A compass made of light
The ability of migratory birds, like the European Robin (Erithacus rubecula), to navigate thousands of kilometers is partly explained by a biological compass. Research that began in the 1960s at Goethe University Frankfurt by zoologists Wolfgang and Roswitha Wiltschko demonstrated that birds possess an internal magnetic sense. Their experiments, which involved placing robins in cages where the magnetic field could be artificially manipulated with Helmholtz coils, showed the birds would orient themselves according to the field. The the avian compass is an "inclination compass." It doesn't detect the polarity (north vs. south) but rather the angle of the Earth's magnetic field lines relative to the ground. Birds perceive "poleward," where the field lines dip downwards, and "equatorward," where they point upwards.
This sense is also light-dependent; it functions only when the bird is exposed to short-wavelength light, from UV to blue or green. Under yellow or red light, the birds' ability to orient themselves vanishes. This light dependency pointed away from simple iron-based magnetoreception and toward a more complex photochemical process, leading scientists to investigate the bird's eye as the location of the sensor. The leading theory is that birds "see" the magnetic field as a visual pattern superimposed on their normal vision, a pattern that changes as they turn their heads.