Magnetic orientation in birds

The magnetic field of the earth is an omnipresent, reliable source of orientational information. A magnetic compass has been demonstrated in 18 species of migrating birds. In all species studied with regard to its functional properties, it was found to be an 'inclination compass', i.e. the birds derive directional information from the inclination of the field lines, and thus distinguish between 'poleward' and 'equatorward' rather than 'north' and 'south'. Such a mechanism means that birds from the northern and southern hemisphere may rely on the same migratory programme. Long-distance migrants, however, face the problem that their magnetic compass gives bimodal information at the magnetic equator. Transfers of information between the magnetic field and celestial sources of directional information have been demonstrated; the two systems interact in a complex way. The data on the use of magnetic parameters for position finding are less clear. The experiments involve releases of homing pigeons; correlations of their orientation with natural variations in the magnetic field and the effects of magnetic manipulation reveal an enormous variability. The role of magnetic parameters in the multifactorial navigational system is poorly understood.