Barn and Long-Eared Owl Diets:
A Comparative Study from Central Pennsylvannia and a
Key for Identification of Prey Items
Suzanne M. Khalafalla and Carlos A. Iudica
Northeastern Naturalist, Volume 17, Issue 1 (2010): 147–154
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2010 NORTHEASTERN NATURALIST 17(1):147–154
Barn and Long-Eared Owl Diets:
A Comparative Study from Central Pennsylvannia and a
Key for Identification of Prey Items
Suzanne M. Khalafalla1 and Carlos A. Iudica1,*
Abstract - Two sympatric owl species, Tyto alba (Barn Owl) and Asio otus (Longeared
Owl), are found in Lewisburg, PA. These species are of similar size, have
similar hunting styles, and most likely use similar prey items. In studying the diets
of these two owl species, we were able to determine their prey selection, the types
and relative abundance of prey in the area, and how these sympatric predators may
be partitioning their resources. A total of 80 pellets (40 from Barn Owls and 40 from
Long-eared Owls) were analyzed to understand their diets. Pellets were collected
from two locations in Lewisburg, PA approximately 2 km apart. Cranial and postcranial
remains within the owl pellets were used to identify species by comparing
them against our reference collections and keys. Typically, the Barn Owl is present
in Lewisburg from late spring through fall and the Long-eared Owl from late fall
through spring. Barn Owl pellets contained a greater number of larger-sized prey
than Long-eared Owl pellets. Both owl species rely heavily upon rodents as their
primary food source, which make up over 90% of their diets. Temporal differences
and differences in prey selectivity are most likely the major factors allowing these
species to share their common resources.
Introduction
Tyto alba Scopoli (Barn Owls) and Asio otus L. (Long-eared Owls) are
both medium-sized owl species found in central Pennsylvania. Both species
are more adapted to open-area hunting, such as in meadows or grasslands,
and select similar prey items including rodents and shrews, which make up
the majority of their diet (Bent 1961, Johnsgard 2002, Maccarone and Janzen
2005). Multiple studies have shown that rodents and shrews constitute
no less than 84% of the diet of Barn Owls and up to 95% in some areas (e.g.,
Virginia [Blem and Pagels 1972], Ohio [Colvin and McLean 1986], and
Pennsylvania [Meyer 1939]). Studies of Long-eared Owl diets from three
locations in the United States have shown that no less than 93% of their
diet is comprised of rodents and shrews (in Oregon [Bull et al. 1989], Idaho
[Craig et al. 1985], and Kansas [Maccarone and Janzen 2005]).
In most cases, Barn and Long-eared Owls do not coexist, showing the typical
sympatric distribution with differential temporal use of the same space.
Barn Owls migrate and are present in central Pennsylvania only during late
spring through autumn, whereas migrations during late autumn bring Longeared
Owls to central Pennsylvania, where they remain through spring.
1Department of Biology, Susquehanna University, Selinsgrove, PA 17870. *Corresponding
author - casaiud@susqu.edu.
148 Northeastern Naturalist Vol. 17, No. 1
The nature of the owl pellets (containing bone materials) makes studies on
owl feeding ecology surprisingly simple; however, due to either disinterest or
population declines (predator and/or prey items), there is a significant void in
the literature on owl diets from PA. Studies of owl diets not only provide information
about the predator/prey relationships or relative abundance of prey
species, but could provide a starting point for further research. In this study,
we describe the diets of sympatric Barn and Long-eared Owls. In addition, we
offer a simplified key to identify small-mammal cranial remains from regurgitated
pellets in central Pennsylvania.
Methods
Pellets from Barn and Long-eared Owls, from Lewisburg, PA were collected
at or near nesting sites. The two study areas are approximately 2 km
apart, which allowed for a close comparison of the feeding ecology of these
two species. Pellets from the Barn Owls were collected from the Stoltzfus
Farm on Col. John Kelly Road (40º59'N, 76°55'W) during August of 2006.
Before they arrived in the spring, we cleaned the area where the Barn Owls
were feeding to ensure that pellets collected were from that spring and
summer. Pellets from the Long-eared Owls were collected from Dale’s
Ridge on the Dale/Engle/Walker property on Strawbridge Road (40º58'N,
76º56'W) in early December of 2004 (produced in winter 2003–2004) and
in May of 2005 (produced during winter 2004–2005). The pellets from the
Long-eared Owls were found in woodland areas exclusively under Juniperus
virginiana L. (Red Cedar) trees. The collection of pellets spanning two
winters allowed for a relative comparison of prey availability and selection
over a two-year period. We did not have the opportunity to survey either
site for total owl counts.
The analysis of the pellets began by soaking each pellet in warm water
to loosen the hairs. The pellets were worked apart gently using forceps, and
skeletal remains were collected and separated into cranial and post-cranial
materials for later identification. Hairs were saved for use in hair casts to
aid in identification if needed, using an ad hoc hair key. The number of prey
items found in each pellet was counted using the skeletal elements. Prey
was counted even if only a portion of the skeleton was found in the pellet in
order to account for prey that was torn apart while ingesting, causing some
portions to be lost in the process.
After the samples were cleaned and dried, each of the cranial remains in
each of the pellets was identified to species using an ad hoc key based on
mammal remains found in Barn Owl pellets of Pennsylvania (Appendix 1).
We also used Roest (1991), Wolniewicz (2001, 2004), and reference collections
from the Museum of Natural History at the University of Michigan,
Carnegie Museum of Natural History, and the Susquehanna University Natural
History Teaching and Research Collections.
Multiple analyses were performed. The first was to determine which of
the two owl species is consuming more prey. The number of prey found in
2010 S.M. Khalafalla and C.A. Iudica 149
each pellet was averaged to determine whether there was any difference in
the amount of food present per pellet. An independent t-test was used to
determine if there was a significant difference in the mean amount of food
consumed. The degree of similarity between the Long-eared and Barn Owl
diets was compared using the similarity index of resource use overlap (Colvin
and McLean 1986):
Oxy = ΣXiYj
√(ΣXi
2 ΣYj
2)
where Xi = number of prey for Long-eared Owls; and Yj = number of prey
for Barn Owls.
A similarity index (Colvin and McLean 1986) was also calculated to
compare the prey selection by Long-eared Owls in Lewisburg, PA over the
course of two winter seasons (2003–2004 and 2004–2005).
Results
Barn Owl pellets contained significantly more prey than Long-eared Owl
pellets despite the fact that both are medium-sized owls. Barn Owl pellets
contained on average 3.2 prey per pellet, while Long-eared Owl pellets presented
only 1.2 prey per pellet (t ≥ 2.00, P < 0.05, α = 0.05).
In addition to the difference in the number of prey per pellet, there was
also a difference in the species found in the pellets of barn and Long-eared
Owls. There were a total of 6 species found over the course of a single
summer season in the pellets of Barn Owls. Prey species and their relative
abundance are shown in Table 1. The majority of the prey taken by Barn
Owls were Microtus pennsylvanicus Ord (Meadow Voles). Meadow Voles
Table 1. Species found in 40 pellets of Tyto alba (Barn Owl) and in 40 pellets (20 for 2004 and
20 for 2005 seasons) of Asio otus (Long-eared Owl). For Barn Owls, there were a total of six
species found. Pellets were collected during August 2006 and were produced during summer
2006. For Long-eared Owls, there were a total of three species found. There was a difference
in the relative abundance over the course of two seasons. Pellets were collected in December
of 2004 (2003–2004 season) and May of 2005 (2004–2005 season). # = number of individuals.
RA = relative abundance.
Long-eared Owls
Barn Owls 2004 2005
Prey species # RA # RA # RA
Meadow Vole (Microtus pennsylvanicus) 108 85.04% 20 80% 24 96%
Short-tailed Shrew (Blarina brevicauda (Say)) 10 7.87% 1 4% 1 4%
White-footed Mouse (Peromyscus leucopus) 4 16%
Meadow Jumping Mouse (Zapus hudsonius) 1 0.79%
Norway Rat (Rattus norvegicus (Berkenhout)) 6 4.72%
House Mouse (Mus musculus L.) 1 0.79%
Microtus sp. A 1 0.79%
Total 127 100.00% 25 100% 25 100%
AMicrotus sp. that we were unable to identify to the species level due to the conditions of the
specimen.
150 Northeastern Naturalist Vol. 17, No. 1
were also the primary prey species for Long-eared Owls; a total of only three
prey species were found in Long-eared Owl pellets. For Long-eared Owls,
there were slight differences between the 2004 and 2005 seasons. During
the winter of 2004, Peromyscus leucopus Rafinesque (White-footed Mouse)
were found in the pellets at a relative abundance of 16% while in the 2005
season, White-footed Mice were not found at all.
The diets of Long-eared and Barn Owls showed a very high similarity
index of 0.99. The similarity between the diets of two consecutive seasons
in the Long-eared Owls was also high with an index of 0.98.
Discussion
The amount of prey contained in the pellets produced by the Barn
Owls was significantly greater than the amount contained in the pellets of
Long-eared Owls. This difference in the amount of prey per pellet could
be due to multiple factors. Small-mammal populations typically experience
large growth during the summer months and large decline during the
winter months (Merritt et al. 2001). Primarily, the Barn Owls are in the
Lewisburg area during the summer months when prey is more abundant
and easier to find. In the winter, when Long-eared Owls are in Lewisburg,
many small mammals are active under the snow cover, which protects them
from generalist predators such as owls. The density of voles and mice tend
to increase during the summer and reach peak numbers in late summer
through autumn. Zapus hudsonius Zimmerman (Meadow Jumping Mouse)
is a true hibernator, which would also explain why this species was only
found in the Barn Owl pellets.
Over the course of two winters, the Long-eared Owl diet did not
change significantly; the primary food source of these owls continued to
be the Meadow Vole. The similarity index of 0.98 shows that there was
not much change from one year to the next, although we noticed some
variations. During the first winter, White-footed Mice comprised 16% of
the Long-eared Owl diet, while in the second winter they were not represented
in any pellets. It is possible that the population of White-footed
Mice in the area declined sharply during that year, or that the Long-eared
Owls excluded this species during prey selection. An ongoing long-term
population study of small mammals (trapping, release, recapture) in this
area has shown that there was a general decline in numbers over the past
couple of years (C. Clinton, A. Janicki, and C.A. Iudica, unpubl. data).
Included in this general decline is the White-footed Mouse. It is difficult
to know if the year-to-year variation in prey consumed is real, without
performing population studies simultaneously to determine if this variation
is due to prey selection of the owls, to changing population of the
rodent species, or simply a matter of the number of pellets sampled. This
variability underscores the need for multi-year sampling.
2010 S.M. Khalafalla and C.A. Iudica 151
There was a high similarity between the diets of Long-eared and Barn
Owls (0.99). This high index is mostly caused by the large amount of
Meadow Voles making up the bulk of both owls’ diets. This species alone
accounted for 88% in Long-eared Owls and 84% in Barn Owls. The fact that
both owls hunted primarily this species suggests high populations in Lewisburg
and that they are available throughout the year as a food source. Voles
are active during all seasons, and do not hibernate or undergo torpor (Merritt
et al. 2001). The majority of both owl diets were made up of rodents, which
totaled 96% in Long-eared Owls and 92% in Barn Owls. These numbers are
on the higher end of the ranges reported previously (Blem and Pagels 1972,
Colvin and McLean 1986, Meyer 1939).
Four species were found in the Barn Owl pellets that were not found in
the Long-eared Owl pellets. Although these species did not comprise a large
percentage of the Barn Owl diet, they were present and captured as a food
source. This difference could be due to seasonal or distributional variations
in the rodent population or could be due to the opportunistic hunting style
of the Barn Owl. Further studies are needed to determine which prey species
are present in the area and the frequencies at which they occur during both
winter and summer seasons. This would give additional insight into differences
in the ecology of Barn and Long-eared Owls.
Acknowledgments
We are grateful to Sarah Ignelzi (a Susquehana University graduate) for her pioneering
contributions. This work was partially funded by a grant from the Degenstein
Foundation and by funds provided to C.A. Iudica from Susquehana University. We
thank anonymous reviewers for their valuable comments and suggestions.
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2010 S.M. Khalafalla and C.A. Iudica 153
Appendix 1. An ad hoc key for identification of small mammal cranial remains in
central Pennsylvania
1a. No diastema (gap between teeth) or canines present 2
1b. Diastema (gap between teeth) present, canines present 8
2a. Small zygomatic arch present, no pigmentation on teeth, 36 or more teeth 3
2b. No zygomatic arch, teeth pigmented dark red or brown color, 32 teeth 5
3a. Number of teeth – 36 Scalopus aquaticus
3b. Number of teeth – 44 4
4a. Lower cheek teeth – 1 Parascalops breweri
4b. Lower cheek teeth – 8 Condylura cristata
5a. Skull length 20.8 to 24.8 mm Blarina brevicauda
5b. Skull length less than 19.2 mm 6
6a. 3rd unicuspid as small as 5th unicuspid or smaller than remainder of unicuspid
Sorex hoyi
6b. 3rd unicupsid larger than 5th unicuspid 7
7a. Unicuspid 1 & 2 are same or very similar in size, unicuspid 3 & 4 are same or
similar in size, 5th unicuspid diminutive Sorex cinereus
7b. All unicuspids differ in size Sorex fumeus
8a. Cusped teeth 9
8b. Flat triangular shaped teeth 13
9a. Grooved notch present in incisors, cheek teeth cusps in a radial pattern, skull
length 20.4 to 22.4 mm Mus musculus
9b. No notch in teeth, cheek teeth cusps arranged in a longitudinal manner 10
10a. Longitudinal groove present in incisors 11
10b. No groove present in incisors 12
11a. 18 teeth present, 4 upper cheek teeth, first cheek tooth small and underformed
compared to other 3 upper cheek teeth, skull length 20.8 to 23.4 mm
Zapus hudsonius
11b. 16 teeth present, 3 upper cheek teeth, skull length 22.7 to 25.2 mm
Napaeozapus insignis
12a. 16 teeth present, skull length 22.4 to 27.8 mm, M3 has a triangular
invagination from the labial face Peromyscus maniculatus
12b. 16 teeth present, skull length 24.1 to 27.4 mm, M3 lacks invagination
Peromyscus leucopus
13a. M3 lacks posterior loops 14
13b. M3 has posterior loops 15
154 Northeastern Naturalist Vol. 17, No. 1
14a. Teeth squared in shape, M3 on cheek teeth has 3 aligning triangles, skull
length 21.7 to 25.4 mm Microtus pinetorum
14b. M3 has a flat anterior face, M3 resembles a “M” at the anterior portion, skull
length 22.5 to 25.1 mm Myodes gapperi
15a. M2 has posterior loop and M3 has crescent shaped anterior face, skull length
25.8 to 28.8 mm Microtus pennsylvanicus
15b. M2 lacks posterior loop and M3 resembles “E” at the posterior portion, skull
length 26.8 to 27.5 mm Microtus chrotorrhinus