THE EFFECT OF EDGE ON AVIAN NEST SUCCESS - HOW STRONG IS THE EVIDENCE

Wildlife biologists historically considered the edge between adjacent habitat types highly productive and beneficial to wildlife. A current dogma is that edges adversely affect a wide range of avian species by increasing depredation and parasitism rates of nests. I critically eva luated existing empirical evidence to test whether there was a gradati on in nest success as a function of distance from an edge. Researchers investigating this question have been inconsistent in their experimen tal designs, making generalizations about edge-effect patterns difficu lt. The majority of studies I examined found nest success varied near edges, with both depredation rates (10 of 14 artificial nest studies, and 4 of 7 natural nest studies) and parasitism rates (3 of 5 studies) increasing near edges. In addition, there was a positive relationship between nest success and patch size (8 of 8 studies). The most conclu sive studies suggest that edge effects usually occur within 50 m of an edge, whereas studies proposing that increased depredation rates exte nd farther than 50 m from an edge are less convincing Prior research h as probably focused on distances too far from an edge to detect thresh old values, and future research should emphasize smaller scales. 100-2 00 m from an edge at 20-25 m increments. Researchers often use relativ ely arbitrary habitat characteristics to define an edge. Therefore, I propose that only openings in the forest canopy with a diameter three times or more the height of the adjacent trees should be included in e dge analyses. This review suggests that fragmentation of eastern North American temperate forests could lead to increased nest predation and parasitism, and there is need to determine if similar processes occur in other forested regions of North America