One of the Whooping Motmot's most distinctive features is the cap of dark blue feathers stretching from the base of the beak to the back of the head, with a small patch of black on the top part of the crown. The blue component of the cap does not extend below the eyes, which are masked with a triangle shaped wedge of black feathers. The rest of the body is green on the back and wings, lighter green to tawny on the underparts. The throat is bluish-green. The Whooping Motmot also has a long tail with a "racquet" tip.
The Whooping Motmot is the only member of the "Blue-crowned Motmot" complex that occurs in the lowlands of Panama and of northwestern South America, and so readily is identifiable within its geographic range. The Andean Motmot (M. aequatorialis) occurs in the Andes of Colombia, at higher elevations than Whooping Motmot, but the two species are generally similar. The Andean Motmot is larger than Whooping, and usually is greener on the underparts, with little if any of the tawny tones that often are present on the underparts of Whooping. Andean Motmot also has a black border to the rear portion of the "diadem" (the blue ring around the lower portion of the crown); Whooping has not black border on the rear of the diadem. The vocalizations of these two species also are different; the song of Andean Motmot is a double hoot, whereas the song of the Whooping Motmot is a single note.
The following description is based on Wetmore (1968):
Adult: Sexes similar. Center of crown black. Crown bordered below by a broad blue band ("diadem"): anterior portion of diadem light greenish blue, becoming violet across the nape. Forecrown, lores and band through eye and auriculars black, terminating in a tuft on the sides of the face; this "mask" narrowly margined with turquoise blue along the lower border of the mask below the eye, and above the mask behind the eye. Back and wings olive-green; outer webs of primaries bluish. Upper surface of tail green to bluish green, shading to blue distally. Racquet tips blue, tipped with black. Underparts more or less dull brown. Feathers in center of breast black, bordered with greenish blue.
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Iris: dull reddish brown
Tarsi and toes: dull brown
Data from Wetmore (1968)
Linear measurements (mm) of Momotus subrufescens subrufescens (Stiles 2009)
| || || wing length (chord)|| tail length|| total culmen|
| male|| mean|| 125.89 ± 3.06|| 226.20 ± 9.75|| 42.05 ± 2.02|
| || range (n)|| 118.5-131.6 (33)|| 202.5-247.5 (30)|| 39.2-45.4 (35)|
| female|| mean|| 123.86 ± 3.86|| 224.62 ± 7.86|| 41.18 ± 2.17|
| || range (n)|| 115.8-131.1 (33)|| 209.5-239.0 (29)|| 35.5-45.4 (32)|
Mass: mean 96.0 ± 12.6 g (range 75-124 g, n=27; Stiles 2009)