Factors Affecting Nesting Success of Wood Thrushes in Great Smoky Mountains National Park

Abstract

Recent evidence suggests that the nesting success of forest-interior Neotropical migrants is lower in fragmented habitat. We examined the nesting success of Wood Thrushes (Hylocichla mustelina) in a large contiguous forest from 1993 to 1997. From a sample of 416 nests we tested for predictors of daily nest survival rates, including activity at the nest and vegetation parameters at the nest site. We tested whether disturbance during nest checks (as measured by the behavior of the adults) was related to subsequent nest predation. Females were more likely to vocalize when brooding chicks than when incubating eggs. However, we found no evidence that observer disturbance or Wood Thrush activity influenced daily nest survival rates. Wood Thrushes nested predominately in small hemlocks, generally surrounded by many other small hemlocks. However, survival rates of nests in hemlocks were not significantly different from those in other substrates. Overall, neither activity at the nest nor habitat in the vicinity of the nest was a good predictor of nesting success, and only one vegetation characteristic, a measure of concealment, was significantly correlated with successful nesting. Brood parasitism by Brown-headed Cowbirds (Molothrus ater) was extremely low (<2% of nests parasitized). However, nesting success was moderate (daily survival rate = 0.958) when compared with other published studies from more-fragmented landscapes. Our results suggest that daily nest survival rates do not increase monotonically from small to very large forest patches.