ORIENTATION ALONG GREAT CIRCLES BY MIGRATING BIRDS USING A SUN COMPASS

The time-compensated sun compass furnishes migrating birds with a possibility of orientating along great circle routes. When moving across longitudes, birds are exposed to a time shift and, if sun compass orientation is used, the course is expected to change along the migration route. These expected course changes are compared with the changing courses along great circle routes. At high latitudes, sun compass courses are expected to change by approximately 1° for each degree of longitudinal displacement, with little daily and seasonal variation. The resulting routes accord well with great circle routes as far away from the poles as lat. 50°-60°. At lower latitudes, birds may travel along approximate great circle routes if they let themselves be guided by sun compass orientation around the time of sunset/sunrise. Generally, sun compass routes at sunset/sunrise are exactly equal to great circles. This also applies to routes where birds, travelling during the night, are guided by stars close to the horizon using a time-compensated star compass. It remains to be elucidated if migrating birds make use of the possibilities, offered by their sun compass, for great circle orientation. © 1991 Academic Press Limited.