Daytime calibration of magnetic orientation in a migratory bird requires a view of skylight polarization

THE orientation of migratory birds is based on a complex of interacting compass mechanisms (the geomagnetic field, stars, patterns of skylight polarization and, perhaps, the Sun)1,2. A magnetic compass develops in birds that have never seen the sky3–8, but the preferred direction of magnetic orientation may be modified during the first three months of life by exposing naive birds to either the clear daytime or night sky under conditions in which magnetic directions differ substantially from true or geographic directions5–7,9. We hypothesized that celestial rotation, which indicates geographic directions both day and night, served as the calibrating reference7, and showed that a rotating pattern of artificial stars provided a sufficient stimulus to calibrate magnetic orientation in young Savannah sparrows (Passerculus sandwichensis)10. During daytime either the Sun's disc or patterns of polarized skylight could provide the reference to geographic compass directions11,12. Here we report that visual access to natural skylight polarization patterns is necessary for calibration of magnetic orientation during daylight.

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