What It's Like to Be a Bird by David Allen Sibley

"Scientists are still working to sort out the remarkable details of the magnetic sense of birds. There is evidence that songbirds have two different systems for detecting the orientation of the magnetic field as well as the slope of the field (which varies with latitude from horizontal at the equator to vertical at the poles). They can also detect polarized light, which gives valuable clues to the position of the sun even when the sun itself is not visible."