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Graham, Scott Alan – Dietary composition and prey selection of Northern Barred Owls (Strix varia) in the Pacific Northwest

Alumni Research

Prior to the early 1900’s the Northern Barred Owl (Strix varia) was a widely distributed resident of eastern North America. During the last century it has expanded its range westward to the Pacific Coast and currently inhabits forests throughout much of western North America (Livezey, in press). This range expansion has been well documented by numerous authors (e.g., Taylor and Forsman 1976, Hamer et al. 1989, Wright and Hayward 1998, Dunbar et al. 1991, Mazur et al. 1997b, Takats 1998, Mazur and James 2000, Hobbs 2005, Gremel 2005, Schmidt 2005, Anthony et al. 2006, Steger et al., in press), but the cause of the range expansion is unknown. As the barred owl has expanded its range westward it has come into sympatry with the closely related, and morphologically similar, Northern Spotted Owl (Strix occidentalis caurina), a federally threatened subspecies (U. S. Fish and Wildlife Service 1990). This has led to concern that the barred owl may represent a threat to the spotted owl because the two species may compete for resources or for space (Dunbar et al. 1991, Gremel 2005, Anthony et al. 2006). Because the diet of the barred owl is almost completely unknown in most of its new range in western North America, I propose to study and describe the diet of barred owls at multiple locations in the Pacific Northwest USA. I will describe dietary composition of barred owls, to include local, seasonal, annual, geographic, and potentially sexual variation in diets during 2007-2008. Comparisons of food-niche breadth (FNB), an index of diversity in the diet, will be examined among and within owl populations. I will also implement an experimental approach to determining prey preference by spotted owls and barred owls based on prey size. A field experiment to determine if preference occurs will provide new information for assessing the relative importance of prey species. This information will help ecologists and land managers to better understand the ecological role played by barred owls in their new environment, including the potential for competition with spotted owls for food. Food habits and prey preference data, in turn, will provide a basis to manage prey populations structured to sustain the 2 owl species, thus promoting the conservation of spotted owl populations in the Pacific Northwest.


Thesis Abstract