In many ways the Antarctic Tern is quite similar to the northern hemisphere Arctic Tern (Sterna paradisea), and in fact the two are found sympatrically in the Antarctic ice pack during the southern summer while the Arctic Tern is spending its non-breeding season there. However, based on the strong barring pattern of the upperparts of juveniles, including the tertials, as well as biogeography, it appears that the Antarctic Tern is in fact quite closely related to the South American Tern (Sterna hirundinacea). Compared to the South American Tern, the Antarctic Tern is smaller, shorter-necked, shorter-legged and shorter-billed. Its plumage is darker than that of the South American Tern. Even so, these two species are extremely difficult to separate and identification criteria are still evolving. Also complex is that immature Antarctic Terns may resemble winter plumages of Arctic Tern, and again, the identification criteria are still being determined. An additional complication of the Antarctic Tern is that it is a circumpolar species which has various separate populations that differ somewhat in plumage, and noticeably in size. One population, the Kerguelen Tern (Sterna virgata), is usually separated as a species, although some of the other populations may also deem separation. More work is needed on the taxonomy of southern Sterna terns. Also complex is that some Antarctic Tern populations are migratory, while others are resident. Furthermore, they do not appear to move north along the Pacific Coast of South America, but may do so on the Atlantic Coast of the continent (Antarctic breeding Wilson’s Storm Petrels show a similar pattern); Antarctic Terns regularly migrate to winter in southern South Africa so definitely some do move north in the Atlantic!