Circannual changes in migratory orientation of the garden warbler, Sylvia borin

Abstract

1. Most migratory birds breeding in the northern hemisphere show regular changes in the direction of their migratory flights from roughly south in autumn to roughly north in spring. To test the hypothesis that these changes are primarily controlled by endogenous physiological changes, we carried out experiments with the garden warbler, Sylvia borin. 2. Eleven birds were hand-raised and subsequently kept under a constant 12-h photoperiod and constant temperature conditions throughout their first fall and spring migratory seasons. At regular intervals, the birds were tested in circular orientation cages for directional preferences of their nocturnal migratory restlessness. During the tests, the birds were exposed to the local magnetic field of the earth, but had no view of the sky. 3. Birds showed a significant preference for a southerly direction in autumn and for a northerly direction in spring (Figs. 2 and 3). These results support the hypothesis that the reversal of the migratory direction results from spontaneous changes in the preferred direction relative to external orienting cues that are controlled by an endogenous circannual clock.