Interaction of magnetic and celestial cues in the migratory orientation of passerines

Cage experiments with various species of migrating passerines indicate two distinct phases in migratory orientation in which magnetic and celestial cues interact in different ways. The first phase during the premigratory period involves the conversion of genetically coded information into an actual compass course. Celestial rotation and the geomagnetic field serve as external references. Celestial rotation alone, mediated by rotating stars at night or by the changing pattern of polarized light during the day, provides a reference direction away from its center, corresponding to geographic South. Population-specific deviations from this reference direction appear to be coded only with respect to the magnetic field. Both cue systems interact to produce the population-specific migratory course. In the role of providing the reference direction, celestial rotation dominates over the magnetic field during this first premigratory phase. Magnetic South, however, can also serve as reference direction when celestial cues are not available. The second phase involves orientation during migration, once the course is set and the birds have left their breeding area. Migrants en route have several options to locate this course, namely a star compass, sunset cues and the magnetic field, and they seem to make use of them all. During this phase, however, the magnetic compass dominates over celestial cues, as indicated by cue-conflict experiments. In case of conflict, the directional significance of stellar cues and sunset cues is adjusted to be in agreement with the ambient magnetic field. Migratory orientation is thus based on an integrated system of celestial and magnetic cues that reverse their dominant role between the premigratory phase and actual migration. The reasons may lie in the changes in the sky and in the magnetic field migrants experience when travelling from higher to lower latitudes towards their winter quarters.