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White Ibis
Eudocimus albus
Order
CICONIIFORMES
– Family
THRESKIORNITHIDAE
Authors: Kushlan, James A., and Keith L. Bildstein
Revisors: Pyle, Peter

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Appearance

Figure 2. Annual cycle of the White Ibis
Juvenile White Ibis, Hatteras, NC, 15 October 2005
Juvenile White Ibis, NC, October
Adult non-breeding White Ibis, Cape Hatteras, NC, December
Adult breeding White Ibis

White Ibis have 10 functional primaries, 9 secondaries (including 3-4 tertials), and 12 rectrices. No substantial geographic variation in plumage aspect or molt strategies has been reported.

Molts

Molt and plumage terminology follows Humphrey and Parkes (1959) as modified by Howell et al. (2003, 2004). White Ibis appears to exhibit a Simple Alternate Strategy (Howell et al. 2003), including complete prebasic molts, a single partial first-cycle molt, and limited prealternate molts in definitive cycles (Fig. 2). The single inserted first-cycle molt may represent a merging of preformative and first prealternate molts formerly present in ancestral species (Pyle 2008); study needed. Individuals can be aged through first part of 2nd cycle, after which definitive aspect assumed.

Prejuvenile (First Prebasic) Molt

complete, May-Jul, in or near nest.

Preformative/First Prealternate Molt

Limited-partial, Oct-Apr, commencing on or near breeding grounds and completing on non-breeding grounds. Includes most to all body feathers, and sometimes 1-2 tertials and 1-4 central rectrices, but no wing coverts or other flight feathers. Begins with greater upper wing coverts and feathers on the back, followed by scapulars, and spreads posteriorly to the tail (Beebe 1914), although considerable overlap occurs. Some head feathers possibly replaced twice, in which case both preformative and first prealternate molts could be defined (Pyle 2008); study needed.

Second Prebasic Molt

Complete, Apr-Oct, variably on or near breeding and/or non-breeding grounds. May start earlier in tropical populations.

Second And Definitive Prealternate Molt

Limited-partial, Feb-Apr, on or near non-breeding grounds. Includes feathers of the head and neck; possibly other body feathers, but no wing coverts or flight feathers.

Definitive Prebasic Molt

Complete, Jul-Sep, on or near breeding grounds, often in dense mangroves. Individuals usually complete 90% of body molt within 60 days; however, initiation of molt can be delayed in breeding individuals. Molt begins on back, spreading to wing scapulars, coverts, remiges, and rectrices. Flight feathers replaced in late summer to early fall: primaries replaced distally (p1-p10), secondaries replaced proximally from s1 and s5 and distally from the tertials, and rectrices likely replaced distally (r1-r6) on each side of tail. Molt appears to complete fairly rapidly and may include brief periods of flightlessness, not affecting feeding or predation risk within dense molting habitats.

Field studies from Heath 2002 showed the following molt details: No significant differences between male and female molt patterns or brood patch development. Of birds captured during courtship display (n = 15), 50% had begun to develop brood patches; by egg production and incubation, 90% showed bare, vascularized brood patches (n = 26). During chick rearing, brood patches were less vascularized and birds tended to groom feathers over the bare area. Some birds captured in the chick-brooding stages showed down growth in the brood patch region (12%, n = 17).

Most White Ibises (85%; n = 27) molted 5 – 85% of their body feathers during the pre-breeding stage. During the breeding season, ibises molted a lower proportion of their body feathers (5 – 20%) than pre-breeding birds, but reproductive birds did show signs of molt in all stages. Of display birds, 33% (n = 18) had at least 5% of feathers in sheaths. In other stages: 16% of birds in egg production (n = 19), 14% of incubating birds (n = 14), and 15% of chick rearing birds (n = 20) were also molting (Heath 2002).

Plumages

See Bent (1926), Palmer (1962), Oberholser (1974), Hancock et al. (1992), and Bildstein (1993) for detailed plumage-aspect descriptions. Sexes show similar aspects in all plumages, with exception of variation to p7 in definitive aspect of some females (see below).

Natal Down primarily black, dark brown and gray, blackest on head and brownest on throat (Bent 1926, Oberholser 1974).

Juvenal Plumage

(Jun-Dec). Aspect with head, neck, and upper breast brownish gray to sandy brown; mantle and uppersides of wings blackish neutral gray (#82); and rump, uppertail coverts, undersides, and underwing coverts white to creamy white. Head and neck streaked Vandyke brown (Smithe 1975–81: color #121) and creamy white, giving an overall impression of glaucous (#80), darkest on head and upper neck (De Santo et al. 1990); streaking becomes narrower on throat and lower neck and is represented by shaft streaks on breast (Beebe 1914). Body feathers with concealed white bases. Primaries, secondaries, and rectrices blackish neutral gray with white bases. By three months, juvenal feathers of mantle, wings, and tail fade to Vandyke brown; then to dilutions of sepia (#119) by eight to ten months, when bird appears dark drab (#119B) or drab-gray (#119D).

Formative/First Alternate Plumage

(Oct-Aug). Early replaced formative feathers of head and neck dark, similar to juvenal feathers; later replaced feathers of back increasingly white. Thus, individuals in second spring and summer retain sandy brown streaking on head and neck but mostly white to white back. Tertials and central 1-4 rectrices sometimes white; otherwise, dark juvenal upperwing coverts and flight feathers retained. If juvenal remiges and rectrices are pulled out before two months old, they are replaced by dark feathers, by lighter feathers from two to four months, and by white feathers after six months (Beebe 1914), indicating how hormonal change affects pigment deposition.

Second Basic Plumage

(Sep-May). Aspect similar to definitive basic plumage (below) but head and neck feathers fringed brownish. Most to all of these replaced by white feathers during 2nd prealternate molt in Feb-Jun; some individuals become indistinguishable from older birds in definitive aspect by Jun.

Definitive Basic Plumage

(Oct-Apr). All feathers white, except for tips of three or four longest primaries, iridescent dark green. Occasional females can have tip of p7 partially or entirely white (Pyle 2008) thus having only three outer primaries (p8-p10) with dark tips. Most to all males appear to have dark tip to p7 and four dark-tipped primaries overall. Extent of black to p7 may correlate with age in females; study needed.

Definitive Alternate Plumages

(Apr-Sep): Aspect similar to that of definitive basic plumage (above) but some to most head and neck feathers apparently replaced, contrastingly glossy when fresh. Head, neck, and back can become soiled buff to brownish in some individuals by summer.

Bare Parts

Bill, Gape, and Facial Skin

Bill pinkish with dusky bands in juvenile, becoming salmon or flesh color (color #5) later in first cycle and in Oct-Mar during definitive cycles. Loral skin blackish in juvenile, becoming pinkish during first cycle. During courtship in Mar-Apr, loral skin and proximal portion of the bill turns geranium (12) distal part turns black in North America: amount variable, usually about two thirds but in some cases the entire bill. Bill returns to salmon color slowly in Jun-Oct; distal portion often remaining gray for several months. Both sexes (not male alone as often reported) also develop a bright red, naked throat pouch, typically larger in females than in males, which expands to 2–3 cm in diameter during pair formation (Kushlan 1973a, Rudegeair 1975a or b). Courtship coloration is enhanced in captive birds maintained on diets including canthaxanthin (S. McDowell and K. Bildstein unpubl. data). Increased vascularization may also play some role (see Frederick 1986).

Iris

Dark in juvenile, becoming sky blue (#66) in all seasons by end of second cycle.

Legs And Feet

Dark in juvenile becoming flesh color by end of second cycle. During adult courtship they turn geranium, along with bill and facial skin.