Minor fitness benefits for edge avoidance in nesting Grassland birds in the northeastern United States

Abstract

Grassland birds are often afected negatively by habitat fragmentation. Outcomes include greater nest predation and brood parasitism, decreased colonization rates of small, isolated patches, and greater nest density in remnant core habitats. These efects have been well documented in the Midwest, but little is known about fragmentation and edge efects on grassland birds in the fragmented agricultural felds within the forested landscapes of the northeastern United States. From 2002 to 2010, we assessed how edges and edge types afected nest-site location and daily nest survival (DNS) of Savannah Sparrows (Passerculus sandwichensis) and Bobolinks (Dolichonyx oryzivorus) breeding in 11 felds (range: 13.2-38.3 ha; mean = 21.1 ha) within a large agricultural region of Vermont. Mean (± SD) distance to edge was 80.3 ± 39.6 m for Savannah Sparrows (n = 995) and 94.5 ± 56.5 m for Bobolinks (n = 652). Both species nested signifcantly less than expected within 50 m of the edge. For Savannah Sparrows nesting within 50 m of the edge, DNS increased with increased distance from the edge. Birds initiating nests later in the season nested closer to edges, but renests were farther from edges than frst nests. Distance to edge had no detectable consequence for Bobolink nest success. Both species used portions of felds near hedgerows less than expected but used wetland, forest, agricultural, road, and developed edges in proportion to availability. For both species, DNS did not vary among edge types. Although edges were used less than expected, nesting near edges had only minor consequences for nest success. © 2013 by The American Ornithologists' Union.