Adverse effects of acid rain on the distribution of the Wood Thrush Hylocichla mustelina in North America

Research into population declines of North American bird species has mainly focused on the fragmentation of habitat on the breeding or wintering grounds [Robinson, S. K., Thompson, F. R., Donovan, T. M., Whitehead, D. R. & Faaborg, J. (1995) Science 267, 1987-1990]. In contrast, research into declines of European species has mainly focused on intensification of agriculture [Donald, P. F., Green, R. E. & Heath, M. F. (2001) Proc. R. Soc. London Ser. B 268, 25-29] and the role played by the atmospheric deposition of pollutants, in particular, acid rain [Graveland, J. (1998) Environ. Rev. 6, 41-54]. However, despite widespread unexplained declines of bird populations in regions of heavy wet acid ion deposition [Sauer, J. R., Hines, J. E. & Fallon, J. (2001) The North American Breeding Bird Survey Results and Analysis 1966-2000 (Patuxent Wildlife Research Center, Laurel, MD)], no North American studies have presented evidence linking such widespread terrestrial bird declines to acid rain. To address the question of the role played by acid rain in population declines of eastern North American songbird species, we combine data from several sources. We use a multiple logistic regression model to test for adverse effects of acid rain on the Wood Thrush, while controlling for regional abundance, landscape-level habitat fragmentation, elevation, soil pH, and vegetation. We show a strong, highly significant, negative effect of acid rain on the predicted probability of breeding by this species, and interactions with elevation, low pH soils, and habitat fragmentation that worsen these negative effects. Our results suggest an important role for acid rain in recent declines of some birds breeding in the eastern United States, particularly in high elevation zones with low pH soils, and show the need to consider other large-scale influences, in addition to habitat fragmentation, when addressing bird population declines.