PRINCETON, N.J. -- Princeton biologist Martin Wikelski is helping to settle a long-standing debate over how migratory birds manage to navigate for thousands of miles in darkness and bad weather.
With two colleagues in Illinois and Germany, Wikelski has found that birds rely on a built-in magnetic compass, which they recalibrate each evening based on the direction of the setting sun. The scientists published their findings in the April 16 issue of Science.
The research, which involved attaching radio transmitters to birds and following them by truck for hundreds of miles across the American Midwest, is the first extensive study of bird navigation in the wild. The results appear to resolve conflicts between earlier laboratory-based studies, which had identified several possible navigational mechanisms, but produced no consensus. Previous theories suggested that birds use some combination of magnetism, stars, landmarks, smells and other mechanisms as navigational aides.
"In the lab situations, all those ideas made sense," said Wikelski, an assistant professor of ecology and evolutionary biology. "In a lab, you can train a bird to recognize almost everything; you can train it to recognize the stars or the magnetic field or whatever. I think what people have really missed is to go out and find what the birds really use."
For his study, Wikelski worked with William Cochran of the Illinois Natural History Survey and Henrik Mouritsen of the University of Oldenburg in Germany to track two species of migratory songbirds. The scientists caught birds just before they left for their overnight migratory flights and placed them in an artificial magnetic field. They then released the birds and followed them throughout the night. Birds that had been in the artificial magnetic field flew in the wrong direction, but recovered their orientation the next night.
"In the morning, shortly before they land, they see the sun and realize they have made a mistake," said Wikelski. "You can see them turn around 90 degrees."
Wikelski concluded that birds rely on the location of the sunset to determine which way to fly. To maintain that heading throughout the night, they sense the Earth's magnetic field, just like a pilot uses a compass at night or in bad weather. "It is the simplest and most foolproof orientation mechanism we can imagine," Wikelski said.
The combination of cues -- sun and magnetic field -- neatly correct each other for possible problems. Migrating at night, birds cannot maintain a fix on the sun and cannot rely on seeing stars because of clouds. A bird's magnetic compass also is not sufficient. The location of the magnetic north pole -- the spot on the globe where a compass points -- is not stable (it is currently in Canada, hundreds of miles from the geographic north pole). The magnetic poles also completely reverse locations every few thousand years, so the north arrow on a compass would suddenly point south.
It makes sense then that birds use the magnetic field only as a guide to keep them on a path that is determined primarily by the sun, Wikelski said. It doesn't matter which way the magnetic field points, so long as it stays steady through the night.
In one set of experiments, the researchers tracked gray-cheeked thrushes, which all tend to migrate in the same direction. Seven of eight birds exposed to an artificial magnetic field flew in a significantly different direction than 14 that were not. The researchers did similar experiments with Swainson's thrushes, whose headings vary considerably from one bird to another. They exposed the Swainson's thrushes to artificial magnetic fields and followed them for at least two days. All the treated birds picked a significantly different direction on their second day, whereas unmanipulated birds kept flying the same direction both days.
In all, the researchers tracked 37 birds over 54 night flights. They also spent many days in the field watching birds that decided to rest a few days before continuing on their migration. The first of these flights took place in 1972 when Cochran, who pioneered methods of attaching transmitters to animals and tracking their movements, first applied the approach to birds (the device sits on their backs, between their wings). Cochran tracked more birds in the late 1970s and in 1984. However, much of the data was not collected until 2003 after Wikelski joined Cochran and was eager for enough data to draw a clear conclusion about how birds navigate.
Wikelski believes the results, while based on just two species, are likely to apply to most migratory birds. "It's such a simple and elegant mechanism that I would say it is widespread," he said.
Even greater advances in understanding the behavior of birds and other animals will come from more sophisticated radio tracking techniques, Wikelski said. With funding from NASA, Wikelski is collaborating on a project to design a space-based system that would be able to track large numbers of individual animals over entire continents. The concept is similar to that of the radio telescopes commonly used to detect faint signals from the distant universe; scientists could track animals by putting such a telescope in space and facing it toward Earth.
"For radio astronomers, it's totally simple," said Wikelski. "It's bread and butter technology. But for ecologists and people doing field work, it's totally new. But I think we're really ready for it."
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