Migratory orientation of European Robins is affected by the wavelength of light as well as by a magnetic pulse

The object of this study was to test the alternative hypotheses of magnetoreception by photopigments and magnetoreception based on magnetite. Migratory European Robins, Erithacus rubecula, were tested under light of different wavelengths; after these tests, they were subjected to a brief, strong magnetic pulse designed to alter the magnetization of single domain magnetite. In control tests under white light, the birds preferred the normal, seasonally appropriate migratory direction. Under 571 nm green light, they continued to be well oriented in the migratory direction, whereas under 633 nm red light, their behaviour was not different from random. The magnetic pulse had a significant effect on migratory orientation, but the response varied between individuals: some showed a persistent directional shift, while others exhibited a change in scatter; one bird was seemingly unaffected.These findings indicate a light-dependent process and, at the same time, suggest an involvement of magnetizable material in migratory orientation. They are in agreement with the model of a light-dependent compass and a magnetite-based map, even if some questions concerning the effect of the pulse remain open.