To test whether celestial rotation or the geomagnetic field served as reference system for the calibration of the star compass, young garden warblers were handraised under the following conditions:
In series I, two groups of birds were raised under an artificial sky the center of which lay in 360° N. For one group, the sky rotated around its center (1 rotation per day), while for the other the sky was stationary. In series II, three groups were raised under a rotating sky with the center of rotation in different position with respect to magnetic north.
Tested under a stationary sky in the absence of meaningful magnetic information, only the birds that had experienced the rotating sky were oriented in series I; their tendency away from the former center of rotation corresponded to their natural southerly migratory direction.—In series II all birds, irrespective of the former relationship between rotation and magnetic north, showed directional preferences away from the former center of rotation.
Our findings demonstrate the crucial role of celestial rotation in the establishment of the star compass. During the first summer there seems to be no transfer of information from the magnetic field to the stars. Thus celestial rotation and the magnetic field serve as independent reference systems for the genetically encoded migratory direction.