Despite its poor reputation, acid rain has not been thoroughly investigated as a cause of declines in bird populations in North America. Instead, research has focused on factors such as habitat loss and fragmentation. Now scientists have determined that acid rain has a negative influence on the breeding habits of at least one North American songbird species. The findings will be published online this week by the Proceedings of the National Academy of Sciences.

Ralph S. Hames of Cornell University and his colleagues combined data on the breeding behavior and distribution patterns of Wood Thrushes with information regarding acid rain deposition and soil acidity. The researchers then tested for any correlation between the birds' breeding and acid rain intensity, taking into consideration population density, vegetation cover and the effects of habitat fragmentation. According to the team's final model, which includes statistics from 653 sites, the probability of Wood Thrushes attempting to breed decreases as the deposition of ions from acid precipitation increases.

Exactly how the contaminated rain affects the birds remains unclear, although the scientists suggest that diminished levels of calcium in the soil could lower the quality and quantity of available prey or lead to increased uptake of other minerals in toxic doses. The results, the authors conclude, "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."