Frequency-dependent predation by birds at edges and interiors of woodland

Structural variations between edge and interior areas within forest fragments may bring about differences in food availability that may influence the selective behaviour of predators and prey population dynamics. The purpose of this paper was to assess patterns of artificial prey selection by wild birds (House Sparrow Passer domesticus and Rock Dove Columba livid) between edge and interior areas of woodland, taking into account differences in prey frequency (10% of one food type and 90% of the other) and density (30 baits/m^-2 and 50 baits/m-²). Experiments were conducted at 24 plots in 3 forest fragments in the city of Madrid, Spain. Selectivity did not vary among parks or between densities. However, selectivity did vary with the frequency and location of baits, showing an anti-apostatic trend (baits were preferred at low rather than at high frequencies) that was more pronounced at interiors than at edges. Two possible factors that may account for stronger anti-apostatic selection at edges are the higher densities of predators and pedestrians found there. However, there are many other possible explanations, and no specific conclusion can be supported with the current data. The results of this study also point out that site heterogeneity should be taken into account in the experimental design of future studies on frequency-dependent food selection by wild birds, particularly in fragmented landscapes.