Acid rain harming song birds?

Acid rain may be responsible for declining numbers of songbirds, claims a new study published by US scientists. The study shows a clear link between acid rain and a change in the breeding habits of the North American wood thrush. Ecologists suggest that calcium depletion in the soil is affecting the bird’s food supply.

Using data collected by thousands of volunteers as part of an ongoing public project, Cornell University’s Lab of Ornithology modelled the breeding patterns of the wood thrush, and found that the bird is less likely to breed in areas receiving high levels of acid rain. The results, published in the Proceedings of the National Academy of Sciences (PNAS Vol 99, No 16), show that mountainous areas receiving greater acid deposits correlate with long-term declines of up to 5% per year in wood thrush populations.

Although the precise reason for the decline is still unclear, ecologists suggest it may be related to the leaching of calcium from the soil induced by acid rain. Some studies have found calcium-depletion to affect breeding in birds, where shortages of calcium rich foods such as snail shells can adversely affect the female and her young during egg-laying and nesting periods.

“They may be finding less good-quality food and having to work harder to find it,” says Dr Ralph Hames of Cornell. “This could potentially lead individual thrushes to attempt breeding elsewhere.” He speculates that birds might assess the available food supplies each spring before deciding where – and whether – to nest and reproduce.

Alternative explanations for the change in breeding habits include a drop in the numbers of soil insects or the increased toxicity of the soil from aluminium and other heavy metals released under acidic conditions.

The ongoing public project Birds in Forested Landscapes links volunteer citizen-scientists and bird-watchers with professional biologists. In this study, BFL participants recorded the presence or absence of breeding wood thrushes, as well as detailed information on the topography, elevation, vegetation and habitat at 650 study sites. “Massive surveys like this one could never be accomplished without the participation of citizen-scientists,” says Dr Hames.

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