Appearance

The molts of plumages of frigatebirds as a group are only partially understood and more is yet to be learned about plumage development in this difficult-to-study family of seabirds. Definitive (adult) plumage apparently develops slowly over many years and is preceded by a distinctive Juvenal plumage and several intermediate subadult plumages. Proposed sequences of plumage development for this species have been most recently described by Nelson (1975), Harrison (1983), and Howell (1994) but studies of marked individuals of known age have yet to verify any of the plumage sequences proposed. Nevertheless, Howell’s (1994) description represents our best current understanding of the sequence of plumage development in this species and is followed below.

Hatchlings

Naked.

Juvenal Plumage

Head, neck and most of underparts white. Remaining upperparts, including upper wing and tail, breast band (often incomplete), flanks, and under tail-coverts blackish brown. Incomplete breast band often appears as a sharp black triangle on each side of the breast, outlining—with the dark flank feathers—a diamond-shaped white belly. Apex of blackish triangles sometimes join at center of breast. Broad light edging to upper wing-coverts extends diagonally inwards towards body from bend on leading edge of wing forming obvious pale tan to dirty white wing-bar, or “alar” bar more evident in females but becoming less so with age. A few may also show a “white spur” extending from the belly onto the axillaries at the base of the underwing (Howell 1994). Sexes alike.

Subadult Plumages

Prebasic molts not described. According to Howell (1994), transition from Juvenal to Basic I plumage coincides with first replacement of remiges. Following Howell (1994), the terms Juvenal, Basic I, Basic II, and Basic III plumages are used here to describe distinctive plumage aspects observed in this species. Their usage here, therefore, does not necessarily correspond to the way they were originally defined to represent the plumage states brought about by separate molts. Dark markings appear on head and neck; well-flying young, still attending colonies and fed occasionally by females in this plumage, suggest it is acquired during first year after fledging (AWD). Main difference from Juvenal plumage is loss of black triangular wedges bordering breast, and dark markings on head and neck (to very varying degrees); Howell (1994) suggests only males show dark feathers on head and neck in this plumage, but this requires confirmation.

Basic Ii Plumage

Males have dusky hood or collar, belly mostly or completely white, and gray feet. Females show dark flecks outlining lower edge of throat patch, and bluish orbital rings. In both sexes, axillary feathers have pale tips giving scalloped appearance.

Basic Iii Plumage

Pale areas on male continue to darken, though much of belly still flecked with white; pale alar bar, present in earlier plumages, no longer evident. Female has head and belly blackish brown or mostly so; last areas to darken are center and sides of throat.

Definitive Basic Plumage

Age at which this plumage attained unclear. Molt poorly known; primary molt proceeds outward, up to 3 series of molting feathers active simultaneously (Cramp and Simmons 1977). Recent field studies confirm conclusion by Stresemann and Stresemann (1966) that adults do not molt primaries while breeding. Important to determine timing and duration of molt in view of its importance in determining length of breeding cycle. Juvenile P10 may be retained for as much as 2 yr (Cramp and Simmons 1977).

Male. Entirely black, with metallic purple sheen especially on back and scapulars, green gloss to crown, nape and wings; lanceolate feathers of crown, nape, back and scapulars. Inner secondaries brownish forming a pale bar or wedge on trailing edge of inner wing (Howell 1994). Wings long, narrow, pointed; tail long and narrow, held either in forked position or closed with overlapping outer rectrices giving appearance of slightly swollen tip to tail.

Female. Blackish brown on head and upperparts, with slight purple and green gloss; lesser wing-coverts, along with inner median and greater wing-coverts, grayish brown, pale tan to dirty white forming distinct pale ‘alar’ bar, extending diagonally inwards towards body from bend on leading edge of wing. Inner secondaries as in male. Sides of neck, whole breast, and sides of upper belly, white, extending as pale scalloping into axillary region. Hind neck paler, forming light collar. Narrow wedge of dark color extends from throat into center of upper breast. Black area on belly protrudes forward into white breast patch, forming a somewhat narrow and tapered forward protrusion and creating a distinctive bilobed (U) shape to white breast. Wing and tail shape as in adult male.

Subadult Stages

Iris dark brown; orbital ring pale gray-blue; bill light gray-blue tipped pinkish flesh to pale horn; legs and feet pale bluish to blue-gray.

Adults

Male. Bill gray or dark horn, sometimes with dark tip; iris dark brown; orbital ring black; feet gray to black. Legs very short, webbing vestigial between all 4 toes. Gular sac (area of bare skin on throat that is inflated like a large balloon during courtship and early incubation period) brilliant red during courtship, fades to pale orange and shrinks rapidly after laying.

Female. Bill pale blue-gray; iris dark brown; orbital ring blue; feet pink or red (not known at what age female foot changes to pink; Howell 1994).