European robins orient themselves using light-sensitive cells in the eye, researchers say. (H. Mouritsen)

Migratory birds navigate in the Earth's magnetic field using cells in their eyes sensitive to light, not cells in their beaks sensitive to magnetic fields, new research says.

Researchers in Germany found that the European robin relies on light-sensing cells in the eye and the visual centre of the brain to orient themselves to magnetic north.

Bird scientists have known for years that birds have an internal compass to help them navigate their long migration paths, but there is debate over how this "sixth sense" actually works.

Some say that cells containing iron crystals in the birds' beaks detect the Earth's magnetic field directly and send that information to the brain. Others say magnetic fields are detected indirectly through light-sensitive cells in the eye to a visual part of the brain called cluster N.

To determine where the magnetic compass resides, Henrik Mouritsen and colleagues at the University of Oldenburg captured 36 robins and performed surgery on them to deactivate one of the two possible pathways.

Cells use light-sensitive proteins

With some of the birds, the researchers severed the trigeminal nerve that carries signals from the cells in the beak, and in the others, they damaged nerve cells in cluster N that process light information from the eye.

The scientists found that the bird with severed trigeminal nerves could still fly in the proper direction, while those with damaged cluster N regions could no longer orient themselves in the magnetic field.

Mouritsen said the cells in the beak could still play a minor role in the magnetic sense of birds, but the research, published this week in the journal Nature, clearly shows that they aren't involved in direction sensing.

Mouritsen said the cells in the eye could indirectly relay information about magnetic fields though light-sensitive proteins called cryptochromes.

When exposed to light, the cryptochromes create negatively charged chemicals called free radicals. The spin of the excess electrons on the free radicals could be sensitive to the Earth's magnetic field, and send signals containing information about the field to the brain.