||The transduction mechanisms and the neurophysiological basis of magnetoreception in birds are still largely unexplained, even though the role of the magnetic compass in the orientation of birds is fairly well understood. The discussion on magnetoreception in birds and terrestrial vertebrates focuses mainly on two mechanisms: small particles of magnetite and biochemical bi-radical reactions of excited macromolecules. When the bi-radical hypothesis was first proposed, magnetic resonance phenomena in the retina were suggested as the primary processes, which led to the question of whether magnetoreception was light-dependent. Homing experiments and electrophysiological evidence from pigeons have produced evidence consistent with such a mechanism. An effect of the spectral composition of light on magnetic compass orientation in amphibians has recently been described: under blue light of 450 nm and below, newts oriented as they did under the full spectrum, whereas they showed a roughly 90Â° counterclockwise shift when tested under wavelengths at or above 500 nm. Here we report the first orientation tests on migratory birds under light of different wavelengths; the results suggest a light-dependent process that appears to differ from that reported in newts.