Evaluating the Roles of Visual Openness and Edge Effects on Nest-Site Selection and Reproductive Success in Grassland Birds

In some species, habitat edges (ecotones) affect nest-site selection and nesting success. Openness, or how visually open a habitat is, has recently been shown to influence grassland bird density and may affect nest-site selection, possibly by reducing the risk of predation on adults, nests, or both. Because edge and openness are correlated, it is possible that effects of openness have been overlooked or inappropriately ascribed to edge effects. We tested the roles of edges and visual openness in nest-site selection and nesting success of two grassland passerines, the Bobolink (Dolichonyx oryzivorus) and Savannah Sparrow (Passerculus sandwichensis), in the Champlain Valley, Vermont. We also evaluated the sensitivity of our results to alternative definitions of edge on our landscape. Bobolink (n = 580) and Savannah Sparrow nests (n = 922) were located on seven hay fields and three pastures from 2002 to 2010. Both species avoided placing nests near edges and in less open habitat compared with expectations based on random placement. When the effects of openness and edge were separated, less open habitats were still avoided, but edge responses were less clear. These results were robust to different definitions of habitat edge. We found no strong relationships between either openness or edges and reproductive success (numbers of eggs and fledglings, percentage of eggs producing fledglings, and nest success), although there may be an edgespecific openness effect on timing of reproduction (clutch completion date). Our results support openness as an important factor in nest-site selection by grassland birds.