Light-dependent magnetoreception: orientation behaviour of migratory birds under dim red light

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Abstract
Magnetic compass orientation in migratory birds has been shown to be based on radical pair processes and to require light from the short wavelength part of the spectrum up to 565 nm Green. Under dim red light of 645 nm wavelength and 1 mW m–2 intensity, Australian silvereyes and European robins showed a westerly tendency that did not change between spring and autumn, identifying it as a `fixed direction' response. A thorough analysis revealed that this orientation did not involve the inclination compass, but was a response based on the polarity of the magnetic field. Furthermore, in contrast to the orientation under short-wavelength light, it could be disrupted by local anaesthesia of the upper beak where iron-containing receptors are located, indicating that it is controlled by these receptors. The similarity of the response under dim red light to the response in total darkness suggests that the two responses may be identical. These findings indicate that the observed `fixed direction' response under dim red light is fundamentally different from the normal compass orientation, which is based on radical pair processes.