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Assessing Prey Selection of Barn Owls in Western Nebraska by Using Pellet Analysis and Small-mammal Trapping

Anisha Pokharel1*, Scott L. Gardner2, and John P. DeLong1

1University of Nebraska–Lincoln, School of Biological Sciences, 1101 T Street, Lincoln, NE 68588 USA. 2University of Nebraska State Museum and School of Biological Sciences, 645 N. 14th Street, Lincoln, NE 68588 USA. *Corresponding author.

Praire Naturalist, Volume 53 (2021):27–35

Abstract
Research on prey selection strategies is important for understanding the connections between predators and their prey communities. Studies on prey selection by Tyto alba (Scopoli) (Barn Owls) show contradictory results; some showed that Barn Owls are random samplers of small mammal communities, whereas others found that they take prey disproportionate to prey abundance. Here, we compared pellet analysis and small-mammal trapping data to assess patterns of prey selection by Barn Owls in western Nebraska. Microtus Schrank spp. (vole spp.) constituted 55.8% of the 1163 prey items. The proportions of several prey types in the diet were significantly different from the expected proportions based on trapping. Microtus spp. occurred more frequently in the diet, whereas Peromyscus Gloger spp. (deer mouse spp.) occurred less frequently in the diet than expected. This pattern may indicate Barn Owls actively select Microtus spp., possibly because they are twice as large as Peromyscus spp. and are, therefore, relatively more energetically rewarding. Alternatively, this pattern may suggest that Microtus spp. are more available to Barn Owls than are Peromyscus spp.

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Prairie Naturalist A. Pokharel, S.L. Gardner, and J.P. DeLong 2021 53:27–35 27 2021 PRAIRIE NATURALIST 53:27–35 Assessing Prey Selection of Barn Owls in Western Nebraska by Using Pellet Analysis and Small-mammal Trapping Anisha Pokharel1*, Scott L. Gardner2, and John P. DeLong1 Abstract - Research on prey selection strategies is important for understanding the connections between predators and their prey communities. Studies on prey selection by Tyto alba (Scopoli) (Barn Owls) show contradictory results; some showed that Barn Owls are random samplers of small mammal communities, whereas others found that they take prey disproportionate to prey abundance. Here, we compared pellet analysis and small-mammal trapping data to assess patterns of prey selection by Barn Owls in western Nebraska. Microtus Schrank spp. (vole spp.) constituted 55.8% of the 1163 prey items. The proportions of several prey types in the diet were significantly different from the expected proportions based on trapping. Microtus spp. occurred more frequently in the diet, whereas Peromyscus Gloger spp. (deer mouse spp.) occurred less frequently in the diet than expected. This pattern may indicate Barn Owls actively select Microtus spp., possibly because they are twice as large as Peromyscus spp. and are, therefore, relatively more energetically rewarding. Alternatively, this pattern may suggest that Microtus spp. are more available to Barn Owls than are Peromyscus spp. Introduction Food-web structure and the patterns of interactions among predator and prey play a key role in setting the stability of ecosystems and their ability to provide ecosystem services (Beckerman et al. 1997, Estes et al. 2011, Fortin et al. 2005). A predator’s diet reflects its connections to the community in which it lives, so documenting diets provides information on food-web structure. Searching predators may make decisions about where to forage, when to forage, and what to attack when they encounter potential prey (Stephens and Krebs 1986). There is extensive literature assessing the degree to which a predator makes decisions that can increase or maximize fitness, including decisions regarding how long to hunt in a patch and what prey to include in the diet (Charnov 1976). Although uncertainty remains about whether predators forage optimally (Sih and Christensen 2001), the degree to which many predators take prey disproportionate to their availability in the environment remains unclear. Such patterns may or may not reflect adaptive strategies but are important in understanding the connections between predators and their prey communities. Tyto alba (Scopoli) (Barn Owls) are widely studied for their prey selection because of their global distribution, conservation status, susceptibility to rodenticides, and use as a biocontrol agent for rodents (Kross et al. 2016, Moore et al. 1998). Barn Owls may be unbiased samplers of the small mammal community upon which they principally prey, with several studies showing that prey in Barn Owl diets are proportional to their abundance in the prey community (Andrade et al. 2016, Bernard et al. 2010, Hawbecker 1945, Heisler et al. 2016, Hucks et al. 2015, Rifai et al. 1998). Moreover, Avenant (2005) demonstrated that Barn Owls are able to sample the small mammal community better than humans are able to sample by trapping. In addition, some diet studies suggest that Barn Owls show little pref- 1University of Nebraska–Lincoln, School of Biological Sciences, 1101 T Street, Lincoln, NE 68588 USA. 2University of Nebraska State Museum and School of Biological Sciences, 645 N. 14th Street, Lincoln, NE 68588 USA. *Corresponding author: anisha.pokharel7@gmail.com. Manuscript Editor: Lawrence D. Igl Prairie Naturalist A. Pokharel, S.L. Gardner, and J.P. DeLong 2021 53:27–35 28 erence towards certain species and select a wide range of prey items that includes not only small mammals (including bats) but also birds, amphibians, arthropods, and fish, depending upon availability (Morton and Martin 1979). In contrast, other studies suggest that Barn Owls take some prey species disproportionate to their abundance in the prey community. Typically, Microtus Schrank spp. (vole spp.) are the principal prey of Barn Owls in many temperate regions (Kopij 1998, Marti et al. 2020, Myers et al. 2009). This pattern is consistent with the results of studies showing that, given a choice, Barn Owls preferred Microtus spp. over other common rodents, such as Peromyscus Gloger spp. (deer mouse spp.), at least in a captive setting (Derting and Cranford 1989, Fast and Ambrose 1976). In the wild, Taylor (2009) found that Barn Owls showed a strong preference for Microtus spp. over mice in Scotland. Similarly, Gubanyi (1989) and Gubanyi et al. (1992) showed that Microtus spp. were taken as prey more often than expected based on their abundance, and Peromyscus spp. were taken less often than expected based on their abundance. Several additional studies showed evidence of selective predation by Barn Owls on Microtus spp., irrespective of their habitat and prey availability (Askew et al. 2007, Colvin 1985, Hindmarch and Elliott 2015, Marti 2010). This higher prevalence of Microtus spp. in the Barn Owl diet compared with other small rodents could result from them being more vulnerable or yielding higher energy intake per unit handling time compared with alternative prey (DeLong et al. 2013, Fast and Ambrose 1976, Stephens and Krebs 1986). Barn Owls hunt mainly using acoustic cues, so it is possible that Microtus spp. are louder and, therefore, more detectable than smaller alternative prey (Derting and Cranford 1989, Taylor 2009). In addition, some studies suggest that Barn Owl prey selection also may be influenced by body mass of the prey (Colvin 1985, Marti 2010, Taylor 2009). For example, Yom-Tov and Wool (1997) showed that, although Barn Owls might choose prey randomly, they prefer larger prey individuals over smaller ones. Such size-based prey selection might explain the Barn Owls’ preference for Microtus spp., which are twice the size of Peromyscus spp. However, Dickman et al. (1991) and Trejo and Guthmann (2003) showed that Barn Owls have some preference towards smaller prey size. In this study, we evaluated prey selection in Barn Owls by using pellet analysis and determined whether prey selection was proportional to the availability of prey. We focused on a site in western Nebraska for which small-mammal community trapping data were available. We tested whether Barn Owl diets reflected prey availability by species, indicating a good sampler strategy, or whether diets were disproportionate to prey’s relative abundance, suggesting either the existence of foraging strategies or differences in prey detectability. We then tested whether the overall distribution of prey body mass in the diet differed from the distribution of body mass in the prey community to determine whether Barn Owls could simply be selecting larger or smaller prey rather than tar geting specific species. Methods Study Area The study area was in western Nebraska (Keith County), in and surrounding the University of Nebraska–Lincoln’s Cedar Point Biological Station (CPBS). Habitats around the station include mixed-grass prairie and dry and irrigated agricultural fields. Bubo virginianus (J.F. Gmelin) (Great Horned Owls), Megascops asio (Linnaeus) (Eastern Screech Owls), Athene cunicularia (Molina) (Burrowing Owls), Asio otus (Linnaeus) (Long-eared Owls), and Barn Owls occur in the area. Prairie Naturalist A. Pokharel, S.L. Gardner, and J.P. DeLong 2021 53:27–35 29 Pellet Sampling We searched for Barn Owl nests and collected pellets from in and under the nests in May to July of 2016–2018 from 5 Barn Owl territories at CPBS and along State Highway 92. We autoclaved and cleaned the pellets, separating the skull and jaw bones. We identified individual prey by the skulls, and we matched jaws to skulls whenever possible, counting additional prey individuals for unmatched jaws (Marti 1973). We measured (in mm) skull length (maximum length) and width (maximum distance between zygomatic arches), as well as the length of both jaws, if available. We only included measurements from bones that were intact and whose dimensions were not altered by digestion. We estimated the mass of prey items identified from pellets through a set of regressions linking bone measurements to wet mass of whole individuals. We estimated the mass (g) of Microtus spp. with the equation , where BL is basilar length in mm (Pagels and Blem 1984). From a subset of Microtus spp. skulls in our sample, we determined that basilar length was 90% of total skull length and, therefore, estimated basilar length as 0.9 × total skull length. For the samples missing skull length, we applied regression equations based on our data, using either or , where y = skull length (mm) and x = jaw length (mm) or zygomatic width (mm), respectively. In the case of samples missing both skull and jaw length measurements (because of skull or jaw damage), we used the mean mass of the specimens of the appropriate species housed in the Division of Zoology in the University of Nebraska State Museum (UNSM). We estimated the body mass (g) of Peromyscus spp. with the regression equation , where a = -2.972, b = 4.146, and ML is mandible length (mm), which is the jaw without incisors or the mandibular condyle (Hamilton 1980). We corrected our jaw lengths to mandible lengths with the correction factor of 0.728, which is the average proportion of mandible length to jaw length in a random sample of 10 jaws from our pellets. In the case of samples that lacked the jaw length, we applied the regression equation80, where x = zygomatic width (mm) and y = jaw length (mm). For Reithrodontomys Giglioli spp. (harvest mouse spp.), we estimated mass using the same mass/mandible length equation as used for Peromyscus spp. but with a = -1.769 and b = 2.958 (Hamilton 1980). We categorized some prey more broadly into the subfamily Neotominae if they could not be classified as either Peromyscus spp. or Reithrodontomys spp., which was the case when prey items were present only as jaws. We estimated the mass of these prey items using the same mass/mandible length equation, with values of a and b averaged across both Peromyscus spp. and Reithrodontomys spp. We estimated the average mass of the other identified small mammal prey, Blarina Gray spp. (American short-tailed shrew spp.), Sorex Linnaeus spp. (red-toothed shrew spp.), Geomys Rafinesque spp. (eastern pocket gopher spp.), and Dipodomys ordii Woodhouse (Ord’s Kangaroo Rat), as the average of locally collected specimen data provided by the UNSM. Small-mammal Trapping We compiled trapping data collected in mid-July to mid-August at CPBS (2012–2016) as a part of a long-term field parasitology course at the University of Nebraska–Lincoln (Genoways et al. 2008, Howell et al. 2016). During the course, we collected small mammals of several different species from different locations in Keith, Arthur, Garden, and Grant counties in western Nebraska: CPBS (41.21°N, 101.65°W; containing Great Plains grassland/ woodland and wetland habitats), Grama Grass (41.19°N, 101.65°W; a fallow mixed-grass pasture), Double Tank (41.20°N, 101.64°W; mixed-grass habitat), Breen’s Flyway (41.18°N, 101.36°W; containing riparian-wetland habitat on the edge of the North Platte River), Ackley Valley (41.33°N, 101.73°W; a short grass horse pasture), and Arapahoe Prairie (41.49°N, Prairie Naturalist A. Pokharel, S.L. Gardner, and J.P. DeLong 2021 53:27–35 30 101.86°W; short grass habitat) (Bailey 1982). We used ShermanTM live traps (H.B. Sherman Traps, Inc., Tallahassee, FL), Museum Special snap traps (Woodstream Corp., Lititz, PA), or pit-fall traps, baited with a mix of vanilla, peanut butter, and oatmeal, to collect specimens (Gardner 1996, Gardner and Jiménez-Ruiz 2009). We checked traps twice daily, in the early morning and late afternoon, and left traps open during the day to capture diurnal mammals, such as Microtus spp. and Sciuridae spp. We prepared standard voucher specimens for all trapped individuals, with weight and measurement data recorded at time of collection, and we deposited specimens in the mammal collection of UNSM (Gardner 1996). Statistical Analyses All statistical analyses were carried out using MATLAB 2019 (Mathworks, Natick, MA). We identified several prey items only to the subfamily Neotominae, which includes Peromyscus spp. and Reithrodontomys spp. To include these taxa in the analysis, we determined the proportions of Neotominae that were identified as Peromyscus and Reithrodontomys and allocated the more broadly categorized items to these 2 genera accordingly to correct for undersampling. We ran chi-square tests to compare the proportion of prey types in the diet to the relative abundance of prey types from the trapping sample at the level of genus. We considered only small mammals in our analyses as we did not have a community sample for other prey types or identifications for the avian prey items. We also compared the distribution of body masses between the trapping and prey item samples by using a 2-sample Kolmogorov–Smirnov test. For the test, we included only the species with masses <200 g and excluded the larger species that were trapped but never found in the Barn Owl diet (e.g., Vulpes Frisch spp., Lepus Linnaeus spp.). Results From all pellet material, we identified 1163 prey items, 11 of which were unidentified birds. Of the 1152 mammalian prey items (Table 1), the majority (649 prey items or 56.3%) were Microtus spp., including Microtus ochrogaster (Wagner) (Prairie Vole; 291 prey items) and Microtus pennsylvanicus (Ord) (Meadow Vole; 336 prey items). The mammals in the diet also included Peromyscus spp. (14.1%), Reithrodontomys montanus (Baird) (Plains Harvest Mouse) and/or Reithrodontomys megalotis (Baird) (Western Harvest Mouse) (21.2%), Blarina brevicauda (Say) (Northern Short-tailed Shrew) and/or Blarina hylophaga Elliot (Elliot’s Short-tailed Shrew) (0.34%), Sorex spp. (3.1%), Dipodomys ordii (2.9%), and Geomys spp. (2.0%). Unidentified Neotominae constituted 13.1% of the prey items. We also found evidence of Faxonius virilis (Hagen) (Northern Crayfish) remains below 1 Barn Owl nest. Mammal trapping yielded 592 individuals representing 14 species, including the 8 found in the owl pellets plus 6 other species not found in owl pellets: Perognathus Wied-Neuwied spp. (pocket mouse spp.), Chaetodipus hispidus (Baird) (Hispid Pocket Mouse), Onychomys leucogaster (Wied-Neuwied) (Northern Grasshopper Mouse), Ictidomys J.A. Allen spp. (lined ground squirrel spp.), Sylvilagus audubonii (Baird) (Desert Cottontail), and Zapus hudsonius (Zimmermann) (Meadow Jumping Mouse) (Table 1). In contrast to the pellet samples, the most abundant prey type in the trapping samples was Peromyscus spp. Microtus spp. and Reithrodontomys spp. occurred significantly more in the diet than expected from their frequencies in the trapping samples, whereas Peromyscus spp. and Dipodomys spp. occurred less frequently in the diet than expected (Table 1). We also found a significant difference between the distributions of the estimated mass of prey items in the diet and the mass of individuals collected by trapping (k = 0.24, P ≤ 0.001; Fig. 1). 31 Table 1. Total number of individuals and percentage of small mammals in trapping samples and Barn Owl pellet samples in western Nebraska. There were an additional 11 bird skulls of unidentified species found in the pellets. The pellet sample numbers include unidentified Neotominae allocated to Peromyscus spp. (41 individuals) and Reithrodontomys spp. (91 individuals) based on their proportions in the identified portion of the prey items. Differences in the frequency of prey types between potential prey and actual prey were analyzed at the genus level through chi-squared (χ2) tests. Prey Items Trapping sample % Pellet sample % χ2 P Microtus Schrank spp. 53 8.95 649 56.33 370.02 <0.001 M. pennsylvanicus Ord 336 M. ochrogaster Wagner 291 Peromyscus Gloger spp. 283 47.80 163 14.14 227.39 <0.001 Dipodomys ordii Woodhouse 49 8.27 33 2.86 24.97 <0.001 Blarina Gray spp. 2 0.33 4 0.34 15.64 NA Geomys Rafinesque spp. 20 3.37 23 1.99 2.96 0.08 Reithrodontomys Giglioli spp. 51 8.61 244 21.18 45.13 <0.001 Sorex Linnaeus spp. 12 2.02 36 3.12 1.85 0.17 Perognathus Wied-Neuwied spp. 37 6.25 – – Chaetodipus hispidus Baird 18 3.04 – – Onychomys leucogaster Wied-Neuwied 31 5.23 – – Ictidomys J. A. Allen spp. 5 0.84 – – Sylvilagus audubonii Baird 7 1.18 – – Zapus hudsonius Zimmermann 24 4.05 – – Total 592 1152 Figure 1. Mass distribution of prey items using trapping data versus estimated mass distribution of prey items in the diet of Barn Owls in western Nebraska. The diet distribution is significantly shifted towards the right of the trapping sample (k = 0.25, P < 0.001). Prairie Naturalist A. Pokharel, S.L. Gardner, and J.P. DeLong 2021 53:27–35 32 Discussion Barn Owl diets have been extensively studied throughout the world (Bernard et al. 2010, Glue 1972, Heisler et al. 2016, Marti 1973), but it remains unclear if Barn Owls generally take prey types in proportion to their abundance in the prey community. Gubanyi (1989) and Gubanyi et al. (1992) recorded ~17 mammal species in the diets of Barn Owls in western Nebraska, with most of the prey being Microtus spp., Reithrodontomys spp., and Peromyscus spp. In addition, Bonner and Geluso (2010) recorded 12 species of mammals in the diets of Barn Owls at Crescent Lake National Wildlife Refuge in northwestern Nebraska, with most of the diets composed of Microtus spp., Dipodomys spp., and Peromyscus spp. Huebschman et al. (2000) found 11 mammal species in Barn Owl pellets collected from 24 counties in Nebraska from 1980–1998, and they found that Microtus spp. and Peromyscus spp. constituted the principle prey items. Our data on diet for western Nebraska indicate a similar array of prey types, with Microtus spp. constituting most of the diet. In our study, the proportions of some species differed significantly between the pellet and trapping samples. For example, Microtus spp. and Reithrodontomys spp. occurred more frequently in the diet than expected from trapping results, whereas Peromyscus spp. and Dipodomys spp. occurred less frequently in the diet than expected, which is a pattern similar to some previous findings (Gubanyi 1989, Gubanyi et al. 1992, Pearson and Pearson 1947). Although covering a wide range of habitats and using multiple trapping techniques, the trapping data may not exactly represent the foraging habitats used by Barn Owls in our study area or may not be a perfectly random sample of the prey base. A mismatch between pellets and trapping also could arise through incomplete spatial overlap of owl hunting and trapping areas, differences in years between the samples, and differences in seasonal timing of pellet and trapping collection. However, a large portion (45%) of our pellet data was collected in 2016, a year in which trapping data was also collected, and both trapping data and pellets came from a broadly overlapping region during the breeding season for the owls, indicating reasonable support for the comparison. Furthermore, the dramatic differences between prey and trapping frequencies for Microtus spp. (56.3% in diet versus 9.0% in traps) and Peromyscus spp. (14.1% in diet versus 47.8% in traps) suggest that, even given some potential error in sampling, Barn Owls showed non-random consumption of potential mammalian prey. Marti (1973) showed that Barn Owls prefer larger prey species over smaller ones, whereas Dickman et al. (1991) found the opposite. Our genus-level differences between prey items and potential prey also reflect that the body mass distribution of the diet is significantly shifted to the right of the body mass distribution of trapped mammals, indicating selection of larger prey (Fig. 1). This result reflects that the most abundant prey, Microtus spp., are about twice the size of Peromyscus spp. This bias toward relatively larger prey species in the diet might reflect the possibility that Microtus spp. are a more energetically profitable option than the smaller Peromyscus spp., suggesting that Barn Owls in western Nebraska forage in a way that is consistent with an optimal foraging strategy (DeLong et al. 2013, Derting and Cranford 1989). Alternatively, Barn Owls might be detecting Microtus spp. more often as they might be louder or more vulnerable and hence more available to Barn Owls than are Peromyscus spp. Individuals of Peromyscus spp. and Microtus spp. also live in different habitats, so Microtus spp. may be more detectable or their locations may be more predictable given a potentially acoustics-focused hunting method by the owls. Raptor diets are likely influenced by prey abundance, seasonal vegetation changes, habitat modification, prey behavior, and prey reproductive patterns (Rosenblatt et al. 2015, Taylor 2009), all of which should be considered when trying to understand prey selection. Prairie Naturalist A. Pokharel, S.L. Gardner, and J.P. DeLong 2021 53:27–35 33 Overall, our results support the idea that Barn Owls select a wide range of prey items but are biased toward certain species (here, Microtus spp.) despite the availability of other prey types. We suggest that, given the equivocal evidence for Barn Owls either being random samplers of the prey community or displaying selective foraging behaviors, future work should consider the conditions under which some degree of prey selectivity would arise for Barn Owls, or for raptors more generally. Acknowledgments We would like to thank T. Labedz from the State Museum of Nebraska for providing the data for mass estimation of small mammals. 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