Diet of the Barn Owl in Northeast and Northwest Oregon.
In terms of habitat, Oregon is a diverse region with clinal changes from the west to the east. Moist coniferous and broad-leaved forests occur mainly in the NW part of the state, while the arid NE is dominated by grasslands and Ponderosa Pine (Pinus ponderosa) forests. In both ecozones, a large proportion of the natural vegetation has been converted into cultivated fields.
The Barn Owl is a common breeder in these ecozones (Marshall and others 2006) and feeds mainly on rodents as it does throughout the world (Taylor 1994; Johnsgard 2002; Marshall and others 2006). However, I hypothesized that the Barn Owl would prey upon different proportions of particular rodent families and prey species in NE and NW Oregon because: (1) the fauna of micromammals is different in these 2 ecozones (Maser and Storm 1970; Verts and Carraway 1998); and (2) the population cycles and dynamics of particular rodent families and species may have different patterns in each ecozone (for example, see Lin and others 2001; Lowell and others 2006).
I tested my hypothesis by analyzing the content of Barn Owl pellets collected during September of 2010. I collected pellets inside barns at the Eastern Oregon Agricultural Research Station (Union County) in NE Oregon (site 1), and at 4 locations in Benton County in NW Oregon (sites 2-5) (Table 1). The Eastern Oregon Agricultural Research Station is located about 1 km southwest from the center of the town of Union. It comprises a pastureland with short grass, bounded by additional cultivated fields on one side and suburbs on the other. The NW sites are located within the William L Finley National Wildlife Refuge (2 sites) and the Oregon Department of Fish and Wildlife EE Wilson Wildlife Area (2 sites), comprising Oak Savannah which has been partly transformed into cultivated fields (mainly grass seed, grass, and hay), with some wetlands. The 2 wildlife areas are approximately 25 km south and north, respectively, from Corvallis, Oregon. Because Barn Owls are known to prey within about I km around their nest/roost sites (Mikkola 1983), I treated each of the NW sites as separate territories.
At the Union County site, I collected 184 pellets from which I removed 328 prey items. A total of 172 pellets collected from the 4 Benton County sites contained 342 prey items. Only compact, entire pellets were used in the analysis. Most of them were dark and slightly glossy, indicating that they were cast within the last few months of summer. Pellets were easily identified as belonging to the Barn Owl by their texture, compactness, dimensions, and glossiness (Mikkola 1983). Barn Owls were also observed at each of the sites. I analyzed the pellets using accepted procedures (cf. Johnsgard 2002). Identification of rodents was based on Maser and Storm (1970). Non-rodent prey were identified with the aid of a reference collection maintained at the US Forest Service, Pacific Northwest Research Station, Forestry Sciences Laboratory, in Corvallis, Oregon.
The numerical percentages reported in this study refer to the proportion of the number of prey items of a given taxon relative to the total number of all prey items collected. Frequency of occurrence was calculated as the proportion of the total number of pellets examined containing a given taxon, expressed as a percentage. Estimation of prey wet biomass in the pellets was based on values given for adult animals in Maser and Storm (1970) and Verts and Carraway (1998). Prey species diversity was calculated using the formula H = -[sup.2][summation][p.sub.i][log.sub.e]pi, where s is the number of species, and [p.sub.i] is the proportion of the total number of individuals which belong to the ith species (Shannon and Weaver 1949).
Barn Owls preyed on a narrow spectrum of prey. In both ecozones, 6 to 8 mammal and 1 to 2 avian prey species were recorded (Table 2), and rodents constituted the bulk of their diet (98.5 and 96.9% of prey items identified from the NE and NW sites, respectively). The most common rodent prey were voles (Microtus spp). They comprised the same numerical percentages in both ecozones (86%) and only slightly different biomass percentages (78% in the NE; 84%, in the NW). The Long-tailed Vole (Microtus longicaudus) was the dominant vole species in NE Oregon, while the Gray-tailed Vole (Microtus canicaudus) was the dominant species in NW Oregon. In terms of frequency of occurrence, voles were more often preyed upon at the NW sites (92%) than at the NE site (73%), as were other non-vole prey (Table 2); these differences, however, were not found to be significant ([chi square] = 2.2; P > 0.05; [chi square] = 0.8; P > 0.05; respectively). The remaining percentages of prey were represented by bats (Chiroptera), shrews (Insectivora: Soricidae), and small- to medium-sized birds (Table 2). Although no invertebrate prey were recorded, using only pellet content analysis to determine Barn Owl diet may completely miss the potential contribution of invertebrates such as earthworms, arachnids, and other soft-bodied prey that are usually completely digested and not present in pellets (Mikkola 1983; Taylor 1994).
The number of prey, prey biomass, and species diversity in pellets were similar between ecozones. The average numer of prey in pellets was 2.0 (n = 172) in the NE and 1.8 (n = 184) in the NW. Voles comprised the sole prey in 72.8% of NE pellets and 83.1% of NW pellets. The average number of voles in these pellets was 1.7 (n = 143) in the NE and 1.9 (n = 134) in the NW, but pellets containing only 1 vole were more frequent in the NW (50.6%) than in the NE (35.9%) (Table 3). Only 9.5% of NE pellets contained both voles and other prey as compared to 18.0% in NW pellets. Less than 15% of pellets in both ecozones contained no voles (NE = 13.0%; NW = 5.2%). The average biomass of prey in pellets was 46.7 g (n = 328 prey items) in the NE and 40.8 g (n = 342 prey items) in the NW. The index of overall prey species diversity (H) was 0.52 in the NE and 0.59 in the NW, although in the NW species diversity varied from 0.34 to 1.12 at 4 sites.
The Deer Mouse (Peromyscus maniculatus) and Northern Pocket Gopher (Thomomys talpoides) were secondary to voles as part of the rodent diet of the Barn Owl. Deer Mice were found more frequently in NW pellets, while Northern Pocket Gophers were found more frequently in NE pellets. Other rodent species, such as the House Mouse (Mus musculus) and Dusky-footed Woodrat (Neotoma fuscipes) were preyed upon only occassionally (Table 2).
The composition of the diet of Barn Owls in both Oregon ecozones in this study was similar to results obtained by Maser and Brodie (1966) through analysis of pellets collected only in northwestern Oregon, 1964-1965. In spring and summer, voles comprised the bulk of the Barn Owl diet (77.3 and 91.0% in 1964 and 1965, respectively, compared to 86.0% in 2010, this study), and although other rodents, shrews, and birds were found to be supplemental in the diet, the Northern Pocket Gopher was not recorded in 1964 and 1965. The diet of the Barn Owl in Oregon studied by Giger (1965), Maser and Brodie (1966), and Bull and Holly (1985) also revealed that voles (Microtus montanus) comprised the majority of prey. The diet was supplemented by mice (mainly Peromyscus maniculatus), birds, and shrews (mainly Sorex spp.). Occassionally, other species such as Neotoma fuscipes, Sylvilagus spp., Eutamias townsendi, Scapanus townsendi, Rattus norvegicus, and Zapus trinotatus were taken. In other parts of the Nearctic Region, for example in Texas (Otteni and others 1972), Colorado (Martini 1974), Arizona (Franzreb and Laudenslayer 1982), and Rhode Island (Johnston and Hill 1987), Barn Owl diet composition has also been reported to be similar to their diet in Oregon (Taylor 1994).
Contrary to my original hypothesis, Barn Owl diet did not vary significantly between eco-zones, and the number of prey, prey biomass, and prey species diversity were relatively similar between the NE and NW Oregon sites. However, because the study sample size was relatively small and limited to a single season (summer), it would not be appropriate to overly generalize the results. It is known that population cycles, as well as population dynamics and patterns vary among rodent species (Lin and others 2001; Lowell and others 2006), and that this variability could contribute to potential differences in the composition of the diet of Barn Owl populations at different times and in different locations. Additional research designed to examine seasonal and multi-year trends would be useful for elucidating any potential associated fluctuations or differences in the composition of the Barn Owl diet.
Acknowledgements.--I greatly appreciate the help of the following people in collecting pellets: Bruce Parks, Patricia Kennedy, and Susan Haig (Oregon State University), and E Monroe (WL Finley National Wildlife Refuge). Eric D Forsman from the USFS Forestry Sciences Laboratory in Corvallis, OR, confirmed identification of some prey items.
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Department of Vertebrate Ecology, Wroclaw University of Environmental & Life Sciences, UI. Kouchowska 5b, 51-631 Wroclaw, Poland; e-mail: grzegorz.kopij@ up.wroc.pl; and Department of Wildlife Management, University of Namibia, Katima Mulilo Campus, Private Bag 1096, Winela Road, Oshakati, Namibia; e-mail: email@example.com. Submitted 22 February 2012, accepted 5 October 2012. Corresponding Editor: Joan Hagar.
TABLE 1. Study site locations, collection dates, and number of pellets collected. Locality Coordinates Eastern Oregon Agricultural 45[degrees]12'36" N, Reasearch Station 117[degrees]51'53" W WL Finley National Wildlife 44[degrees]24'66" N, Refuge-I 123[degrees]19'55" W WL Finley National Wildlife 44[degrees]25'11" N, Refuge-II 123[degrees]19'31" W ODFW EE Wilson Wildlife Area 44[degrees]57'08" N, 123[degrees]27'60" W ODFW EE Wilson Wildlife Area 44[degrees]42'02" N, 123[degrees]12'34" W No. Locality Collection date pellets Eastern Oregon Agricultural 20 September, 2010 184 Reasearch Station WL Finley National Wildlife 29 September, 2010 100 Refuge-I WL Finley National Wildlife 30 September, 2010 35 Refuge-II ODFW EE Wilson Wildlife Area 30 September, 2010 27 ODFW EE Wilson Wildlife Area 30 September, 2010 10 TABLE 2. Contents of pellets of the Barn Owl at northeastern and northwestern Oregon study sites. NE Oregon Frequency of Percent of Prey taxa occurrence prey items Mammalia Microtus spp. 73.0 86.3 Thomomys talpoides 22.2 10.1 Thomomys spp. Peromyscus maniculatus 1.6 1.2 Neotoma fuscipes Mus musculus 1.1 0.9 Sorex spp. Chiroptera spp. Aves Junco hyemalis Picoides villosus Columba livia 1.6 1.2 Passeriformes spp. 0.5 0.3 Number of pellets 184 Number of prey items 328 NW Oregon Frequency of Percent of Prey taxa occurrence prey items Mammalia Microtus spp. 91.9 86.0 Thomomys talpoides Thomomys spp. 2.3 1.2 Peromyscus maniculatus 15.1 9.1 Neotoma fuscipes 0.6 0.3 Mus musculus 0.6 0.3 Sorex spp. 2.9 1.8 Chiroptera spp. 0.6 0.3 Aves Junco hyemalis 1.2 0.6 Picoides villosus 1.2 0.6 Columba livia Passeriformes spp. Number of pellets 172 Number of prey items TABLE 3. Number of Barn Owl pellets containing all prey and voles only with corresponding percentages. In northeastern Oregon, pellets were collected at the Eastern Oregon Agricultural Experiment Station at Union; in northwestern Oregon, pellets were collected in Benton County at 2 locations each in the William L Finley National Wildlife Refuge and Oregon Department of Fish and Wildlife EE Wilson Wildlife Area. NE Oregon Number Pellets--all Pellets--voles prey/pellet prey no. (%) only no. (%) 1 61 (35.5) 78 (54.5) 2 65 (37.8) 42 (29.4) 3 34 (19.8) 17 (11.9) 4 11 (6.4) 5 (3.5) 5 1 (0.6) 1 (0.7) Total 172 143 NW Oregon Number Pellets--all Pellets--voles prey/pellet prey no. (%) only no. (%) 1 87 (47.3) 51 (38.1) 2 61 (33.2) 49 (36.6) 3 26 (14.1) 26 (19.4) 4 9 (4.9) 7 (5.2) 5 1 (0.5) 1 (0.7) Total 184 134
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|Title Annotation:||GENERAL NOTES|
|Publication:||Northwestern Naturalist: A Journal of Vertebrate Biology|
|Date:||Mar 22, 2013|
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