Ontogeny of migratory orientation in the savannah sparrow, Passerculus sandwichensis: calibration of the magnetic compass

Abstract

The ontogeny of magnetic orientation was examined in the savannah sparrow by manipulating the early experience of hand-raised birds with visual and magnetic cues. Tests of magnetic orientation were performed indoors in orientation cages covered by translucent sheets so that the birds could see nothing outside the cages. Control birds, raised entirely indoors, oriented in a magnetic north-northwest to south-southeast axis during autumn migration. Birds raised outdoors, exposed to clear daytime skies in a normal magnetic field, showed north-south magnetic orientation. Three groups were given experience with the natural sky only within a set of coils that shifted magnetic north clockwise to geographical east-southeast: one group saw only the daytime sky, one group saw only the clear night sky, and the third saw both day and night skies. The birds of all three groups oriented magnetic northeast-southwest, significantly different from the controls. Magnetic northeast-southwest corresponded approximately to geographical northwest-southeast within the magnetic coils in which the birds obtained their visual experience. The differences in the orientation directions chosen by the groups show that the primary magnetic compass may be calibrated early in life by some reference to geographical directions. Experience with either the clear daytime or night sky was sufficient to effect this modification. Celestial rotation, which provides a source of geographical directions both day and night, is proposed to be that geographical reference. At night the birds presumably used the stars to assess celestial rotation. In the daytime sky, both the sun itself and patterns of polarized skylight provide a means of determining geographical north. Experiments designed to determine which of these two visual cues was the relevant stimulus in the calibration of the magnetic compass were inconclusive.