A previously tagged Kirtland’s Warbler is captured in its jack pine breeding habitat in Michigan. A little transmitter on its back holds information on where the bird has been from August to May. Karine Aigner

From Audubon Magazine

This Little Warbler Could Lead to Big Discoveries About Migration

After bouncing back from near extinction, the Kirtland's Warbler is helping scientists understand the outsize role of winter habitat for migratory birds.

Nathan Cooper is driving as fast as he dares, through murky April twilight along a twisting road with an unsettling number of pedestrians, free-range chickens, loose dogs, and feral cats. The grandly named Queen’s Highway is a narrow, unmarked strip of potholed macadam that runs the 48-mile length of Cat Island. We need to be at the southern end by sunrise, and we’re late.

Cat Island is well off the main tourist drag in the Bahamas. Shaped like a long, narrow fishhook, it covers just 150 square miles and is so slender that for much of its length it’s only about half a mile wide. Cat is largely flat and featureless, a lot of dry scrub forest bisected by few roads, with barely 1,500 residents. Slash-and-burn farming, raising goats, or fishing for conch are among the only options here.

But what makes Cat Island a tough place for people—its hot, dry climate and hardscrabble soil, the scrubby forest full of highly toxic poisonwood trees, even its herds of ravenous goats—makes it arguably the best wintering spot in the world for the endangered Kirtland’s Warbler. Perhaps a thousand of these half-ounce birds, fully one-fifth­ of the global population, migrate to this relative­ speck of land. And it’s why Cooper, a post-­doctoral fellow at the Smithsonian Migratory Bird Center in Washington, D.C., has returned to the Caribbean with his crew for a third winter. They are capitalizing on the warbler’s unique biology to learn how the effects of wintering ground conditions shape the lives of migratory birds.

Scientists once thought of winter as a respite for a migratory bird, an easy-living, tropical hiatus from the serious work of migration and reproduction. But they’re learning that a bad winter can cast a very long shadow, an ecological hangover that can linger for many months and across thousands of miles. Sparse rain and limited food in a bird’s winter habitat can create a caloric deficit that delays the start of its migration and may even force it to effectively cannibalize its own muscle and organs. It raises the already substantial odds of dying on the journey, and even if the bird arrives on the breeding grounds to find ideal conditions, those shortages can sabotage breeding success. Given that the tropical regions on which hundreds of millions of migratory birds depend are already warming and drying—a trend that is expected to accelerate—this discovery has ominous implications at a time when migrant populations are already in steep decline.

It’s ironic, therefore, that as conservationists turn more and more attention to these so-called carry-over effects and how they may determine the fate of hundreds of species, the bird likely to be most illuminating is the Kirtland’s Warbler, a species nearly extinct just a few decades ago, and routinely hailed as an unparalleled conservation success. Thanks to newly miniaturized tracking devices, Cooper’s team will be able to follow dozens of individual warblers north to their breeding grounds in the jack pines of northern Michigan and back again and, for the first time with any species, directly measure how their condition here at their wintering grounds impacts their later migration and nesting success.

The sun is up by the time we reach the southern end of Cat Island, where we’re surrounded by scruffy, jumbled woodland, the tallest trees maybe 15 feet high, the understory impenetrable. “We work on the roads, mostly,” Cooper says as he shrugs on a bulky pack, sliding an unsheathed machete through the straps behind his back. “We tried to bushwhack through this stuff the other day, and it took us two hours to go 600 meters.”

Cooper is 37, with unruly brown curls, chin whiskers, and the compactly muscular torso of an avid rock climber. The other half of the team today is Chris Fox, quiet and darkly bearded, taking a break from his desk job with an Indiana conservation district. Fox slings a heavy radio receiver over his shoulder and grabs aluminum mist-net poles, while Cooper turns on a handheld caller and strides down the sand road at a brisk pace, blasting the song of a male Kirtland’s Warbler into the brush. (The always reliable David Allen Sibley transliterates its song as a “rich, emphatic flip lip lip-lip-tiptip-CHIDIP rising in pitch and intensity.”)

We trudge up one road, over a small hill past a vacant home with hurricane shutters in place, and down another. A fusillade of angry chips comes from the underbrush, the sign of an aggressive warbler, and minutes after the two biologists set up their net along the edge of the woods, the Kirtland’s launches itself in righteous rage at the sound of what it thinks is an intruder, and right into the mesh.

A male, the warbler weighs 16.5 grams, about three-fifths of an ounce—a couple of grams heavier than normal, and a sign that this bird has been finding lots of fruit and bugs. “He may be ready to go in the next couple of days—I think we’ve caught him as he’s bulking up,” Cooper says. In addition to routine measurements, the researchers take a few drops of blood from a vein in the bird’s wing and scoop a little poop (helpfully splattered on Cooper’s pants)—a colleague is looking at how the warblers’ microbiome changes from winter to summer. Finally, the bird gets a unique set of colored leg bands and a transmitter that sits low on its back.

Known as nanotags, these tiny radio transmitters weigh a fraction of a gram and are tracked by a fast-growing network of more than 350 automated receiver stations set up across the hemisphere. It’s the middle of April, and if all goes well, when the birds leave Cat Island in a few weeks, those receiver sites will allow Cooper to follow them as they migrate north. Once they reach Michigan, 11 receivers covering basically the entire core breeding range of the species will allow him to quickly relocate the tagged birds.

The very aspects of the warbler’s ecology that make the species the ideal lens through which to understand the causes and consequences of carry-over effects—its highly specialized habitat requirements, the incredibly restricted size of its breeding and winter ranges—are the same ones that brought it to within a hairs-breadth of extinction. Naturally rare, its breeding habitat almost disappeared due to fire suppression in the 20th century, while nest parasitism by Brown-headed Cowbirds further shrank its productivity. By 1974 scientists could find just 167 singing male Kirtland’s Warblers, and the species seemed headed to oblivion. But last-ditch conservation measures—the creation of new nesting habitat and aggressive trapping of cowbirds—succeeded beyond almost anyone’s dreams. Today there are roughly 5,000 warblers.

But until recently almost all the scientific and conservation attention has been directed to Michigan; the Bahamian wintering grounds have mostly been ignored. If Cooper and his colleagues are right, though, conditions here may prove to be the species’ Achilles’ heel in a fast-changing world.


The next morning, following overnight rains, the air is muggy and warm even before sunrise. We fishtail back along another sand road, through vegetation that’s barely waist-high—perfect for Kirtland’s Warblers. “This is what they’re eating,” Cooper says, pointing to a low shrub, and I do a double-take trying to see what, exactly, he’s talking about. “Right here—this is black torch, one of their favorite food plants,” he explains, showing me its minute, desiccated fruits. The bird’s other staple, a lantana sometimes known as wild sage, has purple fruits that are about the size of pinheads. The best habitat for these shrubs—and thus for the birds—seems to be abandoned fields and heavily grazed goat pastures, disturbed scrub that is maintained, however accidentally, by the rudimentary farming practiced on Cat.

After hours of effort, we finally tag the crew’s 59th Kirtland’s of the season. With only a few days left, it’s clear Cooper won’t hit his goal of putting nanotags on 100 warblers (In the end, they will tag 63.) But he’s satisfied, even as he looks ahead to months’ more work up in Michigan finding, retrapping, and monitoring these same birds.

That afternoon we sit on the covered deck of the beach house they’ve rented for the season. Cumulus clouds gather as Cooper outlines the history of research into carry-over effects. Soon rain is drumming on the corrugated plastic roof—fitting, since a lot of what he tells me has to do with how precipitation during the traditional winter dry season may ultimately determine a warbler’s chances.

As far back as the 1970s, waterfowl scientists had an inkling that the effects of wintering-ground conditions might carry over into the nesting season. But most experts assumed that the real driving force in a bird’s reproductive success was the quality of its nesting habitat, not what had happened to it months earlier. The breakthrough came in 1998, when Cooper’s boss at the Smithsonian, Peter Marra—then a Ph.D. student at Dartmouth—published research on American Redstarts and sparked an intellectual gold rush into carry-over effects.

Marra and his colleagues found that in Jamaica, redstarts in wet mangrove forest (typically older, more dominant males) fared well, while forcing young males and females into dry scrub habitat, where they generally lost weight. Because at that time there was no way to track individual warblers north, Marra instead captured different redstarts in New England in the breeding season and looked at the ratios of stable carbon isotopes in their blood, which reflected how damp or dry their winter habitat had been. He (and later his students) found that those birds arriving earliest, enjoying the best selection of territories and mates, were those that had wintered in wet forest, while birds with the scrub signature showed up late to the game, weighed less than early arrivals, and were in overall poorer condition. Females from dry habitat had fewer chicks, and those chicks fledged later than those of mothers from good winter habitat. Dry-habitat warblers also had a greater risk of dying in migration.

And it’s not just warblers; many researchers have confirmed the importance of carry-over effects in a range of migratory birds, from auklets to shearwaters. Scientists studying Old World songbirds have shown stark correlations between breeding success in Europe and winter rainfall in the Sahel, the arid southern fringe of the Sahara where millions of the birds winter.

Data collected from recovered nanotags show the spring-migration trajectories of a handful of warblers. Illustration: Katie Peek

Rainfall appears to be the critical factor for Kirtland’s, too. Back in 1981, conservationists noticed a connection between wetter winters in the Bahamas and male warbler numbers in Michigan. More recently another scientist in Marra’s lab, Sarah Rockwell, was able to show specifically that if the rains fail in March, the warblers’ migration mortality rate jumps significantly. In such situations, the birds arrive later on the breeding grounds, are late in starting nests, and fledge fewer chicks.

But as the brief shower ends, Cooper notes that all the carry-over research up to this point has been indirect; the best that researchers can do is make inferences, and our understanding of how one season affects others is still rudimentary. Over the past 20 years there has remained one frustrating disconnect—researchers’ inability to easily study the same individual along its entire life cycle, from winter through migration and breeding and back again.

“There’s so much individual variation in all these things that it’s so much harder to find a signal in the noise,” Cooper says. “But if you’re looking at the exact same bird [each time], you can control for a lot of that variation.”

That’s what makes this project so ground-breaking. With both winter and summer data in hand, Cooper hopes to eventually start teasing apart how a warbler’s fat stores on Cat Island affected its departure date and migration speed, or whether its muscle mass predicted its arrival date in Michigan or the likelihood of its disappearance en route. In the seasons to come, Cooper and Marra want to drill down into the still poorly understood question of how Kirtland’s Warblers are using the Bahamas in winter. For example: Why do some individuals hunker down in one spot all season while others roam widely? Are the strategies tied to age or sex, and is one more successful than the other?