Anguilla, Antigua and Barbuda, Argentina, Aruba, Bahamas, Barbados, Belize, Brazil, Canada, Cayman Islands, Colombia, Costa Rica, Cuba, Dominica, Dominican Republic, El Salvador, French Guiana, Guadeloupe, Guatemala, Guyana, Haiti, Honduras, Jamaica (B), Martinique, Mexico, Montserrat, Netherlands Antilles, Panama, Peru, Puerto Rico, St Kitts and Nevis, St Lucia, St Vincent and The Grenadines, Suriname, Trinidad and Tobago, Turks and Caicos Islands (B) (NB), USA (B), Venezuela, Virgin Islands (British) (B), Virgin Islands (U.S.) (B).
Vagrant to Ecuador, Grenada, St Pierre and Miquelon (P).
Population
Estimated population is 65,000 - 70,000 (2010).
Long considered a subspecies of Little Tern (Sternula albifrons). Proposed form mexicana for birds of Pacific coast of Mexico is not separable.
The following 3 subspecies are recognised:
browni (Mearns, 1916) - Central California (San Francisco Bay) to Baja California and western Mexico. Winters mostly in Central America.
athalassos (Burleigh & Lowery, 1942) - Inland rivers in central North America, from northern Great Plains to northern Louisiana and Texas. Winters to northern Brazil.
antillarum (Lesson, 1847) - Bermuda, and Maine south to Texas (including lower Rio Grande Valley) and Honduras, and on through Caribbean to northern Venezuela. Winters to northern Brazil.
BirdLife International Species Factsheet