"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.
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 therea 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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