Last autumn, Rachel Muheim, a postdoctoral associate in biology professor John Phillips' lab at Virginia Tech, captured Savannah sparrows in the Yukon before they headed south. She was able to demonstrate that the birds calibrate their magnetic compass based on polarized light patterns at sunset and sunrise.
The research appears in the Aug. 11, 2006, issue of Science, in the article, "Polarized Light Cues Underlie Compass Calibration in Migratory Songbirds," by Muheim, Phillips, and Suzanne Akesson. Muheim did her Ph.D. work at Lund University in Sweden with Akesson, who made the Alaska trip possible.
Polarized light is light that oscillates in one plane relative to the direction of propagation. At sunrise and sunset, there is a band of intense polarized light 90 degrees from the sun that passes directly overhead through the zenith and intersects the horizon 90 degrees to the right and left of the sun. Just as the sun location changes with latitude and the time of year, so does the alignment of the band of polarized light.
Muheim and Phillips argue that migratory songbirds average the sunrise and sunset intersections of the polarization band with the horizon to find the north-south meridian (geographic north-south axis), providing a reference that is independent of time of year and latitude. The birds then use this geographic reference to calibrate their other compass systems.
In other wo