MAGNETIC VERSUS CELESTIAL CUES - CUE-CONFLICT EXPERIMENTS WITH MIGRATING SILVEREYES AT DUSK

To assess the relative importance of celestial and magnetic cues for o rientation at dusk, Australian silvereyes, Zosterops l. lateralis, wer e subjected to artificial magnetic fields under the natural evening sk y, beginning 30 min before sunset. Control birds tested in the local g eomagnetic field preferred their normal south-southwesterly migratory direction. Birds tested in a magnetic field with north deflected count erclockwise to 240 degrees WSW showed northeasterly tendencies from th e first test onward. Birds subjected to a corresponding clockwise defl ection to 120 degrees ESE, in contrast, first showed southerly directi ons, but from the 7th test onward shifted towards the northwest. Hence , both experimental groups followed the shift in magnetic north, one i mmediately, the other after a delay. When the birds were later tested in a vertical magnetic field without directional information, the two experimental groups continued in the direction they had preferred in t he artificial magnetic fields, presumably by celestial cues alone. Thi s indicates that they had not simply ignored celestial cues, but had r ecalibrated them according to the altered magnetic fields. The reasons for the initial difference between the two experimental groups remain unclear. Delayed responses to deflections of magnetic north have also been observed in previous studies. They appear to be the main reason why studies that expose birds only once to a cue-conflict situation of ten seem to indicate a dominance of celestial cues, whereas studies ex posing the birds repeatedly usually indicate a dominance of magnetic c ues.