Spatial and temporal variation in diets of Spotted Owls in Washington

2001 
--We studied diets of Northern Spotted Owls (Strix occidentalis caurina) in three different regions of Washington State during 1983-96. Northern flying squirrels (Glaucomys abrinus) were the most important prey in most areas, comprising 29-54% of prey numbers and 45-59% of prey biomass. Other important prey included snowshoe hares (Lepus americanus), bushy-tailed woodrats (Neotoma cinerea), boreal red-backed voles (Clethri0n0mys gapperi), and mice (Peromyscus maniculatus, P oreas). Nonmammalian prey generally comprised less than 15% of prey numbers and biomass. Mean prey mass was 111.4 _+ 1.5 g on the Olympic Peninsula, 74.8 +_ 2.9 g in the Western Cascades, and 91.3 _+ 1.7 g in the Eastern Cascades. Diets varied among territories, years, and seasons. Annual variation in diet was characterized by small changes in relative occurrence of different prey types rather than a complete restructuring of the diet. Predation on snowshoe hares was primarily restricted to small juveniles captured during spring and summer. Mean prey mass did not differ between nesting and nonnesting owls in 19 of 21 territories examined. However, the direction of the difference was positive in 15 of the 21 cases (larger mean fbr nesting owls), suggesting a trend toward larger prey in samples collected f•om nesting owls. We suggest hat differences in diet among years, seasons, and territories are probably due primarily to differences in prey abundance. However, there are other fhctors that could cause such differences, including individual variation in prey selection, variation in the timing of pellet collections, and variation in prey accessibility in different cover types.
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