Calibration of the magnetic compass of a migratory bird by celestial rotation

Abstract

MIGRATORY birds use a variety of environmental stimuli in orientation. Species that migrate primarily at night can use compasses based on the geomagnetic field, stars, the Sun and patterns of skylight polarization^1–4. These compass mechanisms can interact both when migratory birds make day-to-day orientation decisions⁵ and during their ontogeny in young birds^6,7. All of the known compasses used by migratory birds seem to be modifiable by experience during early development. For example, a functional magnetic compass develops in birds that have never seen the sky^8–11. But the preferred direction of orientation by the magnetic compass may be modified during the first three months of life by exposing naive birds to the sky under conditions in which magnetic directions differ substantially from compass directions indicated by the Sun and stars (true or geographic directions)^6,9,10. For hand-raised Savannah sparrows (Passerculus sandwichensis), experience with either the clear daytime or night sky is sufficient to effect this calibration of the magnetic compass¹¹. We therefore proposed that celestial rotation, which provides a source of geographic directions both day and night, is the calibrating reference. Here we report that the rotation of an artificial pattern of 'stars' calibrates the preferred direction of magnetic orientation of young Savannah sparrows.