The magnetic retina: light-dependent and trigeminal magnetoreception in migratory birds

Recent advances have brought much new insight into the physiological mechanisms and required characteristics of the sensory molecules that enable birds to use magnetic fields for orientation. European robins almost certainly have two magnetodetection senses, one associated with the ophthalmic branch of the trigeminal nerve, and one based on light-dependent radical-pair processes in both eyes. The first brain areas processing magnetic information from each of these two senses have been identified. It has been experimentally verified that Earth-strength magnetic fields can affect photo-induced chemical reactions and that these reactions can respond to magnetic field direction. Diagnostic behavioural experiments have provided clues to identify putative magnetoreceptive molecules in the retina. We discuss the implications of these and other recent findings and outline crucial open questions with an emphasis on the light-dependent mechanism.

► Night migratory songbirds have two magnetic senses. ► Light-dependent compass information is sensed in the eye and processed in Cluster N and may be cryptochrome-mediated. ► A second magnetic sense is associated with the trigeminal nerves. ► Light-dependent magnetosensory molecules need not be ordered. ► Important questions for the future are listed.