Distinguishing Characteristics

Largest crested tern; boreal S. m. maxima length 450–500 mm, wingspan 1,250–1,350 mm, mass 350–450 g. Sexes similar. Bill long, orange, with prominent gonys; forked tail. In Definitive Alternate (breeding) plumage, head with full black cap and shaggy occipital crest. Underparts white, upperparts pale gray, rump very pale gray, upper tail-coverts and tail white, trailing edge to wing white, primaries white below with narrow dark trailing edge. Legs and feet black; eye dark brown. Definitive Basic (nonbreeding; winter) plumage similar, but forehead and lores white, crown streaked white anteriorly, and bill paler. Juvenal plumage highly variable; similar to Definitive Basic, but bill smaller and very pale yellow; feet and legs black (uncommon) to green to yellow to pink; underparts as in adults, but upperparts variably spotted and blotched on scapulars and tertials; flight feathers dark, carpal-bar present; tail shorter than adult’s and dusky. Austral S. m. maxima and African S. m. albididorsalis similar but slighter, with longer and thinner bill and less prominent gonys. Most commonly heard call a loud, rolling Keer-reet —lower-pitched than but similar to same call in Sandwich Tern (S. sandvicensis); see Sounds: vocalizations, below.

In Western Hemisphere, only Caspian Tern likely confused with Royal, and normally most immediately separating feature is bill color: crimson with black tip in Caspian, and orange in Royal. However, fewer than 1 in 10,000 North American breeding Royal Terns may have red bill approximating Caspian (Millington and Reid 1995), and in austral S. m. maxima, red bill is perhaps more frequent (R. Escalante pers. comm.), although always lacking Caspian’s dusky tip. Royal Tern also has prominent, shaggy crest, while Caspian has only slight indication of crest. Dusky silver suffusion seen on upper primaries during courtship wears quickly, so that by midincubation, primaries as black as Caspian Tern’s, contra many field guides. However, Caspian Tern’s extensively blackish under-primaries set it apart from Royal, as does its short, shallowly forked tail. Royal Tern’s solidly black crown is also lost early in breeding season (even before arrival at colony), with resulting white forehead, crown, and lores seen for most of year, although solid-black crown occasionally seen in adults in winter.

Outside North America, confusion possible with Great Crested Tern (S. bergii; also known as Swift Tern, Crested Tern, or Greater Crested Tern) especially where sympatric with Royal Tern (S. m. albididorsalis) in w. Africa. Breeding adult Great Crested Tern has white forehead visible at great distance, dorsal color noticeably darker than Royal (especially in Red Sea, where subspecies of Great Crested Tern [S. b. velox]), and its bill is pale greenish yellow and more curved than Royal Tern, with little visible gonydeal angle. In latter feature, Great Crested Tern very similar to S. m. albididorsalis, which, however, is paler still dorsally than boreal S. m. maxima . Some boreal Great Crested Terns approach Royal Tern in having reduced white fore-heads and lighter gray upperparts, but carriage and demeanor remain quite different.

W. African breeders of Royal Tern (S. m. albididorsalis) paler and long-billed than S. m. maxima, in this regard approaching austral-breeding S. m. maxima (Wheeler 1989, Malling Olsen and Larsson 1995). Relationships and species limits among the crested terns generally poorly worked out, and various taxa and populations not yet studied biochemically. Not inconceivable that austral S. m. maxima might some day be described as separate taxon, one possibly more closely allied with S. m. albididorsalis than boreal S. m. maxima .

Males of boreal and austral S. m. maxima and S. m. albididorsalis usually slightly larger than females, but body mass varies greatly, possibly function of fat reserves, recent feeding, health, etc. Juveniles remain slighter, shorter tailed, and more slender-winged than adults for first 1–2 yr.

Detailed discussion of identification of all age classes and currently known/named taxa of North American and Eurafrican crested terns given in Malling Olsen and Larsson 1995 . Many published color photos of boreal S. m. maxima, but we know of none of austral S. m. maxima in English-language publications; for S. m. albididorsalis, only sources Wheeler 1989 and Malling Olsen and Larsson 1995 .