Researchers have long known that birds figure out where to go during migration by using a combination of the earth's magnetic fields, the stars, the sun, the moon, and polarized light pattern cues at the horizon at sunrise and sunset. The exact nature of how each compass system integrates with the others, and how these systems are calibrated and recalibrated has been a mystery. The magnetic compass changes with latitude, and others systems may be altered by weather or time of day. To be effective despite these alterations, it would be necessary for all compasses to be calibrated to a common reference.
A paper published in Science magazine in August, 2006, "Polarized Light Cues Underlie Compass Calibration in Migratory Songbirds", by Muheim, Phillips, and Akesson provides experimental evidence that the polorized light cues at sunrise and sunset are the common reference that other systems calibrate to. The actual reference value is an average of both sunrise and sunset cues. These researchers were able to re-orient the magnetic compass of Savannah Sparrows by presenting a false polarized light cue. Earlier orientation experiments often did not provide the birds a view of the horizon. In these cases, the magnetic compass became the reference, and other systems were calibrated with respect to the magnetic field. It is unknown if all species of birds use this same sequence of cues to orient in migration. Future work will no doubt discover if this system is universal in birds, or if other combinations have evolved in different bird groups.
Photo of Colima Warbler by Greg Lavaty