Acid Rain Leads to Songbird Declines

ITHACA, New York, August 16, 2002 (ENS) - Acid rain may be responsible for the decline of the wood thrush, a songbird, across its North American breeding range. A new large scale study by researchers at Cornell University suggests that the acidic precipitation may be depleting calcium from the birds' food sources.

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 effect was most pronounced in the mountainous areas most affected by acid rain.

wood thrush

Calcium rich foods, required by the adult wood thrush and its young, are harder to find in soils leached by acid rain. (Photo by Mike Hopiak/Cornell Lab of Ornithology, Cornell University)
Acid rain is the broad term used to describe several ways that a weak solution of 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 plants burning fossil fuels such as coal.

Ralph Hames, a postdoctoral associate at the Cornell Lab of Ornithology, and his colleagues analyzed data from a variety of sources to compare patterns of acid rain with the breeding success of wood thrushes. The team found that at high elevations, such as the Adirondack, Appalachian and Great Smokey Mountains and the Allegheny Plateau, where the amount of acid deposited in precipitation could be highest, wood thrush populations showed long term declines of up to almost five percent a year.

Although the reason for the declines is still unknown, the researchers say the problem may be related to the leaching of calcium from the soil by acid rain. European studies of heavy acid rain regions have also linked declining bird populations to depletion of soil calcium by acid rain.

Previous studies by other investigators had shown that calcium depletion can affect breeding birds in a number of ways, Hames said. 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 may decompose more slowly under acidic conditions. Acidic conditions may also increase the amounts of toxic aluminum and heavy metals, such as lead, cadmium and mercury, that the wood thrush ingests.


Red spruce forests on western facing slopes of Adirondack Park's High Peaks region are stunted and dying from acid rain. (Photo courtesy Colgate University)
"They may be finding less good quality food and having to work harder to find it," Hames said. "This could potentially lead individual thrushes to attempt breeding elsewhere."

Hames 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, which 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). BBS is an annual, summer survey begun in 1966 that monitors the species and numbers of birds breeding in selected cross sections of habitat.

Another component of their team's analysis was data gathered by volunteers participating in the Cornell Lab of Ornithology's ongoing Birds in Forested Landscapes (BFL) project. Participants in BFL study species that live in their local areas, visiting survey points in forest patches of different sizes twice during each breeding season to search for selected species, look for evidence of breeding success, and record characteristics about the study site.

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," noted Hames.

Cornell ecologists compiled all of this data into sophisticated statistical analyses, producing a model that predicted where acid rain's effects might be most severe for a bird whose life and reproductive success depend on the food it finds on the forest floor.

power plant

Power plants produce two-thirds of total U.S. sulfur dioxide emissions, and more than a quarter of the nation's nitrogen oxides. (Photo by Carole Swinehart, courtesy Michigan Sea Extension)
The model predicts that, after adjusting for other factors such as soil, vegetation, topography and thrush abundance, the probability of a wood thrush breeding successfully is much reduced at a site with highly acidic precipitation and soil. The negative effects of acid rain may be increased by factors such as high elevation and habitat fragmentation, the researchers found.

Population declines in other songbird species may also be attributable, at least in part, to acid rain, Hames said.

"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," added Hames. "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."

The Cornell study, which appears in the current issue of the journal "Proceedings of the National Academy of Sciences," joins a growing body of evidence that calcium depletion by acid rain may be affecting forests and forest species in unexpected ways.

In July, scientists at the University of California at Riverside reported that acid rain leaches essential metals such as calcium, potassium and magnesium out of topsoil, posing a far graver risk to forests that first estimated.

Martin Kennedy and his colleagues reported in the July 9 issue of the "Proceedings of the National Academy of Sciences" that many plants may be suffering from this nutrient depletion.

"Our work shows that in unperturbed natural ecosystems a very small pool of these nutrients is available and this comes from the atmosphere, mostly as dilute amounts dissolved in rain that then get deposited in topsoil," said Kennedy. "The tight budget of these nutrients is a concern because if the budget is perturbed, the forests are at risk."

And as the forests suffer, so do the birds that depend upon them - like the wood thrush. For more information on the wood thrush study, and other citizen science initiatives at Cornell University, visit: