Distinguishing Characteristics

Medium-sized tern; length 33–36 cm; mass 130–190 g. Sexes similar. Definitive Alternate (adult breeding) plumage mostly immaculate white with black cap, pale gray wings and mantle, pale silvery gray flight feathers (upper surface of remiges silvery white, usually paler than mantle except darker tips to outermost primaries); deeply forked tail (long outer rectrices like streamers); orange legs and variably black-tipped orange bill. Cap barely reaches eye and often shows bulge below eye and ear-coverts. Tail light gray with contrasting white outer margin of outer tail-feathers (inner web of these feathers dark gray, however); white rump contrasts with pale gray of back, but not strikingly so. In Definitive Basic (adult nonbreeding) plumage, black cap replaced by black “mask” covering eye and ear-coverts, not extending to nape or rear crown; forehead white. Bill black, feet dusky orange. Wear and molt results in gradations between these 2 plumages showing varying degrees of brownish to blackish smudginess on nape and worn, dark primaries. Immatures generally have darker primaries than adults, and may show dark centers to tertials.

In the field, resembles and often occurs with Common (Sterna hirundo), Arctic (S. paradisaea), and Roseate (S. dougallii) terns; identification well-covered in recent guides (e.g., Dickinson 1999, Sibley 2000). For details on identification of Forster’s Tern versus Common Tern, also see Wilds 1993 . Dark ear-patch of nonbreeding and immature Forster’s Tern distinctive (even though some Forster’s Terns may become worn and show a dark nape, black eye-patch still apparent); the 3 species noted above have black extending to rear crown and nape. Compared to Common Tern, breeding adult Forster’s Tern generally much whiter, especially wings and underparts (although immature and worn adult Forster’s Tern primaries dark gray), has different tail pattern (Common Tern has whitish tail with dark outer margin), appears long-tailed both in flight and at rest (tail extends well beyond wing-tips on breeding adult Forster’s Tern), has longer, stouter, more orangish bill, and longer legs (tarsus length virtually non-overlapping between species). Immature Common Tern (Juvenal to Alternate I, at least) has dark carpal bar and dark secondary bar. Arctic Tern similar to Common Tern in most features above, additionally being smaller and more delicately built (especially short-necked and short-legged). Roseate Tern appears quite white overall like Forster’s Tern but in flight looks short-winged and has rather rapid, shallow wing-beat; call distinctive 2-syllable CHI-vik totally unlike Forster’s Tern; outer wing dark only on outermost 2–3 primaries; entire tail white; bill mostly black on adults early in breeding season, although bill becomes orangish red as summer progresses and in some populations (e.g., Caribbean) is mostly orangish red even early in breeding season.