Evaluating Great Smoky Mountains National Park as a population source for the Wood Thrush

摘要

The prevailing fragmentation paradigm predicts that large, intact forests are acting as population sources for Neotropical migrant landbirds. We used the Wood Thrush (Hylocichla mustelina) as a model for evaluating the role Great Smoky Mountains National Park (the largest national park in the eastern United States) may play in maintaining regional songbird populations. We estimated the annual productivity of Wood Thrushes in the park by combining observations on the birds' distribution, abundance, and productivity with estimates of habitat availability. We estimated a breeding population of approximately 10,000 nesting pairs using habitat models developed from over 2500 point-count censuses conducted across the park. Data from 426 nests monitored from 1992 to 1997 produced a daily nest survival rate of 0.96. We estimated an annual fecundity of 2.76 fledglings per breeding pair, based on a model that incorporated the re-nesting behavior of Wood Thrushes. Results indicate that the park is producing approximately 3000 surplus female young each year. Daily nest survival rates were below those reported in other studies of Wood Thrushes in large forest tracts. The relatively high productivity of 3.31 nestlings per successful nest suggests that, in the absence of predation, the park provides high-quality nesting habitat for Wood Thrushes, but that it may also support a more diverse and abundant predator community than more disturbed or less contiguous sites. The difficulties of estimating the size of continental breeding bird populations make assessing the significance of the park within a regional landscape context problematic, but our estimates suggest that, although the park is functioning as a substantial population source on a local scale, its potential to sustain regional or continental Wood Thrush populations is limited. Our findings suggest that species such as the Wood Thrush are capable of moderate levels of surplus productivity in high-quality habitat, but that extensive areas of suitable habitat outside protected areas and other public lands will be required to sustain continental breeding populations. [References: 64]