Arctic Tern (Sterna paradisaea)

node · earth · Bipolar - Arctic breeding, Southern Ocean wintering

A small seabird that breeds around the Arctic and winters in the pack ice of the Southern Ocean, making it the one animal that experiences both systems in the same year and the longest known animal migration. Geolocator tracking published in 2010 followed birds from Greenland and Iceland and found annual round trips on the order of seventy thousand kilometres, with routes that are not straight lines but long looping tracks exploiting prevailing wind systems, including a mid-Atlantic staging pause. The tern is the useful corrective to the whole framing of this Journey: the two poles are structurally opposite and biologically almost entirely separate, and a bird weighing about a hundred grams treats them as two ends of one commute.