Spatial and temporal variation in diets of Spotted Owls in Washington

We studied diets of Northern Sported Owls (Strix occidentalis caurina) in t hree different regions of Washington State during 1983-96. Northern flying squirrels (Glaucomys sabrinus) were the most important prey in most areas, comprising 29-54% of prey numbers and 45-99% of prey biomass. Other importa nt prey included snowshoe hares (Lepus americanus), bushy-tailed woodrats ( Neotoma cinerea), boreal red-backed voles (Clethrionomys gapperi, and mice Peromyscus maniculatus, P. oreas). Nonmammalian prey generally comprised le ss 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 restruc turing of the diet. Predation on snowshoe hares was primarily restricted to small juveniles captured during spring and summer. Mean prey mass did nor 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 cas es (larger mean fur nesting owls), suggesting a trend toward larger prey in samples collected from nesting owls. We suggest that differences in diet a mong years, seasons, and territories are probably due primarily to differen ces in prey abundance. However, there are other factors that could cause su ch differences, including individual variation in prey selection, variation in the timing of pellet collections, and variation in prey accessibility i n different cover types.