The magnetic field of the earth is an important tool for navigation among birds. 18 species of birds have magnetic compass to aid them in their seasonal migration and home navigation. The magnetic compass was first described for European robins. Captive individuals of migrants become restless in their cages at the time of the year when their migration usually starts. They also preferred to stay at the side of the cage pointing to their migrating destination. This behavior was used to analyze the orientation of the birds in a laboratory where magnetic fields can be varied in a controllable manner. When the magnetic north was rotated by coil systems, while the field¡¦s total intensity and inclination where held constant, the birds altered their directional preferences according to the change in the magnetic north . This behavior clearly indicates that the birds used the magnetic field for direction finding.
As a result of bird¡¦s dependence on the earth magnetic field for navigation, they have been of particular interest for the study of magnetoreception. Species with magnetoreception functions contain internal chains of either single-domain (SD) magnetite or greigite that produce a magnetic moment large enough to rotate the cell into passive alignment with the geomagnetic field. When a magnetic pulse applied antiparallel to the magnetization direction causes the moment to reverse direction, making a bacterium swim south instead of north, which is a unique property of ferromagnetic material. Suggestions also have been made that deposits of super- paramagnetic (SPM) magnetite detected in some animals may be involved in magnetoreception.
Birds appear to use information from the geomagnetic field in two ways; the first is for position determination while the second as a compass for direction finding. These probably involve separate receptor systems as the biophysical constraints of how both may function differ. In view of these... [continues]
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