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'" /~""'\ ') ~ ~ JI ir \! ~ The peregrine falcon (Falconidae: ~ oerearinus) is a federally listed endangered raptor, which has declined dramatically as a result of the biological concentration of pesticide residues in prey species, and resulting eggshell thinning (U.S. Fish and Wildlife Service 1984) and the increase in food base in Grand Canyon. The population in the Rocky Mountain/southwest region declined from 180 pairs prior to 1975 to 55 pairs in 1983, largely as a result of DDT/DDE thinning of eggshells (U.S. Fish and wildlife 1984) . ~ ;-;, !~ ! Distribution and Abundance The Grand Canyon peregrine population was low in the mid-1970's (Ellis and Monson 1989), but increased dramatically in the 1980's, following recovery efforts by the U.S. Fish and Wildlife Service (1984). At present, the Grand Canyon supports the largest breeding population of peregrine falcons in the coterminous United States (Brown 1991a). 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. J;.: .. :\ ~' "i ,{ \':~ ..:: ;~ 'o}:;' ~ ~~ Il; I.ife Requisites :.;;. l~~~ Peregrine falcons feed on more than 20 species of birds and several small mammals (Porter and White 1973). Hunting areas included marshes or narrow tongues of streamside vegetation, and peregrine falcons may forage up to 17 mi from nest sites. 'peregrine falcon diet at nest sites in national parks in southern Utah included small and medium-sized birds, especially including white-throated swifts, large shorebirds and Clark'S nutcracker (Burnham 1987). ',Sf,:, ';..;, '~'" if'. ;.'", " In Grand Canyon, peregrine falcons feed on waterfowl, swifts, swallows and bats (Brown 1991a, Stevens personal communication), which feed on invertebrate species (especially Diptera) that emerge out of the Colorado River (Blinn et al., 1992). Therefore, dam operations that influence aquatic macro- invertebrate populations exert indirect impacts on peregrine falcons. f\ , . p.-~,: t< ~;. ~ ii;t ~~ ~~~ f? --.:' t}~ T;:'~ ~' ~ ~< t~ -~':i 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). The breeding season in Grand Canyon extends from February to July. The primary reason for the national decline of the peregrine falcon population has been eggshell thinning from DDE and other environmental contaminants, which are biologically concentrated through the food chain. DDE sources to peregrine falcons are derived from their prey, many of which are migratory insectivores. Burnham (1987) reported that swifts, shorebirds, and other migratory insectivores contained 5.8 ppm DDE (wet weight), while mean DDE levels in granivorous migrants, such as grosbeaks and mourning doves, was only 0.14 ppm DDE. Peregrine eggshells from southern Utah parks from 1985 to 1987 were 21% thinner than those from the pre-DDE era, indicating poor viability of eggs (Burnham 1987). Brown (1991b) reported that peregrine falcon eggshells ":':':. . ~;' "I 14