Edge effects on nest predation in the Shawnee National Forest, southern Illinois

Edge habitats may be considered 'ecological traps' for breeding birds if they attract many birds because of apparently favorable nesting conditions but have higher nest predation levels than interior habitats. Four alternative, nonexclusive hypotheses have been suggested to explain why edges might have higher predation levels than interior habitats: (1) predator activity is higher in areas with higher prey density (density-dependent predation); (2) predators are more abundant on edges than in forest interior; (3) the predator community is richer in species on edges than in forest interior; and (4) predators forage along travel lanes (linear geographical features) such as edges. Here we evaluated whether forest-farm edges in southern Illinois are ecological traps, and examined the relevance of these four hypotheses at our study site with several different experiments during May-July 1992 using artificial nests (n = 605) baited with quail eggs and placed on the ground, in shrubs, or in saplings. Our results showed that, in general, the forest-farm edges of southwestern Illinois did not attract significantly more individuals or species of nesting songbirds, but they did have higher nest predation levels than forest interior sites, primarily as a result of higher predation levels on sapling nests. We did not find evidence strongly supporting any of the four hypotheses suggested as explanations for higher nest predation levels near edges. Two data sets showed that predation levels on artificial nests were density-independent. Forest-farm edges had neither more total species of potential nest predators nor more individual predators. However, there were more species of avian predators on edges than in interior sites. Correlations between predator abundance and nest predation levels on individual transects were weak. The travel lane hypothesis was not supported because nest predation levels were either not affected by distance from linear geographical features (roads and ravines) or were significantly less when close to than when far from presumptive travel lanes. High spatial heterogeneity in predation levels, numbers of singing birds, and potential nest predators may have obscured general patterns and suggest a need for larger sample sizes. Edges may be detrimental to some species of singing birds but not to others, and for different reasons.