Distinguishing Characteristics

Large, gull-like tern, 47–54 cm long (Cramp 1985); body mass 530–782 g (see Appendix 1). In Definitive Alternate plumage, upperparts pale gray, underparts white, and rump and tail white; black cap extends distinctly below eye and terminates in only slight nuchal crest. Underside of primaries (except innermost) dark gray or blackish, forming large dark patch that is characteristic of this species in all plumages (Ferguson-Lees 1954, Olsen and Larsson 1995). In Basic plumage, crown streaked and mottled with black or grayish brown and white (may appear gray at a distance); only slightly whiter toward forehead, unlike predominantly white forehead of most other terns in Basic plumage. Sometimes pure white forehead develops, but never as extensively as in Royal Tern (Sterna maxima). Sexes similar throughout year and cannot be distinguished externally (Quinn 1990). In Juvenal plumage, blackish crown distinguishes young Caspian from other young terns, which have mostly white crowns. Immature similar to winter-plumaged adult, with white speckled crown in both winter and summer; juvenile also resembles winter adult, but has dark marks across base of neck, back, and scapulars. Easily distinguished from other terns by large size and massive, dagger-shaped bill that is blood red or scarlet in adults, with dark gray mark near tip and often yellow, orange, or white at extreme tip. In first-year birds, bill is more orange-red (Gantlett 1987). Tail is relatively short and only slightly notched. In North America, most similar to Royal Tern, but about 20% larger; Caspian also has much broader, blunter wings and larger head, and its bill is deeper red and thicker than the orange bill of Royal. Additionally, Caspian Tern’s legs are longer and thicker, and its tail is notched for only one-fourth of its length, versus one-half the length of the Royal’s (Gantlett 1987). In flight, Caspian Tern distinguished from other terns by undersurface of outer 5–6 primaries, which are entirely blackish, whereas uppersurface is whitish. Royal Tern has only dark trailing edge on primaries; uppersides of primaries are slightly dusky on adults and extensively dusky on immatures. Call in flight is a hoarse low croak (kaaa), and a shorter kow or kowk when bird is threatened (Bent 1921); juveniles with adults frequently give high-pitched whistled weee .