COMPARATIVE ORIENTATION EXPERIMENTS WITH DIFFERENT SPECIES OF PASSERINE LONG-DISTANCE MIGRANTS - EFFECT OF MAGNETIC-FIELD MANIPULATION

The orientation of four species of passerine long-distance migrants was studied in spring and autumn by orientation cage experiments during the twilight period after sunset in Sweden. Two groups of migrants from the Palaearctic-African migration system were used: migrants wintering mainly north of the magnetic equator in west Africa (pied flycatcher, Ficedula hypoleuca, and redstart, Phoenicurus phoenicurus) and migrants wintering south of the magnetic equator in southeast Africa (thrush nightingale, Luscinia luscinia, and marsh warbler, Acrocephalus palustris). Orientation experiments were conducted in three magnetic conditions, in the local geomagnetic field and in a deflected and in a vertical magnetic field, under clear and simulated total overcast conditions, respectively. The results did not provide any convincing indications about differences in the orientation system between the two groups of migrants. The responses of all species seemed to be affected in a similarly complex way by celestial as well as magnetic cues, involving conflicting elements of a possible attraction towards the brightest part of the twilight sky as well as orientation in the migratory direction. Shifting the horizontal component of the magnetic field neither shifted nor disrupted orientation. Abolishing the horizontal component of the magnetic field increased the orientational scatter in three species in spring and one in autumn. Simulated total overcast largely abolished orientation, except in two species (in the local geomagnetic field only) that do not migrate across the geomagnetic equator.