Peregrine Falcon (Falco peregrinus) may affect local demographic trends of wetland bird prey species

Abstract

The demographic value of existing habitat can be affected by changes in predator populations. In Finland, wader populations increased when the Peregrine Falcon (Falco peregrinus) declined, due to poisoning by agricultural chemicals during the 1960-1970. Following regulation of DDTusage, nesting Peregrine numbers have increased more than tenfold, while wader populations concurrently declined. The "protection" hypothesis states that although top predators may negatively affect populations of primary prey species, they may benefit other species populations by suppressing activities of mesopredators, such as egg and chick predation. We studied diet preferences of Peregrine Falcons in south-west Finnish Lapland during a period of falcon population growth, based on indices of species consumption versus availability. Preferred and optimal sized prey species, excluding ducks, were equally abundant on bogs with and without Peregrines, and neither did the abundance of non-preferred or non-optimal sized birds differ between bogs with and without Peregrines. Thus, the protection hypothesis was supported for ducks. We found that preferred prey species significantly declined in numbers over time, while populations of non-preferred and non-optimal sized prey species declined less or remained stable, as expected if Peregrines would have direct negative effects on prey species. However, local effects were apparent when comparing local versus national trends of suitable and less suitable sized prey for the Peregrines. While the raptors'role in the population declines of wetland species must have increased after their recovery in the 1970s, this effect might be difficult to separate from other concurrent effects of habitat loss.