Warblers in trouble due to South American deforestation

Golden-winged warbler populations have plummeted in recent decades, even as conservationists have rushed to secure their breeding areas in the Appalachian and Great Lakes regions. Using cutting-edge trackers, researchers recently discovered that the problem could be deforestation on the warblers’ wintering grounds, and they suggest widening habitat conservation to South America to counter the species’ decline.

“Factors in all these places this migratory species is using can limit populations,” said Gunnar Kramer, first author on the paper published in the Proceedings of the National Academy of Sciences. “We found evidence for broad-scale deforestation on nonbreeding grounds — a driver of these population trends. Nonbreeding grounds are a priority, and we should refocus our efforts to assess those issues.”

An estimated 400,000 golden-winged warblers (Vermivora chrysoptera) reproduce around either the Great Lakes — from Manitoba and Minnesota to Michigan and Ontario — or the Appalachian Mountains — from Tennessee and North Carolina up into Vermont. The Appalachian warblers’ numbers have shrunk by up to 98 percent, and they’ve disappeared from sites they once used to inhabit in the eastern United States. The Great Lakes warblers, on the other hand, are stable and currently constitute 95 percent of the species.

From 2013 to 2016, Kramer, a University of Toledo PhD candidate, attached light-level geolocators —devices that weigh less than a gram and track the tiny warblers by registering the sunlight wherever they fly —  to over 70 birds’ backs. He and his colleagues returned to 23 sites in 11 American states and two Canadian provinces each spring, after these warblers had come up from their journeys south for the winter, to recapture them and uncover where they’d traveled.

They found the two populations had very distinct wintering grounds. The Great Lakes populations went to Central America. The Appalachian populations went to northern South America. Unlike other species,  such as the closely related blue-winged warblers (Vermivora cyanoptera), their populations didn’t mix in wintering areas.

“Those differences in where golden-winged warblers are going during winter are linked to the population trends we see,” Kramer said. “Deforestation in South America has outpaced that of Central America by three to one. The timing of those habitat losses is associated with those declines. Deforestation is a problem for Appalachian birds occurring in such small numbers now.”

These findings reflect a “paradigm shift” necessary to save golden-winged warblers, he said, which are considered endangered in many states and receive millions of dollars for breeding site protection.

“Whatever we do on the breeding grounds to boost the production of young might not have any effect if those birds don’t have anywhere to survive during the nonbreeding period,” Kramer said. “We need to reallocate our strategy to areas where the population might be limited. That means focus mostly on nonbreeding grounds management and, to a lesser extent, make sure we have appropriate breeding habitat.”

The findings corroborate and expand those of previous stable isotope investigations into golden-winged warbler migration, he said.

“The mystery now is what’s going on down there specifically?” Kramer said. “That’s going to take more research to look at the wintering ecology of the birds in these South American and Central American areas.”

Header Image: A male golden-winged warbler in Minnesota carries a geolocator. ©Gunnar Kramer