Distinguishing Characteristics

Medium-sized sea-tern: total length 31–35 cm, including elongated outer tail feathers forming 6–9 cm fork; body mass 110–145 g; wingspan 75–80 cm. Breeding adult (Definitive Alternate plumage) characterized by light gray upperparts, pale gray underparts, black cap, orange-red legs, and orange-red bill with black tip. Extensive black on outer primaries conspicuous on closed wing, although covered by white frosting when freshly molted. In flight, both leading and trailing edges of outer primaries are dark; black inner webs of primaries show as dark wedge in spread wing. Outer edge of tail-feathers dark gray; rump and upper tail-coverts white, contrasting with gray back. In May–Jun, bill usually orange-red with black tip, but some birds have extensive black on culmen; a few have bills largely black in May. Black gradually lost during breeding season, so bill largely orange-red by late Jul. S. h. longipennis (e. Asia, migrant in Aleutians) has darker gray underparts, brownish-red legs and black bill.

Adult winter (Definitive Basic) plumage acquired by molt in Jul–Nov. Forehead and lores become white; underparts white; bill black with dark red base, or all black; legs reddish black; dark carpal (cubital) bar on lesser coverts. Unmolted (black) outer primaries retained until arrival in winter quarters.

Juveniles variable: forehead buffy or whitish, contrasting with dark brown crown and ear-coverts; back, scapulars, tertials, and wing-coverts gray with prominent or faint brown bars and dark feather tips; black lesser-coverts forming prominent carpal bar; collar and underparts white; legs pinkish or orange-brown; bill dark with orange or pink base. Primaries grayish brown; secondaries darker, both with whitish edges; tail gray with dark edges, fork shallow (3–5 cm). First winter (Basic I plumage) acquired by molt in Nov–Feb: like adult except more white in forehead, forecrown speckled, back with faint brown bars until Jan, carpal bar more prominent, bill and legs black.

First summer (Alternate I plumage, rare in breeding areas) like adult winter, usually with black bill and legs, and prominent carpal bar. Breeding plumage usually acquired by age 4, but progression variable: 2-yr-olds usually have white foreheads, speckled forecrowns, white underparts, distinct carpal bars, and bills at least one-half black in Jun–Jul. Most 3-yr-olds and some 2-yr-olds have prominently speckled foreheads, bills one-third black, underparts gray with variable white barring; some 3-yr-olds are like adults. Some 4-yr-olds and even a few adults 5–20 yr old have white speckles on the forehead and above the eyes.

Voice has distinctive, sharp, irritable timbre. Descending calls of adults kee-ur or kee-uri (advertising) and kee-arrrr (alarm) separate Common Tern from all similar species (Arctic Tern [Sterna paradisaea] has similar but higher-pitched alarm call). Juveniles in late summer have grating flight call krrrri, and a persistent begging call kri-kri-kri-kri-. . . .

For distinctions from similar species, see Wilds 1993, Olsen and Larsson 1995 . Common Tern most numerous tern in many areas where other species occur and serves as basis for comparisons. Dark wedge in spread primaries distinctive in all plumages. Arctic Tern has much shorter legs, lighter and more grace-ful flight, translucent primaries with very little black on inner webs of outer primaries (i.e., very narrow black line on trailing edge of wing), and higher-pitched, squeakier calls. Breeding adult Arctics have darker gray underparts, uniform dark-red bills, and longer tail streamers. Juvenile Arctics have short black bills, blacker sides to head contrasting more with white forehead and forecrown, and more contrasting wing pattern with dark subterminal bar on pale secondaries. One-year-old Arctics, much more frequent in breeding area than 1-yr-old Commons, have short bill and legs and sometimes have long tail streamers. Downy chicks difficult to distinguish in mixed colonies, but Arctics are dimorphic, with upperparts usually either silver-gray or bright buff; usually have black foreheads and gray bellies. Older Arctic chicks can be distinguished by short tarsi. Forster’s Tern (S. forsteri) resembles Common when in breeding plumage, but has white underparts, lighter primaries, more black on bill, tail with white outer edges and dark on inner webs of outer feathers. In other plumages, Forster’s has distinctive black ear-patch contrasting with white forehead and whitish crown; voice harsh and rasping. Adult Roseate Tern (S. dougallii) much paler, with creamy-white underparts, long white tail streamers, and (in ne. North America) black bill in May–early Jun; Caribbean Roseates and northeastern birds in Jun–Jul have bills partly orange-red, more like those of Commons. Juvenile Roseates have black bills and legs, and prominent black chevrons on gray or buff back-feathers. South American Tern (S. hirundinacea) larger, paler above and below, with less black in primaries and no dark gray in outer tail-feathers; bill larger and all red in breeding season (Harrison 1985, ICTN). For additional details of field characteristics, including plumage development, molts, effects of wear, variations, and hybrids, see Wilds 1993, Olsen and Larsson 1995.