Over 100 animal species are known to be able to perceive the plane of polarization of linearly polarized light; most of these are arthropods but several molluscs and vertebrates, including man, share this ability1,2. At least five aquatic vertebrates (three fish, a salamander and a larval frog) are capable of oriented movement based on the E-vector of linearly polarized light3–9. Pigeons (Columba livia) have been conditioned to discriminate between rotating and stationary polarized light and between widely separated stationary E-vectors10,11, but the behavioural significance of this ability remains unknown. I report here that the migratory orientation of the white-throated sparrow (Zonotrichia albicollis), a nocturnal migrant, is affected by manipulations of the axis of skylight polarization. These data provide the first evidence that polarized light may be a relevant cue in migratory orientation.