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A Communal Roosting of the Great Horned Owl (Bubo
virginianus)
Author(s): Bryce RobinsonCaitlin Davis
Source: Journal of Raptor Research, 48(1):88-89.
Published By: The Raptor Research Foundation
DOI: http://dx.doi.org/10.3356/JRR-13-47.1
URL: http://www.bioone.org/doi/full/10.3356/JRR-13-47.1
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tie a square knot with the trailing thread and the thread
attached to the needle. When I have that first knot tied, I
check the fit on the harness. If the fit is satisfactory, I
continue stitching, tying a knot after every five or six stitch-
es. This ensures that if the thread breaks at some point, the
stitching cannot entirely unravel. When I am done stitch-
ing, I cut off the ends of the harness straps that extend past
the point where the straps are stitched together (Fig. 1B,
Fig. 2), and then cut the template so it can be removed.
This technique has substantially reduced the time it takes
to process a bird, and thus reduces stress on the bird being
tagged(as well as the researcher!). Two anonymous reviewers
provided useful suggestions on the first draft of this letter.—
Richard O. Bierregaard
1
(e-mail address: rbierreg@gmail.
com), UNC-Charlotte, Biology Department, 9201 Univer-
sity City Boulevard, Charlotte, NC 28223 U.S.A.
Received 1 March 2013; accepted 6 October 2013
J. Raptor Res. 48(1):88–89
E2014 The Raptor Research Foundation, Inc.
ACOMMUNAL ROOSTING OF THE GREAT HORNED OWL (BUBO VIRGINIANUS)
KEY WORDS:Great Horned Owl; Bubo virginianus; roost;Utah.
While conducting roadside eagle surveys on 27 Novem-
ber 2012 in west-central Utah, we found two Great Horned
Owls (Bubo virginianus) roosting in a group of six decidu-
ous trees along the southern boundary of a 3-ha farm lot.
Upon investigating further, we found an additional four
owls also roosting in this group of trees. The owls were
dispersed at random in the trees, with the average distance
between owls being approximately 1 m, and the largest
distance between owls approximately 5 m. The number
of owls per tree never exceeded two, and two owls perched
in trees alone. Our proximity to the owls caused three
birds to flush from the trees and retreat to a nearby hayloft
100 m to the west.
The presence of more than two adult Great Horned
Owls is counter to the species’ typical roosting behavior.
The Great Horned Owl is an aggressive and highly terri-
torial species (Houston et al. 1998). They generally roost
alone, except prior to or during breeding season when
members of a mated pair may roost in close proximity
(Houston et al. 1998). Over the course of our field season,
we observed numerous pairs roosted as close as 1 m apart.
Though these birds may tolerate the presence of their
mate, they aggressively defend their territory against other
conspecifics, including juveniles, which are driven away in
autumn (Houston et al. 1998). The Great Horned Owl has
also been reported to engage in cannibalism, a dramatic
example of their aggressive behaviors (Millard et al. 1978).
Considering the known aggressive and territorial nature
of this species, we question why multiple birds were roosting
together. Unmated individual Great Horned Owls that are
unable to establish territories may be solitary wanderers,
often occurring at territory boundaries (Rohner 1995,
1996). Communal roosting has never been reported for
floaters, but factors related to high prey density could con-
tribute to their roosting in close proximity. The farm plot
was surrounded on all sides by agricultural pastures. The
presence of perches (fence posts and power poles) and
potentially high prey availability in the surrounding habitat
could have reduced competition and induced the owls to
remain close to optimal foraging habitat. However, the farm
and surrounding area seemed at least superficially similar to
other farmlands in the region, which also had an abun-
dance of perch sites, roost trees, similar foraging habitat,
etc. Extreme weather might cause unusual behaviors, but
can be disregarded in this instance as our visit was during
mild weather, with typical winter temperatures for the area.
It is possible that parents might tolerate the presence of
their young until near to the following breeding season.
Great Horned Owl clutches containing five eggs have been
reported (Houston et al. 1998), and young may not disperse
the territory till January (Peterson 1979), which suggests the
possibility that these birds may have been a family group.
Still, the presence of six birds would suggest that either the
previous year had an uncharacteristically high fledging rate,
or there were at least two broods of offspring present, which
is highly unlikely.
Three additional visits were conducted to observe the
roosting owls from 3 December 2012 to 8 January 2013,
during which three, two, and three owls were found in the
roost trees. On the second visit, we experimented with call-
playbacks to learn something about the territoriality of the
roosting birds. Hooting between mated pairs is a regular
1
Present address: 421 Cotswold Lane, Wynnewood, PA
19096 U.S.A.
88 LETTERS VOL. 48, NO.1
behavior, used to defend territory boundaries and
strengthen pair bonds (Houston et al. 1998). We observed
no response to call-playbacks, supporting the hypothesis
that these birds were not territorial. On the final visit, we
observed the birds until sunset. Each owl gave at least one
shriek contact call before leaving the roost, with one bird
giving five shrieks, about 30 sec apart, before leaving the
trees. No owls hooted.
We found no prior records of communal roosting in
Great Horned Owls. Because communal roosting is not
characteristic of this species, further research could reveal
whether this instance was unique, or if observed elsewhere,
what specific factors contribute to this behavior. We are
indebted to the Raptor Research Center at Boise State Uni-
versity, and HawkWatch International for their support. We
especially thank Vincenzo Penteriani, Cheryl Dykstra, and
one anonymous reviewer for their detailed edits that
shaped the final manuscript. We are grateful to David L.
Anderson, Shawn Hawks, Steve Slater, Markus Mika, Marc
Bechard, and Jerry Liguori for their helpful guidance, and
for comments and edits on earlier drafts of the manu-
script.—Bryce Robinson, (e-mail address: brycerobinson@u.
boisestate.edu), Raptor Research Center, 1910 University
Drive, SN 100, Boise State University, Boise, ID 83725
U.S.A.; and Caitlin Davis, 35 Old Farm Road, Glen Burnie,
MD 21060 U.S.A.
LITERATURE CITED
HOUSTON,C.S.,E.T.JONES,AND E. PLETZ. 1998. Great
Horned Owls with broods of five. Blue Jay 56:123–124.
———, D.G. SMITH,AND C. ROHNER. 1998. Great Horned
Owl (Bubo virginianus). In A. Poole [ED.], The birds of
North America online, No. 372. Cornell Lab of Orni-
thology, Ithaca, NY U.S.A. http://bna.birds.cornell.
edu/bna/species/372 (last accessed 8 August 2013).
MILLARD, J.B., T.H. CRAIG,AND O.D. MARKHAM. 1978. Can-
nibalism by an adult Great Horned Owl. Wilson Bulletin
90:449.
PETERSON, L. 1979. Ecology of Great Horned Owls and
Red-tailed Hawks in southeastern Wisconsin. Wisconsin
Department of Natural Resources Tech. Bull. 111, Ma-
dison, WI U.S.A.
ROHNER, C. 1995. Great Horned Owls and snowshoe hares:
what causes the time lag in the numerical response of
predators to cyclic prey? Oikos 74:61–68.
———. 1996. The numerical response of Great Horned
Owls to the snowshoe hare cycle: consequences of
non-territorial ‘floaters’ on demography. Journal of An-
imal Ecology 65:359–370.
Received 30 June 2013; accepted 30 September 2013
Associate Editor: Vincenzo Penteriani
J. Raptor Res. 48(1):89–91
E2014 The Raptor Research Foundation, Inc.
COOPERATIVE KLEPTOPARASITISM BY A PAIR OF BALD EAGLES AT LAKE SONOMA,CALIFORNIA
KEY WORDS:Bald Eagle; Haliaeetus leucocephalus; Osprey; Pandion haliaetus; cooperative hunting;kleptopar-
asitism.
The Bald Eagle (Haliaeetus leucocephalus) is well known
for its piratical and kleptoparasitic foraging behavior, tak-
ing prey from other eagles or other birds and mammals
(Stalmaster 1987, Gerrard and Bortolotti 1988, Buehler
2000), particularly Ospreys (Pandion haliaetus; Poole et
al. 2002). Less well documented for Bald Eagles, and also
for other sea and fish eagles in the genus Haliaeetus, is the
extent to which they employ cooperative hunting tactics
to increase capture rate. According to Ellis et al. (1993),
the characteristics of cooperative hunting include a clear
division of labor and the orderly sharing of spoils with
enhanced success, with coordinative signals sometimes
present. When employed by raptors, separate roles are
sometimes evident, but sharing of prey is limited. Coop-
erative hunting by Bald Eagles and other Haliaeetus eagles
has been reported by Thiel (1983), Fischer (1984), Folk
(1992), Berger (1994), Poole (1994), Berkelman et al.
(1999), and Stanley (2002). Cramp and Simmons
(1980) reported cooperative hunting by pairs of White-
tailed Eagles (H. albicilla) in Norway as ‘‘not uncom-
mon,’’ particularly when the eagles were pursuing sea-
ducks (e.g., eiders [Somateria spp.]). In addition, imma-
ture Bald Eagles hunted cooperatively in a group in Ore-
gon and Washington (Buchanan and Watson 2010).
Herein, we report behavior we believe to be previously
undescribed: namely, ‘‘cooperative kleptoparasitism’’ by
a pair of Bald Eagles robbing an Osprey at Lake Sonoma
in northern California.
Lake Sonoma is located on Dry Creek, a tributary of the
Russian River in Sonoma County, about 120 km north of
MARCH 2014 LETTERS 89
... The wide nest dimensions of the Spotted Owlet and narrow nest dimensions of Forest and Jungle Owlets may be explained by the number of individuals that roost in the cavity. Some owl species such as the Eastern Screech Owl Otus Asio (Belthoff and Ritchison 1990b), Great Horned Owls Bubo virginanus (Robinson and Davis 2014), Barn Owl Tyto alba, Indian Scops Owl Otus sunia and the Spotted Owlet have post-fledgling juveniles who stay with parents prior to dispersing (Ali and Ripley 1987). To accommodate more individuals in the same cavity, the nest needs to be wider and deeper. ...
Article
1. The numerical response of great horned owls (Bubo virginianus Gmelin) to the 10-year population cycle of snowshoe hares (Lepus americanus Erxleben) in the boreal forest was examined during 1988-93 in the south-western Yukon, Canada. Demographic parameters were estimated based on censuses (territorial pairs), nest visits (productivity), and radio-telemetry (survival, emigration, and integration of young birds into the population). 2. Hares rose to peak densities in 1990, and almost all resident owl pairs bred and raised large broods during years of increasing and highest prey abundance. In 1991, the first year of hare decline, all breeding parameters including post-fledging survival were reduced, and recruitment in autumn was very low. In 1992 and 1993, reproduction was completely suppressed. 3. Survival of young owls in their first 2 years of life was high during the peak of the hare cycle, and a large number of non-territorial `floaters' were present. These birds were silent, and moved more than territorial owls. Their ranges overlapped broadly with defended territories, and floaters were affected by the hare decline before territory holders. 4. Most ecological studies on birds are based on the territorial fraction of a population. The results of this study show how a large proportion of secretive floaters can delay the detection of population declines in traditional censuses of territorial birds, and can lead to serious underestimates of the impacts of predation.
Article
Predator populations often decline with a time lag after the peak of prey cycles. Theoretical models of predator-prey interactions predict that this delay is caused by a higher rate of population growth in prey, which leaves predators with super-abundant food after the peak and buffers their decline. This situation is met when predator populations have a lower innate capacity for increase than their prey or when the increase is inhibited because of territorial behaviour. Here, I refer to this hypothesis as 'single prey hypothesis' (SPH) in contrast to the 'multiple prey hypothesis' (MPH), which predicts that the delayed decline is caused by high availability of other prey species. Results on population growth rates of great horned owls showed that the predictions of SPH were met, although the predicted difference was small when floaters were taken into account or social exclusion from breeding was removed in a population model. In their diet, great horned owls relied to a large degree on the main cyclic prey (snowshoe hares), and thus the results were not in agreement with the MPH. Inverse density-dependent growth rates in the territorial population, density-dependent accumulation of floaters, and replacements of territorial vacancies were consistent with the hypothesis that social behaviour limited the number of owl territories. Reproduction of resident owls was immediately affected by the prey decline, indicating that there was no buffering effect of super-abundant food. Therefore, neither MPH nor SPH were satisfactory explanations, and I propose a mechanism based on individual behaviour to explain delayed numerical responses: territorial predators monopolize a disproportionately large amount of resources for reproduction during the increase and peak of the cycle, and are then buffered against prey declines by adjusting their breeding activities. Non-territorial floaters have lower access to resources and their numbers are affected more immediately by declining prey.
Liguori for their helpful guidance, and for comments and edits on earlier drafts of the manu-script.—Bryce Robinson, (e-mail address: brycerobinson@u. boisestate.edu), Raptor Research Center Great Horned Owls with broods of five
  • Shawn Anderson
  • Steve Hawks
  • Markus Slater
  • Marc Mika
  • Bechard
Anderson, Shawn Hawks, Steve Slater, Markus Mika, Marc Bechard, and Jerry Liguori for their helpful guidance, and for comments and edits on earlier drafts of the manu-script.—Bryce Robinson, (e-mail address: brycerobinson@u. boisestate.edu), Raptor Research Center, 1910 University Drive, SN 100, Boise State University, Boise, ID 83725 U.S.A.; and Caitlin Davis, 35 Old Farm Road, Glen Burnie, MD 21060 U.S.A. LITERATURE CITED HOUSTON, C.S., E.T. JONES, AND E. PLETZ. 1998. Great Horned Owls with broods of five. Blue Jay 56:123–124.
Great Horned Owl (Bubo virginianus) The birds of North America online, No. 372. Cornell Lab of Orni-thology
  • D G ———
  • Smith
  • C Rohner And
———, D.G. SMITH, AND C. ROHNER. 1998. Great Horned Owl (Bubo virginianus). In A. Poole [ED.], The birds of North America online, No. 372. Cornell Lab of Orni-thology, Ithaca, NY U.S.A. http://bna.birds.cornell. edu/bna/species/372 (last accessed 8 August 2013).
Can-nibalism by an adult Great Horned Owl Ecology of Great Horned Owls and Red-tailed Hawks in southeastern Wisconsin
  • J B Millard
  • T H Craig
  • O D And
  • Markham
  • L Peterson
MILLARD, J.B., T.H. CRAIG, AND O.D. MARKHAM. 1978. Can-nibalism by an adult Great Horned Owl. Wilson Bulletin 90:449. PETERSON, L. 1979. Ecology of Great Horned Owls and Red-tailed Hawks in southeastern Wisconsin. Wisconsin Department of Natural Resources Tech. Bull. 111, Ma-dison, WI U.S.A. ROHNER, C. 1995. Great Horned Owls and snowshoe hares:
Cannibalism by an adult Great Horned Owl
  • J B Millard
  • T H Craig
  • O D Markham
MILLARD, J.B., T.H. CRAIG, AND O.D. MARKHAM. 1978. Cannibalism by an adult Great Horned Owl. Wilson Bulletin 90:449.
Ecology of Great Horned Owls and Red-tailed Hawks in southeastern Wisconsin. Wisconsin Department of Natural Resources Tech
  • L Peterson
PETERSON, L. 1979. Ecology of Great Horned Owls and Red-tailed Hawks in southeastern Wisconsin. Wisconsin Department of Natural Resources Tech. Bull. 111, Madison, WI U.S.A.