Deforestation and Drought in the Tropics Are the Biggest Threats to U.S. Forest Birds

When you pick up a field guide and examine the range map of the Tennessee Warbler, it looks as though North America makes up the bulk of its habitat and that it migrates south for a short winter vacation. But the reality is reversed: This forest species, and others like it, spend the majority of the year in Central America—and there, it might face greater threats in the future.

A new study, published in Global Change Biology, found that 21 forest songbirds that breed in North America spend 60 percent of the year on average in their Central American wintering areas. Computer models showed that by 2050 these species will experience more pressure from land-use and climate change in Central America than at their northern breeding grounds, suggesting that these areas require more attention now in order to protect birds throughout the hemisphere.

The new work was led by Frank La Sorte, a research ecologist at the Cornell Lab of Ornithology in Ithaca, New York, with a suite of international colleagues. He tapped into eBird, a community-driven science database he works with at the lab, to better capture how birds spend their time throughout the year. Unlike more traditional databases like the Breeding Bird Survey or the Christmas Bird Count, which combine comprehensive bird counts taken on a single day from many locations, eBird provides less-comprehensive data for just about every day of the year from wherever birders report. This allowed him to build a “comprehensive, full annual-cycle understanding, instead of just limiting ourselves to little snapshots” of the birds’ lives, La Sorte says.

The resulting analysis found that 21 species of eastern-flyway forest birds well known to U.S. birders—including Least Flycatchers, Tennessee Warblers, and Indigo Buntings—spend up to 200 days per year, on average, at their wintering grounds in Central America. And they really crammed into those southern forests: The migrants occurred in densities three times higher than at their summertime nesting areas. 

The researchers then modeled how changes in land-use (like converting forest to farmland or homes) and climate (like changes in temperature and rainfall) might affect both breeding and wintering areas by 2050. The computer models showed that, within 40 years, deforestation on wintering grounds will pose the greatest threat to these migratory species—even more so than habitat loss where they breed.

As the birds are squeezed by habitat loss, climate change will alter heat and rainfall, according to the model, potentially making food, water, and shelter increasingly unreliable. The model predicted that northern breeding areas will heat up more than wintering areas and, at the same time, see more rain or snow in the winter—a change that might improve habitat when birds are nesting. In the tropics, though, the models predicted less rain in summer, which could affect the growing season and decrease the quality of habitat (including food and shelter) by the time birds arrive in winter. But because temperature and rainfall are related, it's difficult to predict for certain what the combined impacts on birds and their habitat might be.

La Sorte's work is one of the first studies to truly explore the combined threats of climate and land-use change on bird distribution, says Benjamin Zuckerberg, an ecologist at University of Wisconsin, Madison who was not involved in the research. “The role of land-use change on overwintering bird habitat is something ornithologists have long been concerned about, but [it] has rarely been captured,” he says. “This study showcases the power of citizen science for capturing bird ranges and potential environmental threats over unprecedented scales of time and space."

The good news is that both threats to wintering grounds—deforestation and climate change—share a common solution: protecting and keeping intact as much existing forest as possible. This directly prevents deforestation while ensuring refuge for birds coping with the unpredictability of climate change. Currently, and depending on the country, protected areas in Central American wintering grounds are technically abundant, La Sorte says. However, many still legally allow harmful activities to some degree, and laws are frequently ignored to pursue intense, destructive purposes such as logging. “There's the potential there for people to go in and begin extracting large quantities of resources or converting forest to agricultural grassland without much oversight,” La Sorte says. A recent study identified another threat: Up to 30 percent of annual forest loss in Honduras, Guatemala, and Nicaragua is driven by cocaine trafficking—just one of many pressures in the region.

Clearly, declaring an area off-limits and putting a fence around it isn't a realistic solution to protecting vital patches of forest. Organizations—governmental and non-governmental—need to find ways to make forest conservation work for local people so that its benefits outweigh those of deforestation. Throughout Central and South America, local conservation groups (some in partnership with Audubon's International Alliances Program) are working to foster sustainable ecotourism in the region. They're training wildlife guides, developing tour routes, and supporting those opening guest houses and offering hospitality—all to help them make money and improve their lives by taking care of their forests. Taking these steps in the short term may prove to be the best long-term strategy to help birds cope with climate change.

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