Magnetoreception systems in birds: A review of current research

At least two independent systems of magnetoreception are currently believed to exist in birds, based on different biophysical principles, located in different parts of their bodies, and with different neuroanatomical mechanisms. One magnetoreceptory system is located in the retina, and may be based on photochemical reactions on the basis of cryptochrome. Information from these receptors is processed in a specialized part of visual Wulst, the so-called Cluster N. There are good reasons to believe that this visual magnetoreceptor processes compass magnetic information necessary for migratory orientation. The second magnetoreceptory system is probably iron-based (biogenic magnetite), located somewhere in the upper beak (its exact location and ultrastructure of receptors remain unknown) and innervated by the ophthalmic branch of trigeminal nerve. It cannot be ruled out that this system participates in spatial representation and helps forming either a kind of map or more primitive signpost sense (identification of specific geographic regions), based on regular spatial variation of the geomagnetic field. The magnetic map probably enables navigation of migrating birds across hundreds and thousands of kilometres. Apart from these two systems, whose existence has been convincingly shown (even if some details are not fully clear yet), there is evidence for the existence of magnetoreceptors based on the vestibular system. It cannot be ruled out that iron-based magnetoreception takes place in lagena (a part of inner ear in fishes, amphibians, reptiles and birds), and the information perceived is processes in vestibular nuclei. The very existence of this magnetoreception system needs verification, and its function remains completely open