Distinguishing Characteristics

Smallest of North American terns: length 21–23 cm, wingspan 48–53 cm. In Definitive Alternate (adult breeding) plumage, black cap and loral stripe contrasting with white forehead are distinctive. Remainder of upperparts gray, underparts white, outer 2 (rarely 3–4) primaries black; striking dark-tipped yellow or orange bill. Sexes similar. Dark loral stripe considered wider in male (Olsen and Larsson 1995), but sexes more reliably distinguished by behavior.

Basic plumage similar for all ages: white underparts, gray mantle, dark lesser-coverts, marginal or lesser wing-coverts forming a cubital bar, dark nape and crown with variable white flecking in crown, and dark eye-stripe with white flecking at lores. Juvenile recognized by extensive dark U- or V-shaped markings on gray to yellowish brown mantle. Distinguishable from other terns in Basic and subadult plumages by small size.

Gray rump and central part of tail same color as gray back (Olsen and Larsson 1995), as well as vocalizations (Massey 1976), help distinguish Least Tern from Little Tern, a sibling species from which Least Tern was only recently separated taxonomically (Am. Ornithol. Union 1983). In Little Tern—native to Europe, Asia, Africa, and Australia—rump and central rectrices are white or pale gray, always paler in color than the back. Least Tern is slightly smaller than Little Tern and has slightly longer outer rectrices (Olsen and Larsson 1995). Ranges of Least and Little tern do not overlap, although both species may stray to Hawaii.