COMPARATIVE DIETS OF BURROWING OWLS IN OREGON AND WASHINGTON

We studied diets of breeding burrowing owls (Speotyto cunicularia) in the Columbia Basin of southcentral Washington and northcentral Oregon during 1977-78 and 1980-81, respectively. Vertebrates, primarily roden ts, comprised only 9.6 percent of the total prey numbers contained in 6,328 regurgitated pellets but 87.3 percent of the biomass in the two samples combined. Vertebrate prey use was twice as high in Washington (17.1%) as in Oregon (8.4%). We attributed this to differences in habi tat, including soil type, and the effects of annual variations in rain fall on prey populations. Great Basin pocket mice (Perognathus parvus) dominated the vertebrate prey in both states and contributed the grea test overall biomass (35.0%). Orthopteran insects comprised the greate st invertebrate biomass (10.2%), while three families of beetles contr ibuted 49.0 percent of the total individual prey, but only 1.3 percent of the biomass. Soil type influenced differences in vertebrate compos ition in Oregon. Mean dry mass of over 76 prey taxa ranged from < 4 mg to > 40 g, indicating this species has a broad range of prey size.