Avian magnetic compass: Its functional properties and physical basis

The avian magnetic compass was analyzed in bird species of three different orders - Passeriforms, Columbiforms and Galliforms - and in three different behavioral contexts, namely migratory orientation, homing and directional conditioning. The respective findings indicate similar functional properties: it is an inclination compass that works only within a functional window around the ambient magnetic field intensity; it tends to be lateralized in favor of the right eye, and it is wavelength-dependent, requiring light from the short-wavelength range of the spectrum. The underlying physical mechanisms have been identified as radical pair processes, spin-chemical reactions in specialized photopigments. The iron-based receptors in the upper beak do not seem to be involved. The existence of the same type of magnetic compass in only very distantly related bird species suggests that it may have been present already in the common ancestors of all modern birds, where it evolved as an all-purpose compass mechanism for orientation within the home range (Current Zoology 56 (3): 265-276, 2010). The geomagnetic field originates in the earth itself. The field lines leave the earth at the magnetic pole near the geographic south pole, curve around the earth and re-enter at the magnetic pole near the north pole. The inclination or dip, defined as the angle between the field lines and the horizontal plane, is 90° at the magnetic poles and 0° at the magnetic equator; at the latter, the field lines run parallel to the earth's surface. At the northern hemisphere, field lines point downward with a positive inclination, at the southern hemisphere they point upward. The intensity of the geomagnetic field also varies: it has its maximum, about 60 µT, near the magnetic poles and decreases towards the magnetic equator to about 30 µT, with a local minimum in eastern South America (Skiles, 1985). For animals capable of sensing the geomagnetic field, it provides an omnipresent source of directional infor- mation. Its use as a compass is widespread and includes species of all major phyla (R. Wiltschko and Wiltschko, 1995). Birds were the first group where a magnetic compass was demonstrated more than 40 years ago (W. Wiltschko, 1968), and they are still the group where the magnetic compass has been analyzed in most detail, mainly by behavioral methods. In the present paper, we compare the magnetic compass of birds of three orders - passeriforms, columbiforms and galliforms. The respec- tive studies involve three different behaviors, namely migration, homing and small-scale orientation based on directional training.