Bird orientation at high latitudes: flight routes between Siberia and North America across the Arctic Ocean

Bird migration and orientation at high latitudes are of special interest be cause of the difficulties associated with different compass systems in pola r areas and because of the considerable differences between flight routes c onforming to loxodromes (rhumblines) or orthodromes (great circle routes). Regular and widespread east-north-east migration of birds from the northern tundra of Siberia towards North America across the Arctic Ocean (without l andmark influences) were recorded by ship-based tracking radar studies in J uly and August. Field observations indicated that waders, including species such as Phalaropus fulicarius and Calidris melanotos, dominated, but also terns and skuas may have been involved. Analysis of flight directions in re lation to the wind showed that these movements are not caused by wind drift . Assuming possible orientation principles based on celestial or geomagneti c cues, different flight trajectories across the Arctic Ocean were calculat ed: geographical loxodromes, sun compass routes, magnetic loxodromes and ma gnetoclinic routes. The probabilities of these four alternatives are evalua ted on the basis of both the availability of required orientation cues and the predicted flight paths. This evaluation supports orientation along sun compass routes. Because of the longitudinal time displacement sun compass r outes show gradually changing compass courses in close agreement with ortho dromes. It is suggested that an important migration link between Siberia an d North American stopover sites 1000-2500 km apart across the Arctic Ocean has evolved based on sun compass orientation along orthodrome-like routes.