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Bird's eyeEuropean robins, like the one shown here, navigate using the Earth’s magnetic field. New work suggests these birds host their main compass in their eyes, not their beaks.H. Mouritsen

A cell in the eye may be worth two in the beak, at least when it comes to a migratory bird’s magnetic compass. In European robins, a visual center in the brain and light-sensing cells in the eye — not magnetic sensing cells in the beak — allow the songbirds to sense which direction is north and migrate correctly, a new study finds. The study, appearing October 29 in Nature, may improve conservation efforts for migratory birds.

“This is really fascinating science,” says biophysicist Klaus Schulten of the University of Illinois at Urbana-Champaign, who was one of the first to suggest that migrating birds can sense magnetic fields.

Researchers have known that built-in biological compasses tell migrating birds which way to fly, but the details of how birds detect magnetic fields has been unclear.

“This is basically the sixth sense of biology, but no one knows how it works,” says study coauthor Henrik Mouritsen of the University of Oldenburg in Germany. “The magnetic sense is by far the least understood sense in the natural world.”

Some researchers had proposed that iron-based receptors in cells found in the upper beaks of some migratory birds sense the magnetic field and send that information along a nerve to the brain. Other scientists favor the hypothesis that light-sensing cells in birds’ eyes sense the magnetic field and send the information along a different route to a light-processing part of the brain called cluster N.

Special proteins called cryptochromes in the birds’ eyes may mediate this light-dependent magnetic sensing, Mouritsen says. Light hitting the proteins produces a pair of free radicals, highly reactive molecules with unpaired electrons. These electrons have a property called spin which may be sensitive to Earth’s magnetic field. Signals from the free radicals may then move to nerve cells in cluster N, ultimately telling the birds where north is.

To find the location that houses the magnetic compass, Mouritsen and his colleagues caught 36 migratory European robins and made sure that the birds could all orient correctly under natural and induced magnetic fields. Next, the researchers performed surgeries on the birds to deactivate one of the two systems. The team either severed the nerve that connects the beak cells to the brain, or damaged the brain cells in cluster N that receive light signals from cells in the eye.

Birds with the severed beak-to-brain nerve — called the trigeminal nerve — still oriented perfectly, Mouritsen says. “No information from those iron crystals could get to the brain, but the birds oriented just as well,” he says, suggesting that the beak cells are not important for orientation.

On the other hand, birds with damaged cluster N regions could no longer sense and orient to magnetic fields. These robins failed to pick up both the Earth’s natural magnetic field and the artificial fields created by the researchers.

The new study “nicely confirms that the trigeminal nerve is not involved in this direction sensing,” says John Phillips, a neuroecologist at Virginia Tech in Blacksburg. “This is an important advance in what we know about these systems.”

Mouritsen thinks the cells in the beak might play a different role in magnetic sensing, such as picking up minor changes in the strength of the magnetic field along a north-south axis, he says.

Understanding more about how birds navigate and sense the environment may have important conservation implications, Mouritsen says. Migratory birds that humans have relocated often fly back to the original migratory grounds. But if researchers can figure out how the birds navigate, conservationists may be able to trick the birds into staying where it’s safe.

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