Robins can 'see' the Earth's magnetic field which allows them to navigate, scientists believe.
The information, relayed to a specialized light-processing region of the brain called ''cluster N'', helps the robin find its way on migration flights.
Experts know birds possess an internal magnetic compass, but there is disagreement about what form it takes.
One idea is that tiny magnets in the beak wired to the nervous system detect lines of magnetic force.
Another is that magnetic fields are ''seen'' via the eyes using a complex light-sensitive mechanism.
The new research suggests that, for robins at least, the second theory is probably correct.
German scientists studied 36 European robins and found birds with damage to ''cluster N'' were unable to orientate themselves using the Earth's magnetic field.
But damage to another nerve channel necessary for a beak-sensing system had no effect.
The researchers, led by Dr Henrik Mouritsen from the University of
Oldenburg, wrote in the journal Nature: ''The results of the present study
... specifically suggest that cluster N of European robins is an essential
part of a circuit processing light-dependent magnetic compass information
for night-time orientation.
''The exact role of cluster N within this circuit has not been determined, but the present results raise the distinct possibility that this part of the visual system enables birds to 'see' magnetic compass information.''
Other types of magnetic sensor may also exist in birds, said the scientists. There was strong evidence that upper beak magnetosensors were used by pigeons.