South American Tern (Sterna hirundinacea)

Widely distributed along the coasts of southern South America, these terns struck me as somewhat intermediate between Arctic ("S. paradisaea") and Common ("S. hirundo") Terns. They have an all red bill like Arctic, but their overall shape is more like Common Tern. Unlike Common Tern, this species molts its inner primaries only once each cycle, so it does not exhibit the dark wedge (molt limit) usually seen in the primaries of Common Tern. Also wintering Arctic and Common Terns would not be in breeding plumage in January. The similar Antarctic Tern is not known from Argentina.