LIKE most topographic charts, the map on my wall shows the difference between true north and magnetic north, which is nearly 1,500 kilometers (932 miles) from the geographic pole. If I want to use a magnetic compass to find directions, I have to set the instrument to take that difference into account. It now appears that at least some birds do the same sort of thing. They can sense the direction of Earth's magnetic field. This gives them an internal compass. They can find the difference between magnetic north and true north the way the surveyor did in laying out my map. They locate the point on the sky about which the stars rotate. Then they set that difference into their internal compasses.
This adds yet another remarkable capability to the growing list of known mechanisms by which animals find their way around. A number of organisms - including birds, bees, and even bacteria - can use Earth's magnetic field for orientation. Some - again including birds and bees - use celestial cues. They can find direction from star patterns, the angle of the sun, and the polarization of sunlight. Homing pigeons can even smell their way home over long distances.
Charles Walcott, executive director of the Cornell (University) Laboratory of Ornithology in Ithaca, N.Y., works with homing pigeons. He says several decades of research have shown clearly that the pigeons rely on multiple clues for navigation. They will use celestial, magnetic, scent, or other clues, depending on what works best under given circumstances.
The lesson ornithologists are learning, he says, is that the birds' navigation system ``may be much more flexible than previously thought.'' And if that is true of homing pigeons, he adds, it may also be true of migratory birds. The newly demonstrated ability of one migrant to set up an internal compass for long-distance navigation underscores this point.
Migrants often travel at night and use the stars. But when skies are cloudy, a magnetic compass would be a valuable aid. Following up earlier work that showed Savannah sparrows can follow magnetic clues, Kenneth P. Able and Mary A. Able of State University of New York at Albany have found out how the birds set up their compasses to find true headings.
They raised several sparrow flocks in enclosures under artificial skies that showed the celestial pole in different locations. As they reported recently in Nature, studies of the birds' flight showed that each flock set internal compasses according to the artificial star pattern under which it grew up. The birds used the rotation of the stars about the celestial pole as their guide to geographic north.
There still is much to learn about bird navigation. The researchers note, for example, they still don't know how the sparrows cope with changes in the magnetic field over long distances. But it now has become obvious that animal navigation can be complex, flexible, and sophisticated. What's more, the birds don't need the complicated instruments humans use to find true north.