Researchers say they've resolved one of the great mysteries in animal migration: the role that sunlight plays in guiding migratory birds.
Biologists have known for decades that migrating birds use celestial cues and the earth's magnetic field to find their way across continents and oceans.
But researchers from Virginia Tech in Blacksburg and Lund University in Sweden say experiments with savannah sparrows in Alaska show the birds take readings of polarized sunlight at sunrise and sunset and use them to periodically recalibrate their magnetic compasses.
Stars and the sun help guide migrating birds. But cloudy weather can conceal these celestial cues and changes in latitude can alter magnetic fields, making the recalibration a necessity, says Rachel Muheim, a biologist at Virginia Tech and lead author of the study.
Rays of ordinary light vibrate in different directions, but polarized light rays vibrate in only one direction - making them difficult for us to see. Birds that migrate at night use them at sunrise and sunset, when the rays are most evident, Muheim said.
In the study, Muheim caught 50 savannah sparrows in the Yukon Delta National Wildlife Refuge and threw off their celestial compasses by placing polarized light filters over their cages for an hour at either sunrise or sunset.
The filters allowed the birds to see some sunlight and sky, but it shifted the alignment of the polarized light from the sun, making it seem as though the light came from a different direction.
The researchers found that shifting the light made the birds alter or recalibrate their magnetic compasses, so that when the birds were returned to observational funnels, they tried to fly off in the direction indicated by the filtered polarized light.
"We only gave them one hour, either at sunrise or sunset and that small period of time was enough for them to be able to recalibrate their compass," she said.
The findings, reported today in the journal Science, may not apply to all birds, but probably do apply to similar species, she said.
"I wouldn't dare to make a broad conclusion on all birds, but migratory songbirds in general you could say it applies to," Muheim said.
A bird's impulse can be so strong that captive avians will move in a migratory direction in cages during fall and spring migration seasons, researchers say.
Over the past decade, researchers have been trying to track migratory cues by putting birds in specially designed funnels and monitoring their movement patterns in the fall and spring on specially designed floors.
"What's fascinating is that migration is so basic to these animals that they have these redundant systems built in to handle it," said Greg Ball, who studies the brains, hormones and migratory habits of songbirds at the Johns Hopkins University.
Researchers have known since the early 1990s that a bird will use patterns of polarized light as a navigational tool for calibrating its internal magnetic compass.
But previous studies left unanswered exactly when the calibration occurs and showed conflicting results over whether birds rely more on cues from the Earth's magnetic field or from the sun and the stars, experts say.
The study shows the calibration happens at sunset and sunrise, according to Muheim and other experts.
"It demonstrates that sunlight, at sunrise and sunset, is crucial information for birds in the way that they orient themselves," said Verner Bingman, who studies migratory birds at Bowling Green State University in Ohio.
Some bird experts may argue that other bird species use magnetic and celestial cues in different ways, and that the findings apply only to the savannah sparrow, Bingman said. But he thinks other birds are using the same cues in the same ways.
"There will be people who say it's different for different species and for birds in different geographic locations, but I think it applies to other birds as well," he said.
Savannah sparrows migrate from Alaska to California each fall. Mulheim said she used them because they are plentiful, easy to keep, have been studied in the past and, like most birds, migrate at night to avoid predators.
The research was conducted as part of a Swedish polar expedition. The field work was done at the U.S. Fish and Wildlife Service field station in Bethel, Alaska, about 30 miles from the Bering Sea.