Endogenously controlled changes in migratory direction of the garden warbler,Sylvia borin

Summary

Garden warblers breeding in central Europe and wintering in tropical and southern Africa show characteristic changes in flight direction during their fall migration: they initially leave their breeding grounds in a southwest direction but then change to S or SSE in southern Spain or northern Africa. Since it had been demonstrated in previous studies that several features of this species' migratory activity are dependent on endogenous timing processes, the hypothesis was proposed that the changes in migratory direction are also endogenously controlled. To test this hypothesis a total of 59 garden warblers were handraised and subsequently kept under constant temperature and photoperiodic conditions throughout the fall migratory season. At regular intervals the birds were moved to circular orientation cages in which they were tested for directional preferences in their nocturnal migratory restlessness. In these orientation cages the birds had no view of the sky, but were exposed to the local magnetic field of the earth. The analysis of data from 404 nights in which birds showed migratory restlessness revealed a significant concentration of the mean directions in the southern sector of the cage. Moreover the birds showed a counterclockwise shift in the preferred direction from SW or SSW to S or SSE, corresponding with the directional shift in freeliving garden warbler migration. These results suggest that the changes in migratory direction occurring in this species are due, at least in part, to spontaneous endogenous changes in the preferred direction relative to external orienting cues, probably of the earth's magnetic field.