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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. Literature Cited Bent, A.C. 1961. Life Histories of North American Birds of Prey. 2nd Edition. Dover Publications, Inc., New York, NY. 425 pp. Blem, C.R., and J.F. Pagels. 1972. Feeding habits of an insular Barn Owl, Tyto alba. The Virginia Journal of Science 24:212–214. Bull, E.L, A.L. Wright, and M.G. Henjum. 1989. Nesting and diet of Long-eared Owls in conifer forests, Oregon. Condor 91:908–912. Colvin, B.A., and E.B. McLean. 1986. Food habits and prey specificity of the Common Barn Owl in Ohio. Ohio Journal of Science 86:76–80. Craig, T.H., E.H. Craig, and L.R. Powers. 1985. Food habits of Long-eared Owls (Asio otus) at a communal roost site during the nesting season. Auk, Short Communications 102:193–194. Johnsgard, P.A. 2002. North American Owls: Biology and Natural History. 2nd Edition. Smithsonian Institution Press, Washington, DC. 352 pp. Maccarone, A.D., and P. Janzen. 2005. Winter diet of Long-eared Owls (Asio otus) at an urban roost in Wichita, Kansas. Transactions of the Kansas Academy of Science 108:116–120. 152 Northeastern Naturalist Vol. 17, No. 1 Merritt, J.F., M. Lima, and F. Bozinovic. 2001. Seasonal regulation in fluctuating small-mammal populations: Feedback structure and climate. OIKOS 94:505–514. Meyer, W.H. 1939. Analysis of Barn Owl pellets in Pennsylvania. Auk, General Notes 56:187. Roest, A.I. 1991. A Key-guide to Mammal Skulls and Lower Jaws. Mad River Press, Inc, Eureka, CA. 39 pp. Wolniewicz, R. 2001. Field Guide to Skulls and Bones of Mammals of the Northeastern United States (Volumes 1: Skulls and Mandibles). Richard Wolniewicz Publisher, Magnolia, MA. 67 pp. Wolniewicz, R. 2004. Field Guide to Skulls and Bones of Mammals of the Northeastern United States (Volumes 2: The Long Bones). Richard Wolniewicz Publisher, Magnolia, MA. 97 pp. 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