Light-dependent magnetoreception in birds: the behaviour of European robins, Erithacus rubecula, under monochromatic light of various wavelengths and intensities.

To investigate how magnetoreception is affected by the wavelength and intensity of light, we tested European robins, Erithacus rubecula, under monochromatic lights of various wavelengths at two intensities using oriented behaviour as an indicator of whether the birds could derive directional information from the geomagnetic field. At a quantal flux of 7 x 10(15) quanta s(-1) m(-2), the birds were well oriented in their migratory direction east of North under 424 nm blue, 510 nm turquoise and 565 nm green light, whereas they were disoriented under 590 nm yellow light. Increasing the intensity of light at the same wavelengths more than sixfold to 43 x 10(15) quanta s(-1) m(-2) resulted in a change in behaviour: under bright blue and green light, the birds now showed a preference for the East-West axis, with the majority of headings at the western end; under bright turquoise light, they oriented unimodally towards a direction slightly west of North. Under bright yellow light, the birds continued to be disoriented. These findings suggest a rather complex relationship between the receptors involved in magnetoreception. Magnetoreception appears to follow rules that are different from those of vision, suggesting that light-dependent magnetoreception may involve receptors and neuronal pathways of its own.