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 findings, researchers found great interest in determining the existence of magnetic biomaterials in animals through behavioral studies.
In birds, magnetite was found in the head, particularly in the ethmoid region above the beak. The ophthalmic nerve, a branch of the nervus trigminus, inverts these parts of the head. Electrophysiological recordings from this nerve and from the trigeminal ganglion of Bobolinks Dolichonyx oryzivours revealed units that responded to changes in the intensity of the magnetic field. In some studies, treatment with a strong magnetic pulse had a considerable effect on the migratory orientation of Australian silvereyes, deflecting the birds headings from their natural migratory direction approximately 90 degrees towards east for two days. Because the treatment with a strong magnetic pulse usually disorients the birds, it is thought that at least one receptor utilizes a magnetizable material such as magnetite. In a recent study, it was found that the effect of the magnetizing treatment can be abolished by blocking the ophthalmic branch of the trigeminal nerve, but the ability of the bird to select and maintain a certain direction was not affected. These results also support the hypothesis that a magnetizable material is a part of the magnetoreception system.
If magnetite-containing cells are used in magnetoreception, it is reasonable to predict that they should be linked to magnetically responsive nerves. However, understanding the genetic basis of magnetite biomineralization will provide the needed molecular tools for testing the hypothesis of common descent, and for testing magnetite’s role in magnetoreception of all animal groups. The location and structure of the magnetoreceptors that transduce the magnetic information to the nervous system remain unknown mysteries to the interested researcher.
Many studies have been made to determine whether homing pigeons contain magnetic material in their bodies. Most of these studies aimed to prove the existence of such material through behavioral experiments carried on the pigeons. Some studies also proposed the idea of magnetic material influence being integrated by the ophthalmic nerve to guide the birds in their journey. The mechanism of how the pigeons sense the variation in the earth’s magnetic filed and how to use it as a navigational tool is not fully understood.
Models of how the birds can use the earth’s magnetic field have been discussed but weren’t related to the physiological actions. Lengthy researches and studies are the only way to unrevealing the mystery of the neurological mechanisms leading to the precise determination of destination observed in pigeons.