Magnetic compass orientation of birds depends on polarization of light
The magnetic compass of birds requires light to function and has been proposed to be located in specialized magnetoreceptor molecules in the avian retina. Hitherto, biophysical models and behavioral experiments on the magnetic compass sense have not taken into account that natural skylight reaching these receptors can be at least to some degree polarized.
In this study, zebra finches were trained in a spatial orientation task to find a food with the help of their magnetic compass and found that overhead polarized light affects magnetic compass orientation. The birds were only able to reliably find the food reward when the polarized light was aligned parallel to the magnetic field, but not when it was aligned perpendicular to the magnetic field. These findings demonstrate that the magnetic compass of birds, and likely other animals, is polarization sensitive, which is a fundamentally new property of the light-dependent magnetic compass.
It remains to be shown to what degree birds in nature are affected by different alignments of polarized light and the Earth’s magnetic field. It could be a mechanism to enhance the magnetic field around sunrise and sunset, when polarized light is aligned roughly parallel to the magnetic field and when many migratory songbirds are believed to determine their departure direction and calibrate the different compasses with each other for the upcoming night’s flight. During midday, when polarized light and the magnetic field are aligned roughly perpendicular to each other, the magnetic field would be less prominent, thus would be less likely interfere with visual tasks like foraging and predator detection.
Article #15-13391: “Polarized light modulates light-dependent magnetic compass orientation in birds,” by Rachel Muheim, Sissel Sjöberg, and Atticus Pinzon-Rodriguez.