Abstract
Birds can use the geomagnetic field for compass orientation. Behavioral experiments, mostly with migrating passerines, revealed three characteristics of the avian magnetic compass: (1) it works spontaneously only in a narrow functional window around the intensity of the ambient magnetic field, but can adapt to other intensities, (2) it is an “inclination compass”, not based on the polarity of the magnetic field, but the axial course of the field lines, and (3) it requires short-wavelength light from UV to 565 nm Green. The Radical Pair-Model of magnetoreception can explain these properties by proposing spin-chemical processes in photopigments as underlying mechanism. Applying radio frequency fields, a diagnostic tool for radical pair processes, supports an involvement of a radical pair mechanism in avian magnetoreception: added to the geomagnetic field, they disrupted orientation, presumably by interfering with the receptive processes. Cryptochromes have been suggested as receptor molecules. Cry1a is found in the eyes of birds, where it is located at the membranes of the disks in the outer segments of the UV-cones in chickens and robins. Immuno-histochemical studies show that it is activated by the wavelengths of light that allow magnetic compass orientation in birds.
Highlights
In the 1960s, it was first discovered that animals can sense the direction of the geomagnetic field and use it as a compass
The findings suggest that the avian magnetic compass may be a unique development of birds, different from the mechanisms used by other animals
The unusual characteristic of the avian magnetic compass—functional window, inclination compass and its dependency on short-wavelength light—would seem to rule out induction or mechanisms involving permanently magnetic material, because these respond to the polarity of the magnetic field, which birds ignore
Summary
In the 1960s, it was first discovered that animals can sense the direction of the geomagnetic field and use it as a compass. A magnetic compass has been demonstrated in more than 20 other bird species, mostly passerine songbirds [2], and in homing pigeons Columba livia domestica [3,4], sanderlings, Calidris alba (Scolopacidae), a shorebird species [5], and in domestic chicken, Gallus gallus [6]. It was shown in a number of other animals, involving members of all vertebrate classes, insects, crustaceans and mollusks [7]. We will briefly review our present knowledge on how birds detect the direction of the geomagnetic field
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