Without a map

Each fall, billions of birds undertake a migratory odyssey that can span the globe or carry them only a few states away. They fly thousands of miles toward warmer climates or more abundant food -- then reverse their trips each spring.

Despite the regularity of these journeys, scientists know surprisingly little about bird migration. How do birds know where to go? What exact routes do they take to reach their destinations?

Now, scientists have new tools that many believe will revolutionize bird migration research. Tracking technologies like satellite transmitters and GPS units -- formerly too bulky to be placed on any but the largest birds -- now fit on duck-sized birds without hindering their flight.

Scientists can now map complete migration routes for individual birds from the comfort of their offices, in contrast to the demands of previous research methods -- such as bird banding -- which provided only the start and finish-point of a bird's travels.

``There will be a tremendous number of new things to learn from these tracks covering the individual full journey over the years," said ornithologist Thomas Alerstam of Lund University in Sweden.

The new techniques come at a time of increasingly urgent questions about bird migration. Birds' extinction rates now average one species a year, according to a July report in the Proceedings of the National Academy, raising the stakes for protecting migratory habitats and understanding how birds cope with climate change.

Moreover, as bird flu wings its way across Eurasia on the backs of wild swans and bird-carried West Nile virus spreads in the United States, migration routes have a direct impact on human health.

And the miniaturized technology is already shedding light on how birds use signals to navigate.

Birds use both geographic cues like the sun and magnetic cues from the earth's magnetic field to orient themselves as they migrate, Alerstam said. But scientists don't yet know which sense the birds rely on the most, or how birds cope if their different methods provide conflicting information.

``We can make predictions, like, if this is the way they sense the magnetic field, we would expect them to follow the following trajectory," he said.

By placing satellite transmitters on predatory birds like peregrine falcons and osprey -- large, handsome brown-and-white birds -- Alerstam and his colleagues showed that the birds fly in an essentially straight line from breeding to wintering grounds.

If the birds had relied on a magnetic compass, their routes would curve, he said. That's because magnetic north differs slightly from geographic north, and the angle between true north and magnetic north changes as a bird travels.

The finding, published in the current issue of Animal Behavior, suggests that these birds use mostly the sun for navigation. Alerstam's team is repeating the study using GPS transmitters, which provide location data within 5 meters -- far more accurately than ordinary satellite transmitters.

Complete migration data can pinpoint the habitats birds rely on as they travel. Birds need breeding grounds, wintering sites, and a network of food-rich stopover sites where they can rest and refuel.

Bridget Olson, a wildlife biologist with the US Fish and Wildlife Service, studies marbled godwits, long-legged shorebirds that stalk western prairies and marshlands like the Bear River Migratory Bird Refuge in Utah, where Olson works. In April and again in August, Olson and colleague Adrian Farmer of the US Geological Survey attached solar-powered satellite transmitters on two godwits.

The tiny transmitters weigh only 12 grams -- as much as five pennies. At 3 percent of a female godwit's body weight, they're just light enough for the birds to comfortably carry.

By plotting the birds' routes on maps generated by Google Earth, Olson and Farmer discovered that one of the godwits relied heavily on conservation sites linked through the Western Hemisphere Shorebird Reserve Network, a program administered by the Manomet Center for Conservation Sciences. During spring migration, the bird stopped only three times -- each time at a network site.

``She's a really good advertisement for this program, of how important the Western Hemisphere sites are," said Farmer.

This fall, researchers are using larger GPS technology to track whooper swans in western Mongolia -- birds that began mysteriously dying last year of a virulent bird flu strain. Swans aren't likely to transmit the virus directly to humans, who tend to catch bird flu by handling domestic poultry or through the illegal wild bird trade. But swans may serve as carriers for the flu virus, said John Takekawa, a biologist with the US Geological Survey.

The 10 swans Takekawa and his colleagues released in August have only just begun migrating. Surprisingly, some have flown northward into Russia instead of south into China as expected.

``If we can get an idea of where they're going in the winter, we may be able to help shape management plans . . . to make sure influenza doesn't spread to people," Takekawa said.

For now, size still limits the use of satellite transmitters and GPS units to midsized or larger birds. But, as the technology continues to shrink, even tiny songbirds will eventually wear them.

Flocks of these fist-sized migrants wash past biologist Martin Wikelski's office at Princeton University every fall, traveling the eastern US migratory corridor toward South America and other destinations. But, says Wikelski, researchers know almost nothing about songbirds' exact routes and migratory habits.

So far, ``we only have one single track for six days for one individual thrush," Wikelski said.

To bring transmitters down to a size suitable for songbirds, Wikelski and colleague Kasper Thorup , head of the Danish Bird Ringing Center in Copenhagen, are collaborating with engineers to design new satellites that will sit at a lower orbit than current commercial satellites. This low orbit would require less power for transmission, allowing transmitters to be lighter.

Both the European Space Agency and NASA have expressed interest in the projects. Wikelski said. He hopes to have a satellite up and running by the end of the decade. That will free Thorup from sitting on the roof to track the take-off of radio-tagged white-crowned sparrows.

Unlike satellite data, radio transmission travels only a short distance, so Thorup must monitor the birds while colleagues wait below to follow them by car or plane.

``I'm sitting alone on this roof right now," said Thorup while perched atop the Department of Ecology, Princeton University's tallest building, last month. ``We can't wait until the satellite transmitters work."