WINTER FORAGING ECOLOGY OF BALD EAGLES IN ARIZONA

Foraging of Bald Eagles (Haliaeetus leucocephalus) was examined in winter of 1990 and 1991 along the Colorado River in Grand Canyon, Arizona. Eagles acquired food by hunting live prey (86.6%), scavenging (7.7%), and interspecific piracy(5.7%). Most(99.2% of 1,313) foraging attempts were for rainbow trout (Oncorhynchus mykiss). Most foraging attempts occurred in creek habitat where foraging success for live trout was higher than in river habitat, a behavior that presumably increased food intake. Adults exhibited greater foraging success than immatures regardless of attack method, habitat, or prey abundance. Aerial attacks were more successful in creek habitat, and ground attacks were more successful in river habitat. Methods, success, and geographical and hourly patterns of foraging were dependent on prey abundance between years. Eagles modified their foraging strategies to maximize success in a pattern of dynamic optimization under changing conditions.