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002<t85 Wintering and migrant bald eagles have largely left the Grand Canyon region by late March (Stevens et aI. 1997b). The few remaining eagles in April forage opportunistically and may continue to catch trout in the mainstream. The rainbow trout conclude their spawning run in Nankoweap Creek in April as water temperatures warm (Leibfried and Mongomery 1993), and remaining bald eagles no longer have access to that food source. Peregrine Falcons Distribution and Abundance The Grand Canyon peregrine population was thought to be low in the mid-1970's (Ellis and Monson 1989), but apparently increased dramatically in the 1980's, following recovery efforts by the U.S. Fish and Wildlife Service (1984; Glinski 1993). At present, the Grand Canyon supports the largest breeding population of peregrine falcons on a single land-management unit in the coterminous United States (Brown et aI. 1991a, 1992). Surveys for nesting peregrine falcons in 1988 and 1989 revealed 28 and 58 pairs, in 15% and 24% of the park, respectively. Habitat- based estimation of the potential number of peregrine falcons in Grand Canyon suggested that as many as 96 pair existed in Grand Canyon in 1989. Life Requisites Peregrine falcons feed on more than 40 species of birds and several small mammals (Porter and White 1973, Stevens et aI. 1998). Hunting areas included marshes or narrow tongues of streamside vegetation, and peregrine falcons may forage up to 17 mi from nest sites. In Grand Canyon, peregrine falcons feed on waterfowl, swifts, swallows and bats (Brown 1991a, Stevens et aI. 1998), many of which feed on invertebrate species (especially Diptera) that emerge out of the Colorado River (Stevens et al. 1997c). Therefore, dam operations that influence aquatic macro-invertebrate populations exert, at most, only indirect impacts on peregrine falcons. Peregrine falcons breed up to 3,130 m elevation, typically on ledges on steep cliff faces (U.S. Fish and Wildlife Service 1984). The mean distance between nest sites along the South Rim of Grand Canyon varied from 3.5 to 5.0 linear miles, with minimum distances of 1.8 linear miles (Brown 1991a, 1992). The breeding season in Grand Canyon extends from February to July. Most wintering waterfowl on which peregrine falcons feed will have migrated from Grand Canyon by late March; however, mallard and late migrating gadwall and American widgeon are still likely to be common (Stevens et al. 1997a, 1998). Springtime food sources (swifts, swallows and bats) should be present in large numbers at that time of year (Stevens et al. 1998), and are only indirectly influenced by dam operations. Southwestern Willow Flycatcher Distribution and Abundance The willow flycatcher (Tyrannidae: Empidonax trailii) is a Neotropical migrant with a broad breeding range, extending from Nova Scotia to British Columbia and south to Baja California. The Southwestern Willow Flycatcher is a subspecies (SWWF: Tyrannidae: Empidonax trailii extimusl. The SWWF is an obligate riparian insectivore (Hunter et aI., 1987), preferring habitat