Raptor Research
A Quarterly Publication of The Raptor Research Foundation, Inc.
Volume 20, Number 3/4, Fall/ Winter 1986
fLSN GGGG-OGGG)
Contents
Reproductive Biology of Northern Hawk-Owls In Denali
National Park, Alaska, Kenneth Kcrtcl! 91
Roost Tree Characteristics and Abundance of Wintering
Vultures at a Communal Roost in South Central.
PENNSYLVANIA, Anthony L. Wright. Hit-hard II. Vahner and Ccralri R. Stnrm . , ,,,,,,, 302
The Barn Owl Ego: Weight Loss Characters. Fresh Weight Prediction
AND INCUBATION Period. Janies D. Marshall, Chine II. Hager and Gw)'n McKee I OH
Prey and Tropic Ecology of Great Horned Owls in Western
South A m e rig a : An In dication of Latttudi n a l T ren ds .
Fabian M. Jaksic, Jusc L. Van:/ and Jaime R. Ran 113
Impact of a High- Voltage Transmission Line on a Nesting Pair
of Southern Bald Eagles in Southeast Louisiana. David A. Dell and Phillip J Zwank ... 11?
Food of the Booted Eagle (Hierafietus pennafrts) in Central Spain. JomP. Veiga 120
Food of Nesting Bald Eagles in Louisiana.
Joseph A, Dm^uii. Philip J. 7 . wank and Gary C. Furman . , , 12^
Male Food Provisioning and Female Reproduction in
American Kestrels, Timothy J. Cboup 128
Short Communications
Sueecw EUtcof (he Peregrine Filccm {Fake pfirtgrinus) Hisnring Dunlin {Cotidm atpk wU During Winter.
Jitscpli D. Buchanan, Steven 6. I If imen and Tnd M. Jiihniinn 130
Gulden F.a^le Capture rjf an American Cntit. Daniel J. Severson . 1.3 1
^Bilatei Ld Bmnhlelonl in a Wild Red-tailed Hawk. Kevin L. EUlis- 1 32
Dissertation Abstracts ***«.***♦*********..•., 136
News and Reviews 119,133-136
Dedication r T 136
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RAPTOR RESEARCH
A QUARTERLY PUBLICATION OF THE RAPTOR RESEARCH FOUNDATION, INC.
Vol. 20 Fall/Winter 1986 No. 3/4
REPRODUCTIVE BIOLOGY OF NORTHERN HAWK-OWLS
IN DENALI NATIONAL PARK, ALASKA
Kenneth Kertell
Abstract — Two nesting pairs of the Northern Hawk-Owk (Sumia ulula) were studied in 1980 in Denali National Park,
Alaska. Observations began during the incubation phase and ended when the young left the nest and could no longer be
found. During this period information was gathered on food habits and breeding biology. Owls did not return to breed in
the study area until 1984 when a pair layed eggs atanestused in 1980. Failure to breed, at least in 1981, was apparently
the result of a substantial decrease in the microtine population.
Surprising little is known about the status and
biology of the Northern Hawk-Owl (Sumia ulula),
particularly in North America. Walker (1974)
claimed that hawk-owls have been reduced consid-
erably in North America but offered no explana-
tion to account for the reduction. Fyfe (1976) de-
scribed it as rare to low in abundance in eastern
Canada and low to moderate in abundance in cen-
tral and western Canada. In Europe, Mikkola
(1972) believed that hawk-owls had suffered a gen-
eral population reduction in Finland, Norway, and
Sweden, based on small recent invasions. Adequate
raptor data are hard to obtain because of the gener-
ally low densities of raptors and their habit of nest-
ing in remote and inaccessible places (Newton
1976). The fact that owls are secretive and noctur-
nal further compounds the problems of obtaining
adequate data.
Bent (1938) summarized most early information
available on the hawk-owl in North America, and
Gabrielson and Lincoln (1959) summarized infor-
mation on their breeding biology in Alaska. More
recently Smith (1970) published information on
various aspects of the reproductive habits of hawk-
owls near Ottawa, Canada. Information on the
hawk-owl in Europe is more extensive (Mikkola
1983).
Although no studies have provided detailed de-
scriptions of hawk-owl breeding behavior,
similarities in appearance and behavior between
hawk-owls and the diurnal falconiforms are appa-
rent. According to Sparks and Soper (1970), the
hawk-owl is an ecological vicariate of a diurnal fal-
con or accipiter, and behaves like a falconid even
though it is primarily a predator of small mammals.
Harrison (1973) speculated that the hawk-owl may
be filling a vacant diurnal niche.
Here I describe aspects of the breeding biology
and behavior of Northern Hawk-Owls nesting in
Alaska.
Study Area and Methods
Two hawk-owl nests were studied; both were on the
north slope of the Hines Creek drainage at about 670 m
elevation in Denali National Park, Alaska. The 2 nests
were west of park headquarters (R7W,T14S,S7 and
R7W,T14S,S12) and 1.8 km apart. Both were within 100
m of the park road.
The nests were located in open needleleaf forest (Vie-
reck et al. 1982) dominated by white spruce ( Picea glauca).
Aspen (Populus tremuloides) and balsam poplar (Populus
balsamifera) occurred uncommonly. Ground cover con-
sisted largely of willow (Salix spp.) in wet areas and dwarf
birch ( Betula nana and B. glandulosa ) in dry areas. Lab-
rador tea (Ledum palustre) , blueberry ( Vaccinium vitisidaea),
and crowberry (Empetrum nigrum ) also occurred. Sphag-
num was thick in places. Annual rainfall at Denali Park
headquarters averages about 37.5 cm, with summer rains
and occasional summer snow accounting for most of the
total. Daylength varies from 12 hrs in late March and
September to 22 hrs. in late June.
Field Observations - Hawk-owls were observed for 137
hrs between 12 May and 5 July 1980. A 20-45x zoom lens
spotting scope and 9x binoculars were used to observe all
activities. Owls appeared to habituate to observer pre-
sence allowing observations to be made from a distance of
91
Raptor Research Vol. 20 (3/4): 9 1-101
92
Kenneth Kertell
Vol. 20, No. 3/4
less than 60 m from the nest. To reach nests, trees were
climbed directly or with the aid of an aluminum ladder.
Additional observations of hawk-owls were made in 1977
and 1984. In 1977, a nest from which 5 young fledged was
visited twice between 1 1 June and 2 July, and a different
group of 5 fledglings was located on 27 June. In 1984, a
pair of adults was observed between 24 March and 7 April
near a nest used in 1980. Data from 1976 were from the
park observation files.
Clutch sizes in 1980 were measured by climbing to the
nest, while brood sizes were determined from the fact that
all known eggs hatched and young fledged. In 1976 and
1977 the number of young fledged served as an index of
both the minimum clutch sizes and brood sizes.
I noted plumage differences that enabled me to recog-
nize sexes of adults at both nests following observations of
copulation, prey exchange, and egg-laying. Males had
grayish-brown or blackish-brown barring while in females
barring was a lighter chestnut-brown. In the male, the
border between the upper breast and foreneck was de-
marcated by a contrasting blackish band, while the transi-
tion in the female was less distinct. The differences were
more apparent in one pair than in the other and, accord-
ing to Mikkola (1983), these kinds of differences can be
attributable to age.
Food Habits. Information on food habits was obtained
from the analysis of 387 pellets, by direct observation of
prey brought to young, and from discarded prey remains.
Analysis of pellets provided over 95% of cricetid, 100% of
soricid, and about 10% of sciurid and avian prey data. All
remaining data was obtained by observation of prey deliv-
ery and the location of discarded remains. Pellets were
collected beginning on 16 May and it was assumed that all
pellets were cast during the 1980 breeding season. Mic-
rotines were identified to species on the basis of dentition
(Bee and Hall 1956; Hall and Kelson 1959), and a collec-
tion of dentition was sent to the University of Alaska for
verification. When dentition was lacking or badly frag-
mented, prey remains were placed in higher taxonomic
categories.
Most pellets were collected at scattered and often previ-
ously unsearched locations. Since the date when they were
cast could not be determined accurately, trends in food
habits were determined by direct observations of prey
brought to young and by discarded prey remains.
Numerical abundance of prey from pellets was deter-
mined by counting pairs of small mammal jaws and by
examining skeletons of larger mammals and birds. The
biomass contribution of each species was calculated by
multiplying numbers of individuals found by mean prey
wt. Average prey wts were determined from specimens in
the University of Alaska Museum (Appendix 1).
Results and Discussion
Food Habits. A total of 651 prey remains was
recovered from the 2 nests, including at least 4
species of birds and at least 8 species of mammals
(Table 1). Mammalian prey comprised over 94% of
the combined total biomass, with birds contributing
the remainder. Diets of both pairs of owls were
similar qualitatively, but differed quantitatively,
especially in relative use of Clethrionomys rutilus.
Pellets from the 2 nests averaged 1.53 and 1 .72 prey
items, respectively, for an overall average of 1.61
prey items/pellet (range 1 to 4) at both nests. Mik-
kola (1972) found an average of 1.7 prey items/
pellet in Finland.
Microtine voles, particularly C. rutilus and Mi-
crotus sp., were the most important prey of hawk-
owls, contributing at least 70% of the total prey
biomass. Mikkola (1972) found that voles, particu-
larly Clethrionomys sp. and Microtus sp. were ex-
tremely important in the diet of hawk-owls in Fin-
land, Norway, and Russia, contributing 94.8, 98.3,
and 97.7% respectively, of the total prey items.
Clethrionomys sp. was numerically most important in
all countries except Finland, where Microtus sp. was
most prevalent. Although infrequently rep-
resented in European studies, the Water Vole
(Aruicola terrestris) comprised 99,4% of the prey ta-
ken by 2 pairs of hawk-owls nesting on Ulkokrunni
Island, Finland in 1977 (Pulliainen 1978). Thus,
use of microtines by hawk-owls in this study is com-
parable to other areas.
The Varying Hare (Lepus americanus) and Red
Squirrel (Tamiasciurus hudsonicus) comprised over
20% of the total prey biomass, a surprisingly large
percentage considering that the biomass contribu-
tions of sciurids and leporids have not been quan-
tified previously, although hawk-owls are known to
prey on them. Dixon (1938) claimed that the Great
Horned Owl (Bubo virginianus) and hawk-owls were
important predators of Varying Hares in Denali
National Park. Henderson (1919) observed hawk-
owls carrying remains of Varying Hare, but con-
cluded that they probably had been scavenged.
On 27 May the wing of an adult Willow Ptarmi-
gan (Lagopus lagopus) was found near a pile of
hawk-owl pellets. Flesh remaining on the wing was
extremely dessicated, indicating that the ptarmigan
had not been captured recently. Ptarmigan, and
other grouse, apparently are not important prey
items during the breeding season (Table 1), al-
though they are reportedly taken during winter
(Fisher 1893). Birds, especially L. lagopus, were ta-
ken 30 times more frequently during winter than
summer in Finland (Mikkola 1972). During the
time hawk-owls are confined to the vicinity of their
nests, the Gray Jay (Perisoreus canadensis) is probably
Fall/ Winter 1986
Northern Hawk-Owl in Alaska
93
Table 1 . Relative frequency of occurrence and relative biomass of prey in the diet of 2 pairs of Northern Hawk-Owls in
Denali National Park, Alaska. Total number of prey items=651; total prey biomass— 20.641 kg.
Species
% Numbers
% Biom^
Bird
Tetraonidae
Lagopus lagopus
0.15
2.60
L. lagopus or Canachites canadensis
0.31
0.39
Corvidae
Perisoreus canadensis
0.92
2.09
Fringilidae
Spizella arborea
0.31
0.07
Zonotrichia leucophrys
0.15
0.12
Small bird
0.92
0.64
Mammal
Soricidae
Sorex cinereus
1.39
0.18
Sorex hoyi
0.15
0.01
Leporidae
Lepus americanus
0.92
9.43
Sciuridae
Tamiasciurus hudsonicus
2.15
10.85
Cricetidae
Clethrionomys rutilus
49.00
35.54
Microtus miurus
5.84
4.97
M. miurus or Microtus pennsylyanicus
5.53
4.71
Microtus oeconomus
19.82
18.13
Microtus sp.
5.22
4.45
Lemmus sibiricus
0.46
0.49
unidentified microtine
6.76
5.33
Total
100.00
100.00
a more important source of food than grouse (Ta-
ble 1).
Trends in Predation. Hawk-owls exploited
hares, squirrels, and birds in late May and con-
tinued to do so until observations ended on 5 July.
Predation on these larger animals was related to the
availability of large numbers of easily captured
young.
Predation by hawk-owls on Varying Hares was
restricted entirely to juvenile hares, taken between
31 May and 24 June. O’Farrell (1965) estimated
that first litters of hares were born in late May and
that the breeding season ended in late July near
Fairbanks, Alaska.
Red Squirrels were taken by hawk-owls between
17 May and 2 July. Although owls preyed predom-
inantly on juvenile squirrels, they also took adults.
Since Red Squirrel populations do not fluctuate as
widely as those of hares, Red Squirrels probably
represent a more uniform food source from year to
year than do hares.
Juvenile Gray Jays were taken by hawk-owls be-
tween 25 May and 19 June. Young Gray Jays are
generally available as early as 15 April; thus they
may have been taken more frequently prior to the
beginning of observations. Most migrant birds ar-
rived in late May or early June, and fledglings of
migrant species generally appeared during the 2nd
wk of July. Other than the nestlings and occasional
adults of a few migrant, ground-nesting species,
94
Kenneth Kertell
Vol. 20, No. 3/4
Figure 1 . Portion of the bog where male hawk-owl from
nest A frequently hunted in 1980.
such as the American Tree Sparrow (Spizella ar-
borea) and White-crowned Sparrow ( Zonotrichia
leucophrys), owls did not regularly prey on migrant
birds.
Hunting Habitat. Hawk-owls in Denali National
Park frequently hunted in open areas with scat-
tered trees. The male at nest A, for example,
hunted a white spruce bog where 60% of 25 ob-
served hunting strikes took place (Fig. 1). The bog,
located at 0.60 km NW of the nest, was in an area of
widely spaced, stunted white spruce < 4 m tall. The
sparse open understory was composed of willow,
labrador tea, and blueberry. Poor drainage prom-
oted the growth of a thick sphagnum ground layer.
The open understory and sphagnum substrate
apparently enabled the male owl to hunt easily. The
male at nest B was observed also to hunt an area
with short white spruce and a ground cover of scat-
tered shrubs and thick sphagnum.
Foraging Behavior. Hawk-owls captured prey
by pouncing from an elevated perch (Table 2). Ele-
vated perches were always spruce trees, and 92%
(N=25) of the perches were at the top of a tree.
When scanning for prey, owls leaned forward so
that the body and tail were nearly horizontal, and
the head was tilted downward, presenting a very
kestrel-like silhouette. When prey was located the
owl’s head “snapped” into a fixed position and the
body became rigid. When making a strike, owls
launched into a gliding dive. If the strike distance
was great (Table 2), owls flapped their wings a few
times before beginning their descent. Roughtly 2/3
of the hunting strikes of male hawk-owls were suc-
cessful (Table 2). When potential prey was not
properly situated, hawk-owls leaned far forward
while engaged in exaggerated tail pumping, a kes-
trel-like behavior. In extreme cases owls opened
their wings and appeared as if to pounce, almost
falling off the perch before regaining their balance.
At other times owls glided to a lower perch and
waited. On 3 July, for example, a male was perched
atop a 6 m spruce when he apparently located prey
below and immediately flew 3 m and perched at the
top of a 2 m spruce. After 20 sec, he glided to a
perch 0.60 m high and pounced onto a vole. Hov-
ering by hawk-owls has been noted
(C. Collins pers. obs.; Mikkola 1983), but was not
observed in this study.
The young of ground-nesting birds were cap-
tured on the ground. On 22 June a male owl
dropped from its perch atop a 5 m spruce and took
a tree sparrow nestling from the nest. Twenty min
later the male owl returned, descended to the same
nest, and took the remaining nestling. I did not
observe the manner in which owls captured fledgl-
ings or adult birds.
Hawk-owls may take arboreal prey in a different
way. On 15 June, a perched male turned to face a
tree about 7 m away and launched into a rapid glide
directly toward a young Red Squirrel climbing the
trunk. The owl flew directly toward the trunk, and
hit a branch, but the squirrel moved out of range
before contact was made.
Feeding Behavior. Hawk-owls generally “pre-
pared” prey before feeding. Microtines were evis-
cerated prior to, or sometimes after, removal of the
head. Prey items were eviscerated by a tear in the
side, which opened the peritoneal cavity just an-
terior to the hindlegs. Owls pulled out and dis-
carded the intestines and the stomach. The re-
Fall/ Winter 1986
Northern Hawk-Owl in Alaska
95
Table 2: Hunting success, perch height, and strike distance of hawk-owls in Denali National Park, Alaska.
No. OF
Observations
Mean
Success (%)
Mean (M)
Range
S.D.
Hunting success
male
28
68
female 3
5
20
total
33
61
Perch height (male)
25
5.41
0.61—10.61
2.61
Strike distance (male)
18
8.10
0.91—21.21
5.47
a 80% of the female’s strikes occurred while her tail feathers were molting.
mainder of the organs were eaten, and a few times
the intestines were swallowed as they were pulled
from the rodent. Large prey items were not eviscer-
ated, at least not immediately, but the organs may
have been discarded or consumed at a later time.
Varying Hares, Red Squirrels, and Gray Jays often
were partially plucked before they were eaten.
Except for very small prey, such as fledgling
sparrows and young microtines, which were swal-
lowed entire, hawk-owls always began feeding by
removing and eating the head, including the rela-
tively large heads of Red Squirrels. In the case of
microtines, after the head was removed the re-
mainder was usually swallowed intact. Prey larger
than Microtus sp. were dismembered more com-
pletely and eaten in several pieces. Adult owls did
not always completely consume large prey. At vari-
ous perch sites I found the discarded tails and
hindlegs of Red Squirrels, and the legs of Gray Jays.
Owlets were observed swallowing the legs and tails
of squirrels on occasion.
Food Caching. Hawk-owls cached excess food
47 times during observations. Food was cached
more frequently after owlets left the nest than when
they were in the nest. During incubation and
brooding, when the female remained at the nest,
the male conducted all caching and food retrieval.
When brooding of the young was completed the
female also cached and retrieved prey. The male
was twice observed caching prey in a favorite hunt-
ing area about 0.60 km from the nest. Prey almost
always was cached at least 3 m above the ground on
spruce boughs or spruce brooms (caused by the rust
Chrysomyxa arctostaphylii) .
All sizes of prey were cached. Some large prey
items were fed upon periodically for up to 24 hrs.
Three rodents were retrieved and consumed 5 hr, 1
hr, and 15 min, respectively, after being cached.
Smith (1922) first observed food caching by a
hawk-owl during the breeding season, and Collins
(1976) and Ritchie (1978) described the food cach-
ing behavior of captive and wild hawk-owls, respec-
tively.
Nest Trees. In addition to the 2 nests studied in
1980, single nests were found in 1977 and 1982. All
nests were located inside the hollow tops of white
spruce trunks 2 to 10 m above the ground (Table 3).
In Europe, nests were usually 4 to 5 m above the
ground (range 2 to 13) (Glutz von Blotzheim and
Bauer 1980). All nest trees were dead, and in all
cases nest cavities probably formed when the tops of
diseased trees blew off, exposing the hollow upper
trunk (Fig. 2). The nest cavities were characterized
by sections of old trunk projecting 0.3 to 0.9 m
above the nest. Owls entered the nest, cavity over
low points in this shell. Eggs were laid directly on
decomposed sapwood.
Nesting Chronology. In 1980, owls were seen
near nest A on 17 April by park employees. On 18
April, a rodent was passed from one adult to
another at a habitually used perch. In 1984, when
nest B was first reoccupied, owls called near the nest
tree on 24 March. On 27 March, one adult was
perched at the nest cavity entrance and a microtine
was exchanged nearby. According to Eckert (1974),
hawk-owls begin breeding (presumably selecting
nest sites) in mid-March, and sometimes as early as
February. Henderson (1919) observed a pair “in
96
Kenneth Kertell
Vol. 20, No. 3/4
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Fall/ Winter 1986
Northern Hawk-Owl in Alaska
Figure2. Hawk-owl nest tree (nest B) discovered in 1980.
the act of breeding” on 19 February in Alberta,
Canada. Mikkola (1972) found that they began
calling as early as 17 February in Finland, and after
the beginning of March in Russia, with territories
being established a “few weeks” before nesting be-
gan. The initiation of breeding apparently can be-
gin as late as early May (Harrison 1973).
Assuming an incubation period of 28 d (Harrison
1973; Terres 1980), and back-dating from the date
of egg-hatching, the mean date of clutch initiation
in 1980 was calculated to be about 19 April (range
13 to 24 April). Elsewhere in Alaska nests contain-
ing eggs range from 16 April to 18 May (Gabrielson
and Lincoln 1959). In Alberta and central to south-
ern Canada, eggs normally were found between 30
March and 5 June, and in Labrador and New-
foundland between 9 May and 11 June (Eckert
1974). Extreme dates when eggs were found in
Lapland and Finland range from 30 March to 23
June.
The mean date of hatching in 1980 was 17 May
(11 May to 22 May). This estimate was based on the
condition of the young at nest A. On 29 May the
nest contained 4 downy, white young, all with their
eyes closed. Spotted Owls and Short-eared Owls
open their eyes at 8 to 9 and 7 to 8 d after hatching
(Clark 1975; Forsman 1981). Assuming that
hawk-owls open their eyes at about 7 to 9 d, and
considering the different sizes of the young at the
time the nest was examined, I estimated the oldest
young to be about 1 wk old when I first examined
the brood in late May.
Hawk-owls left the nest in early and mid-June (1
to 5 June and 11 to 15 June). If calculations of
hatching dates were correct, the young left the nest
when approximately 20 to 22 d old (Fig. 3).
Roles of Adults During Incubation. Incubation
was performed entirely by the female, while the
male did all the foraging. Mikkola (1972) also found
that females did all incubation. The female at nest A
remained on the nest except for short periods when
she left to receive food, preen, cast, or defecate.
When not foraging the male perched in the tops of
nearby trees about 30 m from the nest.
Food was usually exchanged away from the nest.
Generally the female did not respond immediately
when the male arrived with food and he either
cached the food or, more commonly, flew to the
nest and perched at the cavity entrance. The male
frequently flew to the nest entrance several times
before the female left the nest and accepted food at
a nearby perch.
Roles of Adults During the Nestling Period.
The female at nest A brooded the young almost
constantly for the first 10 d to 2 wks after eggs
hatched. During this time, foraging was conducted
entirely by the male. Until the young were about 2
wks old the female received all food at the nest.
After the 2nd wk the female left the nest to receive
food at nearby perches. Toward the end of the
nestling period the female spent almost all of her
time perched outside the nest. At this time the male
visited the nest only to deliver prey and, when not
foraging, usually perched at least 100 m from the
female.
Roles of Adults During the Post-Nestling
Period. During the first 10 d after the young left
the nest females perched nearby constantly. When
not foraging, males continued to perch about 1 00 m
from females.
Ten to 1 1 d after leaving the nest, owlets moved
98
Kenneth Kertell
Vol. 20, No. 3/4
Figure 3. Owlets approximately 17 days after leaving the
nest, 37 days old on 28 June.
further from nest trees, flying up to 30 m horizon-
tally and frequently landing on the ground. When
they landed on the ground near a potential perch,
they usually would climb. At this time males began
to perch nearer the young and even brought food
directly to them on occasion. Males were observed
to offer only small intact prey to the young, while
females often fed owlets pieces of prey.
The female at nest A was first seen hunting about
2 wks after owlets left the nest and by 27 June, 3
days later, roles of the sexes had changed drasti-
cally. The female was now absent for periods of at
least 5 h and, although presumably hunting part of
the time, seldom brought food to the owlets. The
male fed and guarded the young in the absence of
the female, and owlets were left alone for varying
lengths of time when the male foraged. On 5 July,
the last day of observation, the male at nest A con-
tinued to perch near the young and provided al-
most all their food. The female at nest B was not
observed hunting.
Clutch and Brood Size. Clutch and brood sizes
of nests in this study (Table 4) were similar to those
reported elsewhere. According to Bent (1938),
hawk-owls lay between 3 to 9 eggs, usually 7. Mik-
kola (1972) recorded a mean clutch size of 6.31
(range 3 to 13), and a mode of 5 for 135 completed
clutches in Europe.
Nest Success. Both nesting attempts in 1980
were successful, with no infertile eggs or nestling
mortality. Hawk-owls also nested successfully in
1976 and 1977. Virtually no quantitative informa-
tion is available on nest success or reasons for nest
failure in hawk-owls.
Tail Molt. Mayr and Mayr (1954), and Collins
(1961) summarized information on tail molt of sev-
eral species of small owls, although tail molt of the
hawk-owl has not been well described. Wheelwright
(1863:8443) stated that “the old birds may be seen
in deep moult, without tails, even before the young
are flyers.”
Only the female at nest A molted her tail during
the nesting period. The pair at nest B dispersed
before tail molt was initiated by either adult. Tail
feathers of the female at nest A were first noticed
Table 4. Productivity of hawk-owls in Denali National Park, Alaska.
Year
Nesting
Attempts
Clutch
Sizes
Brood
Sizes
-+- Fledglings /
Successful Nest
1976 a
2
5,6
5,6
5.5 (2)
1977 a
2
5,5
5,5
5.0 (2)
1980
2
4,5
4,5
4.5 (2)
a nesting attempts, clutch sizes, and brood sizes in 1976 and 1977
are represented by minimum numbers, based on family groups located.
Fall/Winter 1986
Northern Hawk-Owl in Alaska
99
missing on 24 June, and only the 2 central tail
feathers remained on 26 June, indicating that the
molt was centripetal; the innermost rectrices were
last to molt. On 29 June all her rectrices were mis-
sing. By 1 2 July her new tail feathers appeared to be
about 20.0 mm long, or about 12% of their total
length (Eckert 1974).
Among smaller owls (those with wing lengths <
210 mm) the tail molt is simultaneous, while among
larger owls (those with wing lengths of > 230 mm) it
is usually gradual or irregular (Mayr and Mayr
1954). Wing lengths of male and female hawk-owls
average 220.9 mm and 226.0 mm, respectively
(Earhart and Johnson 1970). Simultaneous tail molt
in the hawk-owl, then, would extend the upper limit
of wing lengths of owls predicted by Mayr and Mayr
to undergo simultaneous molt. Since the tail feath-
ers of small owls usually are shed over a period of
several days to several weeks, Forsman (1981) has
suggested that the word “simultaneous” be used
sparingly.
Nest Defense and Natural Enemies. Of the in-
terspecific encounters witnessed, a male hawk-owl
defended its nest most vigorously against a North-
ern Goshawk (Accipiter gentilis). On 20 May the owl
attempted to intercept a goshawk that was flying
directly toward the nest tree. The goshawk was 200
m away, and flying rapidly about 35 m above the
ground when the owl left its perch and flew toward
it. The hawk-owl flew past the goshawk without
striking it, and then banked and pursued the
goshawk until the accipiter was about 40 m beyond
the nest.
Other than the goshawk encounter, hawk-owls
remained perched when other raptors flew into
view. The Golden Eagle (Aquila chrysaetos), for
example, soared high over the nest at least once
every 2 observation days, but hawk-owls only
watched until the eagle disappeared from view.
Other raptors elicited a more vigorous response.
On 2 1 May, a perched male hawk-owl stiffened as a
Red-tailed Hawk (Buteo jamaicensis harlani) sailed
rapidly over the nest. Although it remained
perched, the owl called several times and was visibly
agitated.
I observed no instances of hawk-owls being pur-
sued by other raptors and no instances of predation
on adults or young were recorded. Hawk-owls,
however, often were harassed by other birds, par-
ticularly the Gray Jay, American Robin (Turdus
migratorius) , and Varied Thrush (Ixoreus naevius).
Robins and Varied Thrushes attacked hawk-owls
vigorously, diving from above and in 3 to 4 in-
stances struck perched owls. These attacks dis-
rupted the activities of hunting owls, and on several
occasions males flew to the nest area with Robins or
Varied Thrushes in pursuit. A male hawk-owl once
responded aggressively when it was attacked by an
American Kestrel (Falco sparverius). During the des-
cent phase of each of the kestrel’s 10 pendulum
attacks, the owl jumped from its perch into the air
and presented its talons to the falcon.
Cryptic Posture. On 2 different occasions, once
in response to the approach of a goshawk and once
in the presence of a low-soaring Golden Eagle, male
hawk-owls assumed vertically elongated postures.
The owl stiffened and the feathers of the breast,
belly, and back were drawn tightly to the body. The
wings also were pulled tightly against the body and
the leading edge was aligned vertically. The feath-
ers in the facial disc above the eyes were raised,
making the eyes appear very large.
The posture was identical to the “concealing
pose” of the Northern Saw-whet Owl (Aegolius
acadicus) and the Boreal Owl (Aegolius funereus) as
described by Catling (1972), and apparently is the
same posture assumed by several other small
strigids, including the Eastern Screech Owl (Otus
asio), Long-eared Owl (Asio otus), and Elf Owl (Mi-
crathene whitneyi) (Bent 1938; Ligon 1968).
1981 Breeding Season. Hawk-owls were seen
occasionally in 1981, and did not nest in the study
area. Other researchers have noted similar declines
in hawk-owl numbers and reproductive success in
interior Alaska (Dixon 1938; Murie 1963). Even
though hawk-owls were not observed to breed in
1981, there were 1 4 sightings of single owls between
24 March and 15 September, Twice owls were ob-
served < 2 km, and once only 0.3 km from 1980
nest sites.
Although hawk-owls feed on birds, squirrels, and
young hares, they apparently depend on microtines
for successful nesting, thus resembling other strigid
rodent specialists which also respond to low rodent
densities by failing to breed. Among 10 species of
Fenno-Scandian owls, hawk owls were second only
to Snowy Owls (Nyctea scandiaca) in the proportion
of Microtinae in the diet (Mikkola 1983).
Hawk-owls did not breed again in the study area
until 1984 when a pair layed eggs at a nest used in
1980. It was not determined if the owls bred suc-
cessfully.
100
Kenneth Kertell
Vol. 20, No. 3/4
Acknowledgements
Permission to do research in Denali National Park was granted
by John Dalle-Molle. During the Field work I was aided by several
park employees, especially Karen Laing, Rick McIntyre, and Rick
Sladick. Weights of prey species were provided by Daniel D. Gib-
son and Stephen O. Macdonald of the University of Alaska
Museum. Stephen O. MacDonald also provided valuable assis-
tance in the identification of shrews and microtines. James R.
Koplin, Humboldt State University provided guidance and freely
imparted his knowledge of raptor biology throughout the study.
Literature Cited
Bee, J. W. and E. R. Hall. 1956. Mammals of northern
Alaska. Lawrence, Kansas. Univ. of Kansas.
Bent, A. C. 1938. Life histories of North American birds
of prey. Part 2. Smithsonian Inst., U.S. Natl. Mus. Bull.
170.
Catling, P.M. 1972. A behavioral attitude of Saw-whet
and Boreal Owls. Auk 88: 195-196.
Clark, R.J. 1975. A field study of the Short-eared Owl
Asio flammeus Pontoppidan in North America. Wildl.
Mono. 47:1-67.
Collins, C.T. 1961. Notes on the feeding behavior,
metabolism, and weight of the Saw-whet Owl. Condor
65:528-530.
Collins, C.T. 1976. Food caching behavior in owls. .Rap-
tor 7?^. 10: 74-76.
Dixon, J.S. 1938. Birds and mammals of Mount McKin-
ley National Park, Alaska. Wash., D.C.
Earhart, C.M. and N.K. Johnson. 1970. Sizedimorph-
ism and food habits of North American owls. Condor
72:251-264.
Eckert, A.W. 1974. The owls of North America.
Doubleday and Co., Inc., Garden City, New York.
Fisher, A.K. 1893. The hawks and owls of the United
States and their relation to agriculture. U.S. Dept, of
Agric. Div. Ornithol. and Mammal, Bull. 3:1-210.
Forsman, E.D. 1981. Molt of the Spotted Owl. Auk
98:735-742.
Fyfe, R.W. 1976. Status of Canadian raptor populations.
Can. Field-Nut. 90:370-375.
Gabrielson, I.N., and F.C. Lincoln. 1959. The birds of
Alaska. Stackpole Co., Harrisburg, Pennsylvania and
the Wildl. Manage. Inst., Wash., D.C.
Glutz von Blotzheim, U.N. and K.M. Bauer. 1980.
Handbuch der Vogel Mitteleuropas. Vol. 9.
Akademische Verlagsgesellschaft, Wiesbaden.
Hall, R.E. and K.R. Kelson. 1959. The mammals of
North America. 2 vols. New York. The Ronald Press
Co.
Harrison, C. 1973. Hawk owls, pp. 147-163. In Burton,
J.A. (Ed.) Owls of the world. New York. E.P. Dutton
Co. Inc.
Henderson, A.D. 1919. Nesting of the American Hawk
Owl. Oologist 36:59-63.
Ligon,J.C. 1968. The biology of the Elf Owl, Micrathene
whitneyi. Misc. Publ. Mus. Zool. Univ. Mich. No. 136.
Mayr, E. and M. Mayr. 1954. The tail molt of small owls.
Auk 71: 172-178.
Mikkola, H. 1972. Hawk Owls and their prey in north-
ern Europe. Br. Birds 65: 452-460.
Mikkola, H. 1983. Owls of Europe. Vermillion, S.D.
Buteo Books.
Murie, A. 1963. Birds of Mount McKinley, Alaska
Mount McKinley Nat. Hist. Assoc.
Newton, I. 1976. Population limitation in diurnal rap-
tors. Can. Field-Nat. 90:274-300.
O’Farrell, T.P. 1965. Home range and ecology of
snowshoe hares in interior Alaska. J. Mammal.
46:406-418.
Pulliainen, E. 1978. Nesting of the Hawk Owl, Surnia
ulula, and Short-eared Owl, Asio flammeus, and the
food consumed by owls on the island of Ulkokrunni in
the Bothnian Bay in 1977. Aquilo Ser. Zool. 18: 17-22.
Ritchie, R.J. 1978. Food caching of nesting wild hawk
owls .Raptor Res. 14:59-60.
Smith, D.A. 1970. Observations on nesting Hawk Owls
at the MerBleue, near Ottawa, Canada. Can. Field-Nat.
84:377-383.
Smith, F.N. 1922. The American Hawk Owl. Can
Field-Nat. 36:68-71.
Sparks, J. and T, Soper. 1970. Owls: their natural and
unnatural history. Newton Abbot, England. David and
Charles.
Terres, J.K. 1980. The Audubon society encyclopedia
of North American birds. New York. Alfred A. Knopf
Viereck, L.A., C.T., Dyrness and A.R. Batten. 1982
1982 revision of preliminary classification for vegeta-
tion of Alaska. Unpubl. PNW-106, 1980. Inst, of N.
Forestry, Univ. of Alaska, Fairbanks.
Walker, L.W. 1974. The book of owls. New York.
Alfred A. Knopf.
Wheelwright, H. 1963. Notes on the Hawk Owl (Strix
funerea ), and Tengmalm’s Owl (Strix tengmalmi) as ob-
served in Lapland. Zoologist 21:8442-8444.
U.S. Fish and Wildlife Service, Alaska Office of Fish and
Wildlife Research, 1011 E. Tudor Road, Anchorage, Alaska
99503. Present address: 211 La Vida Way, Davis, California
95616
Received: 16 December 1985; Accepted: 25 June 1986.
Fall/Winter 1986
Northern Hawk^Owl in Alaska
101
Appendix 1. Weights of prey species used to compute biomass consumption by hawk-owls.
Species
No. OF
Specimens
Mean
Weight (g)
Source
Birds
Lagopus lagopus
60
550
UA a
L. lagopus or Canachites canadensis
—
40
estimated mean juvenile wt.
Perisoreus canadensis
33
72
UA a
Spizella arborea
—
7
estimated mean juvenile wt.
Zonotrichia leucophrys
26
25
UA a
Small birds
—
22
estimated mean juvenile wt.
Mammals
Sorex cinereus
25
4
UA a
Sorex hoyi
25
3
UA a
Lepus americanus
24
325
mean juvenile wt., UA a
T amiasciurus hudsonicus
29
160 b
mean wt., UA a
Clethrionomys rutilus
25
23
UA a
Microtus miurus
25
27
UA a
M. miurus or Microtus pennsylvanicus
20
27
UA a
Microtus oeconomus
25
29
UA a
Microtus sp.
— .
27
estimated
Lemmus sibiricus
25
34
UA a
Unidentified microtine
—
25
estimated
Specimens in University of Alaska Museum
b Mean weight from a combination of adult and juvenile weights
Third World Conference on Birds of Prey, 1987. An International Conference will be held 22-27 March 1987 at Eilat,
Israel. The Conference will be organized by the World Working Group on Birds of Prey in conjunction with the Israel
Raptor Information Center and the U.S. Hawk Mountain Sanctuary Association. The Conference will consist of seven
paper sessions, each of which may occupy up to one whole day. The themes and organizers are as follows: 1)
Conservation and biology of rare raptors — U.-Meyburg and N. Collar; 2) Conservation and biology of rare owls — R. J.
Clark and H. Mikkola; 3) Raptors on migration and wintering grounds — M. Fuller and J. M. Thollay; 4) Population
biology and breeding — I. Newton; 5) Raptors in polluted environments — R. Risebrough and J. Ledger; 6) Educa-
tion — Y. Leshem and J. Brett; 7) Legislation — P. Robinson. Contributions to these different themes can also take the
form of poster papers.
The Conference will take place within the framework of an international festival, which will include a raptor
photography competition (under the patronage of Eric Hosking), a painting and drawing competition (patron, Roger
Tory Peterson), a film festival and competition, and ornithological and cultural excursions and tours.
During this season, the famous and massive migration movement of raptors over Eilat is in full swing, and in 1985
included 1.1 million raptors of 30 species. For further information, write to the Honorable Secretary of the World
Working Group, Mr. R. D. Chancellor, 15 Bolton Gardens, London SW5 OAL, UNITED KINGDOM.
ROOST-TREE CHARACTERISTICS AND ABUNDANCE OF
WINTERING VULTURES
AT A COMMUNAL ROOST
IN SOUTH CENTRAL PENNSYLVANIA
Anthony L. Wright, Richard H. Yahner and Gerald L. Storm
ABSTRACT — Roost-tree characteristics and abundance of the Black Vulture ( Coragyps atratus) and the Turkey Vulture
0 Cathartes aura) were studied during 2 winters at a communal roost in southcentral Pennsylvania. Vultures selected large
conifers for roosting, which were easily accessible and probably offered a nocturnal microenvironment favorable for
energy conservation. Turkey Vultures left the roost earlier in the morning than Black Vultures. Numbers of vultures
were highest during mid-winter, and Turkey Vultures outnumbered Black Vultures during both winters. Recommen-
dations are to preserve forest stands containing conifers in the vicinity of the roost and minimize human disturbances
near roosts.
Although roosts and perching areas used by
vultures have been described (Coles 1938; Davis
1974; Stewart 1978; Rabenold 1983), quantitative
descriptions of habitat used by vultures during
winter in the northeastern United States are lack-
ing. We examined winter roost trees and abun-
dance of the Black Vulture (Coragyps atratus) and
the Turkey Vulture ( Cathartes aura) at a large com-
munal winter roost at the Gettysburg National
Military Park, Adams Co., Pennsylvania. Our ob-
jectives were to determine (1) characteristics of
roost trees used by vultures at the Big Round Top
(BRT) roost, and (2) within- and between-year
changes in abundance of both species at the roost
during 2 winters.
Study Area and Methods
The study was conducted from 7 December 1982 to 5 March
1983 and from 27 December 1983 to 7 March 1984 at the BRT
roost, which was used nightly by vultures during both winters
(Wright 1984). The Harpers Hill and the Gettysburg Quarry
roosts, used infrequently by vultures, were located within 5 km of
the BRT roost (Wright 1984).
The BRT roost is in the Gettysburg Basin, which is a wide, level
plain, except for low ridges (Socolow 1962). The city of Gettysburg
(population 7,200) lies 3 km from the roost. Forests cover 32% of
Adams County and are composed of 6% conifer ( Pinus spp., Picea
spp.), 81% oak ( Quercus spp.), and 13% northern hardwood (Be-
tula spp., Acer spp., Fagus grandiflora) forest types (Considine and
Powell 1980). Mean temperature from December to February at
Gettysburg is (FC. Annual snowfall averages 73.7 cm, and pre-
cipitation from December to February averages 22.7 cm (Ruffiner
1980).
Description of the Roost. — Trees with at least 25% of the
ground beneath the crown whitewashed by vulture excreta were
defined as roost trees. All roost trees were white pine (Pinus strobus)
located at the base of BRT. Control trees were those receiving little
or no night use by vultures, as indicated by fewer than 2 large
splashes of excreta beneath the tree. Control trees were chosen by
following a 2-m wide transect in a random direction from each
roost tree until an overstory white pine was encountered.
Fifteen variables (Table 1) were compared between roost trees
and control trees with either single-classification analyses of var-
iance or median tests (Daniel 1978; Sokal and Rohlf 1981). Step-
wise logistic regression (BMDPLR, Dixon 1981) was used to pre-
dict use of a tree for roosting based on variables measured at each
tree. The logistic model used was E(s/N) = exp (U)/(l + exp (U)),
where U is the linear combination of one or more independent
variables, s is the sum of the binary (0, 1) dependent variable, and
N is the total sample size. The maximum likelihood method of
estimating variables with default options for remove limit (P >
0. 15) and enter limit (P < 0. 10) was used to build the model.
Counts at the Roosts. — Counts of vultures at the BRT roost
were conducted 2 to 6 d/wk on mornings without measurable
precipitation (< 0.25 mm), beginning 35 min before sunrise and
cominuing until 100 min after sunrise. A cutoff of 100 min was
chosen arbitrarily as birds that did not leave by this time typically
remained in the roost for most of the day. When possible vultures
flying out of the roost were counted and identified to species from
a vantage point that was 280 m from the main roost.
A correction factor (2.2 ± 0.8) was determined to account for
birds that did not leave the roost during a given count. This factor,
based on 5 counts during 1982-83, was the mean ratio of birds
flushed to those visible in the roost before flushing. The number
of vultures visible (both species combined) in the roost at the end
of a count was multiplied by the correction factor to estimate the
number remaining in the roost. When large numbers (2= 60) of
vultures were visible in the roost at the termination of a count, the
count was considered unsuccessful; unsuccessful counts (N — ■
16/68) were discarded from analyses. The total number of vul-
tures in a roost/count was equal to the number of birds leaving plus
the estimated number remaining in the roost ( X = 24 birds/suc-
cessful count). Winter counts were divided into 3 winter
periods: early winter, mid-winter and late winter (see Table 3).
Results
Comparison of Roost Trees with Control Trees.
. . Vultures roosted only in white pines at BRT,
although hardwoods made up to 58% of the over-
story within the roost and 92% of the overstory
within 0.5 km of the roost. Six variables related to
tree size and amount of evergreen foliage were
significantly great (P < 0.05) for roost trees than
for control trees, whereas distance to the nearest
roost tree was less for roost trees than for control
102
Raptor Research Vol. 20 (3/4); 102-107
Fall/ Winter 1986
Vulture Roosts in Pennsylvania
103
T able 1 . V ariables measured at roost trees of Black and Turkey Vultures and at control trees at Big Round T op roost
Adams Co., Pennsylvania (from Wright 1984).
Variable
Description
Diameter at
breast height
Height of tree
Diameter (cm) of tree measured at breast
height (1.5 m) with tree diameter tape.
Height (m) of tree measured with
Abney level and tape.
Height to
lowest limb
Height (m) from ground level to lowest
living limb greater than 6 cm in diameter
at base, measured with Abney level and tape.
Maximum
crown diameter
Maximum horizontal distance (m) between
the ends of living limbs of trees measured
by ocular tube with plumb-bob and tape.
Mid-tree
crown diameter
Horizontal distance (m) between the ends
of living limbs measured midway between
ground level and tree top. Method of
measurement same as crown diameter.
Distance to nearest
roost tree
Distance (m) from roost or control tree
to nearest roost tree measured with a 50-m
Distance to
nearest clearing
tape or taken from a 1:1,600 aerial photo.
Distance (m) from roost or control tree to
nearest area of over 200 m essentially
free of overhead vegetation. Measured by
same method as distance to nearest roost tree.
Number of
overstory trees
Understory stem
density
Number of overstory trees in a 0.04-ha
circular plot.
Density (100’s of stems/ha) of shoulder
height non-overstory, woody stems in 2
perpendicular 22.8-m transects in a 0.04-ha
circular plot.
Percent evergreen
canopy cover
Evergreen canopy coverage (%) based on
56 ocular tube readings evenly spaced on
lines running in 8 main compass directions
from center tree of a 0.04-ha circular plot.
Slope
Maximum ground slope (degrees) from tree to edge
of a 0.04-ha circular plot, measured with Abney level.
Elevation
Canopy height
Elevation (m) taken from USGS 1:24,000 topographic map.
Mean height (m) of trees in a 0.04-ha
circular plot. These are the center tree and
the tree with the greatest diameter at
breast height in each quarter.
Total basal area
Basal area (m) of all overstory trees in a
0.04-ha circular plot.
Basal area of
white pine
Same as basal area, but only for white
pine.
trees (Table 2). Basal area of white pine, understory (understory stem density) + 0.28 (height of tree),
stem density, and tree height were the best variables The model gave 81.2% correct classification of
for predicting use of a tree for roosting: U = trees.
— 10.89 + 6.22 (basal area of white pine) — 0.01
104
Wright et. al.
Vol 20, No. 3/4
Table 2. Means ^ and standard deviation (SD) of 15 variables measured at roost trees of Black and Turkey Vultures
and at control trees at Big Round Top roost, Adams Co. , Pennsylvania, during winters 1 982-83 and 1 983-84.
Roost Tree (N - 33)
Control Tree (N
- 31)
Variable
X
SD
X
SD
Diameter at
breast
height 3
57.42*
10.0
48.6
15.1
Height of
tree 3
28.8*
2.7
25.8
5.2
Height to
lowest limb
17.1
2.5
15.4
4.0
Crown
diameter
9.4
2.1
8.2
3.0
Perpendicular
crown diameter 3
7.5***
1.8
5.6
2.7
Distance to
nearest roost
tree 3
7_g***
7.1
63.4
40.8
Distance to
nearest clearing
109.7
27.7
130.7
89.4
Number of
overstory trees
9.7
2.9
8.4
3.0
Understory
stem density
97.8
75.1
114.6
111.7
Percent ever-
green canopy
cover 3
38.3***
9.0
26.9
9.0
Slope
9.7
2.1
8.9
3.7
Elevation
167.1
0.3
164.7
0.9
Total basal area 3
1.47***
0.35
1.16
0.36
Basal area
of white pine 3
0.90***
0.33
0.43
0.26
a Means or distribution of means varied between roost trees and control trees; *P = 0.05, ***P = 0.001, based on
single-classification analyses of variance or median tests (Daniel 1978; Sokal and Rohlf 1981).
Counts at Big Round Top Roost. — The number
of both vulture species combined was greater in
winter 1982-83 compared to winter 1983-84 (Table
3). Mean number/count varied significantly among
the 5 winter periods (F — 45.3; df = 4, 47; P <
0.001). Paired comparisons of means between
winter periods were significantly different (P <
0.03), except for the comparison of late winter
1982-83 and late winter 1983-84 (Table 3). As a
general trend, numbers increased in early winter,
peaked and remained stable in mid-winter, and
declined in late winter. Several large day-to-day
changes in numbers at the roost also were
documented (Wright 1984).
Turkey Vultures were more common than Black
Vultures at the BRT roost based on all winter
periods combined (Wilcoxon paired-rank test, Z =
- 6.7, n = 63, P < 0.001). The mean percentage of
both Black and Turkey Vultures observed at the
roost differed among periods (F — 7.2; df = 4.58;
P < 0.001); pairwise comparisons of mean percen-
tages of each species observed at the roost were
significantly different between most periods (Table
4).
Fall/ Winter 1986
Vulture Roosts in Pennsylvania
105
Table 3. Means, SD, and coefficients of variation (CV) for counts (N) of Black Vultures, Turkey Vultures, and
vultures of unknown species combined at Big Round Top roost, Adams Co., Pennsylvania, during winter
periods of 1982-83 and 1983-84.
Period
Dates of Counts
N
Means ± SD
CV
1982 - 83 :
Early winter
10 Dec 1982-27 Dec 1982
9
517 ± 239
46.1
Mid-winter
28 Dec 1982-16 Feb 1983
15
719 ± 85
11.8
Late winter
17 Feb 1983-5 Mar 1983
7
199 ± 82
41.4
1983 - 84 :
Early winter
a
a
a
Mid-winter
28 Dec 1983-6 Feb 1984
10
420 ± 74
17.8
Late winter
6 Feb 1984-6 Mar 1984
113125 ±
76 361.0
a A total of 427 and 501 vultures was counted at the roost on 8 December and 17 December, respectively (E. Daniels,
pers. comm.)
Numbers of individual birds departing the BRT
roost/ 15-min time interval in the morning were de-
pendent on species (G = 1,082; df = 8; < 0.001).
Turkey Vultures tended to leave earlier than Black
Vultures (Table 5).
Discussion
BRT, Harpers Hill, and Gettysburg Quarry
roosts are associated with ridges (Wright 1984),
which presumably modify winds (Geiger 1965). Be-
cause both vulture species often use winds when
soaring, ridges may have an effect on roost location
by creating updrafts that were used as travel lanes
(Wright 1984). Topography is known to affect the
distribution of different species of African vultures
according to their flight characteristics and body
sizes (Houston 1975).
Vultures selected mature white pines rather than
hardwoods as roost trees at BRT. Coles (1938) ob-
served that vultures in Virginia abandoned a
hardwood roost site and moved to a conifer roost
site after leaf fall; a similar shift took place at BRT
(J. Coleman, pers. comm.) Both white pines and
hardwoods were used as roost trees at Harpers Hill;
Table 4. Mean ± SD of percent composition of Black and Turkey Vultures between winters and among winter
periods at Big Round Top Roost, Adams Co., Pennsylvania, 1982-83 and 1983-84.
Winter 1982-83
Winter 1983-84
Winter
period
Black
Turkey
Black
Turkey
Early
20.5 ± 8.3 a
79.5 ± 8.3
no data
no data
no data
Mid-
29.4 ± 7.2
70.6 ± 7.2
40.2 ± 11.6
59.8 ± 11.6
Late
33.8 ± 16.0
66.2 ± 16.0
21.3 ± 13.4
78.7 ± 13.4
Combined
28.0 ± 10.5
72.0 ± 10.5
32.5 ± 15.4
67.5 ± 15.4
a All pairwise comparisons for each species were significantly different except between mid-winter 1982-83 and late
winter 1982-83, and between all winter 1982-83 periods combined and all winter 1983-84 periods combined;
Wilcoxon two-sample and Wilcoxon signed-rank tests (Sokal and Rohlf 1981).
106
Wright et. al.
Vol 20, No. 3/4
Table 5. Percentages (numbers) of individual Black and Turkey Vultures departing from the Big Round Top roost,
Adams Co., Pennsylvania, during 9, 15-min morning time intervals in winters 1982-83 and 1983-84
combined.
Time interval (Relative to sunrise)
Percentages (Numbers) of Individual Birds
Black Vultures
Turkey Vultures
35 to 20 min before
0.4 (17) a
1.5 (167)
20 to 5 min before
9.1 (392) a
21.3 (2471)
5 min before to 10 min after
24.7 (1065) a
40.5 (5012)
10 to 25 min after
22.4 (965) a
12.7 (2203)
25 to 40 min after
15.2 (565) a
8.2 (1453)
40 to 55 min after
14.6 (628) a
8.1 (1417)
55 to 70 min after
9.4 (406) a
5.1 (903)
70 to 85 min after
3.5 (151) a
2.1 (360)
85 to 100 min after
0.7 (31)
0.5 (81)
a Numbers of departures per time interval varied between species; P < 0.001, based on 2 x 2 G-tests of independence,
where rows are numbers of vultures/time interval of interest versus numbers/all other time intervals combined and
columns are the 2 species (Sokal and Rohlf 1981).
3 Virginia pines ( Pinus virginiana) were the major
roost trees at Gettysburg Quarry where the forest
type was > 95% hardwood (Wright 1984). Conifers
reduce both wind velocity and nightly drops in am-
bient temperature during winter, suggesting that
vultures lower daily energy requirements by roost-
ing in clusters of large conifers (Francis 1976; Kelty
and Lustick 1977. Stalmaster and Gessaman 1984;
Walsberg 1986). Further strong temperature inver-
sions form in mature forest stands on calm nights
(Geiger 1965); therefore, a perch on an upper limb
in a full conifer would afford a warm microenvi-
ronment to a roosting vulture. Finally, widely-
spaced, horizontal limbs on dominant white pines
enabled vultures to easily alight.
Numbers using the BRT roost may vary by year
according to weather conditions. For example,
mid- winter 1982-83 (January mean temperature,
— 0. 1°C; monthly snowfall, 3.8 cm) was less rigorous
than mid-winter 1983-84 (January mean tempera-
ture, — 3.8°C; monthly snowfall, 18 cm). Numbers
of vultures using the BRT roost were much lower in
winter 1983-84, perhaps due to more vultures mi-
grating farther south than in 1982-83.
The BRT roost presumably provides a favorable
microclimate in mid-winter, but other factors (e.g.;
information centers, Rabenold 1983, 1986; protec-
tion from predation, Weatherhead 1983; abundant
winter food resources, Yahner et al. 1986), also may
be important in explaining high use of this com-
munal roost. Communal roosting by both species
has been observed during summer months (Stewart
1978) and at southerly latitudes (Bent 1937; Coles
1938).
Although our results are based primarily on 1
roost in southcentral Pennsylvania, we recommend
that forest stands containing conifers should be
preserved near communal winter roosts. Efforts
should be made to minimize human disturbances
(e.g., road construction, forest clear-cutting) within
a reasonable distance of a roost. In addition, large
trees at pasture — woodland interfaces within 1 km
of the roost were used readily by vultures at Gettys-
burg National Military Park (Wright 1984) and,
thus, should be retained near roosts.
Acknowledgments
Thanks are given to P. Rabenold, R. Shipmen, K. Steenhof, P.
Stewart, J. Swenson and P. Weatherhead for reviewing an earlier
draft of the manuscript; to H. Greenlee and J. Karish, National
Park Service, for providing logistical support; to E. Daniels and J.
Coleman for sharing information about vultures; to M. Fuller,
U.S. Fish and Wildlife Service, for advice on field techniques; and
J. Grimm for help with statistical analyses. This research was
funded by the Pennsylvania Agricultural Experiment Station, the
U.S. Fish and Wildlife Service, the National Park Service, and the
Fall/ Winter 1986
Vulture Roosts in Pennsylvania
107
Max McGraw Wildlife Foundation. This is Scientific Journal
Series Number 7149 of the Pennsylvania Agricultural Experiment
Station, The Pennsylvania State University, University Park.
Literature Cited
Bent, A. C. 1937. Life histories of North American birds
of prey. Park I. New York. Dover Publications, Inc.
Coles, V. 1938. Studies in the life history of the Turkey
Vulture. PhD Thesis, Cornell Univ., Ithaca, New
York.
Considine, T.J. and D.S. Powell. 1980 Forest statistics
for Pennsylvania — 1980. Northeast For. Exp. Stn.,
Broomall.
Daniel, W.W. 1978. Applied nonparametric statistics.
Boston. Houghton Mifflin Co.
Davis, D. 1974. Roosting behavior of the Turkey Vul-
ture. MS Thesis, Idaho State Univ., Pocatello.
Dixon, W.J. Ed. 1981. BMDP statistical software.
Berkeley. Univ. California Press.
FRANCIS, W.J. 1976. Micrometeorology of a blackbird
roost. J. Wild. Manage. 40:132-136.
GEIGER, R. 1965. The climate near the ground. Cam-
bridge. Harvard Univ. Press.
Houston, D.C. 1975. Ecological isolation of African
scavenging birds. Ardea 63:55-64.
Kelty, M.P. and S.L. Lustick. 1977. Energetics of the
starling in a pine woods. Ecology 58: 1 181-1185.
Rabenold, P.OP. 1983. The communal roost in Black
and Turkey Vultures — an information center? Pages
303-329 In S.R. Wilbur and J.A. Jackson 1 Eds.], Vul-
ture biology and management. Berkeley. Univ. of
California Press.
1986. Family associations in commun-
ally roosting Black Vultures, Auk 103:32-41.
Ruffiner, J.H. 1980. The climate of the states. Vol. 2.
Detroit. Gale Research Co.
Socolow, A.A. 1962. Geology and the Gettysburg cam-
paign. Harrisburg. Pennsylvania Topographic and
Geologic Survey.
Sokal, R.R. and F.J. Fohlf. 1981. Biometry. San Fran-
cisco. W.H. Freeman and Co.
Stalmaster, M.V. and J.A. Gessaman. 1984. Ecological
energetics and foraging behavior of overwintering
Bald Eagles, Ecol. Monogr. 54:407-428.
Stewart, P.A. 1978. Behavioral interactions and niche
separation in Black and Turkey Vultures. Living Bird
17:79-84.
Walsberg, G.E. 1986. Thermal consequences of roost-
site selection: the relative importance of three modes
of heat conservation. Auk 103:1-7.
Weatherhead, P.J. 1983. Two principal strategies in
avian communal roost. Aw. Naturalist 121:237-243.
Wright, A.L. 1984. Winter habitat use and abun-
dance of Black and Turkey Vultures at Gettys-
burg. MS Thesis, The Pennsylvania State Univ.,
University Park.
Yahner, R.H., G.L. Storm and A.L. Wright. 1986.
Winter diets of vultures in southcentral Pennsylvania.
Wilson Bull. 98:157-160.
School of Forest Resources, The Pennsylvania State University,
University Park, Pennsylvania 16802 USA. Address of third
author: Pennsylvania Cooperative Fish and Wildlife Research
Unit, The Pennsylvania State University, University Park,
Pennsylvania 16802 USA.
Received 1 March 1986; Accepted 1 June 1986.
THE BARN OWL EGG: WEIGHT LOSS CHARACTERS, FRESH WEIGHT
PREDICTION AND INCUBATION PERIOD
James D. Marshall, Claire H. Hager and Gwyn McKee
ABSTRACT. — A total of 177 Common Barn-Owl ( Tyto alba pratincola) eggs produced by 14 captive pairs were studied
during the spring of 1985, Initial egg parameters for 75 eggs were fresh weight (26.6 ± 1.4 g), length (43.07 ± 1.24 mm)
and breadth (33.67 ± 0.70 mm). Using these data, a coefficient (K w ) unique to the barn owl egg was calculated for Hoyt’s
(1978) equation for predicting the fresh weight of an egg. (K w = ° .0005453)
For 50 artificially incubated eggs (hatchability = 93.5%) the lay to pip (LP) interval was 28.2 ± 1.4 d, the pip to hatch
(PH) interval was 2. 1 ± 0.5 d and the overall incubation period was 30 ± 1 .5 d. Variance in the latter period (range: 27-35
d) may have been due to an observed delay in initial embryonic development of from 1-7 d.
During incubation, several externally quantifi-
able changes occur in the avian egg. These include:
1) the relatively steady reduction in weight due
mainly to loss of water vapor by diffusion from the
embryonic chorioallantois through the porous shell
and its evaporation at the eggshell surface
(Romanoff and Romanoff 1949; Ar and Rahn
1980); and 2) the equal exchange of O 2 and CO 2
gases through the eggshell by the chorioallantois - a
process not affecting weight loss (Wagensteen and
Rahn 1970, 1971). The mean percentage of fresh
egg weight (Wo) lost during the incubation period
for many avian species ranges from 12-18% (Drent
1970). Proper weight loss is correlated with hatcha-
bility and normal embryonic development
(Walsberg 1980). During artificial incubation, accu-
rate regulation of egg weight reduction is possible
through a variety of methods; (Burnham 1983;
Weaver and Cade 1983).
A mathematical equation (1) based upon egg
length (L) and breadth (B), parameters which are
invariant during incubation, was developed by
Hoyt (1978) to predict avian Wo.
Wo = K W LB 2 (1)
The coefficient (K w ) of this equation interrelates
shell measurements, and may be adjusted to ac-
commodate a single species for accurate Wq for
Peregrine Falcon ( Falco peregrinus ) eggs, and also
observed a reduction in Wq of 15 ± 2% during
incubation of normal eggs. However, our study of
the incubation of common Barn-Owl ( Tyto alba
practincola ) eggs indicates that they cannot be pre-
cisely characterized by values developed for Pereg-
rine Falcon eggs. Our objective was to measure barn
owl egg weight loss and incubation period, and
algin Hoyt’s equation for this species.
Materials and Methods
The barn owl breeding colony of the Raptor Rehabilitation and
Propagation Project, Inc., Eureka, Missouri, was established in
1979 and produced more than 150 juvenile owls yearly through
1986 for release into Missouri. The colony contained non-sibling
breeding pairs collected from eastern North America. Each pair
was housed in an outdoor mew in a natural setting and was fed
daily a diet of fresh rodents ad libitum. Human disturbance was
normally limited to 2 short intervals.
Barn Owls will naturally produce > 1 clutch of 6-8 eggs during
favorable seasons (Eckert and Karalus 1974), and often breed
repeatedly all year in captivity (Mendenhall, pers. comm.). Thus, 2
clutches/pair of owls were assured. The first clutch produced by
each pair was removed for artificial incubation and subsequent
clutches were left with the parents for natural incubation.
Beginning in early January, approximatley 2 wks before initia-
tion of barn owl breeding, each mew was entered daily by 1 or 2
workers and the nest boxes were checked for eggs. This procedure
was completed at a prescribed time every morning through April
to ensure that no egg was older than 24 hr when initially mea-
sured, and to minimize non-random disturbance of the adult owls.
As each freshly laid (4- 0-24 hr) egg was discovered, it was weighed
on an electric field balance to determine Wo, and the dimensions
were measured with a Vernier caliper. Additionally, each fresh
egg was marked with a graphite letter corresponding to its sequ-
ence in the clutch. No egg was ever fully removed from the nest
box and adults were kept at a distance during measurement.
During the subsequent incubation period, each egg was weighed
every other day using similar methods.
To reduce parental stress and promote successful copulation,
no eggs were collected from nest boxes prior to clutch completion
(W.C. Crawford, Jr. pers. comm.), Egg laying interval was ap-
proximately 1 egg every 2-3 d, thus eggs were from 1 to 16 d old
when removed from the nest for artificial incubation. Eggs were
incubated in Roll-X RX2A automatic rolling incubators with a
constant temperature of 37.5° C, and relative humidity of 48%.
Each egg was rolled manually ISO 9 3x/d to supplement automatic
rolling. Throughout the lay-to-pip (LP) interval, each egg was
weighed and candled every other day to determine both weight
loss and corresponding embryonic development. Once an embryo
had pipped its shell, the egg was placed pipped side upwards in
another Roll-X RX2A set at a lower temperature (35° C) but higher
relative humidity (60%). Pipped eggs were not turned. During the
pip-to-hatch (PH) interval, no weight measurements were made
due to shell fragility and difficulty in determining weight at the
instant of hatching. Infertile eggs or eggs containing dead em-
bryos were removed from the incubators to inhibit bacterial
growth.
Eggs undergoing natural incubation were weighed similarly
through pipping, but only occasionally candled to reduce nest
disturbance. No extra care was provided for these clutches (i.e.
cleaning of nest boxes, bad egg removal, etc.) unless a shell failed
108
Raptor Research Vol. 20 (3/4): 108-1 12
Fall/ Winter 1986
The Barn Owl Egg
109
Table 1 . Mean total fraction of grams Wo lost over the 28 d lay to pip interval for Barn Owl eggs a incubated
and naturally.
artifically
Incubation
N b
X
SD
min/max
CASES C
r d
Artificial
39
0.11
0.02
0.07-0.14
441
0.95
Natural
23
0.14
0.04
0.10-0.24
249
0.87
a Only fertile, successful hatching eggs represented,
k Number of eggs.
c Number of points used in generating r values and regression lines give Figure 1.
^ Correlation coefficient relating cumulative fraction of Wo lost to day of incubation.
in a fertile egg; such eggs were removed for artificial incubation
and excluded from the study. To prevent cannibalism, an occa-
sional aspect of barn owl adult-chick behavior, the amount of food
provided for each mew was increased considerably following the
hatch of each egg (W.C. Crawford Jr., pers. comm.).
Statistical analysis was performed using Statistical Package for
the Social Sciences (SPSS) (Nie et. al. 1975). A regression line
developed by the least squares fit was generated plotting the
cumulative fraction of Wo lost by corresponding interval day. The
resulting linear equation was used as a model (assuming 28 d LP
interval) to predict the total fractional weight loss for all cases in
each of the 2 incubation type categories. Other SPSS options were
used to generate F-Test, t-Test, Pearson’s r and Chi-squared (X 2 )
values and probabilities.
Results
The mean total fraction of Wo lost during the LP
interval was significantly different (F — 07.05 df =
P < 0.001) between artificially and naturally incu-
bated eggs which hatched successfully (Table 1).
High degrees of correlation were found between
cumulative reaction of Wo lost and interval day
within each incubation group, implying that eggs
dehydrated similarly in their respective categories
although a wide range of total fraction of Wo lost by
individuals was noted.
We defined hatchability as the percent of fertile
eggs successfully hatched. The hatchability of
naturally incubated eggs was 80.9% (n — 62).
Hatchability between incubation types was signific-
antly different ( x 2 = 4.56; df = P < 0.05).
The relationship between day of incubation and
cumulative fraction of Wo lost was examined (Fig.
1). An increase in the spread of points (statistically
indicated by increasing standard deviations of re-
siduals) from the regression line (Table 2), and
corresponding decrease in correlation coefficients
as incubation progressed through consecutive seg-
ments of LP interval were found. Both incubation
types had this characteristic.
A species specific coefficient (K w = 0.0005453)
was determined using equation (1) for Wo predic-
tion and the measured values of Wo, L and B col-
lected from 75 barn owl eggs (Table 3). Using this
K w a strong correlation was found between directly
Table 2. Increasing deviation of points from regression lines indicated by increasing standard deviation of residuals
and decreasing correlation between fresh Wo lost and incubation day.
Incubation
cases a
r
P
RESIDUAL SD
Artificial
0-10 days
136
0.83
<0.001
0.7906
11-19 days
133
0.77
<0.001
1.0272
20-30 days
172
0.67
<0.001
1.2954
Natural
0- 1 0 days
101
0.75
<0.001
1.3191
11-19 days
85
0.66
<0.001
2.2498
20-30 days
56
0.29
<0.01
3.9209
a Number of points used in generating the r values and regression lines given Fig. 1.
CUMULATIVE FRACTION OF FRESH WEIGHT LOST
110
Marshall et. al.
Vol. 20, No. 3/4
Figure 1. Regression of cumulative fresh weight lost in barn owl eggs by day of incubation.
Fall/Winter 1986
The Barn Owl Egg
111
Table 3. Summary of physical parameters from natural incubation and period of incubation for common Barn-Owl
(T.a.pratincola) eggs incubated artifically.
Parameter
N
X
SD
min/max
Length (1) (mm)
75
43.07
1.24
39.95-47.95
Breadth (B) (mm)
75
33.67
0.70
32.50-35.40
Fresh Weight (Wo) (g)
75
26.6
1.4
24.6 -29.9
Lay to Pip (LP)
Interval (days)
50
28.2
1.4
25-33
Pip to Hatch (PH)
Interval (Days)
50
2.1
0.5
1-4
Incubation Period (days)
50
30.3
1.5
27-35
measured and calculated values of W 0 (r = 0.917; P
< 0.001); the 2 group means were similar (t-Test =
0.39; P = 0.701).
When the coefficient K w ; 0.0005474 developed
by Burnham (1983) was used in equation (1), strong
correlation (r = 0.917; P < 0.001) was also evident
between measured and calculated values of W G ,
although statistical confidence in the similarity bet-
ween the 2 group means was decreased (t-Test =
1.86; P= 0.067).
Discussion
The total incubation period of the barn owl can
be generalized from the literature as 30-33 d, with
extremes of 29 and 34 d (Eckert and Karalus 1974;
Bunn et. al. 1982). Our study indicatd a similar
mean incubation period and range.
The mean Wo value (Table 3) of the barn owl eggs
studied is inconsistent with the mean (Wo) de-
veloped from the single random sample collected
(from the wild) by Sumner (1929), and his values
were reported in other works (Drent 1970; Ar and
Rahn 1980). However, Hoyt (1978) noted that in-
traspecific variability in the values of Wo, L and B
could be expected and we have attempted to account for
such deviation through relatively large samples collected
from many pairs of owls within the subspecies T. a.
pratincola.
Careful, frequent illumination of eggs with cool,
high intensity light provided good visual tracking of
embryonic development. A small fraction of em-
bryos did not achieve the visible blastodisc stage
(indicative of fertility) for up to 7 d following the
date of laying. However, most embryos apparently
began their development immediately, and showed
a blastodisc within 24 hr. A sharp increase in the
rate of egg weight loss in conjunction with abrupt
initialization of embryonic development in dor-
mant-fertile eggs was routinely observed. After an
extremely low rate of daily weight loss, these eggs
suddenly achieved a relatively constant rate of
weight loss which continued for about 28 d until a
normal fraction of Wo was lost. The chicks then
pipped the eggshell. Thus, a specific weight loss
rate occurred for the latter portion of the LP inter-
val, although this interval may have been initially
extended by the dormant-fertile condition. Since
the PH interval was fairly constant, with variance
probably due to observational error, nearly all de-
viation in the barn owl incubation period was due to
the initial dormant-fertile egg. It was unclear
whether the dormant-fertile condition was random
or relative to other eggs’ development within
clutches, but eggs generally hatched in sequence of
their laying. Quantification of this embryological
characteristic was not possible using their sample
and further study is required.
Although hatchability and mean total fraction of
Wo lost was related to incubation type (natural vs
artificial), the 2 incubation methods are very diffe-
rently affected. Factors inherent only during
natural incubation include frequent variation in
nest microclimate and ambient temperature and
humidity, high bacterial exposure, and violent
movement of delicate eggs by disturbed adult owls.
Such relatively uncontrollable variables may have
caused natural incubation weight loss rates to occur
which do not parallel those of eggs in undisturbed
112
Marshall et. al.
Vol. 20, No. 3/4
nests. These adverse factors undoubtedly contri-
buted to the lower hatchability of fertile eggs un-
dergoing natural incubation, although the sample
analyzed includes many eggs from undisturbed
nests.
Regression of weight lost by interval day reveals
an increase in deviation between predicted and ac-
tual egg weights during the LP interval. Since
weight loss is due to expired water vapor, as previ-
ously cited, this unexpected trend may reflect diffe-
rential individual respiratory function, effected by
the chorioallantois in conjunction with the eggshell,
which was not subject to purely passive diffusion.
This result contrasts with recent literature which
cites simple diffusion down concentration gra-
dients as the single force moving gases across the
eggshell (Wangensteen and Rahn 1970, 19721).
Inferences drawn from these results are in-
teresting to both the ecologist and the conser-
vationist propagating this species artifically. Tyto
alba supp. possess extremely favorable reproduc-
tive capabilities. Developmental flexibility is re-
flected in the variable egg weight losses achieveable
during incubation and in the dormant-fertile con-
dition which allows extension of incubation period.
These factors may contribute to the high hatchabil-
ity evident from the data in this study.
Acknowledgments
We thank the Raptor Rehabilitation and Propagation Project,
Inc., and W. C. Crawford, Jr., who has reintroduced over 500 barn
owls in Missouri;
Literature Cited
Ar, A. andH. Rahn. 1980. Water in the avian egg overall
budget of incubation. Amer. Zool. 20:373-384.
Bunn, D.S., A.B. Washburton, and R.D.S. Wilson.
1982. The Barn Owl. Buteo Books, Inc. Vermillion,
South Dakota, p. 220.
Burnham, W. Artificial incubation of falcon eggs. J.
Wildl. Manage. 47:158-168.
Drent, R. 1970. Functional aspects of incubation in the
Herring Gull. Behaviour Suppl. 17: 1-32.
Eckert, A.W. and K.E. Karalus. 1974. The Owls of
North America. Doubleday and Co., Inc. Garden City,
New York. pp. 15-16.
Hoyt, D.F. 1978. Practical methods of estimating vol-
ume and fresh weight of birds’ eggs. Quk 96:73-77
Nie, N.H., C.H. Hull, J.G. Jenkins, K. Steinbrenner
and D.H. Bent. 1975. Statistical Package for the So-
cial Sciences, 2nd Ed. McGraw-Hill Book Co., New
York.
Ramanoff, AL. and A.J. Romanoff. 1949. The Avian
Egg. John Wiley and Sons, Inc. New York, p. 378.
Sumner, E.L. Jr. 1929. Comparative studies in the
growth of young raptors. Condor 31:85-111.
Walsberg, G.E. 1980. The gaseous microclimate of the
avian nest during incubation. Amer. Zool. 20:363-372.
Wangensteen, O.D. and H. Rahn. 1970/71. Respiratory
gas exchange in the avian embryo. Respiration Physiol.
11:1-45.
Weaver, J.D. and T.J. Cade. 1983. Falcon propagation:
a manual on captive breeding. The Peregrine Fund,
Inc. Ithaca, New York.
Raptor Rehabilitation and Propagation Project Inc., Box 193,
Eureka, Missouri 630025. Current address of first author: 1017
Broadway, New Orleans, Louisiana 70118. Current address of
second author: Box 193, Eureka, Missouri 63025. Current ad-
dress of third author: Box 2007, Bartlesville, Oklahoma 70432.
Received 1 February 1986; Accepted 3 November 1986.
Fall/Winter 1986
Ecology of South American Owls
PREY AND TROPHIC ECOLOGY OF GREAT HORNED OWLS
IN WESTERN SOUTH AMERICA:
AN INDICATION OF LATITUDINAL TRENDS
Fabian M. Jaksic, Jose L. Yanez, and Jaime R. Rau
Abstract — Quantitative information on the diet of three Great Horned Owl (Bubo virginianus) popula-
tions along 18 lat. degrees in western South America (Chile) is compared with that of Great Horned Owls
in comparable latitudes along western North America. In Chile, owls preyed mainly on small mammals,
with proportion of birds decreasing, and that of insects increasing, toward southern latitudes. Mean prey
size and diet breadth declined toward southern Chile. These latitudinal trends closely mirror those
documented in western North America.
Although the Great Horned Owl ( Bubo vir-
ginianus) is distributed throughout the Americas, its
food habits have received considerable study
mainly in North America (Burton 1973). The only
published quantitative information on their food
habits in South America comes from central Chile
(approximately latitude 33° to 38°; see Jaksic and
Yanez 1980; Jaksic and Marti 1984). Except for a
preliminary report by Jaksic et ah (1978), no dietary
information was previously available from their
southernmost distribution (see Humphrey et al.
1970). Here we report the prey identified in 125
fresh pellets collected in September (austral spring)
1977 and in 14 other pellets collected in July
(winter) 1978, from under the same nest located at
Torres del Paine National Park (approximately 51°
01'S, 72°54'W; 142 km north of Puerto Natales).
For purposes of comparison we report earlier diet-
ary data published by Reise and Venegas ( 1 974) in a
Chilean journal of very local circulation. Their
study material (an unreported number of fresh
pellets, ±55) was collected under one nest, located
10 km north of Puerto Ingeniero Ibanez (46° 18' S;
71°55'W), in January (summer) 1971. For com-
parative purposes we also use Jaksic and Yanez’s
(1980) report on the prey of the Great Horned Owl
at La Dehesa (33°21'S, 7(L32'W; 20 km east of
Santiago), based on 98 fresh pellets collected dur-
ing September (spring) 1979, beneath one nest.
Although the information analyzed is based on very
small sample sizes, we believe it is useful in con-
solidating new and old information fragmented in
the Chilean literature and not readily available to
ornithologists elsewhere.
Methods
Considering that ca. 95% of the pellets analyzed reflect spring
and summer diet, and that this dietary information covers ap-
proximately 18° latitude, a quantitative comparison seems war-
ranted. We use the following trophic metrics: (a) Geometric mean
prey weight in the diet — essentially the back-transformation of
the mean prey size obtained with log-transformed weight data,
weighted by their relative occurrence in the diet (see Jaksic and
Braker 1983 for formula, justification, and assumptions of this
trophic statistic). Prey sizes are mean weights of small mammals in
Table 1. (b) Diet breadth — the diversity of prey in the diet as
computed by Levins’ (1968) index: Bobs = l/(Spi^)> where/?* is the
relative occurrence of prey taxon i in a given population’s diet
This index generates values between 1 andn (whenn resources are
used equally). Because Levins’ index increases with the number of
prey taxa, a standardization is necessary when comparing popula-
tions in different localities, where the availability of prey taxa may
differ. Colwell and Futuyma (1971) provide a standardized ver-
sion of Levins’ index: Bsta - (Bobs - Bmin)/(Bmax - Bmin), where
Bobs is the observed niche breadth (= Levins’ index), Bmin is the
minimum niche breadth possible (= 1), and Bmax is the maximum
niche breadth possible (= n), which is the number of prey taxa
actually taken by a given owl population (i.e., each of the taxa that
receives a separate line in Table 1; generally, species for mammals
and orders for insects). This standardized index renders values
between 0 and 1 (i.e., between a comparatively narrow niche
breadth, with disproportionately high representation of one or a
few prey items, and a broad one, with a more even consumption of
the available prey categories, respectively).
Results and Discussion
Results are summarized in Table 1, and are here
discussed in a north-south succession. In La De-
hesa, the owls preyed upon all small mammals
known to occur in the locality (see Jacks! c et al.
1981), with the exception of the rodents Octodon
degus (a semi-fossorial species) and S palac opus cy anus
(a truly fossorial one). Jaksic and Yanez (1980)
113
Raptor Research Vol, 20 (3/4): 113-116
114
Jaksic et, al.
Vol. 20, No. 3/4
Table 1. Prey of Great Horned Owls in La Dehesa (33° S), Puerto Ibanez (46° S), and Torres del Paine (51° S), Chile.
Figures are percentages by number of prey individuals; subtotals are in brackets.
Prey Categories
WEIGHT(g)*
33°S
46° S
51°S
Mammalia
[88.6]
[86.0]
[87.5]
Lagomorpha
Lepus capensis
2.000.0
—
5.3
0.6
Oryctolagus cuniculus
1,300.0
15.8
—
—
Marsupialia
Marmosa elegans
40.0
3.5
—
—
Rodentia
Abrocoma bennetti
219.0
18.4
—
—
Akodon lanosus
32.5
—
—
4.8
Akodon longipilis
76/41.0**
16.7
8.7
3.0
Akodon olivaceus
40.0
0.8
—
—
Akodon xanthorhinus
21.5
—
5.3
9.5
Ctenomys cf. magellanicus
271.8
—
15.8
—
Eligmodontia typus
26.5***
—
—
0.6
Euneomys chinchilloides
87.8***
—
26.3
0.6
Notiomys macronyx
—
—
—
2.4
Oryzomys longicaudatus
45/29.8**
4.4
1.8
39.8
Phyllotis darwini
66.0
4.4
7.0
—
Phyllotis micropus
75.0
—
12.3
—
Phyllotis sp.
—
—
3.5
—
Rattus rattus
158.0
19.3
—
—
Reithrodon physodes
81.7
—
—
25.6
Unidentified
—
5.3
—
0.6
Aves
[11.4]
[5.3]
[2.4]
Unidentified
—
11.4
5.3
2.4
Insecta
[0.0]
[8.7]
[10.1]
Coleoptera
—
—
8.7
8.9
Hymenoptera
—
—
—
0.6
Orthoptera
—
—
—
—
Unidentified
—
—
—
0.6
No. pellets
98.00
55?
139. 0C
No. prey
114.00
57.00
168. 0C
Geometric mean prey weight (g)
181.9
104.5
41.1
Twice standard error
0.61
0.83
0.31
Sample size (= prey with weight)
95.00
47.00
142. 0C
Diet breadth (Bobs)
6.90
7.18
4.07
Standardized diet breadth (Bsta)
0.66
0.62
0.24
* Weights with no decimal places are from Jaksic and Marti (1984); all the remaining (except for those marked with
asterisks) are from Jaksic et al. (1983).
**There is a strong latitudinal cline in body size for this species (see Yanez et al. 1978, and Palacios 1982): the first
figure corresponds to its mean weight in central Chile; the second, to its mean in southern Chile.
***From Greer (1965).
Fall/Winter 1986
Ecology of South American Owls
115
suggested that the absence of these 2 species from
the Great Horned Owl diet was due to their diur-
nal-crepuscular activity pattern. In Puerto Ibanez,
owls preyed on essentially all small mammal species
trapped by Reise and Venegas (1974) in the same
locality, and on 2 additional rodents: Euneomys
chinchilloides (a scansorial species) and Ctenomys cf.
magellanicus (a fossorial one). These 2 made up
more than 40% of the owls’ diet (Table 1), but were
neither trapped nor seen in the area (Reise and
Venegas 1974; Yanez et al. in press). In Torres del
Paine, owls preyed on all small mammal species
known to occur there, as well as on 3 other rodents
hitherto not recorded (Rau et al. 1978): the terres-
trial Eligmodontia typus and Akodon lanosus, and the
semi-fossorial Notiomys macronyx. In general, the
three owl populations studied preyed mainly on
small mammals (averaging 87% of their prey). With
increasing latitude, the proportion of birds in the
diet decreased, with the opposite trend seen in the
insect prey (from no insect consumption at all in La
Dehesa, to 10% of the diet in Torres del Paine).
The geometric mean weight of prey declined
monotonically from north to south, with no indica-
tion of a corresponding trend in owl body size
(Johnson 1965; Humphrey et al. 1970). A similar
(but not so consistent) decline in mean prey weight
away from the equator was reported by Knight and
Jackman (1984) for Great Horned Owls along the
Pacific coast of the United States. Comparing areas
at latitudes 30° to 40? between the two hemispheres,
Jaksic and Marti ( 1 984) showed that central Chilean
and California Great Horned Owls did not differ
significantly in body size (1,227 g vs. 1,166 g, re-
spectively), but mean prey weight of California owls
was 59% of Chilean ones. Knight and Jackman
( 1 984) reported mean prey weight of Great Horned
Owls in central Washington (46° N), which coin-
cides with the latitude of Puerto Ibanez. Because
Knight and Jackman (1984) used an arithmetic es-
timate of mean prey weight, we recalculated from
their raw data the geometric estimate, thus making
their results comparable to ours. Washington owls
exhibited a geometric mean prey weight of 22.9 ±
0.21 g (mean ± 2 s.e.; sample size = 872) which
amounted to only 22% of the value reported for
southern Chilean owls at the equivalent latitude
(Table 1). It is difficult to assign causal relations to
these patterns without knowing prey sizes available
to owls in these different localities. Knight and
Jackman (1985), following Herrera and Hiraldo
(1976), speculated that the decrease in mean prey
weight taken by owls at higher latitudes may be
related to smaller prey becoming more abundant as
latitude increases. We have no data to substantiate
this claim.
Diet breadth in Chile also decreased with in-
creasing latitude, in agreement with trends re-
ported by Knight and Jackman (1984) for the Great
Horned Owl along the Pacific coast of the United
States and by Herrera and Hiraldo (1976) for the
Eagle Owl (Bubo bubo) in Europe. Jaksic and Marti
(1984) reported that central Chilean and California
Great Horned Owls have a similar diet breadth at
the class level of prey identification (H’NGG in their
Table 3), but that the former have significantly
narrower diet breadth at the species level of mam-
malian prey H’NM in their Table 3). Knight and
Jackman (1984) documented a diet breadth of 4.12
(which amounts to a standardized diet breadth =
0.12; because Bmax = 26, and Bmin= 1) for
Washington Great Horned Owls. These values
amount to 57% and 19% (respectively) of those
computed for owls at the equivalent latitude in
Chile, and are in fact more similar to observations 5
latitudinal degrees south, in Torres del Paine (Ta-
ble 1). Apparently, both South and North Ameri-
can Great Horned Owls exhibit narrower diets to-
ward higher latitudes, but the latter prey heavily on
relatively few items. In fact, only two rodents
(Thomomys talpoides and Perognathus parvus ) ac-
counted for 73% of the items in the diet of
Washington owls. A similar value in the diet of
Chilean owls was accounted for by the six most
preyed upon rodent species in Puerto Ibanez, and
by three in Torres del Paine (Table 1). The de-
creasing diet diversity away from the equator might
be related to a decreasing number of potential prey
species which is consistent in both hemispheres.
Latitudinal trends in the trophic niche of Great
Horned Owls along the Pacific coast of southern
South America closely mirror trends documented
in northern North America (and of the congeneric
Eagle Owl in Europe). Local estimates of trophic
statistics for latitudinally-matched localities in the
two hemispheres, however, show some marked dif-
ferences. The pattern of decreasing diet diversity
away from the equator could have been expected,
but 'a similar trend in mean prey weights at corres-
ponding latitudes, both related to the local availa-
bility/vulnerability of prey, was unlikely to hold
within/between the two hemispheres.
116
Jaksic et. al.
Vol. 20, No. 3/4
Acknowledgments
We thank Richard J. Clark, Richard L. Knight, M. Ross Lein,
Carl D. Marti, Martin K. McNicholl, Karen Steenhof, and an
anonymous reviewer, for critically reading different versions of
this paper. Jaksic acknowledges the support of grants DIUC
202/83 and 076/85 (awarded by the Pontificia Universidad
Catolica de Chile), and INT-8308032 (awarded by the U.S. Na-
tional Science Foundation) during the several stages of prepara-
tion of the manuscript.
Literature Cited
Burton, J. A. [ed.]. Owls of the World, E.P. Dutton, New
York.
Colwell, R.K., and D.J. Futuyma. 1971. On the mea-
surement of niche breadth and overlap. Ecology
52:567-576.
Greer, J.K. 1965. Mammals of Malleco province, Chile.
Publ. Mus., Michigan State Univ., Biol. Ser. 3:49-152.
Herrera, C.M., and F. Hiraldo. 1976. Food-niche and
trophic relationships among European owls. Ornis
Scand. 7:29-41.
Humphrey, P.S., D. Bridge, P.W. Reynolds, and R.T.
Peterson. 1970. Birds of Isla Grande (Tierra del
Fuego). Preliminary Smithsonian Manual, Smithso-
nian Institution, Washington, D.C.
Jaksic, F.M. and H.E. Braker. 1983. Food-niche re-
lationships and guild structure of diurnal birds of
prey: competition versus opportunism. Can. J. Zool.
61:2230-2241.
Jaksic, and J. L. Yanez. 1980. Differential utilization of
prey resources by Great Horned Owls and Barn Owls
in central Chile. Awk 97:895-896.
Jaksic, F.M., H.W. Greene, and J.L. Yanez. 1981. The
guild structure of a community of predatory verteb-
rates in central Chile. Oecologia 49:21-28.
Jaksic, F.M., J. Rau, andJ. Yanez. 1978. Ofertade presas
y predacion por Bubo virginianus (Strigidae) en el Par-
que Nacional “Torres del paine.” anales del Instituto
de la Patagonia, Punta Arnas (Chile) 9:199-202.
jAksic, F.M., and C.D. Marti. 1984. Comparative food-
habits of Bubo owls in Mediterranean-type ecosystems.
Condor 86:288-296.
Jaksic F.M., J.L. Yanez, and J.R. Rau. 1983. Trophic
relations of the southernmost populations of Dusicyon
in Chile./. Mamm. 64:693-697.
Johnson, A.W., 1965. The birds of Chile and adjacent
regions of Argentina, Bolivia and Peru: volume II.
Platt Establecimientos Graficos, Buenos Aires.
Knight, R.L., and R.E. Jackman. 1984. Food-niche re-
lationships between Great Horned Owls and Common
Barn-Owls in eastern Washington. Auk 101:175-179.
Levins, R. 1968. Evolution in changing environments:
some theoretical explorations. Princeton Univ. Press,
Princeton, New Jersey.
Palacios, O.V. 1982. Morfometria y sistematica de
Oryzomys longicaudatus (Rodentia: Cricetidae). Thesis,
Universidad de Chile, Santiago, 91 pp.
Rau, J.,J. Yanez and F. Jaksic. 1978. Confirmacion de
Notiomys macronyx alleni O. y Eligmodontia typus typusQ.,y
primer registro de Akodon ( Abrothrix ) lanosus T.
(Rodentia: Cricetidae) en la zona de Ultima Esperanza
(XII Region, Magallanes). Anales del Instituto de la
Patagonia, Punta Arenas (Chile) 9:203-204.
Reise, D., and W. Venegas. 1974. Observaciones sobre el
comportamiento de la fauna de micromamiiferos en la
region de Puerto Ibanez (Lago General Carrera),
Aysen, Chile. Boletin de la Sociedad de biologia de
Concepcion (Chile) 47:71-85.
Yanez, J., W. Sielfeld, J. Valencia, and F. Jaksic. 1978.
Relaciones entre la sistematica y la morfometria del
subgenero Abrothrix (Rodentia: Cricetidae) en Chile.
Anales del Instituto de la Patagonia, Punta Arenas
(Chile) 9:185-197.
Yanez, J.L., J.C. Torres-Mura, J.R. Rau, and L.C. Con-
treras. In press. New record and current status of
Euneomys (Cricetidae) in southern South America.
Fieldiana (Zoology).
Departamento de Biologia Ambiental, Universidad Catolica de
Chile, Casilla 1 14-D Santiago, Chile. Address of second au-
thor: Museo Nacional de Historia Natural, Casilla 787 San-
tiago, Chile. Address of third author; Estacion Biologica de
Donana, Apartado 1056, 41080 Sevilla, Spain.
Received 28 March 1986; Accepted 25 July 1986.
IMPACT OF A HIGH-VOLTAGE TRANSMISSION LINE ON A NESTING
PAIR OF SOUTHERN BALD EAGLES IN SOUTHEAST LOUISIANA
David A. Dell and Phillip J. Zwank
Abstract — To evaluate the impact of a 500th kv power transmission line on a pair of nesting bald eagles.
(Haliaeetus leucocephalus) pre- and post-installation observations of eagle area-use were recorded. The mean of
the daily proportion of eagle activity spent in the vicinity of the powerline decreased (P = 0.02) from pre-installa-
tion ( X = 27.6%) to post-installation (X= 18.7%) seasons, indicating that activity patterns were changed after
installation of the powerline. No serious physical threat to nesting eagles could be ascertained. The eagles
regularly flew over and under the powerline, and perched and foraged near it. They never used the powerline
itself for perching.
Wilcox (1979) reported on the success of a pair of
Southern Bald Eagles (Haliaeetus leucocephalus
leucocephalus ) nesting 50 m from a 240th kv power
line, however, quantitative data are unavailable on
the effects of power transmission lines on territory
use by nesting Southern Bald Eagles. The con-
struction of a transmission line through the nesting
territory of a pair of eagles in southeast Louisiana
provided an opportunity to compare area-use by
the eagles within the powerline zone before and
after construction.
Study Area and Methods
The Waterford-Churchill 500-kV line passes through Salvador
Wildlife Management Area (SWMA), St. Charles Parish,
Louisiana, at the northwest shore of Lake Cataouatche, approxi-
mately 14 km south of New Orleans International Airport. The
line consists of steel self-supporting towers of an “H” design. Each
tower is 30.5 m tall and supports 3 phase conductors 9.6 m apart.
The conductors vary from 11 to 21 m above marsh level. Two
smaller static lines are strung approximately 9 m above the phase
conductors. Distances between towers vary, but they are 265-274
m apart in the study area. The powerline is approximately 600 m
north of the eagle nest studied and centered in a corridor ap-
proximately 60 m wide that has been cleared of all trees. Con-
struction occurred during summer (when eagles are absent from
SWMA) 1983.
The eagle nest is in a living bald cypress (Taxodium distichum),
32.9 m high and 107.4 cm in diameter above the swelling at the
base (Dugoni 1980). An observation blind was placed approxi-
mately 320 m north of the nest during 1983-84, between the nest
tree and powerline. In 1979-80, the blind was approximately 100
m closer to the nest (Fig. 1). From the blind, we could observe
eagles flying over an area of about 810 ha. This area was a non-
tidal, permanently flooded, palustrine system (Cowardin et al.
1979) occupied by forested wetland (cypress and Nyssa aquatica),
aquatic bed (Bidens laevis, Eleocham spp., and Sagittaria lancifolia on
floating turf; Nelumbo lutea and Eichhornia crassipes were free-
floating), and unconsolidated organic bottom habitats.
We observed eagles twice weekly from dawn to dusk and re-
corded total minutes spent in various activities and areas. To
analyze the effect of the powerline on the eagles’ area-use, a
“powerline zone” extending 400 m to the south and up to 1000 m
north of the powerline was defined within the study area. The
boundaries of the zone were chosen to include the perch trees and
foraging areas close to the powerline, and because the eagles had
to cross the powerline to reach the most frequently-used foraging
area visible from the blind. The proportion of “eagle-minutes”
(combined number of minutes that both adults were observed)
spent within the powerline zone each day was used as the depen-
dent variable in a randomized-block design analysis of variance to
test for differences between pre-and post-installation seasons
(treatments) and among periods of the nesting season (blocks).
The periods of the nesting season we blocked on were brooding,
pre-fledging (eaglets still in nest, but not brooded), and post-
fledging (eaglets out of nest).
Results and Discussion
Pre-installation observations were conducted
from 3 January 29 to April 1980. During that sea-
son, 25 observation days were completed and
30,651 eagle-minutes were recorded (Shealy and
Zwank 1981). Due to a lawsuit, construction of the
powerline was delayed until summer 1983. Post-in-
stallation observations were for 4 January to 3 May
1984. Thirty-two observation days were completed,
and 45,784 eagle-minutes were recorded.
The mean of the daily proportion of eagle-
minutes spent by the adult eagles in the powerline
zone decreased (P = 0.02) from pre-installation
(x = 27.6x) to post-installation (x=18.7%). Also,
activity varied among the periods of the nesting
season (P = 0.0004) (Table 1).
The eagles spent more of the brooding and pre-
fledging periods in the powerline zone before in-
stallation than after. In the post-fledging period
during both years, the eagles spent almost the same
117
Raptor Research Vol. 20 (3/4): 1 17-1
118
Dell and Zwank
Vol. 20, No. 3/4
Area visible from eagle observation blind, Salvador WMA
Figure 1.
proportion of time in the powerline area.
Activities within the powerline zone consisted of
perching, soaring, foraging, or straight-line flight
between perches. The eagles often flew over and
under the conductors while going between the nest
and various foraging areas. Herrick (1924) re-
ported that one nesting pair of eagles regularly flew
past “wires by the railroad.”
We saw an eagle react to the powerline only once.
While flying in circles 20-40 m above the marsh, an
adult approached the wires several times, then
banked quickly to avoid them. None of the eagles
were ever seen perching on the transmission lines
or towers.
Relocation of the observation blind in 1983 closer
to a perch tree appeared to affect behavior. Use of
this perch tree accounted for 1.3% of total activity
in the pre-installation season (Shealy and Zwank
1981), but was never used during the post-installa-
tion season.
Table 1. Average daily proportion of eagle-minutes spent in the powerline zone in 1979-80 and 1983-84
and averages by nest period.
Period
X
SE
N a
cv%
1979-80
0.276
0.0350
25
63.3
Post-fledging
0.400
0.2006
4
100.3
Pre-fledging
0.254
0.251
14
37.0
Brooding
0.250
0.0386
7
40.9
1983-84
0.187
0.0418
32
126.7
Post-fledging
0.411
0.0806
11
65.0
Pre-fledging
0.082
0.0315
13
139.0
Brooding
0.048
0.0151
8
88.4
Observation days.
Fall/ Winter 1986
Powerline Impacts on Bald Eagles
119
Changes in area-use observed may have resulted
from removal of potential perch trees from the
powerline corridor, blocking of forage flights by
transmission wires or changes in prey availability or
distribution. Replacement of one or both members
of the adult pair could also have influenced be-
havior; we cannot be certain that the same pair
nested in both 1980 and 1983. Also, relocation of
the observation blind changed perching habits, but
its influence on use of the powerline zone could not
be determined.
Based on our observations of eagles during
flight, we do not think the powerline poses a serious
physical threat to the nesting adults. Also, nesting
attempts were successful before and after power-
line installation. Possibly, however, awkward
fledglings could collide with the powerline. Eagle
electrocutions are unlikely because phase conduc-
tors are widely spaced (9.6 m) and we never ob-
served perching on the powerline or towers.
Funding
This article is a contribution from the La. Coop. Wildl.
Res. Unit (USFWS, La. Dept. Wildl. and Fisheries, La.
State Univ., Wildl. Manage. Inst., and School of Forestry,
Wildl., and Fisheries, LSU cooperating). Funding was
provided by Louisiana Power and Light Co.
Literature Cited
Cowardin, L.M., V. Carter, F.L. Golet, and E.T.
LaRoe. 1979. Classification of wetlands and deep-
water habitats of the United States. U.S. Fish and
Wildl. Serv., Off. Biol. Serv. 103 pp.
Dugoni, J.A. 1980. Habitat utilization, food habits, and
productivity of nesting southern bald eagles in
Louisiana. M.S. Thesis, Louisiana State Univ., Baton
Rouge. 151 pp.
Herrick, F.H. 1924. The daily life ofthe American eagle:
late phase. Auk 41:389-422.
Shealy, P.M. and P J. Zwank. 1981. Activity patterns and
habitat use of a nesting pair of southern bald eagles in
southern Louisiana. Pages 127-135. In: R.R. Odom
and J. W. Guthrie, eds. Proc. nongame and en-
dangered wild, symp., Georgia Dept. Nat. Res., Game
and Fish Div. Tech. Bull. WL5.
Wilcox, J.R. 1979. Florida power and light company and
endangered species: examples of coexistence. Pages
451-454. In G.A. Swanson, tech, coord. The mitiga-
tion sym.: a national workshop on mitigating losses of
fish and wild, habitats. U.S. For. Serv., Rocky Mt.
Forest and Range Exp. Sta. Gen. Tech. Rep. RM-65.
Louisiana Cooperative fish and wildlife Research Unit,
School of Forestry, wildlife and fisheries, Louisiana State Uni-
versity, Baton Rouge, Louisiana 70803
Received 15 December 85; Accepted 15 June 1986.
Third New England Regional Hawk Conference - The New England Hawk Migration Committee wishes to announce
the Third New England Regional Hawk Conference will be held 4 April 1987 at the Holiday Inn, Holyoake,
Massachusettes. Registration forms are available from HAWKS, P.O. Box 212, Portland, Connecticut 06480. There are
special rates available for lodging at the Conference center. Registration will be limited.
FOOD OF THE BOOTED EAGLE (HIERAAETUS PENNATUS)
IN CENTRAL SPAIN
Jose P. Veiga
Abstract. — The identification of 202 prey remains of the Booted Eagle (Hieraaetus pennatus) shows that mammals
(41.6% of prey items identified), birds (36.6%) and reptiles (21.8%) are important prey in Central Spain. Most mammals
captured were young rabbits, and the majority of the bird prey were fledglings or juveniles. Lizards were adult or
subadult individuals. Over 90% of the prey captured weighed between 27 and 243 g.
Little is known about the biology of the Booted
Eagle (Hieraaetus pennatus), as it occurs in countries
with little ornithological activity. Most published
accounts of food habits are single enumerations of
prey remains recorded mainly during sporadic vis-
its to nests (Val verde 1967; Araujo 1973; Garzon
1973; Iribarren 1975). This procedure provides an
inaccurate picture of diet, since prey that are large
and leave persistent remains are over-represented
in samples (e.g., Valverde 1967; Delibes 1975). In
spite of this, several recent papers dealing with the
trophic relationships between members of various
raptor communities have made use of such data
(Jaksic and Soriguer 1981; Jaksic 1983; Jaksic and
Braker 1983). In my opinion this has led to errone-
ous conclusions regarding the ecological position of
the Booted Eagle in Mediterranean environments.
The present paper presents more accurate infor-
mation about the diet of this raptor, obtained using
a more systematic data collection procedure. I also
take into consideration some attributes of prey,
such as size and age, that have been overlooked.
Study Area and Methods
This study was carried out in 3 areas, each about 35 km^ in size,
located on the northern slope of the Sierra de Guadarrama
mountains (4(f 35' -4(f 60' N, OP 5'-(f 60' W). Area 1 is about 60%
pasture interspersed with thick scrub. The only arboreal forma-
tions present are 3 small pine groves of between 1 and 5 ha. Area 2
is 1 0 km away and about 40% covered with mature natural pine
trees (Pinus silvestris) over 15m tall. The rest of area 2 is made up of
a sparse evergreen oak grove ( Quercus rotundifolia) with extensive
clearings in v/hich low scrub mixes with pasture land. Area 3, 15
km from area 2 and 30 km from area 1 , is similar to area 1 in that it
has only 2 arboreal formations, one of 2 ha and the other of 25 ha.
Area 1 was visited from 1 978 to 1 98 1 . One pair of Booted Eagles
used the same nest year after year. Area 2 was also visited from
1979 to 1981. In 1979 2 pairs of nesting eagles were present, but in
1980 to 1981 no nests were found. Area 3 was also visited from
1979 to 1981. In both 1979 and 1980 1 pair of eagles was located,
but no eagles were seen in 1981. Visits were made approximately
every 15 d from shortly before incubation (mid-late April) until
after the young left the nest (mid-late August). During the feeding
period nests were occasionally visited every 7 d. Pellets and prey
remains were sought in and around nests and below perches which
were usually within a 200 m radius of the nests.
A total of 110 pellets, containing 130 identifiable prey items,
and 72 prey remains were collected. Each species found in any one
pellet was counted as 1 individual unless it was possible to show
that more than 1 was represented. Therefore, it was necessary to
count pieces of remains such as nails, beaks, teeth, etc. Weight and
approximate age of the prey were estimated by comparing re-
mains with material from zoological collections and with speci-
mens collected in the study areas. In order to establish a frequency
distribution for prey, weight classes were established whose limits
followed a geometric progression (Fig. 1). This insured that the
resulting distribution would be more or less normal (Schoener
1 969; Hespenheide 1971). Only some prey identified in the pellets
could be assigned to one of the established weight categories,
particularly in the case of species, like rabbits and ocellated lizards,
whose weights vary a great deal.
Results and Discussion
Mammals, birds, and reptiles, in decreasing or-
der of capture frequency, comprised the diet of the
Booted Eagle in the study area. Percentage differ-
ences of these taxa in the diet increased considera-
bly when biomass was taken into account. (Table 1).
Among mammal prey, rabbits were the most im-
portant prey species. Birds captured were primarily
species that forage on the ground. The Ocellated
Lizard (Lacerta lepida) was the only reptile prey,
although other lizards are common in the study
area.
The weight of prey items varied between 1 0 and
800 g. However, most were in the 27 to 243 g range
(Fig. 1). A major part of the diet consisted of prey in
the 81 to 243 g weight-class (Fig. 1). Prey-size dis-
tributions do not appear to be the same for the 3
taxa present in the diet: most mammal and lizard
prey weighed between 8 1 and 243 g. Avian prey was
120
Raptor Research Vol. 20 (3/4): 120-123
Fall/Winter 1986
Booted Eagle Diet
121
Figure 1 . Diet of the Booted Eagle. Thick line histogram:
percent of the total biomass supplied by the
prey-items; thin line histogram: percent of the
total number of prey-items. Sample size = 165.
predominantly between 27 and 81 g (Fig. 2). The
majority of birds in this class were the Spotless
Starling, (Sturnus unicolor ) weighing 70 g. Nearly all
rabbits captured were very young individuals. Of
27 bird prey items of known age, the number of
fledgling and juveniles was greater than the
number of adults (22 young vs. 5 adults). All Ocel-
lated Lizards identified were adults or sub-adults.
Prey-size distribution could merely reflect the
size distribution of available prey, assuming Booted
Eagles on the study area selected prey randomly
with respect to size. Nevertheless, the lack of insects,
amphibians, and small reptiles in the diet of some
other raptors of similar size such as the Common
Buzzard (Buteo buteo ), Black Kite (Milvus migrans)
and Red Kite (M. milvus) in the same study area
(Veiga 1982) suggests that prey below a certain
weight were avoided. Prey might also be selected
according to age and experience. This may be par-
ticularly true for avian prey, since the poor flying
abilities of young birds make this age class more
vulnerable to predation by Booted Eagles.
It has been reported that the analysis of pellets
and prey remains for Order Falconiformes tends to
underestimate the amount of some prey while
overestimating others (Valverde 1967; Delibes
1975; Collopy 1983). The absence of small prey
such as insects, amphibians or small reptiles in the
Booted Eagles’ diet could be due to these
methodological biases. However, using the same
methodology, these small prey have been found in
the diet of other similar sized raptors in the same
areas in which the Booted Eagle was studied, Fur-
thermore, by sampling prey remains regularly and
at relatively short intervals the potential bias possi-
bly caused by the greater detectability of certain
prey when collected at longer intervals would be
diminished. The fact that the material to be
analyzed was collected from the nests as well as
from the perches of the adults reduces the possibil-
ity of obtaining a distorted image of diet if it is
assumed that food taken to the nestlings is different
from that of the adults. I have not been able to
demonstrate this in the Booted Eagle.
Earlier studies of Booted Eagle feeding habits
carried out in the Palearctic and in South Africa
describe them as a hunter of small birds and, to a
lesser degree, lizards (Valverde 1967; Araujo 1973;
Garzon 1973; Iribarren 1975; Steyn and Grobler
1981). It is worth noting that although the scarcity
of mammals in the South African Booted Eagles’
diet could be due to a lack of appropriate sized
individuals in the field, the low representation of
this taxon in reports from Spain where rabbits
abound in a variety of sizes is surprising. My results
suggest that the Booted Eagle behaves, in my study
Figure 2. Distribution of the prey remains in the prey-
weight classes in each taxonomic group., Black
circles = mammals; open circles = birds;
squares = reptiles. Sample sizes: mammals =
61; birds = 64; reptiles = 40.
122
Jose P. Veiga
Vol. 20, No. 3/4
Table 1. Prey of the Booted Eagle in central Spain.
Species
Number Occurrence Biomass
of Items Percent Percent
Reptiles
Ocellated Lizard (Lacerta lepida)
44
21.8
14.3
Total
44
21.8
14.3
Birds
Common Kestrel (Falco tinnunculus)
2
0.99
1.0
Quail ( Cotumix cotumix)
2
0.99
0.46
Unidentified Phasianidae
1
0.49
0.46
Little Bustard ( Otis tetrax)
1
0.49
1.76
Stone Curlew (Burhinus oedicnemus)
1
0.49
1.05
Wood Pigeon ( Columbia palumbus )
1
0.49
1.08
Unidentified Columbidae
3
1.48
2.54
Swift ( Apus apus)
1
0.49
0.09
Hoopoe (Upupa epops)
7
3.46
1.03
Green Woodpecker (Picus viridis)
1
0.49
0.39
Unidentified Alaudidae
1
0.49
0.08
Mistle Thrush (Turdus viscivorus)
1
0.49
0.27
Spotless Starling (Stumus unicolor)
28
13.86
5.13
Magpie (Pica pica)
9
4.45
4.33
Jackdaw ( Corvus monedula)
4
1.98
2.1
Carrion Crow ( Corvus corone)
1
0.49
1.18
Unidentified
10
4,95
1.83
Total
74
36.6
24.8
Mammals
Common White-toothed Shrew (Crocidura russula)
1
0.49
0.03
Blind Mole ( Talpa caeca)
2
0.99
0.19
Rabbit ( Oryctolagus cuniculus)
65
32.18
48.71
Hare (Lepus granatensis)
2
0.99
5.0
Unidentified Lagomorpha
1
0.49
0.75
Water Vole (Arvicola sapidus)
8
3.96
5.0
Weasel (Mustela nivalis)
3
1.48
0.94
Unidentified
2
0.99
0.19
Total
84
41.6
60.8
Total Items
202
Fall/Winter 1986
Booted Eagle Diet
123
area, like a taxa-generalist that concentrates on ter-
restrial prey weighing between 70 and 240 g. It is
probable that the general decrease of the rabbit in
Iberian ecosystems in the last decades, resulting
from the effect of mixomatosis, has influenced the
composition of the Booted Eagle’s diet. However,
there are no detailed studies of the population
dynamics of the rabbit and other prey species,
which would be necessary before this could be seri-
ously discussed.
Acknowledgments
I am indebted to G. Bortolotti, C. Griffin and B. Millsap for their
critical comments of an earlier draft.
Literature Cited
Araujo, J. 1973. Falconiformes del Guadarrama sur-
occidental. Ardeola 19: 257-278.
Collopy, M.W. 1983. A comparison of direct observa-
tions and collections of prey remains in determining
the diet of Golden Eagles./. Wildl. Mange. 47: 360-368.
Delibes, M. 1975. Alimentacion del Milao Negro (Milvus
migrans) en Donona (Huelva, Espana). Ardeola 21: 183-
207.
Garzon, J. 1973. Contribucion al estudio del status,
alimentacion y proteccion de las falconiformes en Es-
pana central. Ardeola 19: 279-330.’
Hespenheide, H.A. 1971. Food preference and the ex-
tent of overlap in some insectivorous birds, with special
reference to the Tyrannidae. Ibis 113: 59-72.
Iribarren, J.J. 1975. Biologla del aguila calzada
(Hieraaetus pennatus) durante el periodo de nidifica-
cion en Navarra. Ardeola 25 (Vol. Esp.): 305-320.
Jaksic, F.M. 1983. The trophic structure of sympatric
assemblages of diurnal and nocturnal birds of prey.
Amer. Mid. Natur. 109: 152-162.
Jaksic, F.M. and Braker, H.E. 1983. Food niche re-
lationships and guild structure of diurnal birds of
prey: competition versus opportunism. Can. f. Zool.
61:2230-2241.
Jaksic, F.M. and Soriguer, R.C. 1981. Predation upon
the European rabbit (Oryctolagus cuniculus) in Mediter-
ranean habitats of Chile and Spain: a comparative
analysis./. Anim. Ecol. 50: 269-281.
Schoener, TW. 1969. Models of optimal size for solitary
predators. Am. Nat. 103:277-313.
Steyn, P. and Grobler, J.H. 1981. Breeding biology of
the Booted Eagle in South Africa. Ostrich 52: 108-118.
Valverde, J.A. 1967. Estructura de una comunidad
mediterranea de vertebrados terrestres. C.S.I.C. Mad-
rid.
Veiga, J.P. 1982. Ecologia de las rapaces de un
ecosistema Mediterraneo de montana. Aproximacion
a su estructura comunitaria. Ed. Univ. Compl., Mad-
rid.
Museo Nacional de Ciencias Naturales C.S.I.C. Jose Gutierrez
Abascal, 2. Madrid-28006. SPAIN.
Received 30 March 1985; Accepted 8 April 1986.
FOODS OF NESTING BALD EAGLES IN LOUISIANA
Joseph A. Dugoni, Phillip J. Zwank, and Gary C. Furman
Abstract — During the summer of 1979, remains of 243 vertebrates comprising 3 1 species were collected from 10 nests
that had fledged young during the previous spring to determine the food habits of nesting Bald Eagles {Haliaeetus
leucocephalus) in Louisiana. American Coots (Fulica americana) and freshwater catfish ( Ictalurus spp.) were the most
abundant species, but fish probably constituted a greater portion of the diet than results indicate, due to more complete
digestibility of piscian skeltons.
The Bald Eagle (Haliaeetus leucocephalus) nests in
swamps of southcentral and southeastern
Louisiana. Portions of this habitat are being lost or
altered due to drainage, channelization conversion
of land to agriculture, and industrial development
(Yancey 1970). Loss of swamp habitat may harm
nesting eagles by reducing the availability or abun-
dance of prey. Support for this hypothesis is pro-
vided by McEwan (1977) who found that Bald
Eagles in Florida rely primarily on fish and wetland
birds for food. Foods of nesting Bald Eagles in
Louisiana have not been previously documented.
Study Area and Methods
Fieldwork was conducted in coastal southeastern and south-
central Louisiana, including Terrebone, Jefferson, St. Charles, St.
Tammany, and Assumption Parishes. Climate is subtropical
maritime. Wetlands of 0-2 m elevation predominate; relief is pro-
vided by levees and spoilbanks. Much of the region consists of
permanently or annually flooded baldcypress (! Taxodium distichum)
- tupelogum ( Nyssa aquatica) forests. Dominant land uses include
gas and oil production and industrial development, as well as
hunting, fishing and trapping. Area vegetation and other charac-
teristics are further described by Bahr et. al, (1983) and Chabreck
and Condrey (1979).
Bald Eagle nest locations were determined in 1977 and 1978 by
interviews with private citizens and by using helicopter surveys. In
June and July 1979, immediately following fledging of young and
seasonal departure of parents, prey remains were collected from 9
nests. Additional remains were collected in July from a nest after it
was downed by a hurricane. To ensure as much as possible that
prey remains were those left by 1979 nesters, we collected only
those remains on or near the nest surface immediately after eagles
vacated the nest, prior to possible nest use by other species.
Results
Prey species of nesting Bald Eagles were deter-
mined from remains found in 10 nests during the
summer of 1979. We collected remains of 243 ver-
tebrates, including 4 classes and 31 species (Table
1). Birds comprised the highest percentage of prey
animals (42.4%), followed by fish (41.5%), mam-
mals (15.7%), and a reptile (0.4%). American Coots
(Fulica americana) comprised 40 (47.6%), of the 103
birds, while freshwater catfish (Ictalurus sup.) ac-
counted for 53 (52%) of 101 fish. Muskrat (Ondatra
zihethicus) and Nutria( (Myocastor coypus) combined
comprised 82.2% of mammals, and the reptile re-
mains were those of a Mud Turtle (Kinosternon sub-
rubrum).
Discussion
Remains of 31 vertebrate prey species may sup-
port claims that Bald Eagles are opportunistic feed-
ers (Retfalvi 1970; Todd et. al. 1982; fielder 1982).
However, American Coots and catfish made up
nearly 42% of prey animals, indicating that a pre-
ference for these species may exist. Our findings
agree with those of McEwan (1977), who found that
American Coots and catfish comprised the major
portions of the diet of Bald Eagles in Florida. Fiel-
der (1982) reported that American Coots were the
major prey animal of Bald Eagles at a study site in
Washington, but concluded that availability of prey
dictated usage. Haywood and Ohmart (1986)
found in Arizona that, while catfish and other
benthic-feeding fish comprised the majority of
prey, American Coots were the major avian prey of
Bald Eagles. Benthic fish are common prey proba-
bly because of their high vulnerability to aerial pre-
dators (Todd et. al. 1982). Bald Eagle consumption
of benthic fish, American Coots, and dabbling
waterfowl makes obvious the importance of shallow
wetlands within foraging distance of nest sites. Be-
cause of this importance, proposals to alter such
wetlands should be carefully studied.
A bias toward nonfish prey species probably
exists in our study, because fish skeletal parts can be
more completely digested than those of other ver-
tebrates (Todd et. al. 1982). For instance, although
we observed over 20 Gizzard Shad (Dorosoma
cepedianum) brought to nests and consumed, the
remains of only 2 were recovered.
124
Raptor Research Vol. 20 (3/4); 124-127
Table 1. Species identified from remains collected from 10 Louisiana Bald Eagle nests after the 1978-1979 nesting season.
Fall/ Winter 1986
Foods of Bald Eagles
125
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126
Dugoni et. al.
Vol. 20, No. 3/4
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Fall/ Winter 1986
Foods of Bald Eagles
127
Acknowledgments
Contribution of the Louisiana Cooperative Fish and Wildlife
Research Unit; Louisiana State University, U.S. Fish and Wildlife
Service, Louisiana Department of Wildlife and Fisheries, and
Wildlife Management Institute, cooperating. We wish to thank Dr.
John V. Conner, Professor, Louisiana State University, and Dr.
Royal B. Suttkus, Director, Tulane University Museum of Natural
History, for help in identification of specimens. We thank Mr.
John D. Newsom, Leader, Louisiana Cooperative Wildlife Unit,
retired, for guidance during the early stages of this study, and
thank the U.S. Army Corps of Engineers for financial support.
Literature Cited
Bahr, L.M. Jr., R. Costanza, J.W. Day Jr., S.E. Bailey,
C. Neill, S. G. Leibowitz and J. Fruci. 1983.
Ecologic characterization of the Mississippi Deltaic
Plain region; a narrative with management recom-
mendations, FWS/OBS-82/69. 189pp.
Chabreck, R.H., and R.E. Condrey. 1979. Common
vascular plants of the Louisiana marsh. La. State Univ.
Center for Wetland Res. Sea Grant publ. LSU-T-79-
003. 116pp.
Fielder, P.C. 1982. Food habits of Bald Eagles along the
mid-Columbia River, Washington. Murrelet 63:46-50.
Haywood, D.D., and R.O.Ohmart. 1986. Utilization of
benthic-feeding fish by inland breeding Bald Eagles.
Condor 88:35-42.
McEwan, L.C. 1977. Nest site selection and the produc-
tivity of the Southern Bald Eagle. MlS. Thesis, Univ. of
Florida, Gainesville. 63pp.
Retfalvi, L.I. 1970. Food of nesting Bald Eagles on San
Juan Island, Washington. Condor 72:358-361. Todd,
C.S., L.S. Young, R.B. Todd, C.S., L.S. Young, R.B.
Owen, J.R., and F.J. Gramlich. 1982.
Yancey, R.K. 1970. Our vanishing delta hardwoods. La.
Conserv. 22:30-36.
Address of first and second authors: Louisiana Cooperative
Wildlife Research Unit, Louisiana State University, Baton
Rouge, Louisiana 70803. Address of third author: School of
Forstry, Wildlife and Fisheries, Louisiana State University, Bat-
ron Route, Louisiana 70803.
Received 1 February 1986; Accepted 31 October 1986.
MALE FOOD PROVISIONING AND FEMALE REPRODUCTION
IN AMERICAN KESTRELS
Timothy J. Coonan
While the effects of male raptor nest provisioning
on clutch quality have been documented (Drent
and Daan 1980; Wink et. al. 1980), the effect of
provisioning on later nest success is less well estab-
lished. Male provisioning ability should affect
hatching and fledging success, since the female and
young of many raptor species depend to a degree
on the male for food delivery until fledging (Bal-
gooyen 1976; Snyder and Wiley 1976; Newton
1978; Mueller et. al 1981; Rudolph 1982; and Vil-
lage 1983).
The purpose of this study is to document the
relationship between male provisioning perfor-
mance and pair reproductive success, beyond
clutch size, in the American Kestrel (Falco spar-
verius). Effects of differential male provisioning
performance should be seen in number of young
hatched and number of young fledged from each
nest.
MATERIALS AND METHODS
Six kestrel pairs in wooden nestboxes (Gary and Morris 1980)
were observed from the pre-hatching to fledging stage in the
Coconino National Forest near Flagstaff, Arizona, in June and
July 1982. The study area was primarily ecotonal within the pon-
derosa pine (Pinus ponderosa) forest of the Transition Life-zone
(Lowe 1964). Stands of ponderosa pines were interspersed with
more open areas of one-seed juniper (Juniperus monosperma ),
Gambel’s Oak ( Quercus gambelli), squawbush (Rhus trilobata), prickly,
pear cactus (Opuntia spp.), Parry rabbitbrush ( Chrysothamnus par-
ry i), and blue grama grass (Bouteloua gracilis ). Elevation in the study
area ranged from 2070 to 2160 m.
Observations were made with 7-15x binoculars or 20-60x spot-
ting scope, 100 to 200 m from each nest. Nests were observed in 2
to 8 h shifts between 0800 and 1800 h. Kestrels did not forage
appreciably before 0800 h, perhaps due to the inactivity of or-
thopterans, their principal prey.
Wind velocity can affect kestrel foraging strategies and success
(Rudolph 1982; Village 1983). My observations were confined to
relatively calm days, since windy days added too many variables to
adequately measure its effect on provisioning.
Two hundred fifty-nine male and female provisioning trips to
the nest were documented. Percent male and female provisioning
and male and female prey deliveries/hr were calculated for each
nest. Clutch size (determined just prior to hatching), number of
young hatched and number of young fledged were recorded for
each nest. Male and female prey deliveries/chick/hr were calcu-
lated for each nest.
Spearman’s rank correlation procedure (Zar 1974) was used to
analyze data. A significance level of 0.05 was used in all tests.
Results and Discussion
Males contributed an average of 44.32% of the
food deliveries to the nest from hatching to fledg-
ing. Individual males, however, varied in their
contributions relative to the female (Table 1). Per-
cent male provisioning ranged from 18.0 to 67.1%.
Male prey deliveries/hr ranged from 0.60 to 2.80
and was significantly correlated with percent male
provisioning (r § = 0.943, P < 0.025). Clutch size,
number of young hatched and number of young
fledged were each significantly correlated with
male prey deliveries/hr (r s = 0.843, P < 0.05; r s =
0.843, P < 0.05; r s - 0.929, P < 0.025).
Balgooyen (1976) found that the female pro-
vided 71.1% of the food deliveries to one nest dur-
ing the period after hatching when both male and
female hunt. Females in this study during the same
period provided an average of 54.02% of the prey
deliveries (n = 6), though there was high individual
variation in female provisioning. Female prey de-
liveries/hr ranged from 0.64 to 2.73 (Table 1).
Neither female prey deliveries/hr nor total prey
deliveries/hr correlated significantly with number
of young fledged (r s = 0.500, P > 0.10; r s =
0.014 P > 0.25). Spearman r g between total prey
deliveries/chick/hr and persent fledged was 0.629.
While not significant (P > 0.10), this suggests that
higher feeding rates result in higher chick survival.
Spearman’s r g between male and female prey
deliveries/hr was -0.657 (P = 0.10), suggestive of a
negative correlation. Males and females of indi-
vidual pairs may adjust their prey delivery rates
relative to their mate’s abilities.
The individual variation in male provisioning
correlated with several measures of nest reproduc-
tive success. Those males which delivered a greater
number of prey/hr appeared to realize a higher
immediate reproductive success of young hatched
and fledged.
Other factors could account for the observed re-
sults. First, data were insufficient to evaluate quality
of prey delivered by males or females. Some males
may have provided relatively more vertebrates to
their mates and broods than did others. The
128
Raptor Research Vol, 20 (3/4): 128-129
Fall/ Winter 1986
American Kestrels
129
Table 1. Male and female nest provisioning performance and female reproduction for six American Kestrel nests,
northern Arizona, 1982.
Percent
Provisioning Prey Deliveries/hr Prey Deliveries/chick/hr
Nest
Male
Female
Male
Female
Total
1
67.1
22.9
1.31
0.64
1.95
2
52.4
47.6
0.85
0.77
1.62
3
18.0
82.0
0.60
2.73
3.33
4
25.0
75.0
0.71
2.13
2.84
5
43.8
56.2
0.82
1.05
1.87
6
59.6
40.4
2.80
1.90
4.70
energetic advantage this could confer is substantial;
Bird et. al. (1982) estimated 1 vole contained about
125 times more energy than 1 grasshopper.
Second, clutch size and brood size were each cor-
related with male prey deliveries/hr. Perhaps males
adjusted their respective rates of prey delivery ac-
cording to the brood size for which they had to
provide. Testing for such a functional response
would require comparing male delivery rates be-
fore and after hatching. Data were insufficient to
evaluate this possibility. However, male prey de-
liveries/chick/hr was not correlated with brood size
(r s -0.129, P > 0.50), indicating that male prey
delivery rates to individual chicks were the same for
large and small broods. This suggests that males
respond functionally to larger broods by providing
prey items at a higher rate. In contrast, female prey
deliveries/hr was not significantly correlated with
brood size (r s = 0.554, P > 0.10), indicating that
females do not respond functionally to larger
broods by increasing their rates of prey delivery.
The variability in male and female prey delivery
rates could have resulted from other factors, such
as age or experience of the bird (see Newton 1979)
or territory quality (Newton 1976; Rudloph 1982).
A final possibility is that smaller size and lower
wing-loading of some males provided them with
greater energetic efficiency (Balgooyen 1976; von
Schantz and Nilsson 1981). These factors were not
measured.
Literature Cited
Balgooyen, T.G. 1976. Behavior and ecology of the
American Kestrel (Falco sparverius L.) Univ. Calif. Publ.
Zool. 103:1-83.
Bird. D.M., S. Ho and D. Pare. 1982 Nutritive values of
three common prey items of the American Kestrel.
Comp. Biochem. Physiol. 73A: 513-515.
Male
Female
Total
Clutch
Size
Young
Hatched
Young
Fledged
0.262
0.128
0.390
5
5
4
0.170
0.154
0.324
5
5
3
0.200
0.910
1.110
4
3
2
0.355
1.065
1.420
3
2
2
0.273
0.350
0.623
4
3
2
0.560
0.380
0.940
5
5
4
Drent, R.H., and S. Dann. 1980. The prudent parent:
energetic adjustments in avian breeding. 2Ardea 68:
225-252.
Gary, H.C., and M.J. Morris. 1980. Constructing
wooden boxes for cavity-nesting birds. Research Note
RM-381, USD A Forest Service.
Lowe, C.H. 1964. Arizona’s natural environment. Tus-
con; Univ. of Ariz. Press.
Mueller, H.C., N.S. Mueller, and P.G. Parker.
1981. Observation of a brood of Sharp-shinned
Hawks in Ontario, with comments on the functions of
sexual dimorphism. Wilson Bull. 93: 85-92.
Newton, I. 1976. Breeding of sparrowhawks in diffe-
rent environments. /. Anim. Ecol. 45: 831-849.
1978. Feeding and development of Spar-
rowhawk Accipiter nisus nestlings. J. Zool. Lond. 184:
465-487.
. 1979. Population ecology of raptors. Ver-
million, South Dakota; Buteo Books.
Rudolph, S.G. 1982. Foraging strategies of American
Kestrels during breeding. Ecology 63: 1268-1276.
Schantz, T. von, and I. N. Nilsson. 1981. The reversed
size dimorphism in birds of prey: a new hypothesis.
Oikos 36: 129-132.
Snyder, N.F.R., and J.W. Wiley. 1976. Sexual size di-
morphism in hawks and owls of North America. Or-
nithol. Monogr. 20: 1-96.
Village, A. 1983. Seasonal changes in the hunting be-
havior of Kestrels. Ardea 71: 117-124.
Wink, M., C. Wink, and D. Ristow. 1980. Biology of
Eleanora’s Flacon (Falco eleonorae). 8. Clutch size in
relation to hunting success and weight of the parent
falcons./. Ornithol. 121: 387-390.
Zar, J. 1974. Biostatistical analysis. Englewood Cliffs,
N.J.; Prentice-Hall, Inc.
Department of Biology, Northern Arizona University,
Flagstaff, AZ 86011. Present addresss: San Antonio Missions
National Historical Park, 2202 Roosevelt Ave., San Antonio, TX
78210.
Received 8 October 1985; Accepted November 1986.
130
Short Communications
Vol. 20, No. 3/4
Short Communications
Success Rates of the Peregrine Falcon ( Falco peregrinus)
Hunting Dunlin ( Calidris alpina) During Winter
Joseph B. Buchanan, Steven G. Herman and Tod M. Johnson
The Peregrine Falcon ( Falco peregrinus)
utilizes a wide variety of prey types (Ratcliffe
1979; Cade 1982) throughout its nearly cos-
mopolitan distribution. In addition, it also
exhibits a huge range (7-83%) of success rates
for hunting flights (see Roalkvam 1985 for
review). Success rates of hunting flights dur-
ing winter periods have been reported by
Lindberg (1975), Clunie (1976) and Roalkvam
(1985); however, data on success rates for
specific prey species or prey type are lacking.
Here we present data for rates of success when
Peregrines hunted Dunlin ( Calidris alpina )
during the winter (December-March).
During the winters of 1979-1981, while
studying the ecology of the Dunlin in western
Washington, we observed hunting flights di-
rected at this species by peregrines. Hunting
flights were observed at the Samish River De-
lta, in northern Puget Sound, and at Bower-
man Basin and 2 other estuarine sites located
in Grays Harbor on the outer coast. Subadult
and adult male and female Peregrines of 2
subspecies, F. p . pealei and F. p . anatum, were
observed hunting Dunlins. A description of
behavioral interactions between Dunlins and
their falcon predators [Peregrines and Mer-
lins (F. columbarius)] will be presented
elsewhere.
We define a hunting flight as a perch-to-
perch flight involving one or more capture
attempts at suitable prey. A capture attempt is
defined as an individual effort to capture a
specific individual during a hunting flight.
We observed 17 hunting flights directed at
Dunlins, 15 of which had known outcomes.
Peregrines were successful at capturing Dun-
lins from a stoop and when pursuing indi-
viduals in a low direct chase. Stoops were used
in 11 (65%) hunting flights. In 2 flights these
were high stoops, originating from heights of
1000 m or more. Other stoops originated from
50-80 m. Feint stoops were observed only
twice. Of 47 capture attempts, 33 (70%) were
stoops at flocks, 6 (13%) were low chases of
single Dunlins and 8 (17%) were horizontal
attacks of flocks.
The success rate for hunting flights was 47%
(n= 7) while the rate for capture attempts was
14.6%. Five flights (33%) were successful on
the first capture attempt. A stoop was used in 3
of these hunts, and low chase after a single
Dunlin in the other 2. Seventy-one percent of
the successful hunts were successful on the
first capture attempt. All but one successful
hunt involved in-flight prey capture.
The success rate (47%) which we observed
was significantly greater than the winter rate
of 9.6% reported by Clunie (1976) (x^ = 7.9,
df = 1; 0.001 < P < 0.005) or the 13.7%
reported by both Lindberg (1975) and
Roalkvam (1985) (x 2 = 1 1.6; df = 2; 0.001 <P
< 0.005). Dekker (1980) presents data for suc-
cess rates in regard to fall hunting flights di-
rected at shorebirds (9.0%), although his data
obviously refer to migrant falcons.
The high success rate of hunting flights
which we observed, relative to these studies,
might be explained in two ways. First, if a
falcon is unfamiliar with a site that it seldom
visits (e.g., during migration) success rates for
hunting flights might be expected to be lower
than for falcons which maintain winter ter-
ritories. Secondly, the high success rate we
observed was related to the success rate of the
initial capture attempt, which at 7 1 % for sue-
Fall/ Winter 1986
Short Communications
131
cessful hunts means that few falcons con-
tinued to hunt after the first attempt. This is
not surprising since a certain trade-off must
exist between reasonable energy expenditure
and profitable caloric intake.
Cade (1982) states that Peregrines will not
stoop into a massed flock of birds. We found,
however, that a stoop into a compact flock was
the most common technique used by Pereg-
rines hunting Dunlins in western Washington.
The rarity of the high stoop may be related to
the apparent ease with which Peregrines cap-
ture Dunlins using other techniques. Also,
such an effort may be energetically inefficient
considering the probable low caloric value of a
small species such as the Dunlin.
Acknowledgments
We thank L, A. Brennan, A. M. Cahall, M. A. Finger, and C. T.
Schick for their assistance in the field. C. M. Anderson contributed
in many ways and provided valuable discussion on the topic of
definitions for hunting flights and capture attempts. Funding
during the winter of 1980-81 was provided by NSF-SOS Grant
SPI80-04760. The manuscript was improved by comments from
D. A. Boyce, S. K. Sherrod and C. M. White.
Literature Cited
Cade, T.J. 1982. The Falcons of the World. Cornell
University Press, Ithaca, New York.
Clunie, F. 1976. A Fiji Peregrine (Falco peregrinus) in an
urban-maritime environment. Notornis 23:8-28.
Dekker, D. 1980. Hunting success rates, foraging habits,
and prey selection of Peregrine Falcons migrating
through central Alberta. Can. Field-Nat. 94: 371-382.
Lindberg, P. 1975. Pilgrimsfalken i Sverige. Svenska
Naturskyddsforeningen. Stockholm, Sweden.
Ratcliffe, D. 1980. The Peregrine Falcon. Buteo Books.
Vermillion, South Dakota.
Roalkvam, R. 1985. How effective are hunting Pereg-
rines? Raptor Res. 19:27-29.
1409 W. 7th Avenue, Olympia, Washington 98501. Address of
second author: The Evergreen State College, Olympia,
Washington 98505. Address of third author: 12508 S.E. 49th
Avenue, Bellevue, Washington 98806.
Received 7 April 1986; Accepted 25 July 1986.
Golden Eagle Capture of an American Coot
Daniel J. Severson
Previous food habit studies of the Golden
Eagle (Aquila chrysaetos) list the American Coot
(Fulica americana) as a prey item (Dixon, Con-
dor 39:49-56, 1937; Olendorff, Am. Midi. Nat.
95:231-236, 1976; Bloom and Hawks, Rap tor
Res. 16:110-115, 1982; Marr and Knight,
Murrelet 64:73-77, 1983). Dixon (1937) re-
ported several observations of Golden Eagles
capturing coots. The eagle positioned itself
between the lake and the coots, using natural
barriers to conceal its approach, and then
quickly attacked its prey. Dixon believed that
this was a learned method that was often used.
On 14 April 1982 at ca 0900 H near
Malheur Lake in southeastern Oregon, ac-
companied by M. Rule, I observed an esti-
mated 500 coots feeding in a partially flooded
alfalfa field bordering Malheur Lake. I also
observed a Golden Eagle flying directly to-
ward the coots at a height of 20 m. The coots
began running toward the lake when the eagle
was within ca 200 m. The eagle flew over the
coots, turned into the wind and briefly
hovered, holding its position as they passed
underneath. The eagle made 2 unsuccessful
dives at the coots before capturing one on the
132
Short Communications
Vol. 20, No. 3/4)
third attempt starting from a height of ca 2 m.
The eagle then stood on the coot for about 30
sec before flying north for 50 m and accidently
dropping its prey in the water, where the coot
was observed splashing in the water. The eagle
immediately flew down, picked up the coot
and flew to a fence post 50 m away and 3 min
later began feeding.
Collopy (Auk 100:747-749, 1983) found that
Golden Eagles typically orient into the wind,
presumably to reduce flight speed and to at-
tack prey from close quarters. The same
technique apparently was used by the eagle in
this observation.
I thank M. Collopy, D. Garcelon and J.
Houk for reviewing and commenting on the
manuscript.
Malheur National Wildlife Refuge, P.O, Box 113, Burns, Ore.
97720. Present address: San Luis National Wildlife Refuge, P.O.
Box 2176, Los Banos, Calif. 93635
Received 20 February 1986; Accepted 15 June 1986
Bilateral Bumblefoot in a Wild Red-Tailed Hawk
Kevin L. Ellis
Bumblefoot is the falconer’s term for any
abnormal enlargement of a raptor’s foot or a
portion thereof. It is the most common clinical
condition associated with captive raptors
(Riddle, K., Recent Advances in the Study of
Raptor Diseases, London, 1980). Raptors with
high wing-loading, such as the larger fal-
conids, seem more prone to the condition than
other hawks with lower wing-loading (Hal-
liwell, W Zoo. Anim. Med . 6 (4): 8-10 1975).
Bumblefoot in captive raptors is most often
management related; usually caused by im-
proper perches (Riddle, 1980).
Bumblefoot often starts when the integu-
ment of the metatarsal pad is injured and in-
flammation results. Inflammed areas then
may become infected resulting in formation of
an abscess. Continued trauma to the infected
area may cause the fissure of abscess material
into surrounding uninfected areas. The dis-
ease will often continue to spread until much
of the foot is infected. In many cases
bumblefoot is followed by osteomyelitis
and/or septicemia, endocaritis, and finally de-
ath (Riddle, 1980). Bumblefoot can also be
directly caused if the foot is punctured by a
talon or other sharp object. Bacterial agents
associated with bumblefoot as secondary
agents include Staphylococcus spp .,Escheria coli.
Streptococcus spp. and Clostridium spp. Fungi
have also been associated with bumblefoot in-
Figure 1 ; Swollen metatarsal pad afid digits on both feet.
Fall/Winter 1986
Short Communications
133
fections (Cooper, J.E., Veterinary Aspects of
Captive Birds of Prey, Standfast Press, Eng-
land, 1978).
The condition may be unilateral or bilateral
and in less severe cases can heel without clini-
cal treatment if the underlying cause is re-
moved. If the lesion becomes infected, how-
ever, surgical treatment is often necessary,
and the success rate may be very high (Riddle,
1980).
To my knowledge no case of bumblefoot in
a wild raptor is documented in the literature,
however, it has been observed. Brian Cade
and Clayton White (pers. comm.) banded a
female nestling Peregrine Falcon (Falco pereg-
rinus) on the Colville River, Alaska, in July that
had severe bumblefoot. It was trapped on
Holly Beach, Louisiana, in October of the
same year. The trapper made no mention of
its swollen feet at that time. Pat Redig (pers.
comm.) has seen bumblefoot in wild Bald
Eagles ( Hallaeetus leucocephalus) 1 Prairie Fal-
con ( Falco mexicanus), 1 Rough-legged Hawk
( Buteo lagopus), and Red-tailed Hawks (. Buteo
jamaicensis). In all cases the birds had an injury
to the other leg; usually a missing foot from a
trap. The purpose of this note is to report
bilateral bumblefoot in a wild Red-tailed
Hawk.
On 10 March 1985 I received a report of an
injured hawk along the Pine River (La Plata
County, Colorado). Upon arriving to the area,
I located an adult Red-tailed Hawk which was
barely able to fly. Upon capturing the bird, I
noted that the metatarsal pad, as well as all
digits, of the bird’s feet were severely swollen
(Figure 1). The right foot had a scabbed over
puncture wound above and between digits III
and IV. The hawk was severly emaciated and
could not move the digits of either foot. Im-
mediately after euthanasia, I made an incision
into the metatarsal pad of the right foot. A
large amount of casious exudate was located
around the tendon. Culturing of this material
yielded E. Coli. Although not known for sure,
it is probable that the infection entered
through the aforementioned puncture wound
and spread to other areas, becoming so acute
that the bird was unable to catch prey items
and thus its physical condition deteriorated.
Acknowledgements
I wish to thank Clayton White and Pat Redig for their
comments on the manuscript. Linda Eskridge and Dixie
Simmons also provided assistance.
Colorado Division of Wildlife, 151 E. 16th St., Durango,
CO. 81301
Received 15 April 1986; Accepted 1 September 1986.
News and Reviews
Report — 1st International Symposium on the Golden Eagle. On 14-15June 1986, the 1st International Symposium
on the Golden Eagle was held in Brunissard, French Alps. It was organized by The Alpine Research Centre for
Vertebrates, the Queyras Regional Nature Park, the Ecrins National Park, and the Mercantour National Park. The
organizing committee was chaired by Samuel Michel from the Alpine Research Centre for Vertebrates.
In France, the concern about the Golden Eagle has lasted since several years ago, and in 1981, during a meeting in
Montpellier, an Interregional Working Group was formed, with Roger Mathieu as a chairman. The Group accepted a
common research programme, adopted standardized methods and terminology, and established contacts between
Golden Eagle specialists from neighbouring countries- — Italy, Spain and Switzerland.
The aim of this year’s meeting was to discuss the present situation of the Golden Eagle in Europe and to widen the
contacts within European countries. About 200 people arrived in Brunissard. France was represented most numer-
ously, and a fairly large group arrived from Italy. There were also representives from Great Britain, Yugoslavia,
Poland, Spain and Switzerland. The audience listened to 26 lectures, 14 of which were from France, 3 from Italy, 2 each
from Great Britain and Switzerland, and 1 each from Yugoslavia, Poland and Spain. Additionally, 2 communications
were read, having been sent from Austria and Norway. The organizers plan to publish the proceedings in the languages
in which the papers were presented during symposium (4 in English, the rest in French). Anyone interested in this
publication should contact Samuel Michel, Le Coin, 05390 Molines en Queyras, France.
134
News and Reviews
Vol. 20, No. 3/4
The symposium was divided into 3 sessions. The first session “Status of Golden Eagle in Europe” lasted the entire first
day and was chaired by Roger Mathieu in the morning and by Paolo Fasce in the afternoon. During this session 16
papers were presented. Four of them discussed the status of the eagles in different countries (Britain — Roy Dennis;
Italy — Paolo Fasce; Poland — Wojtek Krol; Switzerland — Heinrich Haller), and other concerned smaller administrative
or geographical units (e.g. Macedonia — Bratislav Grubac; the Ecrins National Park — Christian Couloumy;
Pyrenees — Michel Clout; Sicily — Salvatore Seminara). In most of the papers the authors gave breeding numbersd and
density of the eagles in a given area, data on breeding results, nest site selection, food habits, numbers trends and threats
for the species.
In the morning of 15 June, there was a session “Biology of the Golden Eagle,” chaired byJean-Francois Terrasse. The
first speaker, Roger Mathieu, described “Relationships between age, plumage, behaviour and sexual maturity in the
Golden Eagle.” In the second lecture Jeff Watson discussed “Land use changes in the Highlands of Scotland and their
effects on the Golden Eagle population.” Two papers in this session showed the results of investigation on food habits of
Golden Eagles — in Spain (Fernandez Leon Carmelo) and in France (Rick Huboux). One lecture concerned breeding
biology of the eagle in the Appenines (Bernardo Ragni et al.), and in the last paper Daniel Simeon and Michel Belaud
described habitat use by eagles in the sample area of Southern Prealps in France.
In the afternoon Michel Clouet was the chairman. The first speaker, Michel Gillone, a French ethno-ornithologist,
gave a lecture “The Golden Eagle in Mexico: a rare and worshipped bird,” followed by a session entitled “Management
of the Golden Eagle” with 4 papers. Rick Huboux commented on breeding results of Golden Eagles in France in
1964-1984, Jean-Marc Cugnasse described “Management techniques of the Golden Eagle during breeding season,”
Christian Couloumy talked on “The programme of research on Golden Eagles in the Ecrins National Park, and finally
Rick Huboux described “Methods of numbers estimation of Golden Eagles in the Mercantour National Park.”
The last point of the programme was the general discussion. It was initiated by Michael Clouet, who pointed out that
there is quite a lot of information on eagles breeding in western Europe, while little is known about birds living in the
Balcan Peninsula, which feed principally on turtles (Testudo sp.), and not on marmots and lagomorphs like their
conspecifics in western Europe. French ornithologists will encourage their colleagues from Yugoslavia and Greece to
begin Golden Eagle research by giving them field equipment and methodological advice. As a most important task for
the territory of France, Michel Clouet discussed research on the survival and dispersion of young eagles, which can be
studied by means of marking birds with wing-tags (this method is already used in Scotland). It turned out however, that
for ethical and aesthetical reasons this method has many opponents in France. Among ornithologists present at the
symposium, the strongest opposition came from Jean-Francois Terrasse and Michel Gillome. Other research methods
were then discussed — telemetry, which is however unpractical in the mountains, and the Argos system (reading
transmitter signals by satellite) which is very costly. Little by little the subject of the discussion was changed, and no
decision was taken on the matter of wing-tag marking.
The data provided during the symposium show that the situation of the Golden Eagle in Europe is good. Its numbers
are estimated to be 3250-3500 pairs (excluding USSR): 450-500 pairs in Scotland; 400 pairs each in Italy, Norway,
Sweden and Spain; 200 pairs each in France and Greece; 100-150 pairs each in Bulgaria and Finland; and Albania,
Austria, Czechoslovakia, Yugoslavia, Poland, Rumania and West Germany each have <100 pairs. In some countries an
increase of breeding population has been found (e.g. Norway, Scotland, Switzerland) and in others. As a result of recent
inventories, many more eagles were discovered than was previously suspected. — Wojtek Krol, 13, rue Daubenton,
75005 Paris, FRANCE.
The Grouse and the Goshawk as Prey and Predator. Airing Thursday, 26 February 1987, 8:00 PM EST on PBS (check
local listing). A film by Marty Stouffer Productions, Ltd., Aspen, Colorado, as part of the Wild America series, which
begins its fifth season in 1987. A half-hour program filmed in the Cascades of Washington and Oregon and hosted by
Marty Stouffer.
L’Aquila Reale in Italia: Ecologia e Conservazione. By Paola and Laura Fasce. Lega Italians Protezione Uccelli, Serie Scientifica,
1984: 66 pp., 15 figs., 14 tables, numerous black and white plates and line drawings, summaries in Italian, English and French, Lira
10,000 excl. of postage. — This work on the Golden Eagle in Italy has chapters entitled, Introduction, Materials and Methods, Biology of
the Species, Status of the Species in Italy, Population Dynamics and Problems of Conservation. The first part of the book is on the general
biology of the species and the second part contains technical information based on 1 1 years data collected by thirteen collaborators in the
western Italian Alps and northern Apennines. There are estimated to be 183 pairs in the former area (except the Sesiaand Ossola valleys)
and between 329 and 389 in the latter area. Although I have seen few major works on raptors from Italy this one appears impressive for
its thoroughness. — Richard J. Clark.
Fall/ Winter 1986
News and Reviews
135
HOME FREE: Return of the Bald Eagle. A film produced by the Massachusettes Audubon Society, copyright 1984. Length 28 min.,
color, sound. Available from The New Film Company, Inc., 7 Mystic Street, Suite 118, Arlington, Massachusettes 02174, U.S.A. Price:
$450.00 U.S. on 16 mm, $400.00 U.S. on video; $5.00 U.S. shipping. Video available in W and %" VHS. Review copy received in W
VHS format. Rental rates for free admission showings are $50.00 for one day and $15.00/day for each additional day for 16 mm rental;
$35.00 for three days and $10.00/day for each additional for video. Special rates are available for fundraising and other paid admission
showings.
The apparent decline of the Bald Eagle in parts of North America
has been of concern to conservationists and wildlife biologists for
the past several decades, and the species is still listed as either
threatened or endangered in the contiguous United States. The
film presents a brief story of the Bald Eagle’s decline and attempts
to reintroduce the species in the area of Quabbin Reservoir in
western Massachusettes.
The film begins with an aerial view of Quabbin Reservoir set to
music with narration by Jack Swedberg. Besides the Bald Eagle,
Swedberg is the main character of the film. Additional narration
by Joyce Zinno describes Swedberg’s hobby and later profession as
a wildlife photographer which eventually leads him to the Bald
Eagle. Swedberg was a nature photographer for some 20 years
before eventually landing a job as wildlife photographer for the
Massachusettes Division of fisheries and wildlife. Swedberg began
photographing Bald Eagles during wintering periods, primarily
to document their presence in the Quabbin Reservoir area, the
first part of the film tells Swedberg’s story and contains footage of
Swedberg preparing to photograph eagles from a blind.
The film then goes into an encapsulated version of the overall
decline of the Bald Eagle, with Swedberg’s particular thoughts as
to why the birds ceased to breed in western Massachusettes. Dur-
ing this sequence, the film uses black and white still photographs
of hunter-killed eagles, of nests, eggs, and young, a close-up shot
of a cracked egg in a nest containing one young, and additional
narration by Zinno to describe man’s impact upon the species.
According to the film Swedberg considers loss of habitat to be the
primary reason for the extirpation of breeding Bald Eagles in
western Massachusettes, but unfortunately this is not expanded
upon. Instead, the DDT issue receives a considerable portion of
the discussion. The film then shifts back to Swedberg in his blind,
and this part of the film ends with some nice footage set to music,
although brief, of interractions around a deer carcass between an
adult Bald Eagle and a two-year-old immature. The introduction,
DDT sequence and photography sequence make up the first 7 min
of the film.
The remainder of the film is devoted to documenting the rein-
troduction of the Bald Eagle into the Quabbin Reservoir area. The
initial part of the sequence briefly depicts construction of a hack
tower and nest, with narration by Swedberg and Zinno which
includes a brief explanation of hacking procedures. Credit for
financial support of the project is given to the Massachusettes
Audubon Society and the Bank of Boston. The sequence gives a
good impression of overall size of a Bald Eagle hack tower, and a
good idea of the man-power involved in order to raise such a
structure.
Next, Swedberg and members of the U.S. Fish and Wildlife
Service are shown preparing for a trip to Canada to take young
wild eagles for reintroduction at Quabbin Reservoir. As the flight
departs the Quabbin area, there is a brief aerial sequence of the
terrain and surrounding regions. Next, the film shifts to a Bald
Eagle nest in the Province of Manitoba. The sequence on the
ascent of the nest tree is complete with narration of the climber’s
labored breath, which will bring back memories to anyone who has
evrr used tree climbers to scale a tree to a raptor nest. A young
eaglet is removed from the nest and lowered to Swedberg waiting
at ground level. Some close fottage of young eagles, both in the
nest and while being handled on the ground, is shown. The sequ-
ence ends with brief aerial footage of a portion of the return flight,
and a final landing on Quabbin Reservoir.
After the arrival of the young eagles, the film introduces David
Nelson, a Wildlife Biology graduate student at the University of
Massachusettes. Nelson is the hack-site attendant, and several
aspects of behavior, such as preening, stretching, (mantling), wing
exercise, etc., are shown as Nelson narrates from his notes on each
individual bird. A portion of the tower enclosure was furnished
with one-way glass in order that photographs and observations
could be made while the young eagles were growing and develop-
ing. Next, there is a brief discussion by Swedberg and Nelson
concerning preparations for the release of the young eagles to
include a change in their diet. Up to this point, only Fish are
mentioned as being part of the diet of the Bald Eagle.
The film then shifts to close-up fottage of the young eagles
being examined and affixed with patagial markers and radio
transmitters. The radio transmitters are shown being attached
(sewn) to the central rectrices by an unintroduced individual who
narrates part of the sequence. Another individual comments upon
the health of the young eagles, including a brief discussion on the
physiology of the avian eye. The discussion ends with a comment
on the eagle being the “champion visual animal on the earth.” The
footage of the hack tower and preparation for release of the young
eagles lasts approximately 17 min.
The remaining 4 min of the film documents the release of the
young eagles from the hack tower set to music. The eagles’ enclo-
sure on the hack tower is slowly opened, the young eagles step
outside, and finally take flight over Quabbin Reservoir. Following,
there is discussion by Swedberg and Nelson concerning the success
of the release operation, and a brief shot of Nelson monitoring the
eagles’ movements with a receiving unit for the radio transmitter.
The film ends with a short narration by Swedberg and more aerial
footage of Quabbin Reservoir. The credits list numerous other
individuals involved with the production of the film, including
Media Music and Sound for the musical background.
A brochure which accompanied the film contained several black
and white photographs. The brochure also contained some very
favorable comments on the film by the Editor of the National
136
News and Reviews
Vol. 20, No. 3/4
Geographic Society, the President of the National Wildlife Feder-
ation, and CBS News. I agree in part with those comments, in that
the photography is excellent, the film does hold your attention,
and it does tell an environmentally important story. The dedica-
tion of Jack Swedberg to returning the Bald Eagle to Quabbin
reservoir as a breeding bird cannot be denied after viewing the
film. There are portions of the film, however, which require
additional comment.
It is unfortunate that the issue of habitat alteration in western
Massachusetts was not dealt with more intensively. Instead of the
recurring discussion of DDT’s impact, educational though it may
be, it would have been equally educational and interesting to
discuss “before and aftger” documentation of nesting locations
within the Bald Eagle’s former range in Massachusettes.
At one point in the discussion of the eagle’s extirpation in
Massachusettes, Swedberg makes the statement that “at the time,
no one thought of the idea of hacking eagles.” Although an in-
formed viewer would not support such a statement, the unin-
formed viewer possibly would. Perhaps Swedberg was the first
person to think of such an idea in Massachusettes, as was probably
the statement’s purpose, but the reintroduction of the Bald Eagle
into areas elsewhere in the United States has been carried out for
several years by numerous organizations and individuals.
Nevertheless, the viewer is left with the impression that the film is
documenting somewhat of a first. This would be true only on a
local level.
Another point of concern is Swedberg’s statement that “the
eagle is primarily a fish-eater.” It has been well documented that
the Bald Eagle’s diet can be much more varied, and in some
wintering areas the diet may consist primarily of mammalian or
avian prey, and carrion. Again, such statements might be mis-
leading to an uninformed viewer learning about the Bald Eagle
for the first time. General statements concerning the biology of the
species should have been carefully edited.
The cost of the film is not warranted from a scientific point of
view. However, I would recommend the film on a junior high
school or high school level as an introduction to man’s efforts to
return raptors to former nesting grounds. Although not the qual-
ity of the National Audubon Society’s 1981 film “Last Stronghold
of the Eagles,” HOME FREE provides an educational and infor-
mative documentary of the effort that is required in attempting to
restore the Bald Eagle into its former nesting localities. Although
most raptor biologists are well familiar with such undertakings,
the general public is not. Education about raptors should be a
primary area of concern as we close out the 1980s and move into
the 1990s. Films such as HOME FREE are important as a means of
educating the public at an early age, an education that hopefully
will be remembered later in life. — Jimmie R. Parrish.
Dissertation Abstracts
The Feeding Ecology and Breeding Biology of a
Cape Vulture Colony in the Southwestern Cape Province
A dissertation submitted to the Faculty of Science,
University of Witwatersrand, Johannesburg,
for the degree of Master of Science.
Johannesburg 1983
Cape Vultures (Gyps coprotheres) at Potberg obtain their food from stock farms, within a limited area surrounding the colony. The size
of the foraging range was determined by means of a postal survey, and the quantity of food available within it was estimated to exceed the
colony’s requirements. Data pertaining to daily feeding forays of individuals, monthly foraging patterns and the growth of nestlings
indicated no seasonal shortages in the amount of food obtained.
One complete breeding cycle and another two post- fledging dependence periods were observed during 165 days. Results concerning
deferred maturity, frequency and success of breeding, a sex-linked difference in behaviour, nestling parasites, behaviour of dependent
juveniles, aggressive terminations of the post-fledging period and survival of marked individuals, were obtained. It is suggested that the
transition to feeding exclusively on sheep carcasses has not been achieved with equal success by all age groups. — Robertson, Alistair
Stuart, 1983. M.S. thesis, Faculty of Science, University of the Witwatersrand, Johannesburg, South Africa.
DEDICATION — William Ruttledge wishes to dedicate in retrospect his paper entitled “Captive Breeding of the
European Merlin (Falco columbarius aesalon), which appeared in Volume 19(2/3), 1985, pp. 68-78 of Raptor Research, to
the memory of his wife, Dorothy, and in recognition of the very great contribution that she made to the preparation of
the manuscript which, sadly, she did not live to see in print.
Mr. Ruttledge takes the opportunity also to give more adequate recognition to the very extensive and invaluable
advice given throughout his work by Dr. L. H. Hurrell from his wide experience in captive breeding, including the
preparation of MK I and MK II mice (see page 70 of the article).
RAPTOR RESEARCH
A Quarterly Publication of The Raptor Research Foundation, Inc.
EDITOR: Clayton M. White, Department of Zoology, 161 Widtsoe Building, Brigham Young University, Provo,
Utah 84602
ASSISTANT EDITOR: Jimmie R. Parrish, Department of Zoology, 159 Widtsoe Building, Brigham Young Univer-
sity, Provo, Utah 84602
ASSOCIATE EDITORS
Jeffrey L. Lincer - Environmental Chemistry and Toxicology
Richard Clark - Order Strigiformes
Ed Henckel - Family Cathartidae
Gary E. Duke - Anatomy and Physiology
Patrick T. Redig - Pathology, Rehabilitation and Reintroduction
Jim Mosher - General Ecology and Habitat Analysis
INTERNATIONAL CORRESPONDENT: Richard Clark, York College of Pennsylvania, Country Club Road,
York, Pennsylvania 17405
Raptor Research (ISSN 0099-9059) welcomes original manuscripts dealing with all aspects of general ecology, natural
history, management and conservation of diurnal and nocturnal predatory birds. Send all manuscripts for considera-
tion and books for review to the Editor. Contributions are welcomed from throughout the world, but must be written in
English.
INSTRUCTIONS FOR CONTRIBUTORS: Submit a typewritten original and two copies of text, tables, figures and
other pertinent material to the Editor. Two original copies of photographic illustrations are required. Raptor Research is
published in a double-column format and authors should design tables and figures accordingly. All submissions must
be typewritten double-spaced on one side of 8 1 /? x 1 1 -inch (21 14 x 28 cm) good quality, bond paper. Number pages
through the Literature Cited section. The cover page should contain the full title and a shortened version of the title (not
to exceed 30 characters in length) to be used as a running head. Author addresses are listed at the end of the Literature
Cited section. Authors should indicate if present addresses are different from addresses at the time the research was
conducted. When more than one author is listed, please indicate who should be contacted for necessary corrections and
proof review. Provide an abstract for each manuscript more than 4 double-spaced typewritten pages in length. Abstracts
are submitted as a separate section from the main body of the manuscript and should not exceed 5% of the length of the
manuscript. Acknowledgments, when appropriate, should immediately follow the text and precede the Literature
Cited. Both scientific and common names of all organisms are always given where first appearing in the text and should
conform to the current checklists, or equivalent references, such as the A.O.U. Checklist of North American Birds (6th
ed., 1983). Authors should ensure that all text citations are listed and checked for accuracy. If five or fewer citations
appear in the text, place the complete citation in the text, following these examples: (Brown and Amadon, Eagles,
Hawks and Falcons of the World. McGraw-Hill, New York. 1968), or Nelson {Raptor Res. 16(4):99, 1982). If more than
five citations are referenced, each should include author and year (e.g., Galushin 1981), or in a citation with three or
more authors, the first author and year (e.g., (Bruce et al. 1982)). Citations of two or more works on the same topic
should appear in the text in chronological order (e.g., (Jones 1977, Johnson 1979 and Wilson 1980)). Unpublished
material cited in the text as “pers. comm.,” etc., should give the full name of the authority, but must not be listed in the
Literature Cited section. If in doubt as to the correct form for a particular citation, it should be spelled out for the Editor
to abbreviate.
Metric units should be used in all measurements. Abbreviations should conform with the Council of Biology Editors
(CBE) Style Manual, 4th ed. Use the 24-hour clock (e.g., 0830 and 2030) and “continental” dating (e.g., 1 January 1984).
Tables should not duplicate material in either the text or illustrations. Tables are typewritten, double-spaced
throughout, including title and column headings, should be separate from the text and be assigned consecutive Arabic
numerals. Each table must contain a short, complete heading. Footnotes to tables should be concise and typed in
lower-case letters. Illustrations (including coordinate labels) should be on 814 x 1 1 -inch (21 14 x 28 cm) paper and must
be submitted flat. Copies accompanying the original should be good quality reproductions. The name of the author(s)
and figure number should be penciled on the back of each illustration. All illustrations are numbered consecutively
using Arabic numerals. Include all illustration legends together, typewritten double-spaced, on a single page whenever
possible. Line illustrations (i.e., maps, graphs, drawings) should be accomplished using undiluted india ink and
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requires that prior arrangements be made with the Editor and the Treasurer.
A more detailed set of instructions for contributors appeared in Raptor Research , Vol. 18, No. 1, Spring 1984, and is
available from the Editor.
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