Songbird population declines linked to acid rain, Cornell ecologists report

A large-scale study has for the first time shown a clear link in North America between acid rain and widespread declines across the breeding range of a songbird, the wood thrush. Calcium depletion affecting the birds' food is a possible cause, Cornell University ecologists say.

Using data collected by thousands of volunteer citizen-scientists in the Birds in Forested Landscapes project, scientists at the Cornell Lab of Ornithology showed that the wood thrush is less likely to attempt to breed in regions that receive high levels of acid rain. The finding is reported in the current Proceedings of the National Academy of Sciences (PNAS Vol.99 No. 16) by Ralph S. Hames, a postdoctoral associate at the Cornell Lab of Ornithology who conducted the research with colleagues Kenneth V. Rosenberg, James D. Lowe, Sara E. Barker and André A. Dhondt.

Acid rain is the broad term used to describe several ways that a weak solution of inorganic acids, such as nitric and sulfuric acid, falls out of the atmosphere as rain, snow, mist and fog. Sulfur dioxide (SO2) and oxides of nitrogen (NOx) are the primary causes of acid rain. In the United States, about two-thirds of all SO2 and one-fourth of all NOx come from electric-power generation that relies on burning fossil fuels, such as coal.

High elevations, such as the Adirondack, Appalachian and Great Smokey mountains as well as the Allegheny Plateau, where the amount of acid deposited in precipitation could be highest, show long-term declines of up to nearly 5 percent annually in wood thrush populations. Although the exact mechanism leading to the declines is still unknown, it may well be related to the leaching of calcium from the soil by acid rain, according to Hames. European studies of heavy acid-rain regions similarly have linked declining bird populations to acid-rain-induced depletion of soil calcium.

Previous studies by other investigators had shown that calcium-depletion can affect breeding birds in a number of ways, Hames notes. In particular, shortages of calcium-rich foods, such as snails and snail shells, might be critical at egg-laying time, when calcium demand is highest for female birds, or during the nesting period, when calcium supplements are often provided to growing young.

However, low levels of soil calcium might also affect a wide range of prey, such as earthworms, millipedes and centipedes, pillbugs and other insects that adult birds need to nourish themselves and feed their young. Fallen, decaying leaves and other natural litter on the forest floor could decompose more slowly under acidic conditions. At the same time, acidic conditions could also increase the amounts of toxic aluminum and heavy metals (such as lead, cadmium and mercury) that the wood thrush ingests.

"They may be finding less good-quality food and having to work harder to find it," Hames says. "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.

The Cornell scientists set about modeling the effect of acid rain on the wood thrush by predicting the probability of a bird attempting to breed at a given location, based on the amount of acid rain falling there. First they gathered existing data from sources such as the National Atmospheric Deposition Project's National Trends Network that monitors pollution in rainfall, as well as detailed soil maps from the Natural Resources Conservation Service. Next, the scientists combined the precipitation and soil data with information about the regional abundance of the wood thrush, as reported by the Breeding Bird Survey (BBS). A critical component of their analysis was data gathered by the volunteer citizen-scientists participating in the Cornell Lab of Ornithology's ongoing Birds in Forested Landscapes (BFL) project.

BFL participants had recorded the presence or absence of breeding wood thrushes, as well as detailed information on the topography, elevation, vegetation and habitat fragmentation at more than 650 study sites across the geographic range of the species. "Massive surveys like this one and the BBS could never be accomplished without the participation of citizen-scientists,"says Hames.

Cornell ecologists used the data collected in sophisticated statistical analyses to produce a model that predicted where acid rain's effects might be most severe for a bird whose life and reproductive success depend on food it finds on the forest floor. The model predicts that, after statistically adjusting for several other factors (soil, vegetation, topography, thrush abundance), the probability of a wood thrush breeding is much reduced at a highly acidified site. The negative effects of acid rain might also be heightened by such factors as high elevation and habitat fragmentation.

Population declines in other songbird species also could be attributable – at least in part – to acid rain, Hames says. "There are a number of other factors that we know can hurt populations of particular species. This is also true in the case of the wood thrush," he adds. "However, in some places, there also appear to be many fewer birds than there used to be, and these often appear to be the same places most severely impacted by acid rain."

Funding for the study was provided by the National Fish and Wildlife Foundation, the U.S. Department of Agriculture Forest Service; the Archie and Grace Berry Charitable Foundation, the Florence and Joan Schumann Foundation, the Packard Foundation and an Institute for Ecosystem Studies-Cornell University Human Accelerated Environmental Change grant.

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