Have magnets, will travel

Homing pigeons seem to possess at least two direction sensors. Years of experiments with released birds have proved that they use sun compasses on sunny days but have magnetic backups for cloudy days. But how do they sense the earth's magnetic field? Paired-coil tests suggested that the pigeon compass resided in the neck or back of the head. Narrowing the search with sensitive magnetometers and two dozen dissected pigeons, the authors discovered tiny bits of tissue containing magnetite crystals. The same tissues contained yellow crystals likely made by the iron-storage protein ferritin, which was probably used in the biological synthesis of the magnetite.

(Walcott, Charles, et al; "Pigeons Have Magnets," Science, 205:1027, 1979)

Comment. Many species of mud bacteria also synthesize magnetite for purposes of orientation, indicating that nature or some directive force used the same strategy in two widely separated species.