FORAGING ECOLOGY OF PEREGRINE FALCONS (FALCO PEREGRINUS) ALONG THE COLORADO RIVER, GRAND CANYON, ARIZONA

We compiled 355 observations of foraging events of peregrine falcons (Falco peregrinus) in Grand Canyon, downstream from Glen Canyon Dam on the Colorado River during 1973-1995, and documented abundance of prey, foraging behavior, and success in foraging. Peregrine falcons attempted to forage on >= 44 species, including >= 40 species of birds: swifts and swallows (29%), waterfowl (Anseriformes, Podicipediformes, and similar birds, 27%), other waterbirds (waders and other large, non-anseriform waterbirds, 11%), bats (10%), large wasps (8%), and other species (15%). Frequency of foraging during the breeding season was positively related to overall abundance of prey, but relative intensity of predation (ratio of relative frequency of attack on a species to mean abundance of that species) increased on most species of prey in the usually turbid lower reach, where prey were less abundant. Air-to-air strikes were most common (94% of 271 cases for which data were available), followed by air-to-water strikes (4%), and air-to-ground strikes (2%). Tandem attacks occurred in >= 43% of cases. Overall foraging success was >= 52%, and was highest on large wasps (100%) and bats (87%), intermediate on large waterbirds (63%), swifts, swallows, and small terrestrial birds (42%). and waterfowl (40%), and lowest on belted kingfishers (Ceryle alcyon; 18%) and small shorebirds (<3%). Foraging success was related to abundance of prey and number of co-foraging falcons, and was negatively related to relative body mass of prey. Most dictary biomass was derived from waterfowl (64%) and large waterbirds (25%), whereas swallows and swifts contributed only 3%. The low and variably turbid segment near the dam had greater abundance of prey and lower relative intensity of predation compared to the usually turbid segment downstream, but distribution and overall frequency and success of foraging by peregrine falcons were not affected by regulation of flow.