Mechanisms of magnetic orientation in birds.

Behavior and electrophysiological studies have demonstrated a sensitivity to characteristics of the Geomagnetic field that can be used for navigation, both for direction finding (compass) and position finding (map). The avian magnetic compass receptor appears to be a light-dependent, wavelength-sensitive system that functions as a polarity compass (i.e., it distinguishes poleward from equatorward rather than north from south) and is relatively insensitive to changes in magnetic field intensity. The receptor is within the retina and is based on one or more photopigments, perhaps cryptochromes. A second receptor system appears to be based on magnetite and might serve to transduce location information independent of the compass system. This receptor is associated with the ophthalmic branch of the trigeminal nerve and is sensitive to very small (<50 nanotesla) changes in the intensity of the magnetic field. In neither case has a neuron that responded to changes in the magnetic field been traced to a structure that can be identified to be a receptor. Almost nothing is known about how magnetic information is processed within the brain or how it is combined with other sensory information and used for navigation. These remain areas of future research.