Prey of Ferruginous Hawks Breeding in Washington

We collected and analyzed pellets and prey remains from breeding territories of 39 ferruginous hawks (Buteo regalis) in the Columbia Basin of Washington between 1992 and 1995. Among 4402 identified items were remains of 12 mammal species, primarily northern pocket gophers (Thomomys talpoides). Orthopterans, primarily Tettigoniidae, outnumbered all other identified prey items. Birds and snakes were difficult to count accurately, but contributed significantly to diets at some territories. Prey items of ferruginous hawks at the Hanford Nuclear Reservation were almost exclusively pocket gophers and insects. Hawks elsewhere in Washington often had more varied diets incorporating small mammal species, primarily Great Basin pocket mice (Perognathus parvus). At the 4 territories where prey were quantified each year (all at the Hanford Nuclear Reservation), the relative contribution of insect prey varied. Few lagomorphs were encountered, suggesting a dietary shift since the 1920's for ferruginous hawks nesting in Washington.