Magnetoreception in birds: two receptors for two different tasks

Birds can use the geomagnetic field as a source of navigational information in different ways: the magnetic vector provides a compass; magnetic intensity and/or inclination play a role as a component of the navigational ‘map’, and magnetic conditions of certain regions act as ‘sign posts’ or triggers, eliciting specific responses. Two hypotheses on magnetoreception are currently under discussion. One proposes a chemical compass based on a radical pair mechanism involving direction-dependent reactions in specialized photopigments; the other postulates processes involving iron-rich particles, such as magnetite. Behavioral evidence suggests that birds use both mechanisms, with radical pair processes in the right eye providing directional information and an iron-based mechanism in the upper beak providing information on position as a component of the navigational ‘map’. Electrophysiological data support this view. The interactions of the two mechanisms, however, are still poorly understood.