The Arctic Tern (Sterna paradisaea) is perhaps best known for its extraordinary annual migration, which is often cited as the longest seasonal distance traveled by any animal. It has long been known that the Arctic Tern breeds in the Arctic and migrates each year to spend the northern winter at high latitudes in the Southern Ocean. Until recently, what has been known about the Arctic Tern's migration has come from limited banding recoveries and at-sea observations. Thanks to new technology, however, far more detailed data on this small (<125 grams) bird's annual migration are now available. Egevang et al. (2010) fitted 11 Arctic Terns with miniature (1.4 gram) geolocators. They found that the annual distances traveled are, in fact, even greater than previously believed--more than 80,000 km annually for some individuals. All tracked terns migrated south to spend the austral summer (December–March) south of 58° S and between 0 and 61° W in the Atlantic sector of the Southern Ocean. This region, which includes the Weddell Sea, is particularly productive, and supports higher densities of a key prey for many seabirds, Antarctic krill (Euphausia superba), than do other regions of the Southern Ocean. All birds began the return migration to breeding colonies in early–mid April, always traveling over deep water at considerable distance from continental shelf margins.
Egevang et al. note that the routes used for both the southbound and northbound migrations showed a high level of congruence with parts of those taken by Manx Shearwater (Puffinus puffinus) and Cory’s Shearwater (Calonectris diomedea), which also winter in the South Atlantic (although considerably farther north than the Arctic Tern). Thus, despite their small size, Arctic Terns apparently exploit the prevailing global wind systems (clockwise in the North, and counter-clockwise in the South Atlantic) much as the substantially larger shearwaters do, as has been previously suggested. These new geotracking studies found that the main wintering region was the marginal ice zone around Antarctica, which agrees with at-sea observations. The mean duration of the northbound migration was about 40 days (range 36 to 46). Mean duration of the southbound migration was about 93 days (range 69 to 103). (Egevang et al. 2010)
Egevang et al. (2010) provide the following summary of key statistics derived from their study, showing mean (range):
Total distance traveled on migration: 70,900 km (59,500–81,600 km)
Distance traveled on southbound migration: 34,600 km (28,800–38,500 km)
Distance traveled per day on southbound migration: 330 km per day (280–390 km per day)
Distance traveled on northbound migration: 25,700 km (21,400–34,900 km)
Distance traveled per day on northbound migration: 520 km per day (390–670 km per day)
Distance traveled within winter site: 10,900 km (2,700–21,600 km)
Perhaps most striking, the tracked birds were found to travel nearly twice the total distance generally cited for the annual Arctic Tern migration (typically quoted as ~ 40,000 km). Given that Arctic Terns can live for more than 30 years, the total distance traveled in a lifetime may exceed 2.4 million km, equivalent to approximately three round-trip journeys to the Moon. (Egevang et al. 2010)
