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.
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.