Chapter 8 - Magnetoreception in Birds and Its Use for Long-Distance Migration

Abstract

The Earth’s magnetic field provides potentially useful information, which birds could use for directional and/or positional information. It has been clearly demonstrated that birds are able to sense the compass direction of the Earth’s magnetic field and that they can use this information as part of a compass sense. Magnetic information could also be useful as part of a map sense, and there is a growing body of evidence that birds are able to determine their approximate position on the Earth on the basis of geomagnetic cues. In addition to direct uses for orientation and navigation, magnetic information also seems to be able to influence other physiological processes, such as fattening and migratory motivation, as a trigger for changes in behavior. Although the behavioral responses to geomagnetic cues are relatively well understood, the physiological mechanisms enabling birds to sense the Earth's magnetic field are only starting to be understood, and understanding the magnetic sense(s) of animals, including birds, remains one of the most significant unsolved problems in biology. It is very challenging to sense magnetic fields as weak as that of the Earth using only biologically available materials. Only two basic mechanisms are considered theoretically viable in terrestrial animals: iron-mineral-based magnetoreception and radical-pair based magnetoreception. On the basis of current scientific evidence, iron-mineral-based magnetoreception and radical-pair-based magnetoreception mechanisms seem to exist in birds, but they seem to be used for different purposes. Plausible primary sensory molecules and a few brain areas involved in processing magnetic information have been identified in birds for each of these two types of magnetic senses. Nevertheless, we are still far away from understanding the detailed function of any of the at least two different magnetic senses existing in some if not all bird species, and, at present, no primary sensory structure has been identified beyond reasonable doubt to be the source of avian magnetoreception. This is an exciting but challenging field in which several major discoveries are likely to be made in the next 1–2 decades.