By Ben Hirschler Tue Sep 25, 7:05 PM ET

LONDON (Reuters) - Migrating birds, it seems, can "see" the Earth's magnetic field which they use as a compass to guide them around the globe.

Specialized neurons in the eye, sensitive to magnetic direction, have been shown for the first time to connect via a specific brain pathway to an area in the forebrain of birds responsible for vision, German researchers said on Wednesday.

Scientists have known for many years, from behavioral experiments, that birds use an internal magnetic compass to navigate on their epic annual journeys. But exactly how the system works has been a mystery.

Now work by Dominik Heyers and colleagues at the University of Oldenburg in Germany has started to unravel the mechanism at a neuroanatomical level -- and it shows the eye is key.

Magnetic sensing molecules in the eye, known as cryptochromes, appear to stimulate photoreceptors depending on the orientation of the magnetic field.

This strongly suggests migratory birds perceive the magnetic field as a visual pattern, the researchers said.

"It's a pity we cannot ask them, but what we imagine is that it is like a shadow or a light spot on the normal vision of the bird," Heyers said in a telephone interview.

The German team, which published their findings in the online Public Library of Science journal PLoS ONE, based their research on laboratory studies of the garden warbler, a highly migratory bird.

Warblers from Germany and Russia were held in captivity and their nerve patterns traced and analyzed to establish the direct functional link between cells in the retina and the Cluster N forebrain region.

Garden warblers, which are estimated to number around 10 million worldwide, breed in northern Europe and spend the winter in Africa.