Calibration of the Magnetic Compass by Polarized Light Cues

Birds can determine their migratory direction with the help of different compass systems (magnetic compass star, sun and polarized light compass). Because of the changing relationship between the magnetic and geographic reference systems underlying these compasses, birds have to calibrate the different systems with each other on a regular basis.
In experiments on Savannah sparrows (Passerculus sandwichensis) in the Yukon Delta National Wildlife Refuge, Alaska in 2005 we showed that birds recalibrate their magnetic compass using polarized light cues from the sky near the horizon at sunset and sunrise. To avoid having to compensate for differences in topography and horizon height, they used polarized light cues only from the sunrise or sunset sky, and not from other times of day. Moreover, to deal with the problem that sunrise/sunset positions shift with season and latitude, we believe that they average the information from successive sunrises and sunsets.
 
In experiments carried out at Braddock Bay Bird Observatory with white-crowned sparrows, we could confirm our results from Alaska. Also these birds recalibrated their magnetic compass by sunrise/sunset polarized ligth cues.
 
Currently, we are carrying out cue calibration experiments at Stensoffa Ecological Field Station and Falsterbo Bird Observatory with European robins.

 

However, in species like the Savannah sparrows that use both sunrise and sunset cues to recalibrate their magnetic compass, failure to average the information from both times of day would produce an “zig-zagging” migratory path, depending on whether the clear skies necessary to see the polarization pattern occurred most recently at sunrise or at sunset. Therefore, the opportunistic use of sunrise and sunset is not adaptive.