Magnetic compass information has a key role in bird orientation, but the physiological mechanisms enabling birds to
sense the Earth's magnetic field remain one of the unresolved mysteries in biology. Two
biophysical mechanisms have become established as the most promising magnetodetection candidates. The iron-mineral-based hypothesis suggests that magnetic information is detected by magnetoreceptors in the upper
beak and
transmitted through the ophthalmic branch of the
trigeminal nerve to
the brain. The light-dependent hypothesis suggests that magnetic field direction is sensed by radical pair-forming
photopigments in the
eyes and that this
visual signal is processed in cluster N, a specialized, night-time active, light-processing
forebrain region. Here we report that European robins with bilateral
lesions of cluster N are unable to show oriented magnetic-compass-guided behaviour but are able to perform
sun compass and star compass orientation behaviour. In contrast, bilateral section of the ophthalmic branch of the
trigeminal nerve in European robins did not influence the birds' ability to use their magnetic compass for orientation. These data show that cluster N is required for magnetic compass orientation in this species and indicate that it may be specifically involved in processing of magnetic compass information. Furthermore, the data strongly suggest that a vision-mediated mechanism underlies the magnetic compass in this migratory songbird, and that the putative iron-mineral-based receptors in the upper
beak connected to
the brain by the
trigeminal nerve are neither necessary nor sufficient for magnetic compass orientation in European robins.
