Light sets the compasses
Migratory birds tune their different endogenous compasses by repeatedly determining the intersection of the polarized light of the central star occurring at sunrise and sunset with the horizon and thus obtaining a geographic north-south axis.
This comparison provides the animals with a reference point that is independent of the time of year and the degree of latitude at which they are currently located. They then use this line to calibrate their magnetic and celestial compass, the bases of which – such as the sun or the constellations – fluctuate over the course of the day or can be influenced by the weather, summarizes Rachel Muheim from Virginia Polytechnic Institute and State University in Blacksburg and her colleagues.
Like other migratory birds, the grass buntings (Passerculus sandwichensis) they studied use the earth's magnetic field and celestial bodies to maintain their migration direction. However, so that they are not misled, they have to reset them immediately before they take off and constantly on the way. Exactly how they do this was unknown to scientists, however, as experiments have only found that the birds use the magnetic compass to calibrate the celestial compass and vice versa. The scientists traced this confusing circular argument back to the structure of the previous experiments, which, with two exceptions, prevented the animals from seeing the horizon.
During sunrise and sunset, a band of highly polarized light occurs that intersects the horizon line at a 90-degree angle to either side of the Sun. The birds can perceive this light and use it to spatially determine their current position.