Effects of predation on community assembly and spatial dispersion of breeding forest birds

In a landscape context, nesting avian predators cause variation in predation risk with respect to the distance to their nests, which may have both direct (predation) and indirect (predation risk) effects on the prey community. We studied the community assembly and spatial distribution of a breeding forest bird community in a structurally complex forest landscape surrounding forest-hunting Sparrowhawk (Accipirer nisus) nests. Birds were censused on transect arrays within 1-ha study plots divided into four subplots. We determined whether the distribution of species within study plots was clumped, random, or uniform. Because predation risk is higher nearer a hawk's nest than farther away, we predicted that species would show a more clumped distribution within study plots near the Sparrowhawks' nests than farther away. We also expected densities and community patterns to vary with respect to predation risk. The results suggest that predation and/or its risk has a complex and local effect on the prey community. The predation risk negatively affected the number of species and densities of birds only in the first year (1996) of study. The relationship between the density of large birds (greater than or equal to 20 g, preferred prey) and predation risk was also negative in 1996, but not in 1997, whereas the density of small birds (<20 g) was not affected by predation risk in either year. There was no relationship between predation risk and clumping for all species together. When we analyzed separately plots that contained large and small birds or large birds alone, however, increasing predation risk positively affected the index of dispersion, but only in thicket (one of the forest types), which is probably the preferred hunting habitat for Sparrowhawk. These results are well in line with studies made on predator-prey interactions in farmlands. In those studies, however, the effects of predators on prey communities were much stronger and extended farther in the landscape from the nest of predators than in this forest study. A possible reason for this difference is higher habitat structural complexity in forests, which may mitigate the effect of predators.