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.
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.