Light-dependent magnetoreception in birds: analysis of the behaviour under red light after pre-exposure to red light.

In previous experiments, migratory birds had been disoriented under 635 nm red light, apparently unable to use their magnetic compass. The present study with European robins, Erithacus rubecula, confirms these findings for red light at the levels of 6 x 10(15) quanta s(-1) m(-2) and 43 x 10(15) quanta s(-1) m(-2), suggesting that the disorientation under red light was not caused by the test light being below the threshold for magnetoreception. However, pre-exposure to red light for 1 h immediately before the critical tests under red light of 6-7 x 10(15) quanta s(-1) m(-2) enabled robins to orient in their seasonally appropriate migratory direction in spring as well as in autumn. Pre-exposure to darkness, by contrast, failed to induce orientation under red light. Under green light of 7 x 10(15) quanta s(-1) m(-2), the birds were oriented in their migratory orientation after both types of pre-exposure. These findings suggest that the newly gained ability to orient under red light might be based on learning to interpret a novel pattern of activation of the magnetoreceptors and hence may represent a parallel to the previously described enlargement of the functional window to new magnetic intensities. Mechanisms involving two types of spectral mechanisms with different absorbance maxima and their possible interactions are discussed.