Fragmentation of grassland habitat may increase predation rates on grassland passerine nests and contribute to population decline of several species. Studies that simultaneously document the nest predator community and associate predator species with edges created by fragmentation have not been conducted for grassland habitats. The purpose of our study was to evaluate the effects of using miniature video camera systems to document predation events, identify grassland passerine nest predators in grazed pastures, and determine whether predation patterns of nest predators known to prefer wooded edges differed from those of other nest predators. In 1998–2000, we deployed cameras at 89 nests of Savannah Sparrows (Passerculus sandwichensis), Grasshopper Sparrows (Ammodramus savannarum), Song Sparrows (Melospiza melodia), meadowlarks (Sturnellaspp.), and Bobolinks (Dolichonyx oryzivorus) in southwestern Wisconsin pastures 16–169 ha in size. Abandonment rates were higher for nests with cameras than for nests without cameras (P = 0.04). Trampling rates did not differ between nests with and without cameras. There was limited evidence of differences in predation rates between nests with and without cameras. Predation rate was high in the early incubation stage. Grassland passerine nests were depredated by at least 11 different species in that system, and the predator community differed from those documented in similar studies in other regions. Raccoon (Procyon lotor), thirteen-lined ground squirrel (Spermophilus tridecemlineatus), and snakes (Thamnophisspp. and Elaphespp.) were most common. Over one-third of documented predation events were caused by species that prefer wooded edges. Those species usually depredated nests located closer to wooded edges than to any other type of edge, but there was no evidence that those species restricted their movements to depredate nests within a certain distance from wooded areas in the landscape compared to grassland specialist species (P = 0.28). Predators known to prefer wooded edges traveled up to 190 m into pastures and up to 150 m from wooded areas. Effects of edge predators in pastures are likely to extend beyond the 50 m suggested by other grassland passerine studies.
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Vol. 120 • No. 2