How this magnetic sense works, however, has been frustratingly difficult to figure out.

Now, two researchers at Baylor College of Medicine, Le-Qing Wu and David Dickman, have solved a central part of that puzzle, identifying cells in a pigeon’s brain that record detailed information on the earth’s magnetic field, a kind of biological compass.

“It’s a stunning piece of work,” David Keays of the Institute of Molecular Pathology in Vienna wrote in an e-mail. “Wu and Dickman have found cells in the pigeon brain that are tuned to specific directions of the magnetic field.”

Their report appeared online in Science Express on Thursday. Kenneth Lohmann at the University of North Carolina at Chapel Hill, who also studies magnetic sensing, said in an e-mail that the study was “very exciting and important.”

Navigating by magnetism includes several steps. Birds have to have a way to detect a magnetic field, and some part of the brain has to register that information; it seems likely that another part of the brain then compares the incoming information to a stored map.

The Baylor researchers have offered a solution to the middle step. They identified a group of cells in the brainstem of pigeons that record both the direction and the strength of the magnetic field. And they have good, but not conclusive, evidence to suggest that the information these cells are recording is coming from the bird’s inner ear. Dr. Dickman said this research “is still something we want to pursue.”

They did not work on the third step, but Dr. Dickman said a good candidate for the location of that map was the hippocampus, the brain region involved in memory of locations in both birds and humans.