Migration patterns of tundra birds: Tracking radar observations along the northeast passage

Bird migration was recorded by tracking radar and visual observations at 15 study sites, situated between 50 degrees E and 170 degrees E along the Nor theast Passage, during a ship-based expedition in July and August 1994. A t otal of 1087 radar tracks (average duration 220 s) of bird flocks on postbr eeding migration were recorded. Migration was dominated by waders and to a certain degree also skuas (especially pomarine skua Stercorarius pomarinus) . Terns, gulls, ducks, and geese were also among the migrants tracked by ra dar. The radar data revealed a major migratory divide at about 100 degrees E (Taymyr Peninsula), with mainly eastbound migration to the east of this d ivide, and mainly westbound migration to the west of it. The main stream of eastbound migration was directed toward the sector 90-120 degrees and that of westbound migration toward the sector 240-270 degrees; these directions are broadly in parallel with the coasts of the Arctic Ocean east and west of the Taymyr Peninsula, respectively. There was also important ENE migrati on, which provided strong indications of long-distance flights along orthod rome-like routes directly between Siberia and North America, across vast ex panses of the Arctic Ocean pack ice. Analysis of flight directions in relat ion to wind indicated complete compensation for wind drift. Mean flight alt itude was 1.3 km, and the birds regularly travelled at high altitudes above 3 km (9% of the tracks) up to a maximum height of 4.8 km. They preferred t o migrate on occasions and at altitudes with following winds; such conditio ns provided an average gain in speed of 4.6 m/s. There were also recurrent cases of birds migrating in tailwinds of gale force, between 18 and 24 m/s. The birds' airspeed varied between 8 and 22 m/s, with a mean of 14 m/s. Ai rspeed was significantly correlated with altitude, wind, and vertical speed and seemed to be intermediate between the speeds for minimum power and max imum range predicted by aerodynamic theory.