Orientation Behavior of Garden Warblers (Sylvia borin) Under Monochromatic Light of Various Wavelengths

A little more than 30 years ago, a study revealed that migratory European Robins (Erithacus rubecula) could use the earth's magnetic field for directional orientation (Merkel and Wiltschko 1965, W. Wiltschko 1968). Today, magnetic compass orientation has been demonstrated in more than 15 other species of migratory birds and in homing pigeons, and it seems to be a rather widespread mechanism among birds (R. Wiltschko and Wiltschko 1995). Yet, it is still unclear how birds perceive the geomagnetic field and obtain directional information. One hypothesis under investigation is that magnetoreception occurs via light-dependent processes parallel to the processes of vision in the avian eye (Leask 1977, 1978; Schulten and Windemuth 1986). This hypothesis assumes that incident light elevates certain macromolecules into an excited state, where they may be transferred to an excited triplet state and where further reactions are determined by the direction of the ambient magnetic field. This would mean that light is essential for magnetoreception. Results from early experiments with homing pigeons are in agreement with this prediction. Inexperienced young pigeons derive their home direction from information obtained with their magnetic compass during the outward journey (R. Wiltschko and Wiltschko 1978); pigeons transported to the release site in total darkness respond like birds transported without meaningful magnetic information, which indicates that light is indeed necessary for magnetic compass orientation (W. Wiltschko and Wiltschko 1981).