Why Are These Rare Female Thrushes Disappearing?

On a hazy day in late September 2025, Desiree Narango walked the sandy paths on the Foreman’s Department Fowl Observatory in coastal Maryland on the lookout for the secretive and uncommon Bicknell’s Thrush. The songbird—arduous to establish and few in quantity—solely stops right here briefly to relaxation and refuel on its migration south to the Caribbean. However she hoped to get fortunate and catch a number of to assist clear up a two-decade thriller: Why have been there so few females? 

Within the early 2000s, Jim Goetz, a conservation scientist on the Vermont Heart for Ecostudies (VCE), printed his statement that male Bicknell’s Thrushes outnumbered females at two websites in Vermont, the place they breed. Concerningly, the ratio may very well be as skewed as eight males for every feminine. “It doesn’t matter what hen species you’re finding out, females are the limiting issue for inhabitants dimension,” says Narango, one other biologist at VCE. “It doesn’t matter what number of males you could have, if there isn’t a feminine, they’ll’t breed.”

Traditionally many ornithologists have centered their consideration on extra colourful, vocal, and easier-to-observe male birds, usually overlooking how intercourse variations might affect avian conservation. However for Bicknell’s Thrushes, determining what’s occurring to the females could also be crucial. In accordance with the 2025 State of the Birds report, the species has declined by an estimated 50 p.c in 50 years.

The VCE group is working to trace the lacking feminine birds to their final recognized handle. They began, on the time of Goetz’s discovering, with the simplest speculation to check—that maybe males outnumbered females proper from the beginning of life. The scientists sought out nests in scrubby conifers on mountaintops within the northeastern United States and at decrease elevations in Canada. Since men and women look alike, Goetz and different scientists drew nestlings’ blood for a DNA take a look at to differentiate their intercourse. However it turned out nests have been hatching the identical variety of men and women. Their questions deepened.

Subsequent, they regarded to see if the females have been disproportionately dying the place the birds spend the winter—primarily on the island of Hispaniola, which incorporates Haiti and the Dominican Republic. In 2007 and 2011, VCE biologists and their native companions found that Bicknell’s females tended to make use of poorer-quality habitat on the middle of the island, whereas males have been extra seemingly present in richer habitat, resembling within the Dominican nationwide park Sierra de Bahoruco. Nonetheless, it appeared that males outnumbered females residing on the island, as they did in Vermont. What’s extra, the Dominican Republic nonprofit SOH Conservation discovered the general variety of Bicknell’s Thrushes utilizing these habitats held regular, each via a single season and year-to-year. 

Taken collectively, the findings satisfied the scientists that the females have been in all probability not dying in the course of the bulk of their time on Hispaniola. “For us, they don’t seem to be disappearing,” says Jorge Brocca, director of the SOH Conservation. 

That left one other chance: The females have been dealing with better dangers alongside their fall migration journey. However that’s a more durable scenario to review. Early on, the scientists didn’t know the species’ route or the place the thrushes stopped to relaxation whereas touring—not to mention threats female and male birds confronted on the transfer. Neighborhood observations from birders, resembling eBird knowledge, that usually would possibly assist, have been much less helpful for these thrushes. Not solely do the sexes look alike, however Bicknell’s Thrush appears to be like almost equivalent to the extra frequent Grey-cheeked Thrush, making them simple to misidentify, says Michael Hallworth, one other of the undertaking’s researchers. (Scientists as soon as thought the 2 have been the identical species.) 

To get a clearer image of the place and why the thrushes have been dying, the scientists first wanted higher migration knowledge. An early trace of their route got here when a VCE scientist used tiny geolocators to point out that the birds that left Vermont paused within the mid-Atlantic states. However the knowledge might solely level to the overall space. Later, GPS tags helped extra precisely pinpoint the birds’ stopover within the mid-Atlantic states earlier than they launch over the ocean from alongside the North Carolina coast. Nonetheless, to retrieve the information, scientists needed to recapture birds that returned to their breeding grounds after surviving each the autumn and spring migrations. 

Ideally, they need to get info from each hen they tag, even those that don’t make it again to Vermont, Hallworth says. To map the birds’ actions and monitor the place their sign goes quiet, they turned to a more recent expertise: a worldwide community of stationary towers, called Motus, that picks up radio indicators from tagged birds that cross inside 12-mile and delivers knowledge in actual time. Over the past decade, many Motus towers have been constructed within the jap United States. After affixing tags to the birds that bred on Vermont’s Mount Mansfield, they discovered each sexes have been spending as much as three weeks on the Delmarva Peninsula and in close by Delaware and Virginia.

That’s what introduced Narango and Hallworth to the area for a better look final fall. Foreman’s Department Fowl Observatory was Narango’s first cease. This banding station has recorded extra Bicknell’s Thrushes than every other website on the species’ migration route: a mean of 4 a yr. At first, the flat coastal plain appeared a wierd place for thrushes that breed and winter in mountains, and he or she puzzled what attracted them to relaxation there. On her first day of subject work, Narango discovered that she might discover the thrushes within the coastal equal of their mountain habitat—moist, closed-canopy forests of ample tupelos and sweetgums—and over three weeks within the space, the VCE scientists managed to seize and take blood samples from 23 doable Bicknell’s Thrushes. (Later genetic evaluation confirmed three have been Bicknell’s Thrushes, and the remaining have been the look-alike Grey-cheeked species).

The majority of the investigation on this necessary stopover area remains to be to return, Narango says. The group now has a fairly good thought the place, when, and learn how to discover these two carefully associated thrush species. As they pattern extra birds, they hope to determine what key threats have an effect on each sexes that cease right here—and whether or not females face better threat. For one, they need to take a look at blood samples to see whether or not pesticides or heavy steel ranges are increased than they have been on the breeding grounds.

Total, nonetheless, they think habitat loss and fragmentation is probably the most crucial problem, and there’s purpose to suppose females are extra susceptible to this risk on each their migration stopover and wintering areas. Within the Dominican Republic, they theorize, males arrive first and declare one of the best habitat. If females migrate later than males, the scientists wonder if one thing comparable may very well be occurring at stopover websites in Maryland, too. In the future, Hallworth would additionally prefer to see Motus towers put in on the Dominican Republic’s north shore, to assist study extra about what number of avian migrants make it to the island every fall.

Within the meantime, these scientists have discovered extra about this elusive species than appeared doable only a few years in the past. Maren Gimpel, the affiliate director of the Foreman’s Department Fowl Observatory, a program of Washington Faculty, says that for years, she thought the worth of the positioning for finding out migrating birds was that it’s simply an strange place alongside a preferred migration route, offering scientists a take a look at what’s typical and common. The rising analysis displaying how necessary the world is for this thrush has modified her view. “This can be a particular place for these birds.”