The flexible migratory orientation system of the savannah sparrow (Passerculus sandwichensis)

The orientation system of the Savannah sparrow (Passerculus sandwichensis) is typical of nocturnal migrant passerine birds. It is based on a system of interacting compass senses: magnetic, star, polarized light and, perhaps, sun compasses. The magnetic compass capability develops in birds that have never seen the sky, but the preferred direction of magnetic orientation may be calibrated by celestial rotation (stars at night and polarized skylight patterns during the day). This ability to recalibrate magnetic orientation persists throughout life and enables the bird to compensate for variability in magnetic declination that may be encountered as it migrates. The polarized light compass may be manipulated by exposing young birds to altered patterns of skylight polarization. There is some evidence that the magnetic field may be involved in calibration of the polarized light compass. In short-term orientation decision-making during migration, visual information at sunset overrides both stars and magnetic cues, and polarized skylight is the relevant stimulus in dusk orientation. The star pattern compass seems to be of little importance. This extremely flexible orientation system enables the birds to respond to spatial and temporal variability in the quality and availability of orientation information.