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| Several studies have investigated the importance of different orientational
cues that pre-migratory, naïve 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. | |
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