After-Effects of Exposure to Conflicting Celestial and Magnetic Cues at Sunset in Migratory Silvereyes Zosterops l. lateralis

Abstract

Migratory birds can use celestial and magnetic cues to locate their migratory direction. The relative importance of the two cue systems was analyzed by testing Australian Silvereyes for their orientation behavior in a cue-conflict situation during spring at dusk. Control birds tested under the natural sky in the local geomagnetic field headed in their southerly migratory direction. The experimental birds, tested under the natural sky in a magnetic field with magnetic North turned to 2400 WSW, followed the shift in magnetic North and preferred ENE directions that corresponded to magnetic South. Subsequently, all birds were tested under the sky in the absence of magnetic information. Both groups continued in their previous direction, control birds south and experimental birds northeast, by celestial cues alone. Tested indoors with the magnetic field as the only cue, both groups headed southward. These findings indicate a dominant role of magnetic cues: in case of conflicting cues, the birds maintained their magnetic heading and recalibrated the celestial cues on the basis of the altered magnetic field. This is in contrast to recent findings with Savannah Sparrows Passerculus sandwichensis; one possible reason for these differences may be that the Savannah Sparrows were tested in their breeding area at high latitudes where celestial cues might generally be of greater importance, whereas our Silvereyes were tested at lower latitudes where the magnetic field appears to play a more important role.