SEASONAL-VARIATION IN THE DIET OF GREAT HORNED OWLS (BUBO-VIRGINIANUS) ON SHORTGRASS PRAIRIE

We collected and analyzed pellets from known roosts of great horned ow ls (Bubo virginianus) on shortgrass prairie in N-central Colorado betw een 1992 and 1994 to examine seasonal trends in owl diets. We also com pared the species composition of small rodents in owl diets to the rel ative abundances of rodents on the study area. In all seasons, lagomor phs (desert cottontails Sylvilagus audubonii, black-tailed jackrabbits Lepus californicus, white-tailed jackrabbits Lepus townsendii) compri sed most of the diet biomass. Northern pocket gophers (Thomomys talpoi des) were also preyed upon in all seasons, with the highest percentage consumed in spring. Most of the individual prey items taken were rode nts, primarily deer mice (Peromyscus maniculatus). Comparisons of diet composition with measures of rodent abundance suggested that owls for aged over a relatively restricted part of our study area, especially i n winter Most prey apparently were taken from roadsides, buildings and an adjacent four-wing saltbush (Atriplex canescens) floodplain, where rodent densities remained relatively higher in all seasons than in th e surrounding vegetation. We speculate that the preferential use of th ese foraging habitats may reflect the availability of suitable hunting perches on the site.