The following paper might be of interest
Regards, Laurie.
Polarized Light Cues Underlie Compass Calibration in Migratory Songbirds
Rachel Muheim,1,2* John B. Phillips,2 Susanne Åkesson1
Science 11 August 2006: Vol. 313. no. 5788, pp. 837 - 839
Migratory songbirds use the geomagnetic field, stars, the Sun, and
polarized light patterns to determine their migratory direction. To
prevent navigational errors, it is necessary to calibrate all of these
compass systems to a common reference. We show that migratory Savannah
sparrows use polarized light cues from the region of sky near the
horizon to recalibrate the magnetic compass at both sunrise and sunset.
We suggest that skylight polarization patterns are used to derive an
absolute (i.e., geographic) directional system that provides the
primary calibration reference for all of the compasses of migratory
songbirds.
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