The Blue-crowned Motmot is a large, long-tailed bird; the tail also has a distinctive "racquet" tip. The plumage generally is green. One of the Blue-crowned Motmot's most distinctive features is a broad, black line (or "mask") that extends from the base of bill across the sides of the head, terminating in elongated, lanceolate feathers on the auriculars. There also is a cap of dark blue feathers stretching from the forecrown along the sides of the crown to the nape, In most subspecies, this blue band surrounds a small patch of black on central portion of the crown, but in the northernmost subspecies the center of the crown is blue as well.
The Blue-crowned Motmot is the only species of motmot that occurs in eastern Mexico north of southern Veracruz, and so readily is identifiable in this region. From southern Mexico (the Yucatan Peninsula, and the Pacific slope of Chiapas) south to Costa Rica, the distribution of the Blue-crowned Motmot overlaps with that of the similar Turquoise-browed Motmot (Eumomota superciliosa). The Turquoise-browed Motmot is smaller than Blue-crowned, but has longer bare shafts on ohe central rectrices. Additionally, the Turquoise-browed Motmot has blue remiges that contrast with the wing coverts and the body plumage; the nape and the center of the crown are green; the center of the back is rufous, not green; and has a black and turquoise stripe down the center of the throat (lacking on Blue-crowned Motmot).
The following description is of lessonii, and is based on Ridgway (1914); for characters of other subspecies, see Geographic Variation.
Adult: Sexes similar. Lores, orbital region and auriculars, extreme anterior portion of forecrown, upper margin of malars, and center of crown black. Forehead and sides of bright turquoise blue; this blue band ("diadem") continues to the rear crown, becoming more violet, and bordered posteriorly by black. The black of the auriculars (part of the "mask") narrowly is margined above with bright turquoise. Nape, back, scapulars, and rump olive-green. Wings greener, becoming greenish blue on the primaries and primary coverts. Upper surface of tail green or bluish green, shading more bluish distally; racquets blue, broadly tipped with black. Chin, throat, and lower portion of malars dull light bluish green. Breast variably olivaceous-tawny to light olive-green; belly lighter and greener. A tuft of narrow black feathers in center of breast, these feathers narrowly margined with light greenish blue.
Immature: "Similar to adults, but colors much duller, the blue of forehead replaced by grayish brown or dusky (sometimes tinged with verdigris green), black crown-patch larger, under parts of body more rufescent, black tuft in center of foreneck wanting, middle rectrices without terminal spatules, and texture of plumage much looser."
Iris: dull red
Bill: black (base of gonys and lower edge of rami dull brownish white)
Tarsi and toes: tarsi fuscous, toes fuscous-black
Data from Wetmore (1968)
Total length: 38-43 cm (Howell and Webb 1995), 39 cm (Stiles and Skutch 1989)
Linear measurements of Momotus coeruliceps lessonii (Stiles 2009)
| || || wing length (chord)|| tail length|| total culmen|
| male|| mean || 136.79 ± 4.07|| 229.77 ± 8.23|| 44.50 ± 2.47|
| || range (n)|| 130.5-142.3 (24)|| 215.0-245.0 (21)|| 40.6-50.5 (25)|
| female || mean|| 134.23 ± 3.05|| 224.95 ± 8.58|| 43.33 ± 2.32|
| || range (n)|| 128.4-140.0 (20)|| 206.0-233.5 (18)|| 39.6-46.4 (20)|
Mass: mean 115.5 g ± 11.4 (range 90-135 g, n=32; Stiles 2009)