.

PRE-NESTING BEHAVIOR OF THE SWALLOW-TAILED KITE
{ELANOIDES FORFICATUS), INCLUDING INTERFERENCE BY
AN UNMATED MALE WITH A BREEDING PAIR

Lawrence Kilham
Department of Microbiology
Dartmouth Medical School
Hanover, New Hampshire 03755

Snyder (1974) has described the breeding biology of the Swallow-tailed Kite (Ela-
noides forficatus). My observations of their pre-nesting behavior, made while staying at
the Archbold Biological Station at Lake Placid, Florida, confirm his on copulations,
courtship feeding, and nest-building. I report two aspects of behavior, one, the activities
of an unmated male and the other relating to courtship feeding, that have not been re-
ported previously, or at least not described in detail as far as I am aware. My observa-
tions were made between 16 March and 1 April 1979 at Parker’s Island, Highland
County, Florida, a colony known to have been in existence for at least 25 years. Past
nests, from local reports, were located in the tops of tall slash pines {Finns elliotii) as
were two of the three nests in 1979. A third nest was apparently located in a swamp
(bayhead) located 1.1 km away. The colony consisted of seven individuals, three pairs
and an unmated male. The two pairs in the pines nested close to one side of a road and
did their courting and perching on bare trees on the other. This situation was favorable
to making observations, and I did not use a blind. Determination of sex was possible
during much of the first 90 to 120 min. of a day, when the females spent most of their
time perching on the dead trees where they were subject to copulations and courtship
feedings.

Activities of an Unmated Male

Intrusions of an unmated male (hereafter designated IM for intruding male) were
largely upon pair A. Pair A completed a nest 10 days later than the other two pairs of
the colony. The latter became relatively inactive once incubation began. The intrusions
of IM involved conflicts with pair A and interference with copulation, courtship feed-
ing, and nest building.

Copulations. IM interfered with copulation 14 times. On 18 March, when in flight,
IM knocked male A (MA) from the back of female A (FA) twice in 13 min. But physical
interference was not always needed. On 26 March MA mounted FA five times in succes-
sion, breaking away each time when IM circled within 5 m. The intruding male tried to
copulate with FA four times. She never assumed her horizontal invitation pose, and all
he was able to do was to alight on or brush over her back, momentarily.

Courtship Feedings. As with the copulations, interference took two forms, with IM
either trying to prevent MA from feeding FA (N = 3) or to feed her himself (N = 11). On
27 March IM swooped and knocked MA away two times in succession when the latter
was about to feed his mate. On the following day he knocked FA from her perch after
she had received an anole {Anolis carolinensis) from MA. Although IM carried anoles to
feed FA, she never assumed her horizontal invitation pose and never accepted prey

Raptor Research 14(l):29-3l

RAPTOR RESEARCH

Vol. 14 No. 1

from him. On one occasion female B (FB), who had just left off incubating, accepted an
anole from the unmated male.

Interference with Nest Building. The chief interference of IM was to perch within 2
to 4 m of the nest. On 28 March he twice swooped at FA when she came in carrying
nest material, forcing her the first time to leave and the second time to drop the stick
she was carrying.

Conflicts. IM persisted in remaining close to pair A for hours each day. MA was also
persistent in trying to drive him away. The result was that much time was taken up with
the two kites circling over the nest area. This conflict circling differed from the circling
seen at times when the other members of the colony gathered, by the tighter circles,
more flapping of wings, and dives as MA partly closed his wings to dive on IM. Many
per-weat calls accompanied these conflicts. I never saw physical contact although I
twice saw the kites come within centimeters of each other. These conflicts diverted
MA’s attention from his mate and nest-building. Evidence for this was suggested by ob-
servations on 28 March when IM was absent between 06:25 and 08:08. During this in-
terval MA copulated five times and fed his mate anoles nine times, the most attention
that he had paid to her on any single morning. As soon as IM returned at 08.08, the
diving and circling began again. I never saw aerial maneuvers that I could interpret as
courtship although the question of their being so is raised both by Brown and Amadon
(1968) and by Snyder (1974).

Courtship Feeding and Displacement Activity

Of 84 observed transfers of prey from the males to the females of the three pairs, the
prey was an anole 6-7 cm in length in 83 and an unidentified object in one. All the
females flattened with wings slightly out in an invitation pose (Snyder 1974) on seeing
their mates coming and accepted the offerings readily. Three quarters of the observa-
tions were made on pair A. Male A brought anoles to his mate four times on 19 April
between 06:48 and 07:27. Then at 07:34, for the only time by any male, he brought a
small frog. When he alighted by his mate, she made no move to take it. Male A flew off,
circled, and tried to feed her 12 times in 34 min., carrying the frog the whole time and
shifting it from bill to feet and back. On four of these attempts he held his wings up and
partly open for periods of 15 to 60 sec. The wings-out maneuver appeared to reflect a
conflict situation in which the male was uncertain whether to stay or to fly. Although
FA always accepted anoles from MA, she would not, it seemed, accept a frog. The un-
mated male circled near the perching tree on three occasions carrying an anole. FA as-
sumed no invitation pose and refused his offering the two times he alighted beside her.
He then rested with wings partly out in what I considered, for both males, to be a dis-
placement activity.

Discussion

I have found no previous accounts of interference by an unmated bird with pre-nest-
ing Swallow-tailed Kites. Skutch (1965) speaks of as many as six additional birds at a
nest but gives no explanation. 1 saw extra birds by nest A but could see from lines of
flight that they were from the other pairs nesting in the same colony, one pair coming
from as far as a kilometer away. There were never more than seven kites at Parker’s
Island and one of these, the unmated male, behaved differently from the others. Snyder
(1974) speaks of a single extra kite but gives no further details.

A question is whether female Swallow-tailed Kites always demand anoles in courtship
feeding. Snyder (1974), in the two courtship feedings he describes, noted an anole in one
and a lizard in the other.

Although Swallow-tailed Kites are not considered a threatened .species, they have
been extirpated from most of their range within the United States (Bent 1937). The
Parker’s Island colony, consisting of 20 pairs in the 1950s (local report), had dwindled to
three pairs by 1979. Any factors that may contribute to further decline would seem wor-
thy of study. The persistent time- and energy-consuming intrusions of the unmated male
must have been a handicap to the mated pair. Yet interpretations are difficult. Pines
suitable for nesting appeared to be scarce, and pair A might have experienced delay
even without the intruder. Although I left Florida on 1 April, subsequent observation by
Fred E. Lohrer .showed that pair A did .succeed in establishing a ne.st.

Acknowledgments

I am obliged to my wife, Jane Kilham, for aiding in my observations; to Fred E. Loh-
rer for checking on the colony after I had left; and to James N. Layne for reading and
commenting on my observations.

Literature Cited

Bent, A. C. 1961. Life histories of North American birds of prey. 2 vols. Dover, New
York.

Brown, L. H., and Amadon, D. 1968. Eagles, hawks, and faleons of the world. 2 vols.
McGraw-Hill, New York. 945 pp.

Skutch, A. F. 1965. Life history notes on two tropical American kites. Condor
67:235-246.

Snyder, N. F. R. 1974. Breeding biology of Swallow-tailed Kites in Florida. Living Bird
13:73-97.

JUVENILE PRAIRIE CHICKEN PREDATION BY MARSH HAWK
by

W. Daniel Svedarsky

Agriculture Division of Technical College and
Northwest Experiment Station
University of Minnesota
Crookston, Minnesota 56716

While Marsh Hawks {Circus cyaneus) are known to prey upon Prairie Grouse
(Grange, Wisconsin Grouse Problems. Wise. Gons. Dept., Madison, 1948), no published
account could be located in the literature detailing an observed attack. During a study
of the reproductive ecology of Greater Prairie Ghickens {Tympanuchus cupido pin-
natus) in northwest Minnesota near Grookston, I observed the predation of a 29-day-old
chick by an adult female Marsh Hawk.

At 19:15 on 24 July 1975 I located a radio-tagged hen with a brood of 4 chicks feed-

Raptor Research 14(l):31-32

RAPTOR RESEARCH

Vol. 14 No. 1

ing in a hayed alfalfa field 15 cm tall. The hen and brood were 25 m from a 6-m-wide
strip of small grain 40 cm tall. I remained in a mobile-receiving vehicle some 110 m
from the brood and observed with binoculars. I gave a chick distress call to determine
the exact location of the hen. When the radio signal became constant at 19:20 (in-
dicating no movement), I observed the hawk which had apparently just landed about 5
m from the hen near the edge of the grain strip. The hawk preened until 19:22 when it
flew away from the brood, over the grain strip, and landed 100 m away. At 19:27 the
hawk returned, flying near to the ground, landed, and sat motionless at the edge of the
grain strip near the crouched hen and brood. At 19:30 the hawk flew toward the brood,
hovered briefly about 1 m off the ground and then dropped sharply, apparently attack-
ing a chick. Immediately the hen flew at the hawk, and feathers were observed flying
about from an apparent struggle on the ground. The radio signal indicated considerable
activity for 1 or 2 minutes and then became constant as though the hen had been killed.
At least 2 chicks then flushed and flew 100 m out into the alfalfa. At 19:34 the hawk
took flight, carrying a chick, and once again the hen flew at the hawk, following it for
10-15 m, but no aerial contact was noted. The hawk flew 150 m and landed, probably
to eat the chick.

It is probable that the strip of small grain provided ambush cover for the hawk. I had
not seen it approach initially, and perhaps the hen had not either. The short cover of the
hayfield likely enhanced detention of the brood by the hawk and made possible the sub-
sequent predation.

ANNOUNCEMENTS

WANTED: RECORDINGS OF RAPTOR VOCALIZATIONS

We are in the process of developing and testing a technique for censusing diurnal and
nocturnal raptors. The technique is based on responses to recorded raptor vocalizations.
We are interested in access to recordings of all North American woodland hawks and
owls for which the following information is available: Location and behavioral context
within which the recording was made and, if possible, age and sex. We would be happy
to respond to any inquiries and to establish correspondence with anyone who is involved
in related work. Direct correspondence to: Dr. James A. Mosher, Appalachian Environ-
mental Laboratory, University of Maryland, Frostburg State College Campus, Frost-
burg, Maryland 21532.

The Migratory Bird and Habitat Research Laboratory, U.S. Fish and Wildlife Service,
has been asked to determine the population status of the Cooper’s Hawk and the Golden
Eagle in the eastern United States and to identify research and management needs. We
would appreciate historic and current information about productivity, habitat use, and
sources of mortality and disturbance. Please contact:

Dr. Mark Fuller

Migratory Bird and Habitat Research Laboratory
Laurel, MD 20811
301/776-4880 x281

THE RAPTOR RESEARCH FOUNDATION, INC.
OFFICERS

President Dr. Richard R. Olendorff, Division of Resources (C-932),

2800 Cottage Way, Sacramento, California 95825

Vice-President Dr. Joseph R. Murphy, Department of Zoology, 167 WIDE,
Brigham Young University, Provo, Utah 84602

Secretary Jeffrey L. Lincer, P.O. Box 8, Sarasota, Florida 33578

Treasurer Dr. Gary E. Duke, Department of Veterinary Biology, College of
Veterinary Medicine, University of Minnesota, St. Paul, Minnesota
55101.

Address all matters dealing with membership status, dues, publication sales,
or other financial transactions to the Treasurer.

Send changes of address to the Treasurer.

Address all general inquiries to the Secretary.

See inside front cover for suggestions to contributors of manuscripts for Rap-
tor Research, Raptor Research Reports, and special Raptor Research
Foundation publications.

BOARD OF DIRECTORS

Eastern Dr. Mark R. Fuller, Migratory Bird Lab., U.S.F.W.S., Patuxent Re-
search Center, Laurel, Maryland 20811

Central Dr. James Grier, Department of Zoology, North Dakota State Uni-
versity, Fargo, North Dakota 58102

Pacific and Mountain Dr. Joseph R. Murphy, Department of Zoology, 167
WIDE, Brigham Young University, Provo, Utah 84602

Canadian Eastern: Dr. David Bird, Macdonald Raptor Research Center, Mac-
donald College, Quebec, H9X ICO, Canada

At Large Dr. Stanley Temple, Department of Wildlife Ecology, Russell Lab-
oratory, University of Wisconsin, Madison, Wisconsin 53706

At Large Dr. Thomas Dunstan, Biology Dept., Western Illinois University,
Macomb, Illinois 61455

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