Cynanthus are medium sized hummingbirds with relatively straight, black-tipped bright red bills, and tails are moderately long and slightly notched. Dusky Hummingbird is a dull colored Cynanthus. The upperparts are dull green to golden green, with a distinct white or whitish postocular stripe and blackish auriculars. The underparts are dusky gray or gray brown. The sexes are similar, but the female has a duskier bill (the maxilla is blackish), paler underparts, and the outer rectrices of the female have whitish tips and a blackish subterminal band.
Dusky Hummingbird is very similar to female Broad-billed Hummingbird (Cynanthus latirostris); the distributions of these two species are largely allopatric, but they overlap locally. Female Broad-billed Hummingbird is brighter, more emerald, green above, with a more bluish black (not dull blackish) subterminal band on the outer rectrices, and whiter, more contrasting, tips to the outer rectrices. "Some individuals, however, may not be safely identified, even in the hand" (Howell and Webb 1995: 404).
The following description is based on Ridgway (1911), Binford (1985), and Howell and Webb (1995):
Adult male: Upperparts dull bronze green or greenish bronze, usually much duller (dull brownish gray or grayish brown) on forecrown, crown, and uppertail coverts. Rectrices dull greenish bronze or grayish brown glossed with greenish bronze, dusky basally. Remiges dusky, the outermost primary narrowly edged with pale gray or grayish white. Pale gray or grayish white postocular streak; auriculars dusky. Underparts deep sooty gray, deepest on the throat; throat sometimes flecked with bluish green or greenish blue. Undertail coverts mostly buffy. Leg tufts white.
Adult female: Similar to the adult male, but the four pairs of lateral rectrices are bronzy green with a broad subterminal dull blue blackish band and pale brownish gray tips (these tips broader on the outermost pair of rectrices). Underparts paler.
Immature: Similar to adult (of respective sex), but outer rectrices narrowly tipped with pale buff (Howell and Webb 1995).
Iris: dark brown
Bill: bright red with black tip (male) or maxilla blackish, mandible reddish with dark tip (female)
Bare parts color data from Ridgway (1911) and Howell and Webb (1995).
Total length: 9-10 cm (Howell and Webb 1995), 9.8-10.6 cm (Ridgway 1911)
wing length, mean 55.9 mm ± 2.0 mm (range 52.9-58.6 mm, n = 20; Binford 1985)
tail length, mean 34.4 ± 2.3 mm (range 31.5-37.6 mm, n = 18; Binford 1985)
culmen length, mean 21.5 mm ± 1.4 mm (range 19.5-24.3 mm, n = 21; Binford 1985)
female (n = 2; Ridgway 1911)
wing length, 55 mm
tail length, mean 31.7 mm (range 31.5-32 mm)
bill length, 29 mm
Mass: male, 4.3 g (n = 2; 4.3 g, 4.3 g, Binford 1985); male 4.2 g (n = 1; Colwell 2000), female 3.4 g (n =1; Colwell 2000).