Prey Selection by Peregrine Falcons during the Nestling Stage

We determined species and relative abundance of prey brought to eyries by peregrine falcons (Falco peregrinus) during the nestling stage and compared these with the relative abundance of birds along a portion of the Yukon River, Alaska in 1985 and 1986. We identified >55 taxa (species and groups of species) during bird surveys. Prey remains included 1,536 individuals from 77 taxa. Of 47 prey taxa selected (taken in greater frequency or biomass than available within 3 km of eyries), 8 were selected by frequency and biomass during both years. Lesser yellowlegs (Tringa flavipes) were the primary prey. Riparian habitats (lakes, ponds, and rivers) produced the largest numbers of key prey and should be managed to maintain or enhance prey populations. J. WILDL. MANAGE. 52(4):730-736 Food supply may regulate populations of peregrine falcons (Ratcliffe 1980) and limit distribution and density of nesting pairs (Bond 1936, Hickey 1942, Newton 1976). Population levels are dependent upon survival of young, so food supply is especially important during the nestling stage. Prey species used by peregrines differ regionally (Beebe 1960, Cade 1960, White and Cade 1971, White 1975, Ratcliffe 1980). In Alaska, peregrines prey on >75 species, primarily birds (Cade 1951, 1960; Cade et al. 1968; White and Roseneau 1970). Diets of peregrines have been described, but quantitative data on prey selection (frequency or biomass of prey in the diet in relation to prey availability) are limited. Numbers of each species taken may reflect only prey abundance. Prey selection reveals those species taken in greater proportion than available. Effective management of peregrine falcons, which are listed as endangered or threatened by the U.S. Fish and Wildlife Service (1986), include management of adequate prey. Information on prey selection provides managers with the means to identify prey species that may be managed to benefit peregrine falcons. Our objectives were to determine species and number of prey brought to eyries by peregrines during the nestling stage and to compare these data with relative abundance of prey. Our research was supported as part of a cooperative agreement between the U.S. Fish and Wildlife Service and Oregon State University. R. G. Anthony, C. J. Henny, R. L. Knight, K. McGarigal, S. K. Nelson, and C. M. White reviewed drafts of the manuscript. We appreciate the help of M. W. Britten, D. D. Gibson, B. Kessel, P. A. Liedberg, B. A. Long, C. L. McIntyre, R. T. Ratcliffe, and S. R. Ulvi. This is Technical Paper 8227 of the Oregon Agricultural Experiment Station. STUDY AREA AND METHODS The study was conducted in eastern Alaska along a 265-km portion of the Yukon River between the Alaska-Yukon Territory border and Circle, Alaska. Most (75%) of the study area was within the Yukon-Charley Rivers National Preserve. Land adjacent to the Yukon River ranges in elevation from 180 to 950 m. In June 1985 and 1986, active eyries (a pair of peregrine falcons present at a nest ledge) were located by observing sites from a boat or river bank. Successful eyries (>1 young surviving to fledging age) were identified in July and August. Diets of peregrine falcons were determined from prey remains (Errington 1932) collected at 16 successful eyries each year, of which 11 were the same sites both years (Hunter 1987). To minimize loss of identifiable parts, prey remains were collected twice: 3 and 6.5 weeks after hatching. Young had fledged by the second collection. The 2 collections were combined to represent the diet of peregrines during the nestling stage. Except for 1 eyrie where the pair nested unusually late in 1985, all first collections were made 1-14 July and all second collections were made 1-10 August. Preliminary analyses IPresent address: 18622 N. 45th Avenue, Glendale, AZ 85308.