The influence of patch and landscape characteristics on grassland passerine density, nest success, and predators in southwestern Wisconsin pastures

Fragmentation of grasslands leads to smaller patches of grassland with greater amounts of edge in the landscape, which have been implicated in grassland bird population declines. Many grassland passerines are considered to be area sensitive, preferring or requiring larger patches of grassland over smaller patches for nesting. However, the mechanisms underlying area sensitivity are not well understood, and may be influenced by the surrounding landscape matrix. Furthermore, predation patterns, believed to be a cause of area sensitivity, have not been linked to the composition of the local predator community. I evaluated how environmental characteristics at three scales (within-patch, patch, and landscape) influence the density and nest predation rates of five grassland passerines in Wisconsin pastures, and simultaneously documented the nest predator community using miniature video cameras. Bird density was frequently higher in larger pastures and in landscapes with more grassland and less wooded area. There was little evidence that bird density differed between pastures in more versus less fragmented landscapes. Nest density was lower near edges, but there was no difference in density between nests located near wooded versus non-wooded edges. Distance to edge generally was not important to nest survival or predation, and edge parameters were generally poor predictors of nest survival and predation compared to other factors such as vegetation structure around the nest. Grassland passerine nests were depredated by at least 11 species. One-third of predation events were caused by species preferring wooded edges, and the remaining events were caused by generalists or grassland specialists. Edge-associated species traveled into pasture interiors, and usually depredated nests located closer to wooded edges than other edge types. Nests in pasture interiors and edges were equally vulnerable to predation because of the ease with which predators can move within pastures, the high percentage of non-edge nest predators, and the relatively small size of grassland patches. Edge avoidance by grassland birds may not be a beneficial behavior in pastures in a highly fragmented landscape. Management priorities should be developed using a multiscale approach, with a focus on large pastures and in landscapes with a relatively high proportion of grassland.