Effects of forest patch size on nesting success of wood thrushes

Declines of many forest-dwelling Neotropical migrants have been attributed, in part, to fragmentation of forest habitat on the breeding grounds in North America. During 1990-1991, we determined reproductive success of Wood Thrushes (Hylocichla mustelina) nesting within contiguous forest habitat (>10,000 ha) and in nine forest fragments ranging in size from 9.2 to 126.5 ha in Berks County, Pennsylvania. We located 171 Wood Thrush nests. Nesting success differed significantly among forest size categories, with 86% of the nests successful in contiguous forest, 72% successful in large fragments (>100 ha), and 43% successful in small fragments (<80 ha). The variable that best predicted nest survival was forest area (R(2) = 0.86). Rates of predation differed significantly among forest size categories, and predation was the primary cause of nesting failure. We found 56% of the nests in small fragments were lost to predators as compared to 22% in large fragments and 10% within the contiguous forest. Visitation by mammalian predators to scent posts was significantly different between small and large forest sites (41 vs. 14%, respectively), and relative abundance of avian nest predators was significantly higher in small forest fragments than in the large forest sites ((x) over bar = 1.04 vs. 0.41 birds per census point). Brown-headed Cowbirds (Molothrus ater) parasitized 9% of the nests. Rates of brood parasitism did not differ significantly among forest size categories and had little influence on nesting success. Our results suggest that reproductive success of Wood Thrushes nesting within contiguous forest is high and that severe reproductive dysfunction as a result of high rates of nest predation is an important consequence of forest fragmentation.