Migratory direction established in inexperienced bird migrants in the absence of magnetic field references in their pre-migratory period and during testing

Several studies have investigated the importance of different orientational cues that pre-migratory, na´ve bird migrants might use to develop their appropriate migratory orientation. We tested the hypothesis that, without any interplay with the magnetic compass in the pre-migratory period, celestial rotation alone cannot lead to any migratory orientation that differs significantly from due south, i.e. celestial rotation is used as a reference only and it is set by the geomagnetic compass to the species-specific migration direction. In the present study, juvenile whitethroats, Sylvia communis, trapped in the field soon after fledging, developed appropriate migratory orientation when held in outdoor cages in full view of celestial cues, but in a strong, heterogeneous magnetic field without any meaningful, magnetic directional information and tested in a strong and approximately vertical magnetic field. The migratory orientation of these birds did not differ from that of birds held in an undisturbed magnetic field, and both differed significantly from south. Thus, the birds established a deviation from south (away from celestial rotation) in the absence of meaningful magnetic information in the pre-migratory phase. This indicates that magnetic information is not necessary for establishing the appropriate migratory direction when natural celestial cues are available in the pre-migratory period.