PREY USE AND SELECTION IN RELATION TO REPRODUCTION BY PEREGRINE FALCONS BREEDING ALONG THE YUKON RIVER, CANADA

Some raptors are considered to be generalist predators and switch among different species of prey according to their availability in the environment. In contrast, other raptor species are specialists with narrow dietary niche widths, selecting only one or a few types of prey; reproductive success in these species is often closely related to variability in abundance of preferred prey species. Determining whether raptors are generalist or specialist predators is important for understanding the degree to which fluctuations in their population sizes are influenced by individual responses to changes in prey availability. We assessed diet components based on prey remains collected at nest ledges to determine whether Peregrine Falcons breeding along the Yukon River in northern Canada were specialist feeders or, conversely, whether they took prey according to its relative abundance. We also examined whether the use of certain types of prey had consequences for reproductive success. Compositional analyses showed that peregrines exhibited nonrandom prey selection, with shorebirds, jays and thrushes, and waterbirds being used more than expected based on their abundance, whereas sparrows and other small passerines were used significantly less than expected. Similarly, medium-sized prey was selected by peregrines, but small (< 50 g) and large (> 1000 g) prey were used significantly less than expected. Pairs occupying territories in which the diet. included large numbers of waterbirds, and few sparrows and small passerines, fledged more young. Although prey > 1000 g were among the least selected by pairs in our study population, pairs that used these larger prey species fledged more offspring. These patterns may relate directly to the contribution of female parents to provisioning. Females that are able to capture large prey items may deliver larger amounts of biomass to their nests and, as a consequence, may have high reproductive output. We conclude that Peregrine Falcons in the Yukon are selective predators and that some of the variation in reproductive rates in this population may be attributed to variation in the types of prey available to and utilized by individual pairs.