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Diet of the Great Horned Owl, Bubo virginianus, along a vegetation gradient in Oregon.

Key words: Bubo virginianus, diet, environmental gradient, food niche breadth, Great Horned Owl, Oregon

Trophic ecology of the Great Horned Owl (Bubo virginianus) has recently received considerable attention (Houston and others 1998; Johnsgrad 2002). Although its diet has been extensively studied throughout North America (for example, Kittredge and others 2006), little is known about variation of the diet composition along environmental gradients. Therefore, this note contributes further to our knowledge about the Great Horned Owl diet in its very wide geographical range. In this study, I analyzed the diet of the Great Horned Owl along a vegetation gradient from a farmland (an artificial grassland and parkland), through a natural grassland, to a semidesert with sparse grass and shrub vegetation. This constitutes a precipitation gradient from high rainfall in the farmland to low rainfall in the semidesert. To date, Great Horned Owl diet has been analyzed along a latitudinal gradient in Chile (Jaksic and others 1986), and along an altitudinal gradient in Argentine Patagonia (Donazar and others 1997).

Material for this study comprised pellets collected in 3 localities: Union, Hells Canyon, and Summer Lake, Oregon. The farmstead at Union, Union County (45[degrees]12'N, 117[degrees]52'W; elevation = 849 m), comprised a pastureland with short grass and some cultivated fields further afield on one side and suburbs on the other side. Pellets were collected on 20 September 2010, below trees growing in a clump on the campus of Eastern Oregon Agricultural Research Station. In Hells Canyon, Wallowa County (45[degrees]09'N, 116[degrees]43'W; 2128 m), the vegetation is represented by natural grassland with some sparse trees and shrubs. Pellets were collected from a barn located in the Hells Canyon National Recreational Area on 25 September 2010. The vegetation around Summer Lake, Lake County (42[degrees]97'N, 120[degrees]78'W; 1264 m), represents a semidesert with only sparse grass and vegetation. Pellets were collected on the shore of the lake on 28 September 2010. Pellets of the Great Horned Owl were easily distinguished from Barn Owl (Tyto alba) pellets by their size, texture, and color. In addition, at each site, the presence of owls casting the pellets was confirmed.

Diet composition was determined through pellet content analysis. Only compact entire pellets were used in the analysis. The pellets were analysed using a widely accepted procedure (see Yalden 2009). Identification of rodents was based on Maser and Storm (1970) and Verts and Carraway (1998). Other prey were identified with the aid of a reference collection maintained by the College of Forestry at Oregon State University, Corvallis, Oregon.

Prey species diversity was calculated using the following formula: H = -[sup.s][summation][p.sub.i][log.sub.e]pi, where [P.sub.i] is the proportion of the total number of individuals which belong to the [sub.i]-th species (Shannon and Weaver 1949). Dietary Overlap (DO) was calculated according to Pianka (1976): DO = 1/[summation][p.sup.2] x (1/n), where p is the proportion of a given prey taxon and n is the number of prey taxa.

Diet of the Great Horned Owl in Oregon consisted of mammal and avian prey. Only one other prey type, a dung beetle (Scarabaeidae spp.), was recorded (Table 1). Among mammals, rodents (Rodentia), especially voles (Microtus spp.), comprised the bulk of the diet, with the White-tailed Jackrabbit (Lepws townsendii) and bat (Chiroptera spp.) also being occasionally predated. At least 10 avian prey species were identified (Table 1). They ranged in size from the Western Bluebird (Sialia mexicana) to Great Blue Heron (Ardea herodias).

The contribution of rodents, especially voles, to the Great Horned Owl diet, both by numbers and by biomass, clearly declined along the vegetation gradient, being highest in the farmland and lowest in the semidesert. On the other hand, the contribution of birds increased along this gradient, being 8.2% by number in the farmland, 22.3% in grassland, and 33.8% in semidesert. Also, avian prey species diversity varied along this gradient, being lowest in the farmland and highest in the semidesert.

The Shannon and Weaver index of prey species diversity (H) was 0.67 for the farmland location, 0.93 for the grassland location, and 1.27 for the semidesert. Therefore, the range of prey in the diet of the Great Horned Owl was more diverse in the arid habitat. Similarly, the diet breadth index (DB) increased along this environmental gradient, from 5.95 in farmland, 10.51 in grassland, to 22.31 in semidesert.

The Great Horned Owl is a generalist predator with a wide range of mammalian and avian prey (Mart and Kochert 1996; Kremer and Belk 2003; Kittredge and others 2006). In Oklahoma, 16 bird and 30 mammalian prey species were identified out of 17,744 prey items (Kittredge and others 2006). In this study, out of 189 prey items, at least 10 avian and 9 mammalian prey were identified (Table 1). When these results are combined with earlier studies of the Great Horned Owl diet in Oregon (n = 875 prey items; Maser and Brodie 1966; Maser and others 1970), the number of mammalian prey = 25 and avian prey = 12.

In most states, lagomorphs constitute more than 50%, and small rodents <23% of prey biomass in the Great Horned Owl diet (Houston and others 1998). In Oregon (this study), only at the semidesert location, lagomorphs comprised >50%, while small rodents comprised <23% of prey biomass. In North American studies, birds comprised <12% of prey biomass (Houston and others 1998); in Oregon (this study) their contribution approximately doubled at the grassland location and tripled at the semidesert location.

The Great Horned Owl is an opportunist hunter, taking whatever prey is most abundant and most easily available; but it becomes a generalist predator when densities of prey are low (for example, at the semidesert location). At least in Oregon, prey diversity in the diet and the breadth of diet niche declines with increase in precipitation. This is probably because under semidesert conditions prey densities are usually much lower than in more humid habitats.

Acknowledgments.--I would like to thank Dr P Kennedy (Eastern Oregon Agricultural Research Center, Oregon State University) and Dr S Haig (US Geological Survey and Oregon State University) for their guidance during this study; and Dr E Forsman (US Forest Service and Oregon State University) for confirming prey identification.


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Department of Vertebrate Ecology, Wroclaw University of Environmental and Life Sciences, Kozuchowska 5b, 51-631 Wroclaw, Poland, grzegorz.; Department of Wildlife Management, University of Namibia, Katima Mulilo Campus, Winela Rd, Private Bag 1096, Katima Mulilo, Namibia, Submitted 5 March 2015, accepted 3 July 2015. Corresponding Editor: Joan Hagar.
TABLE 1. Summer diet of the Great Horned Owl along a
vegetation gradient in Oregon. F = frequency of occurrence
expressed as the percentage of pellets containing a given
taxon relative to the total number of pellets analyzed; N =
percentage of prey items of a given taxon in relation to the
total number of prey items; W = percentage of biomass of
prey items of a given taxon relative to the total biomass of
all prey (in grams).

         Prey taxa                  Farmland             Grassland

                              F      N      W      F      N      W

  Microtus spp.              94.0   90.2   64.0   65.2   74.2   34.7
  Thomomys talpoides         25.4   14.6   21.0    --     --     --
  Peromyscus maniculatus     11.9    7.3    2.7    --     --     --
  Sciurus spp.                1.5    0.8    4.1   17.4   12.9   43.1
  Lepus townsendii            --     --     --     --     --     --
  Chiroptera spp.             --     --     --     --     --     --

  Perdix perdix               --     --     --     4.3    3.2   10.8
  Falco sparverius            --     --     --     4.3    3.2    4.3
  Fulica americana            --     --     --     --     --     --
  Colaptes auratus            --     --     --     4.3    3.2    4.7
  Picoides villosus           --     --     --     4.3    3.2    2.5
  Sialia mexicana             --     --     --     --     --     --
  Unidentified large birds    --     --     --     --     --     --
  Unid. medium-sized
    birds                     1.5    1.6    8.2    --     --     --

  Scarabaeidae                --     --     --     --     --    0.0
Number of pellets/prey
  items/biomass                67    123   7290    23     31   2786

         Prey taxa                  Semidesert

                              F      N       W

  Microtus spp.              52.2   65.7    8.9
  Thomomys talpoides          --     --      --
  Peromyscus maniculatus      --     --      --
  Sciurus spp.               4.3    2.9     2.8
  Lepus townsendii           8.7    5.7    55.3
  Chiroptera spp.            4.3    2.9     0.2

  Perdix perdix               --     --      --
  Falco sparverius            --     --      --
  Fulica americana           4.3    2.9     3.7
  Colaptes auratus           4.3    2.9     1.2
  Picoides villosus           --     --      --
  Sialia mexicana            4.3    2.9     0.2
  Unidentified large birds  13.0    8.6    27.7
  Unid. medium-sized

  Scarabaeidae               4.3    2.9     0.0
Number of pellets/prey
  items/biomass               23     35  10,845
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Title Annotation:GENERAL NOTES
Author:Kopij, Grzegorz
Publication:Northwestern Naturalist: A Journal of Vertebrate Biology
Article Type:Report
Geographic Code:1USA
Date:Mar 22, 2016
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