INFLUENCES OF FLUCTUATING RIVER FLOWS ON BALD EAGLE FORAGING BEHAVIOR

We examined habitat use, prey capture, and foraging success of Bald Ea gles (Haliaeetus leucocephalus) in winters of 1990 and 1991 to evaluat e influences of hourly fluctuating river flows from Glen Canyan Dam al ong the Colorado River in Grand Canyon National Park Arizona. Patterns of habitat use were strongly dependent upon fluctuating Bows in both years. Foraging in river, shore, and isolated pool habitats decreased to 0% at flows > 568 m(3) sec(-1), whereas foraging in adjacent creek habitat increased to 100%. More foraging attempts occurred farther fro m the river in adjacent creek habitat as river flows increased, but th is did not influence foraging success in creek habitat. Rainbow trout (Oncorhynchus mykiss) stranded by fluctuating river flows comprised 12 % and 19% of eagle prey captures in 1990 and 1991, respectively. Forag ing success in river habitat decreased in 1990 as river flows increase d from < 284 m(3) sec(-1) (74%) to > 284 m(3) sec(-1) (39%); foraging success in river habitat was independent of fluctuating flows in 1991. Low river flows exerted neutral or positive influences on eagle habit at use and prey capture, whereas high river Bows reduced eagle foragin g habitat diversity, lowered foraging success in river habitat, and re stricted foraging opportunities. Management strategies to limit high r iver Bows and sustained flooding during peak eagle concentration may b enefit wintering eagles.