Magnetic information calibrates celestial cues during migration

Migratory birds use celestial and geomagnetic directional information to or ient on their way between breeding and wintering areas. Cue-conflict experi ments involving these two orientation cue systems have shown that direction al information can be transferred from one system to the other by calibrati on. We designed experiments with four species of North American songbirds t o: (1) examine whether these species calibrate orientation information from one system to the other; and (2) determine whether there are species-speci fic differences in calibration. Migratory orientation was recorded with two different techniques, cage tests and free-flight release rests, during aut umn migration. Cage tests at dusk in the local geomagnetic field revealed s pecies-specific differences: red-eyed vireo, Vireo olivaceus, and northern waterthrush, Seiurus noveboracensis, selected seasonally appropriate southe rly directions whereas indigo bunting, Passerina cyanea, and grey catbird, Dumetella carolinensis, oriented towards the sunset direction. When tested in deflected magnetic fields, vireos and waterthrushes responded by shiftin g their orientation according to the deflection of the magnetic field, but buntings and catbirds failed to show any response to the treatment. In rele ase tests, all four species showed that they had recalibrated their star co mpass on the basis of the magnetic field they had just experienced in the c age tests. Since release tests were done in the local geomagnetic field it seems clear that once the migratory direction is determined, most likely du ring the twilight period, the birds use their recalibrated star compass for orientation at departure. (C) 2000 The Association for the Study of Animal Behaviour.