Modeling the Impact of Edge Avoidance on Avian Nest Densities in Habitat Fragments

In fragmented landscapes, many species of birds are absent from, or have reduced densities in, small habitat fragments. This pattern may result, at least in part, because birds avoid placing their nests near habitat edges where nest success often is low. We sought to clarify the role played by edge avoidance in producing these patch size effects. Using a numerical approach, we modeled nest densities in patches of different sizes and shapes both for species displaying edge avoidance (i.e., “edge-sensitive” species) and for those not displaying this characteristic (i.e., “edge-insensitive” species). Edge avoidance in our model was defined as a reduced probability of nest placement occurring near a habitat edge. Our model produced the expected result that edge avoidance reduced nest densities in patches of all sizes compared to densities of edge-insensitive species. Surprisingly, however, edge avoidance did not reduce nest densities in small patches relative to large patches, and nest densities actually increased exponentially as patch size decreased for edge-insensitive species. Also unexpected was the result that nests of edge-sensitive species were found in the edge habitat at frequencies only slightly below those expected based on edge area, whereas edge-insensitive species actually had higher than expected nest densities in edge habitat. However, in our model, edge-sensitive species displayed a greater reduction in nest densities near edges when their overall patch density was reduced by half, suggesting that edge avoidance is density dependent. Finally, both types of species showed marked increases in nest densities in linear habitat patches compared to square patches. These patterns were a direct result of our settlement rule that required a female's nest location to be a minimum distance from other nests. This study suggests that knowledge of the settlement rules used by female birds may be a key to accurately demonstrating the existence and assessing the potential consequences of edge avoidance. Detailed observations of marked females immediately following arrival at habitat patches, as well as a comparison of nest densities, territory sizes, polygyny levels, and use of habitat off territory, would greatly help our understanding of this interesting and important phenomenon.