It has been known for many years that birds possess a magnetic "inclination compass," which essentially allows birds to obtain directional information from the magnetic field by interpreting the angle of magnetic-field lines with regard to the horizon rather than by interpreting the magnetic field's polarity. Previous work by Dr. Ritz had suggested that in interpreting magnetic signals, birds employed a so-called chemical compass that worked by way of chemical reactions in specialized photopigments in their eyes. The chemical-compass idea implied that magnetoreception was light dependent, and this possibility was subsequently given support by work from the Wiltschko team showing that the orientation of European robins, a night-migrating species, was influenced by the intensity of light in the blue-green spectrum.
In the present study, the Ritz and Wiltschko groups teamed up to analyze the orientation behavior under turquoise light in detail and revealed an unexpected phenomenon: Increasing the intensity of turquoise light changes the birds' orientation significantly, in comparison to dimmer light levels. The researchers found that in dim turquoise light, similar to that found about 33 minutes after sunset, the birds show normal migratory orientation, with the seasonal shift between southerly directions in autumn and northerly directions in spring. Tests under specific magnetic conditi ons clearly showed that this orientation involved the inclination compass and suggested that it is based on the type of "chemical compass" processes predicted by the Ritz model.
However, the researchers also found that under brighter turquoise light, corresponding to light levels found 20 min after sunset, the birds still orient by the magnetic field, but they no longer show the seasonal change between spring and autumn and instead head north in both seasons. This behavior did not appear to involve the normal inclination-compass and chemical-compass mechanisms.
The new findings show that bright-colored light interferes with magnetoreception such that migratory birds can no longer obtain the information required to head into their migratory direction. The findings point to the existence of two distinct mechanisms of magnetoreceptors in the birds--an inclination compass and a polarity-driven compass. It is especially intriguing that under some conditions, birds appear to switch to the polarity-type magnetic response, which is based on a mechanism of a very different nature than that thought to contribute to the inclination mechanism.