Studies have suggested that migrating birds rely on an internal map and compass to traverse large distances, though just where these senses reside is unclear. Now, scientists say they have the strongest evidence yet that map sense is associated with the beak.

Researchers have long suspected that migrating birds navigate by sensing the Earth's magnetic field. The idea was that their beaks, which contain a lot of iron, worked like real magnets, with the metal aligning itself relative to the field.

Supposedly, the so-called trigeminal nerve transmitted this information to the brain.

In 2010 and 2011, a team led by Henrik Mouritsen of the University of Oldenburg in Germany captured 57 Eurasian reed warblers near Kaliningrad, Russia. Every spring, these birds migrate northeast to their breeding grounds in southern Scandinavia, more than 600 miles away.

Once again, the scientists snapped the trigeminal nerve, in half of the birds. But then they also moved all 57 birds more than 600 miles to the east, where the magnetic field differs from their home site, and released them.

The warblers that had their beak-to-brain connection cut flew northeast, as if they had departed from near Kaliningrad - they had lost their "map sense" and could no longer determine their location.

Those with the nerve intact, on the other hand, quickly oriented themselves and turned northwest, toward their breeding grounds, the team reported this week in PLOS ONE.

This meant that the beak-to-brain system, which, according to the earlier tests, had no impact on the "compass sense," did matter for the "map sense" of the birds - if the link was damaged, the birds simply did not know they had been displaced.

This is the first study that involves both displacing migratory birds long distance and severing the trigeminal nerve, with conclusive results, the scientists say.

Co-author Dmitry Kishkinev, now at the University of Guelph in Canada, says that the next step will be to determine where in the beak the magneto-receptors might be.