 It has been shown that songbird migrants can use several compass cues for orientation (e.g. sun position at sunset and possibly sunrise and related polarised light cues, stars and the geomagnetic field); therefore, the obtained information is redundant. This suggests that compasses of migratory birds must have certain hierarchical relationships and be calibrated. Currently, it is not known how avian compass calibration is accomplished. We report the results of our experiments with Garden Warblers Sylvia borin, long-distance songbird migrants. We tested the birds in two experimental conditions: in a local magnetic field with access to a starry sky (Control group) and in a vertical magnetic field that does not provide magnetic compass information with access to stars (Clear sky experimental group) or without it (Overcast experimental group), and analysed locomotor activity and orientation in all three groups. For the Garden Warblers from the control and experimental groups, we revealed two periods of activity separated by a quiescent period: twilight and nocturnal periods. The average direction for both periods of activity showed no significant difference in the control group. Birds from the experimental group were disoriented in both periods. Birds from the clear sky and overcast groups were also disoriented. These data suggest that long-distance songbird migrants, particularly the Garden Warbler, need information from the geomagnetic field, but not from the stars, at sunset and during twilight in order to choose the correct migratory direction. The nocturnal period of migratory activity probably represents actual migratory flight, while the nature of the twilight period remains unknown. The results of the present work and data from prior cue-conflict experiments on other species suggest that the twilight period may correspond to compass calibration activity.
