EFFECT OF GEOMAGNETIC-FIELD ON ORIENTATION OF THE MARSH WARBLER, ACROCEPHALUS-PALUSTRIS, IN SWEDEN AND KENYA

The orientation of juvenile marsh warblers during autumn migration was investigated at two widely different latitudes, in Sweden and in Kenya, by cage experiments in manipulated magnetic fields during the twilight period after sunset. The objective was to compare responses by birds exposed to different geomagnetic conditions, particularly to the shift in magnetic inclination between the northern and southern hemispheres. Orientation experiments were performed under clear skies and under simulated total overcast. The marsh warblers from the two sites differed markedly in their orientation under clear skies. Marsh warblers in Sweden showed an average orientation in the expected migratory direction (southeast), while in Kenya they oriented towards west-northwest by northwest (significantly dilferent from the sunset point), a direction clearly different from the expected migratory direction in this area (south-southwest). The warblers did not respond in a consistent way to shifts in the magnetic field and they failed to show significant directional tendencies under total overcast conditions, indicating the importance of visual rather than geomagnetic cues for the birds1 orientation. In Kenya activity in the orientation cages was strongly reduced under overcast skies in comparison with clear sky conditions. Mean mass and fat deposits of the marsh warblers were much larger in Sweden than in Kenya. These results indicate that migratory marsh warblers captured and tested at different sites along the migration route show markedly different orientational dispositions associated with differences in body condition and migratory strategy. © 1993 The Association for the Study of Animal Behaviour.