Along with the Snowy-crowned Tern (Sterna trudeaui), the South American Tern is the most widespread tern in the now restricted “Sterna” group of mid-sized terns. Both are temperate in distribution, and are largely limited to the Southern Cone, although not entirely. The South American, along with its close relative from farther south the Antarctic Tern (Sterna vittata), is extremely similar in look to the Common (S. hirundo) and Arctic (S. paradisea) terns of the north. This would not be an issue if it wasn’t for the fact that these species all overlap, when northern breeding Common and Arctic migrate through or winter in South America’s southern Cone. Needless to say that identification criteria are subtle and still evolving. One would think that the difference in molt timing between northern and southern hemisphere birds alone would be enough to separate these species as they should be off by approximately 6 months. However, it is now clear that South American Terns have northern and southern breeding populations, the northern ones are timed much more closely to the timing of northern hemisphere breeders! Little is known about the northern breeders, and it appears that there is a sizeable range gap between the northern birds and the southern breeders.