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

Abstract

The arctic tern Sterna paradisaea completes the longest known annual return migration on Earth, traveling between breeding sites in the northern arctic and temperate regions and survival/molt areas in the Antarctic pack‐ice zone. Salomonsen (1967, Biologiske Meddelelser, Copenhagen Danske Videnskabernes Selskab, 24, 1) put forward a hypothetical comprehensive interpretation of this global migration pattern, suggesting food distribution, wind patterns, sea ice distribution, and molt habits as key ecological and evolutionary determinants. We used light‐level geolocators to record 12 annual journeys by eight individuals of arctic terns breeding in the Baltic Sea. Migration cycles were evaluated in 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 westward across 120–220 degrees of longitude toward 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 traveled 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, molt, flocking), and effects of predation/piracy and competition.

Use of sea ice by arctic terns Sterna paradisaea in Antarctica and impacts of climate change

“…Light-level geolocators can be fitted to plastic leg rings and seem, in recent studies, to be well tolerated by terns Bevan 2019a, Kürten et al 2019). Use of these devices on arctic terns (Egevang et al 2010, Fijn et al 2013, Alerstam et al 2019 has revealed the extent of their global migration strategies, confirming inferences from observations and ringing recoveries (Salomonsen 1967) of migration routes. However, for technical reasons, such studies have not analysed geolocator data for the periods when birds are in 24-h daylight during the Antarctic summer.…”

“…The general migration strategies of arctic terns breeding in Europe are well known (Egevang et al , Fijn et al , Alerstam et al ), and birds from Western Europe traverse east for variable distances across the southern Indian Ocean before moving south to the Antarctic (Fijn et al , Alerstam et al ). Daily mean minimum temperatures recorded by the geolocators rapidly decreased as the birds approached the Antarctic and generally remained between 0 and −5°C (Fig.…”

“…As with arctic terns from Baltic and Dutch populations (Fijn et al 2013, Alerstam et al 2019, birds from the Farne Islands entered the Antarctic further east than Greenland birds (Egevang et al 2010). However, although the mean longitudes of entry into the Antarctic may differ between arctic tern breeding populations, perhaps in relation to longitudinal location in the Northern Hemisphere and differential migration costs (Alerstam et al 2019), individuals from the Farne Islands displayed a wide range of strategies varying from directly south through the Atlantic, to the most-easterly bird which entered Antarctica between the Ross and Amundsen Seas after traversing the southern Indian Ocean to the western Pacific. This latter bird (G82) was also one of the most easterly in 2017, suggesting behavioural repeatability by individuals with respect to broad migratory strategy.…”

“…This latter bird (G82) was also one of the most easterly in 2017, suggesting behavioural repeatability by individuals with respect to broad migratory strategy. Potential interactions between weather systems at the Antarctic Convergence and the likelihood of good foraging conditions in the southern Indian Ocean (Alerstam et al ) as determinants of the longitudinal points of entry into the Antarctic require more detailed analyses.…”

“…At a broader scale, atmospheric currents are likely to be important factors in the overall migration strategy of arctic terns, with prevailing westerlies aiding their movement across the southern Indian Ocean to East Antarctic longitudes, a southerly movement to the Antarctic coastline aided by westerlies and longitudinal cyclonic air flows (Parmelee ), and subsequent movement back along productive coastal Antarctic waters with the easterly coastal winds (Greene et al ) in preparation for northward return to breeding colonies via the Atlantic (Salomonsen , Alerstam et al ). Dates at which migratory stages were achieved, such as entry into 24‐h daylight, showed little evidence of repeatability in re‐tagged birds, but the repeatability of longitudes at entry and exit from 24‐h daylight suggested that individuals followed particular migratory routes to the Antarctic.…”