Appearance

Sexes alike in all plumages. Following descriptions based on Ridgway 1919, Cramp 1985, Wilds 1993, and Olsen and Larsson 1995, but information from these sources not based on studies of birds of known age and remains unconfirmed.

Hatchlings

Upperparts light buffy brown or avellaneous, irregularly spotted with black; underparts slightly paler buffy brown, nearly brownish white; throat usually sooty.

Juvenal Plumage

No information on timing and sequence of Prejuvenal (postnatal) molt.

Juvenal plumage similar to Definitive Basic plumage except feathers of forehead, crown, back, scapulars, tertials, and all wing-coverts edged white to pale gray; forehead and crown washed with variable amount of grayish, yellowish, or cinnamon-brown, and back-feathers, distal halves of scapulars, and tips of tertials, lesser, median, and inner greater wing-coverts, mottled with same color. Secondaries with dark ends and white tips, producing a dark secondary bar along trailing edge of wing, and narrow, inconspicuous, grayish ‘carpal’ bar present on lesser wing-coverts. Brownish wash usually extends from back to sides of lower neck and upper breast, and often to rump. Primaries duskier, less whitish than in fresh Definitive Basic plumage. Tail shorter than in Definitive Basic; rectrices with tips mottled brownish and edged white, except outer-most rectrix (R6) with gray inner web and white outer web. By late summer and early fall, most brownish color lost on head, back, and scapulars through wear and fading, leaving crown and nape whitish (or whitish with some black speckling), and back and scapulars largely gray. Brownish color on wing-feathers also reduced. Tips of rectrices, centers of tertials, and innermost secondaries, and tips and margins of primaries also become darker gray.

Basic I Plumage

Prebasic I (postjuvenal) molt complete. Molt begins in autumn, with back and median upper wing-coverts; much of body plumage and central rectrices (R1) replaced by midwinter; primaries replaced sequentially from P1, beginning Nov–Jan, with 3–6 feathers replaced by May–Jun, and all feathers may be replaced by Jul–Aug, although some Juvenal secondaries and outer primaries may be retained into autumn. Greater wing-coverts often not replaced until late spring. Tail molt completed by May–Jun.

Basic I plumage similar to Definitive Basic plumage except forehead and crown may be mottled and spotted more heavily with black (but black patch through eye always darker than crown or nape) and tail shorter. During active molt, presence of secondary bar and indistinct carpal bar on lesser upper wing-coverts, heavily worn upper greater wing-coverts, and dark primaries and tertials (which become blackish before they are replaced) diagnostic of birds 1 yr old.

Basic Ii Plumage

Although some birds show increased amounts of blackish on crown by summer of their second calendar year (perhaps indicative of a limited Prealternate molt; Wilds 1993), most apparently molt directly from Basic I plumage to Basic II plumage during this period.

According to Wilds (1993: 107), Prebasic II molt occurs from at least May through at least Aug of second calendar year, producing a plumage virtually identical to Definitive Basic, although “some show gray-centered tertials and gray bar on trailing edge of inner wing,” but very few additional details available regarding this molt. According to Cramp (1985), a second wave of primary molt occurs Apr–Sep of the second calendar year, before the first wave (initiated the previous Nov–Jan) has completed. Wilds (1993), however, describes only one wave of primary molt in second-year birds. All primaries appear new Sep–Oct of second calendar year but outer primaries begin to darken by Jan of third calendar year.

Alternate I Or Ii Plumage

Forster’s Terns undergo an incomplete Prealternate molt during the spring of their third calendar year from Mar to Jun which includes head and other body-feathers, and at least the outermost rectrices (R6; streamers). This molt produces a plumage similar to Definitive Alternate, except forehead largely white and outer (worn) primaries blackish.

Definitive Basic Plumage

Definitive Prebasic molt complete; may begin by mid-Jun on head but usually begins mid-Jul to mid-Aug with inner primaries, head, body, and rectrix molt usually beginning by Aug and usually ending Sep or early Oct (although a few black head-feathers may be retained to late Oct). Primaries replaced mostly Jul–Oct. Molt ends Nov–Dec with secondaries and outer rectrices. Primary molt occurs earlier (Apr–Sep) in birds undergoing this molt for the first time (in their third calendar year), producing a distinct contrast between white, new inner primaries and blackish, older outer primaries during the summer months.

Definitive Basic plumage similar to Definitive Alternate plumage but with forehead white, crown white with fine black streaks or dots, nape streaked and mottled off-white, gray, and black; large black patch around eye to ear-coverts dusky to black, dusky bar across nape; primaries also uniformly pale and whitish, and tail shorter.

Definitive Alternate Plumage

Definitive Pre-alternate molt incomplete; including head, underparts, portions of back and scapulars, at least the outermost rectrices (R6), and inner primaries (stopping at P2–P5); molt occurs Jan–Apr but mainly Feb–Mar.

Forehead, upper portion of lore, crown (down to bottom of eye), and nape black, forming a black cap on the head. Remainder of head (including white wedge of 3–6 mm between gape and cap which is broadest at center of lore), sides of neck, and remaining underparts white. Nape-feathers not elongated. Back, scapulars, tertials, and most wing-coverts pale neutral-gray, appearing slightly bluish. Rump white. Tail forked with outermost rectrices (streamers) greatly elongated, extending up to 3 mm beyond end of primaries on perched birds. Upper tail-coverts and rectrices same color as back; rectrices with diffuse white margins, except outermost pair (R6) with tip and outer web white. When fresh, primaries and secondaries silvery white, paler than wing-coverts and upperparts. Secondaries edged on inner web and broadly tipped with white; inner primaries similar but with narrower white edges. Inner webs of outer primaries dull gray with silvery white streak along shaft and pale gray wedge near base. Greater upper primary-coverts light gray becoming silvery white at tips. Leading edge of wing, axillars, and under wing-coverts white.

As season progresses, upperparts gradually fade and become more whitish and some feathers of the wing and tail darken through wear (centers of tertials become more brownish, inner web of R6 and tips and margins of the outermost 5–6 primaries become dark gray to blackish). Primaries often begin to darken by late May or Jun (Cramp 1985, Wilds 1993).

Bill And Gape

Breeding adult described as dull orange with terminal third black or dusky (Ridgway 1919); adult, orange to orange-red at base with black tip; immature and wintering adult, usually black (Sibley 2000); chick, dark pinkish at base, dusky at tip, upper mandible darkening before lower mandible (McNicholl 1971).

Iris

Described as dark brown (Ridgway 1919).

Legs And Feet

Breeding adult, intense orange or orange-red (Ridgway 1919, Cramp 1985); immatures and winter adults, red-brown (Cramp 1985); chick, pink (McNicholl 1971).