Avian Predation Risk Modifies Breeding Bird Community on a Farmland Area

In farmland areas of western Finland more than four—fifths of breeding birds are migratory species, mostly small passerines. Migrants arrive at their breeding areas later than their main avian predators, European Kestrels (Falco tinnunculus). We provide evidence that avian predators are a major factor modifying the breeding bird assemblage of a large field. The distribution of kestrels (hence, predation) was manipulated using nest boxes, which are the only nesting places in open farmland. We counted birds on nine 25—ha plots in which a kestrel pair bred, and on nine control areas situated > 1000 m from the nearest kestrel nest in large agricultural fields during early summer 1992. We also counted birds on 15 transect lines (length 1000 m, width 100 m) radiating away from kestrel nests. Observations of the hunting behavior of breeding kestrels showed that kestrels mostly captured their prey items < 1000 m from their nest. Breeding bird density was higher on control areas than on areas close to a kestrel nest. In particular, densities of migratory and small birds (body mass < 140 g), which are potential prey of kestrels, were lowest near the predator nests. In contrast, the densities of nonmigratory and large—sized birds (body mass > 140 g) were independent of the presence of kestrel nests. Skylarks (Alauda arvensis), the most abundant migratory species in the study area, bred less often near kestrel nests than farther away. The results of line transect counts corroborated results of area counts. Our conclusion is that predation risk influences habitat selection of small migratory species in open habitats where scarce vegetative cover gives little protection.