Foraging Ecology of Peregrine Falcons (Falco Peregrinus) Along the Colorado River, Grand Canyon, Arizona (Report‪)‬

Peregrine falcons (Falco peregrinus) are versatile, generalist predators, well known for hunting a broad array of species using diverse foraging behaviors, and often achieving high levels of foraging success (e.g., Monneret, 1973; Rogers and Leatherwood, 1981; Smith, 1981; Tucker, 1981; Cade, 1982; Ratcliffe, 1993). Peregrine falcons prey on a wide variety of birds that are usually, but not always, smaller than themselves (Cade, 1951; Campbell, 1975; Dekker, 1980, 1987, 1988; PruettJones et al., 1981; Verlarde, 1993; Rosenfield et al., 1995; Ellis et al., 2002), and occasionally on large invertebrates, reptiles, and mammals (Stager, 1941; Porter and White, 1973; Bird and Aubry, 1982; Ritchie, 1982; Sherrod, 1983; White and Brimm, 1990; Bradley and Oliphant, 1991; Jenkins and Avery, 1999; Lee and Kuo, 2001). Peregrine falcons employ a wide array of foraging techniques (Hunt et al., 1975; Baker, 1980; Schenk et al., 1985; Dekker, 1980, 1995). Foraging success is 7-100%, depending on sex and age of falcons, season, prey, and number of cooperatively hunting birds (Cade, 1982; Roalkvam, 1985; Dekker, 1990), but foraging studies often are constrained by the number of independent observations. Peregrine falcons have large home ranges, often in canyon-bound or otherwise topographically complex terrains, factors that limit opportunities for observation of their foraging behavior. Consequently, foraging studies have focused on nest provisioning (e.g., Cade, 1951; PruettJones et al., 1981; Schenk et al., 1985; Skaggs et al., 1986; Oro and Tella, 1995; Rosenfield et al., 1995; Ellis et al., 2002), behavior of single falcons or family groups at few localities (e.g., Cade, 1951; Jenkins and van Zyl, 1994; Rosenfleld et al., 1995), migrating peregrine falcons (e.g., Dekker, 1980, 1988), and on reviews (Roalkvam, 1985; Schenk et al., 1985; Dekker, 1990; Ratcliffe, 1993). While this research has provided considerable insight into foraging ecology of peregrine falcons, the full range of foraging behavior and diet remains unclear, particularly among non-nesting, post-nesting, and adult peregrine falcons foraging far from nests in complex landscapes. As opportunistic generalists, peregrine falcons are expected to forage in a prey-density-dependent fashion. Based on nest remains at 159 upland aeries in Greenland, observations of two active nests, and surveys to determine density of prey, Rosenfield et al. (1995) calculated the contribution of biomass of prey to the diet of nestlings. They reported that peregrine falcons generally forage in a density-dependent fashion, often differentially taking younger prey, with some variation among species of prey potentially related to flight behavior (e.g., snow buntings, Plectrophenax nivalis, Meese and Fuller, 1989). However, studies investigating the extent of prey-density-dependent foraging, and those presenting comprehensive data on foraging success, are few. There also are few studies of foraging ecology in topographically complex terrain that are based on a large number of independent observations, or that relate foraging attempts and success to availability of a broad spectrum of prey. No study has described responses of foraging behavior of peregrine falcons to dam-induced alteration of riverine ecosystems.