The adult is mostly black with a yellow bib covering the throat and chest. Sexes are similar in color but the male is larger. Males average 554 mm in length, females average 520 mm (Van Tyne 1929). Keel-billed Toucans are similar in plumage to, but smaller than, "Chestnut-mandibled Toucan" (Ramphastos ambiguus swainsonii).
The species of Ramphastos toucans are similar to one another in their mainly black plumage, large bills, and light colored throats (white or yellow). Of those with yellow throats, only the "Chestnut-mandibled Toucan" (Ramphastos ambiguus swainsonii) overlaps geographically with the Keel-billed Toucan (R. sulfuratus). Both species have similar plumage patterns and coloring to the orbital skin. The Keel-billed Toucan is smaller, however, and has a distinctive colorful bill, which explains the alternative name, Rainbow-billed Toucan.
Adult: R. s. brevicarinatus: Sexes similar in color pattern. The crown and hind neck are black, the latter strongly washed with maroon; upper tail-coverts are white; the rest of the upper surface including wings and tail glossy greenish black. The under tail-coverts are red (black basally) and the rest of the underparts slight1y greenish black. The malar and auricular regions, throat, upper breast, and small spot between the eye and nostril are bright yellow (Van Tyne 1929). The convex lower margin of the yellow breast has a broad curved band, 5-12 mm wide, of bright red (Ridgeway 1914, Van Tyne 1929).
Juvenile: Similar to adults but duller; red in the breast and under tail-coverts is decidedly duller, the black body feathers a dull brownish black, those of the belly tipped with red; less maroon on the nape feathers. The red breast band is more diffused over the black of the lower breast (Van Tyne 1929).
An incomplete formative ("post-juvenile") molt begins within a month after fledging, and involves all of the body plumage except the remiges and rectrices. All feathers changed at first basic molt.
In all subsequent years, there is a single annual basic molt. In Panama, this occurs after nesting (February-May). Variation occurs in the exact time of molting, some still molting during the last of August.
Method and order of molt is regular, beginning at proximal primaries and almost immediately afterwards in the body plumage, first on the head, and then in the various tracts over all the body.
Primaries molt outward in regular sequence, major upper coverts in the same order, but several places ahead of their respective primaries. Major lower coverts lost at about the same time as primaries. Secondaries and terials begin to molt at the same time, when the fifth or sixth primary is lost. Secondaries proceed inward. Tertials molt is completed before secondaries because they are fewer in number.
Tail molt is nearly unique among birds, beginning from the outer and continuing toward the central feathers. This molt begins simultaneously with the molt of the second primary and is usually completed by the time the sixth primary is molted. The upper and lower tail coverts molt at the same time as the tail, in no particular order. The molt of the remiges and rectrices is usually very symmetrical; corresponding feathers on the two sides are lost at exactly the same time (Van Type 1929).
There is no natal down. The juvenile is well covered at fledging (about forty-five days).
The iris is olive yellow to green; the bare orbital skin yellow green about the eye to lemon yellow above and posteriorly.
The bill is light greenish yellow along the culmen becoming yellow green toward the tomia and the basal third of the mandible; terminal fourth of the bill maroon. The wedge-shaped lateral area on the tomial half of the maxilla is orange; remainder of mandible pale cerulean blue. Bill margined basally by a sharply defined black line about two millimeters wide.
Tarsi and toes pale grey-blue, becoming pale green on upper tarsi; claws black (Van Tyne 1929).
Juvenile (up to the molt of the juvenile plumage):
Iris pale gray, becoming lime green; orbital skin yellow (bright green-yellow about nostrils), becoming pale blue below the eye.
Maxilla apple green centrally to olive-yellow on culmen. Anterior part becoming yellow-green centrally to yellow on culmen; lateral wedge-shaped area is light orange, becoming; terminal fourth of bill light red.
Mandible green, becoming light grey-blue on anterior part and green toward the base.
At the base of the bill, in contrast to adult, there is no sharply defined black line but a broader blackish area nearly ten millimeters in width. This becomes more restricted by the end of the juvenal period (Van Tyne 1929). Bill does not have serrations on cutting edge (Wetmore 1968).
Tarsi and toes blue, with many of the large scutella of the tarsus blackened centrally in the younger birds (Van Tyne 1929).
Keel-billed Toucan males on the average are larger than females, especially in bill size, but their measurements overlap (Skutch 1971). Average measurements (mm) are listed below. Sources: a = Short and Horne 2001, b= Ridgway 1914, c = Van Tyne 1929, d = Wetmore 1968.
Wing Length Male Female Tail Length Male Female
sulfuratus (a) 208.4 199.3 166.2 159.4
sulfuratus (b) 208.6 204.3 164.8 158
brevicarinatus (b) 200 193.2 155.2 148.5
brevicarinatus (c) 202.3 196.5 157.1 148.6
brevicarinatus (d) 197.1 189.4 156.5 151
Culmen Male Female Tarsus Male Female
sulfuratus (a) 157.4 134.7 49.6 47.5
sulfuratus (b) 150.3 131.4 49 47
brevicarinatus (b) 136.1 120.4 47.7 46.4
brevicarinatus (c) 142.5 119.9
brevicarinatus (d) 137.3 124.4 49.3 48
Mass (g) Male Female
sulfuratus (a) 433.1 401.4
brevicarinatus (c) 427.1 380.5