Magnetic orientation is critical to the migratory success of many bird species. By studying the influence of light on the ability of migratory birds to orient to magnetic signals, researchers have found clues to suggest that birds orientation abilities may be more complex than previously thought and that birds may be able to interpret magnetic signals by more than one mechanism. The work is reported in Current Biology by a team including Thorsten Ritz, of the University of California, Irvine, and Wolfgang and Roswitha Wiltschko, of the University of Frankfurt, Germany.
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 fields 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 conditions 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.
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