Physiology. The ability to detect magnetic fields. It is most commonly observed in birds but also in turtles, honeybees, and certain types of bacteria. Detection of the magnetic field of the Earth is essential for the orientation and navigation of migratory birds. The underlying mechanism has not been determined. A radical pair reaction is discussed as possible mechanism in which cryptochrome acts as receptor molecule in the eye. Also, biological magnetite could be involved in magnetoreception.