High-Level Problems for Songbirds
In the Southern Appalachians there’s been a substantial decline in eastern hemlock due to an exotic insect pest, the hemlock woolly adelgid. The loss of hemlock will bring changes in forest composition and structure that may affect the species composition, densities, and nesting success of high-elevation bird communities—especially when combined with the continuing effects of acid deposition.
Acid deposition—more familiarly known as acid rain—occurs when sulfur and nitrogen compounds from fossil fuel combustion combine in the atmosphere with water. Though the Clean Air Act reduced the emissions that cause acid rain, it’s still a problem in the high-elevation forests of the Southern Appalachians.
Though much is unknown about how high-elevation ecosystems are affected by continuing acid rain, ecologists have started to suspect that high levels of acid deposition, which leach calcium from the soil, may be one of the causes for unexplained population declines in neotropical migratory birds, especially highelevation bird species.
Female songbirds need large amounts of calcium to produce eggs. Birds get some of their calcium from insects and worms, but mostly from snail shells, which may be in short supply when calcium is leached from the soil.
When it comes to studying birds, Kay Franzreb, SRS research wildlife biologist, has always been in the thick of things—studying forest birds in the Southwest, scrub-jays in Florida, red-cockaded woodpeckers in South Carolina, and neotropical migratory birds in southeastern forests.
Impacts of Hemlock Loss
In 2007, Franzreb, with University of Tennessee collaborator David Buehler, began a field study to assess the possible impact of hemlock loss on birds in the Southern Appalachian Mountains. The researchers already have data on the nesting success and provisioning rates of these species. They will revisit the areas where that data was collected to determine the extent of the hemlock loss and how this loss has affected the bird community by measuring avian densities, nest success, parental provisioning rates, and vegetative structure and composition. Reproductive success of three target species will be monitored: dark-eyed junco, blue-headed vireo, and blackthroated blue warbler.
Once the results are in, they hope to use the data to develop landscape-based models that can predict the effects of hemlock loss on bird communities in the Southern Appalachians.
Snail Shells and Nesting Success
In 2006, with North Carolina State University collaborators Rebecca Hylton and Ted R. Simons, Franzreb began a study on the effects of acid deposition and calcium depletion on high-elevation songbird and snail populations in the Great Smoky Mountains National Park. The project focuses on dark-eyed juncos and on six species of snails.
For the study, Franzreb and collaborators are surveying land snail populations across a gradient of low-to-high acid deposition in sites within the national park. They’re also examining museum specimens collected from the area to see if snail shell size, thickness, and calcium content have changed over time.
The information on snails will then be compared to data from the same time frame and location on the nesting success of juncos and the calcium content of their eggs. Blood from nesting juncos has already been analyzed for both calcium and mercury (a chemical pollutant that accumulates in living tissues) levels. The first year of data collection for the project has been completed, with researchers anticipating at least two more years of field work.
“Once we’ve established this study on sites in the park, we’ll expand the research to nearby high-elevation sites on national forest lands,” says Franzreb. “If we can better understand the connections between air pollution and the ecology of songbirds and snail populations in the Southern Appalachians, we can provide better recommendations for conservation and management.”