Why Are These Rare Female Thrushes Disappearing?

On a hazy day in late September 2025, Desiree Narango walked the sandy paths at the Foreman’s Branch Bird Observatory in coastal Maryland looking for the secretive and rare Bicknell’s Thrush. The songbird—hard to identify and few in number—only stops here briefly to rest and refuel on its migration south to the Caribbean. But she was hoping to get lucky and catch a few to help solve a two-decade mystery: Why were there so few females? 

In the early 2000s, Jim Goetz, a conservation scientist at the Vermont Center for Ecostudies (VCE), published his observation that male Bicknell’s Thrushes outnumbered females at two sites in Vermont, where they breed. Concerningly, the ratio could be as skewed as eight males for each female. “No matter what bird species you are studying, females are the limiting factor for population size,” says Narango, another biologist at VCE. “It doesn’t matter how many males you have, if there isn’t a female, they can’t breed.”

Historically many ornithologists have focused their attention on more colorful, vocal, and easier-to-observe male birds, often overlooking how sex differences may affect avian conservation. But for Bicknell’s Thrushes, figuring out what’s happening to the females may be critical. According to the 2025 State of the Birds report, the species has declined by an estimated 50 percent in 50 years.

The VCE team is working to track the missing female birds to their last known address. They started, at the time of Goetz’s finding, with the easiest hypothesis to test—that perhaps males outnumbered females right from the start of life. The scientists sought out nests in scrubby conifers on mountaintops in the northeastern United States and at lower elevations in Canada. Since males and females look alike, Goetz and other scientists drew nestlings’ blood for a DNA test to distinguish their sex. But it turned out nests were hatching the same number of males and females. Their questions deepened.

Next, they looked to see if the females were disproportionately dying where the birds spend the winter—mainly on the island of Hispaniola, which includes Haiti and the Dominican Republic. In 2007 and 2011, VCE biologists and their local partners discovered that Bicknell’s females tended to use poorer-quality habitat on the center of the island, while males were more likely found in richer habitat, such as in the Dominican national park Sierra de Bahoruco. However, it appeared that males outnumbered females living on the island, as they did in Vermont. What’s more, the Dominican Republic nonprofit SOH Conservation found the overall number of Bicknell’s Thrushes using these habitats held steady, both through a single season and year-to-year. 

Taken together, the findings convinced the scientists that the females were probably not dying during the bulk of their time on Hispaniola. “For us, they are not disappearing,” says Jorge Brocca, director of the SOH Conservation. 

That left another possibility: The females were facing greater risks along their fall migration journey. But that’s a harder situation to study. Early on, the scientists didn’t know the species’ route or where the thrushes stopped to rest while traveling—let alone threats male and female birds faced on the move. Community observations from birders, such as eBird data, that typically might help, were less useful for these thrushes. Not only do the sexes look alike, but Bicknell’s Thrush looks nearly identical to the more common Gray-cheeked Thrush, making them easy to misidentify, says Michael Hallworth, another of the project’s researchers. (Scientists once thought the two were the same species.) 

To get a clearer picture of where and why the thrushes were dying, the scientists first needed better migration data. An early hint of their route came when a VCE scientist used tiny geolocators to show that the birds that left Vermont paused in the mid-Atlantic states. But the data could only point to the general area. Later, GPS tags helped more accurately pinpoint the birds’ stopover in the mid-Atlantic states before they launch over the ocean from along the North Carolina coast. However, to retrieve the data, scientists had to recapture birds that returned to their breeding grounds after surviving both the fall and spring migrations. 

Ideally, they want to get information from every bird they tag, even the ones that don’t make it back to Vermont, Hallworth says. To map the birds’ movements and track where their signal goes quiet, they turned to a newer technology: a global network of stationary towers, called Motus, that picks up radio signals from tagged birds that pass within 12-mile and delivers data in real time. Over the last decade, many Motus towers have been built in the eastern United States. After affixing tags to the birds that bred on Vermont’s Mount Mansfield, they learned both sexes were spending up to three weeks on the Delmarva Peninsula and in nearby Delaware and Virginia.

That’s what brought Narango and Hallworth to the region for a closer look last fall. Foreman’s Branch Bird Observatory was Narango’s first stop. This banding station has recorded more Bicknell’s Thrushes than any other site on the species’ migration route: an average of four a year. At first, the flat coastal plain seemed a strange place for thrushes that breed and winter in mountains, and she wondered what attracted them to rest there. On her first day of field work, Narango learned that she could find the thrushes in the coastal equivalent of their mountain habitat—wet, closed-canopy forests of abundant tupelos and sweetgums—and over three weeks in the area, the VCE scientists managed to capture and take blood samples from 23 possible Bicknell’s Thrushes. (Later genetic analysis showed three were Bicknell’s Thrushes, and the rest were the look-alike Gray-cheeked species).

The bulk of the investigation in this important stopover region is still to come, Narango says. The team now has a pretty good idea where, when, and how to find these two closely related thrush species. As they sample more birds, they hope to figure out what key threats affect both sexes that stop here—and whether females face greater risk. For one, they want to test blood samples to see whether pesticides or heavy metal levels are higher than they were on the breeding grounds.

Overall, however, they suspect habitat loss and fragmentation is the most critical challenge, and there’s reason to think females are more vulnerable to this threat on both their migration stopover and wintering areas. In the Dominican Republic, they theorize, males arrive first and claim the best habitat. If females migrate later than males, the scientists wonder whether something similar could be happening at stopover sites in Maryland, too. One day, Hallworth would also like to see Motus towers installed on the Dominican Republic’s north shore, to help learn more about how many avian migrants make it to the island each fall.

In the meantime, these scientists have learned more about this elusive species than seemed possible just a few years ago. Maren Gimpel, the associate director of the Foreman’s Branch Bird Observatory, a program of Washington College, says that for years, she thought the value of the site for studying migrating birds was that it is just an ordinary place along a popular migration route, providing scientists a look at what is typical and average. The emerging research showing how important the area is for this thrush has changed her view. “This is a special place for these birds.”