Birds use multiple compasses for orientation during migration, which include magnetic, star, and sun compass. The relationship between the different reference systems changes along the migratory journey as a consequence of changing magnetic declination. Since cue availability changes with weather conditions, time of day, and latitude, birds must regularly calibrate the different compasses with respect to a common reference to avoid navigational errors.
We have previously shown that migratory songbirds recalibrate their magnetic compass with respect to sunrise/sunset polarized light cues from the region of sky near the horizon. Since sun and star compass calibrations are secondarily derived from magnetic compass and polarized light cues, we proposed that polarized light cues near the horizon at sunrise and sunset provide the primary compass calibration reference, which is independent of season and latitude.
Open questions which remain to be answered include: Why is a view of the polarized light cues near the horizon important for calibration? Which information from the skylight polarization pattern do the birds use? Why do they average at sunrise/sunset and not during other times of the day? How do they average sunrise and sunset information? To answer these questions, we use a variety of approaches, like experimental exposures of migratory birds to conflicting information from magnetic and polarized light cues and consecutive tests in orientation funnels and the observations of free-flying migrants tracked with radio telemetry.