| Summary: | The arctic tern Sterna paradisaea completes the longest known annual return migration on Earth, travelling between breeding sites in the northern arctic and temperate regions and survival/moult areas in the Antarctic pack ice zone. Salomonsen (1967) put forward a hypothetical comprehensive interpretation of this global migration pattern, suggesting food distribution, wind patterns, sea ice distribution and moult habits as key ecological and evolutionary determinants. We used light-level geolocators to record twelve annual journeys by eight individuals of arctic terns breeding in the Baltic Sea. Migration cycles were evaluated in the light of Salomonsen's hypotheses and compared with results from geolocator studies of arctic tern populations from Greenland, Netherlands and Alaska. The Baltic terns completed a 50,000 km annual migration circuit, exploiting ocean regions of high productivity in the North Atlantic, Benguela Current and the Indian Ocean between southern Africa and Australia (sometimes including the Tasman Sea). They arrived about 1 November in the Antarctic zone at far easterly longitudes (in one case even at the Ross Sea) subsequently moving westwards across 120 – 220 degrees of longitude towards the Weddell Sea region. They departed from here in mid-March on a fast spring migration up the Atlantic Ocean. The geolocator data revealed unexpected segregation in time and space between tern populations in the same flyway. Terns from the Baltic and Netherlands travelled earlier and to significantly more easterly longitudes in the Indian Ocean and Antarctic zone than terns from Greenland. We suggest an adaptive explanation for this pattern. The global migration system of the arctic tern offers an extraordinary possibility to understand adaptive values and constraints in complex pelagic life cycles, as determined by environmental conditions (marine productivity, wind patterns, low pressure trajectories, pack ice distribution), inherent factors (flight performance, moult, flocking) as well as effects of ...
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