The Swainson’s Warbler is prized by birdwatchers and difficult for scientists to study because of the dense habitat it occupies across South Carolina and the southern U.S. Think lowland swamps, canebrakes, and mountainous rhododendron ravines.
Even if you bushwhack close enough, you’re more likely to hear the bird’s sweet, ringing song than catch a glimpse of it flitting through the understory.
“I call it machete habitat,” said Jen Tyrrell, community science and research program manager at Audubon South Carolina. “They are in places that are thick and hard to get to, usually occupied by snakes and mosquitoes.”
That secrecy has left the species relatively understudied. But a recent research paper by Audubon and partners, published in Ornithological Applications, used new technology to map the warbler’s movements to their wintering grounds in Latin America.
Migration Journey Revealed
The work was led by Garrett Rhyne, a Louisiana State University-trained avian ecologist now at the American Bird Conservancy. Researchers tagged birds at Audubon’s Francis Beidler Forest Center and Sanctuary and other locations across South Carolina, Louisiana, Arkansas, Missouri, Tennessee, and Virginia.
“We really just had no handle on where these populations are going, how they’re connected, what migratory routes they’re taking,” Rhyne said. “It’s because of their secretive nature.”
Researchers found that eastern breeders from Appalachia and the Atlantic Coastal Plain take a land-based route through Florida before making a short hop to the Bahamas and Cuba. Birds from the Mississippi Valley and Missouri Ozarks take a riskier path, crossing the Gulf of Mexico to the Yucatán Peninsula.
“This kind of migratory connectivity data can help identify threats to habitat and target conservation efforts throughout the species’ range,” Tyrrell said.
Tracking Barometric Pressure
The study was made possible by advances in lightweight tracking technology that can be carried by small songbirds.
Traditional light-level geolocators estimate location based on day length, which varies depending on longitude and latitude. But this method is less reliable for species like the Swainson’s Warbler that spend their lives under dense forest cover.
New devices combine light and barometric pressure data, giving more accurate location data and new information on altitude and length of migration, as researchers are able to see spikes in pressure when birds are moving high overhead. Rhyne’s study was the first in the U.S. to use barometric pressure tracking devices on birds.
For a species as elusive as the Swainson’s Warbler, that added precision is helping fill major gaps in our understanding of its lifecycle.
Creating Swainson’s Warbler Habitat in South Carolina
On their nesting grounds, Swainson’s Warblers thrive in young, overgrown habitat that often emerges after storms, timber harvests, and other disturbances. Audubon staff saw this firsthand at Silver Bluff Sanctuary after a tornado tore through the property.
Dense regrowth created ideal habitat and Swainson’s Warblers quickly moved in. Over time, however, the forest canopy closed and the birds disappeared.
“That led us to look at how we could create habitat for this species on an ongoing basis,” said Tim Evans, land conservation director at Audubon South Carolina.
Audubon staff began experimenting with forestry techniques that mimic natural disturbances, creating small openings within otherwise uniform habitat and then allowing these holes to regenerate into thick understory.
As staff refined these techniques, Audubon expanded the work through partnerships with private landowners, helping demonstrate how active forest management can support birds while maintaining healthy, productive forests.
“When landowners can selectively harvest timber for birds and earn income at the same time, they’re more likely to keep those forests intact,” Evans said. “That’s critical in South Carolina, where forest conversion remains one of the biggest threats to wildlife habitat.”
This latest research is the next chapter in a two-decade effort by Audubon South Carolina to better understand and conserve this secretive species. Together, habitat management, scientific research, and partnerships are helping shed light on the conservation needs of Swainson’s Warblers across the hemisphere.
