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Migrating Birds Reset "Compasses" at Sunset, Study Says

John Roach
for National Geographic News
April 15, 2004
 
Sunset is more than a thing of beauty for Swainson's thrushes and gray-cheeked thrushes. It keeps them on course when migrating between their winter and summer homes, according to an international team of scientists.

The finding sheds further light on a question that has vexed scientists for years: How do birds navigate between nesting areas separated by thousands of miles with pinpoint accuracy?


"Whatever that mechanism is, it might have great applications for navigation for mankind," said Sidney Gauthreaux, who studies bird migration, orientation, and navigation at Clemson University in South Carolina.

Over the years laboratory experiments have shown that birds orient themselves based on cues from the sun, stars, Earth's magnetic field, and by memorizing landmarks while migrating. But the relative importance of such cues was unknown and a source of scientific debate.

"In particular, nobody knew what birds are really doing in the wild," said Martin Wikelski, a professor of ecology and evolutionary biology at Princeton University in New Jersey.

To learn more, Wikelski teamed up with wildlife biologists William Cochran, of the Illinois Natural History Survey in Champaign, and Henrik Mouritsen, of the University of Oldenburg in Germany. The trio designed an experiment that allowed them to see, for the first time, how migratory songbirds behave in the wild.

The team describe their findings in tomorrow's issue of the journal Science. The research was supported by a grant from the National Geographic Society's Committee for Research and Exploration.

Wild Thing

Cochran and colleagues captured songbirds during their natural northward migration, exposed them to an altered magnetic field at sunset, and then tracked them for several nights as the birds flew through Illinois and Iowa.

The night the birds were exposed at sunset to the altered magnetic field, which was pointed east (instead of magnetic north), they flew the wrong direction—to the west. The next night the songbirds flew off in the right direction—northward.

This led the researchers to conclude that the thrushes used cues from the setting sun to update their internal magnetic compass and get back on the right track.

Gauthreaux, the Clemson University researcher (who was not involved in the study), said he is not surprised by the finding, adding that it brings some resolution to the scientific debate over which cues—visual or magnetic field—the birds use as their primary navigational tool.

"There are two findings out there," he said. "One is that visual cues are calibrated against magnetic compasses, the magnetic compass is the primary [cue]. And the other is just the opposite, that the visual is the primary and the magnetic [compass] is calibrated based on the visual cues."

The new finding by Wikelski and colleagues suggests that the songbirds' magnetic compass is calibrated, perhaps on a daily basis, by visual cues. According to Gauthreaux, other bird species may do the opposite, depending on their evolutionary history.

Compass Calibration

The researchers are still uncertain exactly how songbirds detect Earth's magnetic field. "We just know they can," Wikelski said. But animals like sea turtles, salamanders, and tuna accomplish the feat using internal stores of the mineral magnetite, which acts like little magnetic needles in the animals' cells.

Gauthreaux said there is also evidence to suggest that some birds may detect Earth's magnetic field with receptors associated with their sense of smell. Other studies suggest birds may be able to detect the magnetic field with receptors in their eyes.

Even though the birds can sense Earth's magnetic field, they need to calibrate their internal compass. Relying on Earth's magnetic field alone could nudge the songbirds off course as the field shifts direction, which occurs in the songbirds' North American breeding grounds.

Enter the sun, which always sets in the west.

Under natural conditions, Wikelski said, "The birds use the setting sun as westerly direction, then decide they want to fly 90 degrees clockwise from there—a direction that is normally aligned with magnetic north."

During the team's experiments, however, the songbirds were placed inside a cage with a magnetic field pointing eastwards. Relative to the direction of the setting sun, the songbirds wanted to fly in a direction that was 90 degrees counterclockwise from the detected magnetic field in order to stay on a northward track.

"When they are released from the [cage] and the sun is gone, magnetic north shifts back to real north and the birds still want to fly 90 degrees counterclockwise from magnetic north," Wikelski said. "This is why they go west during the experimental night."

The next day, at sunset, the songbirds were left undisturbed in the wild, where they appeared to recalibrate their compasses and take off that night in the right direction.

The researchers believe that other migratory songbirds are likely to employ this system of a twilight-calibrated magnetic compass as they migrate between the continents.
 

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