Unlocking Migration's Secrets

Unlocking Migration's Secrets

For centuries the study of bird migration has been riddled with mystery and unanswered questions: Where do birds go in winter? How far do they fly? Can they navigate a hurricane? Scientists are tapping new technologies to find the answers, and transforming everything we know--or think we know--about birds.

By Scott Weidensaul/Photography by Joel Sartore
Published: March-April 2012

Last August a female whimbrel took off from the treeless tundra of Southampton Island, which guards the iceberg-choked entrance to Hudson Bay in the Canadian subarctic, and set a course southeast.

Long-limbed and gray-brown, she was the size of a small duck, bearing the field marks that make this shorebird instantly identifiable--dark stripes on the crown of her head, and a long, thin, drooping crescent of a bill. It gives the whimbrel its genus name: Numenius, Greek for "new moon."

Southampton had merely been a way station for her; some weeks earlier she had arrived there from her breeding grounds on the Mackenzie River delta in the Northwest Territories, some 1,500 miles and 52 hours of flying time to the northwest. Now her bill was stained purple from weeks of gorging on the autumn tundra's bounty--blueberries, crowberries, and cloudberries, all of which her body had converted to thick layers of fat, fuel for the incredible journey ahead.

Tapered wings pumping without pause or rest, she flew east across Hudson Bay, passing thereafter over the rugged Ungava Peninsula of northern Quebec, then above the trackless boreal forests and wild rivers of Labrador. After 1,500 miles of unbroken flight, averaging 43 miles per hour, she left land behind, flying out into the open ocean somewhere south of Newfoundland.

Connectivity Map
Illustration by Peter Hoey
High Hope: A Whimbrel's Migration

It was here that she ran into trouble. Her flight path brought her into the remnants of Tropical Storm Gert, which was roiling its way up the north Atlantic. Fighting ferocious headwinds, the bird struggled to stay aloft as she plowed ahead at a painful crawl; for the next 27 hours her average ground speed slowed to a mere nine miles an hour.

Finally she broke through to the storm's far side. Now, with the gale at last at her back, she rocketed along at 92 mph. In little more than 90 minutes she flew 150 miles to make landfall at Cape Cod--her first rest since she'd taken off from Southampton Island four days and 2,325 miles earlier.

That we know about this migratory drama at all, much less in such extraordinary detail, is thanks to a small satellite transmitter nestled among the feathers on the whimbrel's back--part of a transformation in wildlife tracking that is making it possible for scientists to understand the movements of migratory animals with a level of precision unimaginable even a few years ago. They can comb through the chemical isotopes in a bird's feather and learn where it spent the winter, sift the DNA in its genes to tease out how migratory populations are related to one another, attach minute devices that faithfully record its location for years at a stretch by just observing the time the sun rises and sets each day, or track threatened eagles in the Appalachians by tapping into the ubiquitous cell phone network (and thus help planners avoid conflicts with new wind farms). They have found that neighboring populations of the same species may take wildly different routes to far-flung wintering areas, and that many birds show an almost incomprehensible degree of fidelity to particular stopover and wintering sites.

With that new knowledge has come a recognition of what we've badly underestimated: the profound, almost universal role that migration plays in virtually every aspect of a wild bird's life--and, with it, a new effort by dozens of researchers, agencies, and organizations like Audubon to better map these movements, better understand their significance, and harness this greater understanding for the benefit of birds.

"This new generation of telemetry has enormous potential for understanding bird biology and conservation," says Steve Kress, Audubon's vice president for bird conservation, and the founder of Project Puffin. "The new data loggers and transmitters could be far more important than bird bands, and future ornithologists may someday think of placing metal rings on bird legs as being as primitive as we might think of John James Audubon's method of tying a silver thread to a phoebe leg. This is because these new devices give us the ability to track individual birds throughout the year--something that a band was never intended to do." (To learn about the migratory secrets of some other individual species, from long-tailed ducks to pink-footed shearwaters and golden eagles, go to audubonmagazine.org.)

But by lifting the veil that has masked much of where and when birds travel, conservationists not only realize how little they know about migration. They've also learned, sometimes to their grief, that they have badly underestimated, too, the dangers facing these global wanderers--birds like the whimbrel named Hope.

 

Hope--whose migration from the Northwest Territories to Cape Cod last fall was followed in exquisite detail--was tagged with her satellite transmitter in May 2009, when scientists from the Center for Conservation Biology at the College of William and Mary captured her on the tidal mudflats along Hope Creek, on Virginia's Eastern Shore.

For the next two and a half years, they and their partners followed Hope as she migrated back and forth on an annual round-trip of more than 12,000 miles--the equivalent of five trips between New York and Los Angeles. She was one of 19 whimbrels fitted with such sophisticated transmitters; these birds' carefully traced journeys exemplify a newly holistic approach to understanding bird migration.

Yet nothing prepared the scientists for the story that unfolded in 2011, when the whimbrels found themselves not only battling exhaustion, hurricanes, and tropical storms but a final and (for some) fatal barrage that underscores how increasingly perilous such passages can be--and how understanding the full extent of a migratory bird's travels is essential for protecting it.

It was a gnawing worry about whimbrel populations, and a lack of information on how they spent most of their lives, that first moved Center for Conservation Biology director Bryan Watts to put satellite transmitters on the elegant shorebirds. "We were doing weekly aerial surveys in the spring on the Eastern Shore [of Virginia], and by the early 2000s we started to see significant declines in whimbrels," Watts says. "We didn't feel it was a local issue, because the birds are feeding on fiddler crabs and marine worms, which are super-abundant. We didn't know if the problem was on the breeding grounds or the wintering grounds, but where were they breeding? Where were they wintering? We realized we needed to get more of a full, annual-cycle picture."

The first whimbrel Watts's team had tagged, a bird nicknamed Winnie that they captured in May 2008, stunned researchers by flying more than 3,200 miles in just six days to the Mackenzie River delta in the Northwest Territories and then, after a brief rest, continuing on to the North Slope of Alaska to breed.

"That was just a complete shock," Watts recalls. Scientists had assumed the eastern and western populations of whimbrels were discrete, with their own migration routes and wintering areas. "That took the roof off of what we thought we knew, and told us for the first time that there was all this crossover and mixing going on."

As Watts's team and their partners deployed more transmitters, there were more surprises. "We assumed these birds were coming back through the Delmarva every year, but the peninsula's a hundred kilometers long. We had no idea that the same bird would come back to the same creek, to exactly the same mudflat, year after year," Watts says. As the whimbrels roved the world--from the high Arctic, where they nest, to the mangrove swamps and mudflats of the Caribbean and northeastern South America, where they spend the winter--they were showing an astounding level of what biologists call site fidelity.

Hope is a good example. Following her daily satellite fixes, Center for Conservation Biology scientists like Fletcher Smith saw her scribe looping flight paths across half the Western Hemisphere--and came to realize how utterly dependent this bird was on a mere handful of locations where she could rest and feed after epic flights.

"Hope is basically using just four sites in the world, while crossing almost 12,000 miles a year," Smith says, his voice tinged with equal measures of wonder and worry. It is both a migratory miracle and a conservation nightmare, because a bird that depends on so few stopover places is in deep trouble if any of those spots proves dangerous. This, too, would be a lesson that the whimbrels would teach.

 

The ability to pinpoint the movements of a single bird across thousands of miles and over many years, thanks to ever-smaller and more advanced tracking devices, is a triumph for wildlife research. But it has also revealed how woefully shortsighted our understanding of birds has been since--well, since the beginning of ornithology. The study of birds arose in the Northern Hemisphere, in Europe and North America. For centuries, ornithologists there wore the equivalent of geographic and seasonal blinders when it came to their research, concentrating largely on what happened to migratory birds during the breeding season, when the birds, too, were in the north.

As a result, we knew little about what happened to birds during the seven or eight months--most of the year--when they were migrating or on their wintering grounds. Many scientists dismissed those nonbreeding periods as basically unimportant.

"You can't look at this discrete breeding period alone and expect to understand the biology of birds--but that's what we've done as biologists," says Peter Marra, a research scientist at the Smithsonian Migratory Bird Center within the Smithsonian Conservation Biology Institute in Washington, D.C. "One of the obvious reasons why we've done so is because it's in our own backyard. The second one is that we haven't really been able to follow these birds throughout the year, so there's been a disconnect between where these birds breed and where they winter."

But that is changing rapidly, as ornithologists like Marra embrace the notion of "migratory connectivity"--linking individuals and populations throughout their annual cycle. Only by following migratory animals during the entire year can scientists comprehend what physiological feats wild travelers are accomplishing--and begin to tease out the layers of selection pressure that drive their evolution.

And it's only through such all-encompassing approaches, Marra and others argue, that conservationists can hope to stem the declines and disappearances of migratory species, from songbirds to salmon and whales. That's why Smithsonian's Marra and Sue Haig from the U.S. Geological Survey, along with dozens of other scientists, and with support from Audubon, have created the Migratory Connectivity Project to encourage research into this poorly understood area.

The notion of connectivity has changed the way Marra looks at almost everything related to migratory birds. Whether as a cause or a consequence, Marra is now one of the leaders in the growing field of migratory connectivity research, which grew out of his decades of studying American redstarts on their wintering grounds in Jamaica. He found, to his surprise, that what happens in the Caribbean doesn't stay in the Caribbean.

Redstarts practice something called sexual habitat segregation--common in many birds--where adult males grab the best winter territories, and on down the dominance ladder to immature females, which get the worst. "Best" means wettest and buggiest, mangrove swamps being preferred in Jamaica. By testing stable chemical isotopes in redstart blood, muscle, and toenails in the breeding area, Marra and his colleagues have found that the birds that winter in the wetter habitats are able to head north on their spring migration earlier and arrive on the nesting grounds first.

What's more, the quality of their winter territory has what Marra calls "carryover effects" right into the breeding season. "When you track those birds and follow their reproductive success, you find that birds that are originating in wet habitat have significantly higher reproductive success than birds coming back from dry habitat," Marra says. "It doesn't matter what happens on the breeding grounds--most of the variation in the number of young fledged is controlled by winter habitat."

These findings, which made the ornithological world sit up and take notice, are of more than academic importance. Winters in Jamaica have been getting steadily drier for the past 16 years, and if--as climate models predict--that trend continues, it could mean that no matter how much management we do here in North America, birds like redstarts could be in trouble.

That makes the timing perfect for the Migratory Connectivity Project. The goal is to harness interest in connectivity with incredible advances in technology and analytical techniques like isotope studies. While satellite transmitters like the ones used on the whimbrels are still relatively bulky, restricting their use to larger species, new inventions are allowing scientists to track smaller birds with undreamed-of precision.

Take light-sensitive geolocators, for instance, which weigh less than a penny and record moment-to-moment changes in sunlight to calculate a rough latitude and longitude for two years or more. In 2007 a team of scientists including Iain Stenhouse affixed geolocators to Arctic terns nesting in Greenland and Iceland. They found that some of the terns were traveling more than 47,000 miles in a single year--nearly double the distance ornithologists thought they migrated.

"Had someone told me just 10 years ago that we would be able to track small seabirds like terns across whole oceans and entire years--never mind that I'd be involved in such a project--I would have been completely incredulous," says Stenhouse, who studies seabirds at the Biodiversity Research Institute in Maine.

Nor did they stop with terns. In 2007 he and two colleagues used geolocators to track Sabine's gulls from their Arctic breeding grounds to wintering sites off southern Africa--at almost 25,000 miles, the longest migration known for any gull. "Over the last decade or so we have seen an all-out revolution in the study of avian migration," Stenhouse says. "Seeing the actual tracks of individual birds traversing the face of a globe still gives me shivers."

Using these new techniques, scientists are redrawing the maps of migration, discovering how regional populations are often taking very different routes to disparate wintering grounds, or discovering new over-winter areas entirely. One of Marra's students has used geolocators to discover the previously unknown destination for eastern willets (they travel to southern Brazil, as it turns out), while other researchers have used the devices to track the ever-shifting movements of bobolinks after they leave the grasslands of North America.

"The bobolink stories are just amazing," Marra says. "These birds are in an almost constant circuit of movement, from their breeding areas in Vermont or Nebraska down to their wintering area in northern South America, then back up to what might be multiple breeding areas. It's just a fascinating story we're learning from these light-level geolocators."

Not all the techniques being marshaled to understand migratory connectivity involve high-tech gadgets. Bird bands, which scientists have used for more than a century, may seem unsophisticated, but they also provide a treasure trove of untapped data. Last year Marra and two colleagues combined banding data and information from geolocators to map the connections between different populations of gray catbirds, showing that those from the Midwest take a circum-Gulf route to winter in Central America, while those from the mid-Atlantic region flit down to Florida and the Caribbean each fall.

Building on that, the Migratory Connectivity Project is undertaking the first comprehensive analysis of millions of band-recovery records, with the aim of creating an atlas of migratory connectivity for hundreds of species.

 

For Hope the whimbrel--resting on Cape Cod after surviving her passage through Tropical Storm Gert, her transmitter sending regular reports back to Fletcher Smith and Bryan Watts at the Center for Conservation Biology--the journey was far from over. After pausing just a day, she flew down the coast to the Eastern Shore, lingered a month on the same mudflats and salt marshes where she was originally tagged, then on September 9 flew out over the sea. Late summer was a bad time to be aloft on the western Atlantic, however. Another whimbrel tagged by Watts and his partners, a female named Goshen, normally puddle-jumps from the Eastern Shore to the northeastern coast of South America near Belem, Brazil. Instead, on August 22, she hit the violent eastern edge of Hurricane Irene, then a Category 3 storm, and three days later made an emergency landing on the island of Antigua.

A third female whimbrel, named Machi, which also normally flies nonstop from Virginia to Suriname and eastern Brazil, rode out Irene on the Eastern Shore, then took off only to get tangled up with Tropical Storm Maria before making a safe landfall on the island of Montserrat. Back in Virginia, Smith and Watts were watching the satellite tracks with worry and breathless wonder--seeing how the fragile birds cut through some of the most ferocious storms on the planet and survived. Through the Internet, on which the birds' travels could be followed in real time as colorful dots traversing a map of the hemisphere, uncounted others were able to share in the drama, too.

"Most of these birds are entering the storms on the northeast quadrant, where the winds are in their faces, and then going straight through the eye of the hurricane," Smith says. "It looks like these whimbrels are strong enough to negotiate even these huge storms."

Strong but not invulnerable. Wrung out by their journeys, both Machi and Goshen began island-hopping down the Lesser Antilles instead of flying the rest of the way to South America in a single leap. On September 12, both birds independently approached Guadeloupe, a French-controlled island--which, like several islands in the Antilles, maintains a tradition of legal, basically uncontrolled shorebird hunting.

Tired and hungry, the two birds zeroed in on some of the only wetland habitat on the island--so-called "shooting swamps," where middle- and upper-class Creole hunters watch the weather, and wait for storms to ground exhausted shorebirds. There are 3,000 such hunters on the tiny island, dozens of whom ring the shooting swamps at a time, sitting in lawn chairs with shotguns ready for yellowlegs, dowitchers, semipalmated sandpipers, and other migrants. Other gunners patrol freshly cut sugarcane fields, where upland sandpipers and American golden-plovers feed.

Machi and Goshen both died before they could even land on Guadeloupe. Their deaths enraged conservationists--and provided a tragic reminder that even the briefest portion of an annual cycle can have devastating consequences for a migratory bird.

Are the steady declines in whimbrels, semipalmated sandpipers, and several other shorebirds due to the heavy but--until now--little-known hunting pressure these species face on a few Caribbean islands and along the South American coast?

"Clearly, what's going on down there is a throwback to an earlier time," Watts says. "We hunt woodcock and snipe up here, but it's a managed hunt, and we have good data to back it up. This is an open-ended hunt--there's no bag limit. In just one fallout last year, 2,000 to 3,000 golden-plovers were shot; one hunter said he killed a hundred in a single day. This is legal open-ended hunting."

If there's a bright side, it's that the very public deaths of two well-known migrants is finally spurring an examination of the unregulated shooting on Guadeloupe and elsewhere in the Caribbean. The local newspaper even covered the whimbrels' death, labeling the hunters "murderers," and there are indications that whimbrels and red knots may be added to the list--widely ignored by gunners--of protected species. On Barbados (an independent country where the last Eskimo curlew--the whimbrel's closest relative--was shot in 1963, and which is also a gunning hotspot), one shooting swamp is being bought and converted to a sanctuary.

And what of Hope? She also had to contend with Tropical Storm Maria, her second major gale of the autumn, but she averaged 31 mph through even the worst parts of the storm. On Sept. 14--83 hours and 1,624 miles after leaving Virginia, and nearly 6,000 miles from her nesting site in the Northwest Territories--she made landfall on the island of St. Croix.

There she found a very different world than the gauntlet that awaits migrant shorebirds on islands like Guadeloupe, Martinique, and Barbados. As one of the U.S. Virgin Islands, St. Croix is subject to U.S. laws, including the international treaty that protects shorebirds and to which French possessions like Guadeloupe are not signatories.

Yet even though Audubon and BirdLife International recognize Hope's favored wintering site--a St. Croix mangrove swamp known as Great Pond--as an Important Bird Area, a huge resort development has been proposed that would essentially envelop the bay. Local environmentalists have stalled that project thus far, but disturbance by four-wheel-drive vehicles and illegal dumping are ongoing problems. There are many dangers for a migratory bird, and only a few come out of the barrel of a gun.

But for the time being, at least--and true to her name--Hope was safe from all but the mostly natural dangers to which shorebirds have always been subject. And true to her nature, which depends on a few extraordinarily small, localized places in her hemispheric wanderings, Hope would defend a tiny winter territory on St. Croix. Every time she has been observed at Great Pond, it has been within 10 yards of the same spot--a small section of a mangrove-rimmed mudflat rich in fiddler crabs, which she defends against other whimbrels. For the next seven months this would be her universe while she rebuilt her reserves of fat and strength for the arduous return trip that awaits her.

About the time you read these words, Hope will be winging northwest again, retracing the thousands of miles that will bring her back to Hope Creek on the Virginia tidelands, there to gorge on still more crabs, then across the Canadian Shield to the flower-specked tundra on the Mackenzie Delta--spanning the continent on a journey whose complexities, triumphs, and dangers, like those facing all migrants, grow ever more astonishing and awe-inspiring as we decipher them.

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