Data from: Hypotheses and tracking results about the longest migration: the case of the arctic tern

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 predation/piracy and
competition.