BEASON, R. C.: Mechanisms of Magnetic Orientation in Birds

The ability of birds to navigate long distances during migration and to return home following displacement involves use of the Earth�s magnetic field for many if not all species. Behavioral and physiological studies support the existence of a magnetic compass in birds and the possibility of a map-type of reference system, also based on the geomagnetic field. The characteristics of receptors for a magnetic map differ from those needed for a magnetic compass. Birds use an inclination compass rather than a polar compass. Receptors for such a compass do not need to be sensitive to small changes in the magnetic field’s intensity or orientation. Behavioral and physiological evidence support the idea that the receptor for the magnetic compass is light dependent and wavelength (color) sensitive. The general consensus is that visual pigments, therefore, are involved; whether they are associated with retinal, pineal, or other photoreceptors is unknown. A receptor for a magnetic map system needs to be sensitive to small changes in some aspect of the magnetic field, most likely intensity. Magnetite-based receptors have the theoretical sensitivity to detect such minute differences and have been proposed as the map magnetoreceptors. Behavioral and physiological data indicate a sensitivity to changes of 30�50 nT, sufficient to determine location based on magnetic information. Pulse magnetization of pigeons and migratory birds change the direction selected by the bird but does not produce disorientation as would be expected if a magnetizable material were part of the magnetic compass receptor. Information from these two systems must come together within the nervous system of the animal and be integrated with other sources of information for the animal to select its directional heading.