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Since the began in 1966, Golden-winged Warblers have declined by as much as . Today, the remaining 400,000 breeding adults make up one of the smallest populations of any songbird outside the Endangered Species List.

The downturn is a tale of two populations. Along the Great Lakes, where the birds nest beneath grasses near forest edges, the population has remained stable. But the only other population—found summering in young forests of the Appalachian Mountains—has declined by 98 percent. For years, these uneven trajectories have puzzled scientists. A species’ success has long been tied to the condition of its breeding habitat, yet they couldn’t identify clear drivers of the decline in Appalachia.

Now the mystery is starting to unravel. By tracking the migratory pathways of individual warblers, scientists learned that the birds' biggest threats aren’t found where they breed in North America. Instead, disturbance in their winter habitat in South America is likely responsible for the population declines recorded in the summer survey. “It could be habitat fragmentation, it could be degradation—but it’s something that’s associated with where warblers go during the nonbreeding period,” says Gunnar Kramer, the paper’s lead author and a graduate student at the University of Toledo. The results,  this month in the Proceedings of the National Academy of Sciences, suggest that the traditional conservation framework, which focuses on protecting breeding habitat, may not be sufficient for these birds.

To track down the cause of the species' decline, Kramer and his team first had to figure out where Golden-winged Warblers migrate each winter. And so during breeding season, they netted a few hundred warblers and strapped them with tiny tracking devices that weighed less than half a gram (each bird weighs around 10 grams). As the warblers flew south, the lightweight devices recorded the time of sunrise and sunset, which allowed the team to calculate location—but only after the locators, which stored the information, were collected. That meant that his team had to recapture the same birds in nets the following year.

It might seem that such a task would be nearly impossible—like catching the same fish in the ocean two years in a row. But fortunately, Golden-winged Warblers—like many other migratory songbirds—have what’s called “high site fidelity.” That is, they will return faithfully to the same breeding territory and the same wintering territory year after year. The predictable vacationers will even revisit the same patch of trees.

Sean Graesser is a field biologist and conservation photographer who travels to South America annually to band birds. He has seen the Golden-winged Warbler's strong site fidelity firsthand. “They travel a thousand miles round trip and come back to the very same dot in a sea of green," Graesser says “within probably 100 feet.” In fact, he has netted the same Golden-winged three years in a row in a band of forest in Costa Rica, where some birds of the Great Lakes breeding population spend the winter.

When Kramer downloaded the geolocator data, he found a surprising trend: The two breeding populations remain separate when they migrate south to their nonbreeding habitat in Central and South America—like high school cliques that never mingle, whether at the cafeteria or during summer break. And they face quite different conditions where they winter. The Great Lakes birds migrate to Central America, a region with less habitat loss, whereas the warblers that breed in Appalachia—which have long been in decline—spend nonbreeding months in a region of Venezuela that shows signs of both current and historic exploitation.

“We know that deforestation is disproportionately affecting areas [in Venezuela], relative to Central America,” Kramer says. “When you lose the majority of native forest cover in these areas, that’s going to be meaningful to these populations.”

The findings have dire implications for the birds. Because the species has such high site fidelity, warblers that breed in Appalachia are unlikely to change their migration route to seek better habitat in Central America. That’s because migration is based in genetics, and evolution takes hundreds, if not thousands, of years. Meanwhile, human-caused change is happening quickly. “The level of anthropogenic change is something that you wouldn’t see naturally,” Kramer says. “It’s such a broad change that there’s not enough flexibility there genetically to escape.” Warblers that migrate to these degraded forests in Venezuela are stuck in what he calls an “ecological trap.”

This ecological trap has likely contributed to the Golden-winged Warbler’s downturn observed in Appalachia—though they may not be the only birds caught. In the study, the researchers identified 16 other neotropical migrants that might also be affected by changes to their nonbreeding, winter habitat. “If you look at where we know they overwinter, a lot of them follow this same basic trajectory,” says Curtis Smalling, an author on the paper and director of land bird conservation for . “It does seem to be broadly applicable.”

The new research is a leap forward, though there’s still much that remains unknown. For example, scientists still aren’t sure why these breeding populations of Golden-winged Warblers began using separate wintering areas in the first place. And they also still wonder how quickly Appalachian warblers might be able to evolve a more favorable migration strategy if their winter habitat continues to degrade. “That’s one of the remaining mysteries,” Smalling says.

But one thing has become clear: For the conservation of migratory species, the boundaries between countries or continents don’t apply. Indeed, if a migrant like the Golden-winged Warbler is wintering in habitat that’s at risk to deforestation, protecting its breeding grounds is only half the equation.

“There’s always an intuitive argument that everything we do to make baby birds is better,” Smalling says. “But our efforts here might be relatively futile if there’s nowhere for these birds to go during the nonbreeding period. There has to be a balance.”