The physical rigors and many other dangers of migration cause about 85 percent of all songbird deaths, and humans are adding to the dangers, warns Cornell University ornithologist Miyoko Chu.
Fewer birds return to their breeding grounds every year because of poisoning by pesticides, habitat loss resulting from destructive agriculture, and other human pressures.
Climate change can damage habitats and alter the ecological rhythms birds depend on, including peaks in their food sources.
In the Americas, more than half of all migrating bird species have dwindled in number in the past 40 years, according to BirdLife International, a U.K.-based conservation group.
The National Audubon Society notes that some North American species have plummeted by as much as 80 percent in the last four decades.
Birds flying between Europe, the Middle East, and Africa have declined as well—by 40 percent. But it's still unclear exactly why. So ornithologists are working to find out where birds go and what trials they face each season—before their journeys get any tougher.
By examining molecules in birds' tissues, fitting them with location-sensing tags, and even studying how migrants learn survival strategies from local birds along their routes, scientists have begun seeing how conditions birds face in summer might affect their survival in winter, or vice versa.
(See related: "Memory Aids Birds in Migration, Study Finds" [April 29, 2003].)
The Case of the Mysteriously Vanishing Bobolink
The bobolink has declined by nearly 50 percent since 1966, according to the North American Breeding Bird Survey.
Rosalind Renfrew, a biologist at the Vermont Center for Ecostudies, is hoping to pinpoint some causes of this loss by deciphering the bird's diet.
She's doing this by collecting bobolink feathers and analyzing their stable isotopes—one of the most promising strategies for interpreting how circumstances on wintering grounds affect birds' success on breeding grounds and their overall survival.
Scientists began studying isotopes (versions of one element with different numbers of neutrons) in migratory birds during the 1990s to monitor the diets of large groups.
Bobolinks, which gather in huge flocks, grow new breeding plumage containing carbon and nitrogen isotopes from food they eat in South America, and the information scientists glean from the feather isotopes "tells you what winter habitats they're in, which determines what threats they might be facing," said Renfrew, who is currently analyzing feathers snipped last summer.
Those threats include pesticides in Bolivia and, to a lesser extent, control efforts throughout the bobolink's winter range by South American farmers, who consider the birds vermin. (North American farmers used to take potshots at bobolinks until a U.S. bird protection law forbade it).
As farmland has replaced the South American grasslands bobolinks historically used for food, their diets have shifted.
With support from the National Geographic Society's Committee for Research and Exploration, Renfrew is analyzing the extent to which bobolinks rely now on South American rice instead of wild seeds.
Rice might fatten birds better, but it also comes with risks, such as pesticides, and may not result in ideal body condition for migration and reproduction.
Tracking a 10,000-mile Migration
Scientists are also using satellite transmitters to directly track larger birds' movements, and are able to find out for the first time the exact routes and extraordinary distances some birds travel.
The lives of many seabirds and shorebirds remain perfect puzzles for these one-ounce (30-gram) satellite tags.
Marine biologist Andrew Westgate, a National Geographic grantee, has taped and glued transmitters to gull-like greater shearwaters, which appear to be declining in the Atlantic Ocean.
Westgate's project leader, biologist Rob Ronconi of Dalhousie University in Nova Scotia, tracked six greater shearwaters for more than 10,000 miles over three ocean crossings—the first-ever record of their Atlantic migration, which took them from North America to Africa, back west to South America, and finally to their nesting grounds between South America and Africa.
Oil spills, fishing lines, and predators introduced to certain breeding spots might be contributing to the shearwaters' apparent decline, but scientists have yet to gather strong evidence of how many birds are dying.
Satellite-tagging programs like this one may help shed new light. Important protections have already resulted from the information other seabirds have relayed.
By satellite-tagging endangered albatross, ornithologists have shown that many die after getting tangled up in fishing gear. This helped lead to a 12-nation treaty, the Agreement for the Conservation of Albatrosses and Petrels, which President Bush proposed the U.S. join in October 2008.
Time Running Out
Despite advances in research and understanding of migratory birds and their routes, entire avian migratory systems remain ignored, warn University of Florida zoologists Douglas Levey and Alex Jahn.
They're spearheading DNA studies of the Southern Hemisphere's largest migration, which spans South America—Earth's most bird-rich continent—and about which little is known so far.
"We believe that with modern technology like genetic techniques and stable isotopes, migration research in South America can come a long way in a very short time," said Levey, who receives support from National Geographic for his research.
And time is running out: destructive agriculture in Latin America is decimating protective and food-rich forest cover for birds including the fork-tailed flycatcher, one of the birds Levey is studying.
As these forests shrink, so do the numbers of birds who depend on them for shelter and food. They have to search elsewhere for what they need, and in some cases become more vulnerable to predators.
But new research shows that locals can provide migrating birds with life-saving information about unfamiliar predators along their long-distance routes.
Biologist Joseph Nocera, at the University of New Brunswick, has shown for the first time that migrating songbirds identify and avoid these predators while traveling south. They learn which animals are dangerous by heeding local birds' alarm calls.
With support from National Geographic, Nocera found that songbirds like the red-eyed vireo and Swainson's thrush, migrating from Canada to Belize, quietly observe and learn from local birds, just "sitting back and gaining the benefits of the enhanced information."
He compared the traveling birds' learning process to his own experience as a first-timer in Belize: "I never knew what animal to avoid and what not to," he recalled. "I was jumpy around every snake I saw. So I thought, what must a migrating bird go through?"
Getting inside bird brains as Nocera has done is a major frontier for migration science, said Greenberg of the Smithsonian Migratory Bird Center. He anticipates future research into how individual migrants make choices and use spatial learning during their journeys will help scientists piece together more of the migration puzzle.
Conservation approaches are sometimes complicated because protections vary from country to country and data is scarce from vast geographical swaths including much of Africa and southern Asia.
But conservationists are relying on current research to fine-tune their strategies.
If certain pesticides are harming some birds, other alternatives may be available. For some birds, preserving summer or winter grounds may be the key.
For others, "It's the tens of sites in between that are incredibly important," said Martin Fowlie, a BirdLife International spokesperson.
"It's only over the last few years, using these new technologies, we have seen how important some of these places are."
Even now, despite declines, migrants still flood the skies, giving ornithologists and conservationists hope.
"The time to protect a bird is when it's common," Greenberg said. "What we love about migration is that you can go out on a spring day and see it."
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