How do birds that use Earth's magnetic field to navigate get their directions straight? For a long time, rival theories have sought to explain the mechanism — with some scientists arguing that magnetic particles in nostrils are the key, while others proposing magnetic eyesight is the answer. 

A new study on robins shows the eyes appear to be responsible. Researchers took two groups of robins and cut signals to the brain from the magnetic particles in the nostrils in one, and a cluster of cells that interpret magnetic data from the eyes in another. New Scientist reports on what happened, saying,

The researchers then exposed the surgically treated and untreated robins to the Earth's natural magnetic field, and also to a field which artificially rotated magnetic north 120 degrees anti-clockwise. The robins lacking their nostril-to-brain connection weren't tricked, locating the true and artificial magnetic norths just as well as the controls. But the robins without cluster N were unable to navigate. "The results raise the distinct possibility that this part of the visual system enables birds to 'see' magnetic compass information," conclude the researchers.
While the particles in the beak are not the driving force behind a bird's sense of direction, they may help detect minor changes in the strength of the magnetic field along a north-south axis. Understanding how this "sixth-sense" works is important to conservationists, since it may help one day in "tricking" birds into migrating to places that are safe, rather than those under threat from humans.

via New Scientist