Here's an intriguing notion for bird-watchers. Birds may actually see Earth's magnetic field when they use it for navigation.

This possibility goes to the heart of the mystery of how some animals sense Earth's magnetism. And, remarkable as it may seem, it has begun to receive some indirect experimental support.

Some biologists, like Klaus Schulten of the University of Illinois at Urbana-Champaign, think the visual sense of some animals may be involved in their ability to sense magnetic fields.

Schulten theorizes that pigment molecules in the eyes of migratory birds combine magnetic information with certain light colors to give visual clues.

He speculates that the birds may actually see a visual distortion cased by Earth's magnetic field when they fly.

Biologist John B. Phillips at Indiana University at Bloomington says such theorizing is "on the right track."

Work in his laboratory using male Eastern red spotted newts has produced the "first clear demonstration that the animal magnetic compass is light-dependent," he says.

Reporting this research in the Sept. 10 issue of Nature, Phillips and colleague S. Chris Borland explain how certain wavelengths of light directly affect the compass sense of these semiaquatic salamanders.

For example, long-wavelength light rotates the animal's magnetically derived directional sense 90 degrees counterclockwise from what it is under full-spectrum light.

Commenting on this finding in a telephone interview, Phillips says that establishing the light-dependence of the animal "compass" is the "first step" in the long process of demonstrating the Schulten thesis.

He added that his laboratory is getting the same kind of light-dependent response in the magnetic sense of certain species of flies. He believes there is little doubt that the magnetic field is affecting the flies' photoreceptors.

He calls the accumulating evidence of a visual-magnetic link "pretty compelling evidence that Schulten is on the right track."

Evidence is abundant that a wide variety of life forms use Earth's magnetism for orientation. They are as diverse as bacteria, birds and salamanders. Yet biologists have been unable to identify a specific magnetic sense organ in any species.

Many life forms, including humans, do have tiny magnets in their bodies. Yet there's little evidence that these magnets, by themselves, confer a magnetic sense.