Sandwich Tern has long been considered a single species with three subspecies (North American acuflavidus, South American eurygnathus, and European sandvicensis). These populations are morphologically very similar, but recent genetic studies show significant divergence between European and American populations (Efe et al. 2009). In fact, American populations are genetically closer to Elegant Tern than they are to “European” Sandwich Tern. Given this, the British Ornithologists’ Union and other authorities split Sandwich Tern into two species as early as 2011. The American Ornithological Society and Clements/eBird continue to treat it as a single species, but I believe recognition as a full species will surely come to North America.

Nomenclature

Currently all authorities that split these into separate species use the name Cabot’s Tern for the American population (including South America), but they continue to use Sandwich Tern for the European populations. This is confusing on several levels: It restricts the name Sandwich Tern to just part of the whole complex, and the name Cabot’s Tern (which has been used commonly to distinguish the North American subspecies from the South American “Cayenne” Tern) is applied to the whole American range. Furthermore, there is a widespread desire to move away from patronymic and honorific names for birds, so using a descriptive name for these terns is preferable.

For greatest clarity and continuity, I recommend simply adding the modifiers American and European to the species names, as follows:

A cladogram illustrating the evolutionary relationships of the Sandwich Tern complex. Note that the closest relative of American Sandwich Tern is Elegant Tern—not European Sandwich Tern. Image © David Sibley.

American Sandwich Tern

Thalasseus acuflavidus

• Northern (i.e., Cabot’s), T. a. acuflavidus

• Southern (i.e., Cayenne), T. a. eurygnathus

European Sandwich Tern

Thalasseus sandvicensis

The Southern (Cayenne) subspecies of American Sandwich Tern is distinguished from the Northern subspecies mainly by bill color, and will not be discussed further in this paper. For more information see Pranty 2022.

Identification

Normally, the ranges of European and American Sandwich Terns do not overlap, and it is generally safe to identify them by location. But each has been recorded several times within the range of the other, and records show that both taxa are possible almost anywhere.

David Sibley has been watching and drawing birds for as long as he can remember. At age 18 he began traveling full-time throughout the United States and Canada, all the while studying and sketching. Eventually, he wrote and illustrated multiple books about the birds (and trees) of the region. The goal of David’s work is to reveal and help others connect to the rhythms and patterns of the natural world.