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Graham, Scott Alan – Diet composition, niche and geographic characteristics, and prey size preference of Barred Owls (Strix varia) in the Pacific Northwest. 2012.


Photo of Barred OwlThis thesis comprises three chapters describing my investigations of dietary composition, niche and geographic characteristics, and prey size preference of Barred Owls (Strix varia) following their recent range expansion into the Pacific Northwest.

In the first chapter, I examine annual, seasonal, within-breeding season, and local variation in the diet and evaluate reproductive success as a function of dietary composition in western Oregon during 2007–2009. Diets were based on 3,686 prey individuals identified in 1,127 regurgitated pellets collected from 26 owl family areas. Prey identified in pellets included ≥ 85 taxa (33 mammals, 25 birds, 4 reptiles, 4 amphibians, 1 fish, 3 gastropods, 1 diplopod, 1 collembolan, 12 insects, and 1 crustacean). Based on percent of prey numbers in pellets, owl diets comprised 64.8% mammals, 2.9% birds, 1.0% reptiles, 9.8% amphibians, 0.3% fish, 6.6% gastropods, 0.2% diplopods, < 1% collembolans, and 14.4% insects. Mean mass of prey in pellets was 55.8 g. Diets varied between years and seasons and among within-breeding season periods and owl family areas, but were generally dominated by coleopteran beetles, mammalian insectivores, and northern flying squirrels (Glaucomys sabrinus). Taxonomic richness of the diet provided an indication of the versatility of Barred Owls capable of preying on diverse kinds of prey in their expanding geographic range. Estimated food-niche breadths were generally narrow to moderate, indicating use of comparatively few to a variety of taxa in large numbers. Spatiotemporal variations in diet appeared to reflect this species’ adaptation andopportunistic feeding strategies in an area of range expansion. These results will enable ecologists and land managers to better understand the ecological role played by Barred Owls in their new environment, including potential effects such as competition for food with other native fauna of the Pacific Northwest, especially the threatened Northern Spotted Owl (Strix occidentalis caurina).

In the second chapter, I compare diet composition among three geographic populations of Barred Owls in the Pacific Northwest, including the central Coast Ranges in Oregon, Olympic National Park in northwest Washington, and eastern Cascades in central Washington during the breeding seasons of 2007–2009, 1997–2009, and 2004–2006, respectively. For this analysis, I examined 1,021 regurgitated pellets from 25 owl family areas in the central Coast Ranges, 48 pellets from 20 areas in Olympic National Park, and 57 pellets from 9 areas in the eastern Cascades. The estimated number and total biomass of prey in pellets was 3,463 prey and 192,951 g in the central Coast Ranges, 187 prey and 11,444 g in Olympic National Park, and 336 prey and 12,871 g in the eastern Cascades. The number of taxa owls used as prey differed among the study areas, with 81 taxa in the central Coast Ranges, 36 in Olympic National Park, and 32 in the eastern Cascades. Diets were similar between the central Coast Ranges and Olympic National Park areas in that diets mainly included forest mammals (64.3% and 71.7% of prey numbers, respectively), with a variety of shrews, coast moles (Scapanus orarius), and northern flying squirrels predominating in prey numbers and biomass. Owl diets differed in the eastern Cascades where insects were the most numerous taxa in the diet (47.0% of prey numbers), with beetles predominating by prey number (45.2%) followed by frogs(18.8%) and flying squirrels (12.2%). Flying squirrels were the primary source of biomass across all areas (24.8% in the central Coast Range; 34.0% in Olympic National Park; 41.4% in the eastern Cascades) and occurred in pellets most of the time. Mean mass of individual prey was 55.7 g in the central Coast Ranges, 61.2 g in Olympic National Park, and 38.3 g in the eastern Cascades. Food-niche breadth values indicated that although many prey taxa were taken, the food-niche dimension for each population of Barred Owls was narrow. Factors contributing to differences in diet among geographic locales likely included disparities in prey distributions, differences in the number of pellets collected, and likely temporal and local variation in prey use and prey availability.

In the third chapter, I investigate prey size preference behavior by sympatric, wild Northern Spotted Owls and Barred Owls in Oregon and Washington. I describe a controlled experiment in which I used feeding experiments to test the hypothesis that owls will select a larger prey animal when given a simultaneous choice between a small prey animal (Mus musculus) and a larger prey animal (Rattus norvegicus or Rattus rattus). I performed 30 independent feeding trials with Northern Spotted Owls (11 females, 19 males) and 17 independent trials with Barred Owls (12 females, 4 males, 1 gender unknown) during 1 March–31 August 2008. Northern Spotted Owls preferred the smaller prey in 24 trials and the larger prey in 6 trials. Barred Owls preferred the small prey in 9 trials and the larger prey in 2 trials. Both species exhibited significant preference for the smaller-sized mice. There was no difference in prey size selection between female and male Northern Spotted Owls; both sexes preferred smaller prey. Sample sizes for Barred Owls were too small to test for sexual differences. There was no interspecific difference in prey size selection between Northern Spotted Owls and Barred Owls. My results for Northern Spotted Owls were not expected because diets of Northern Spotted Owls are typically dominated by medium-sized mammalian prey such as northern flying squirrels and woodrats (Neotoma cinerea and N. fuscipes). Wild Barred Owls preferentially selected small prey during feeding trials, which is consistent with Barred Owl diet in different regions of North America, including the Pacific Northwest.

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