Species of Eupherusa are medium sized hummingbirds with straight black bills and square-tipped tails; all species also have extensive white in the tail, and a rufous wing patch. The sexes of Eupherusa are dichromatic.
Male Black-bellied Hummingbird is emerald green above, but is black on the sides of the head and on the entire underparts. The secondaries are rufous. The inner rectrices are green, but the outer two pairs of rectrices are black, with extensive white on the inner webs. The underparts, and the sides of the face, of the female are pale gray.
Male Black-bellied Hummingbird has a highly distinctive plumage pattern and is unlikely to be confused with other species. Female Black-bellied Hummingbird is similar to the female of Stripe-tailed Hummingbird (Eupherusa eximia), but female Stripe-tailed is slightly larger, has a somewhat more contrasting rufous patch on the secondaries, and the sides of the breast and the flanks of female Stripe-tailed Hummingbird are more heavily spotted with green.
The following description is based on Wetmore (1968); see also Ridgway (1911):
Adult male: Upperparts, from the crown to the rump, bronze green; feathers of the crown narrowly edged with dull black. Uppertail coverts dull bronze. Central rectrices dull black, glossed with bronze; outer rectrices white. Wing coverts bronze green. Remiges dusky, with a purplish bronze reflection; secondaries basally cinnamon brown, with the tips more or less dusky. Forecrown, sides of the head, breast, and belly black; sides and undertail coverts green. Undertail coverts white.
Adult female: Upperparts metallic bronze green; uppertail coverts bronzier. Wings as in male, but with reduced cinnamon on the secondaries. Underparts grayish white; undertail coverts white.
Little information. Molt typically occurs following breeding, and is intitiated earlier in males (February) than in females (March) (Stiles 1985).
Iris: dark brown
Toes: dusky flesh
Bare parts color data from Ridgway (1911) and Stiles and Skutch (1989).
Total length: 7-8 cm (Wetmore 1968), 8 cm (Stiles and Skutch 1989)
Linear measurements (from Wetmore 1968):
male (n = 11)
wing length: mean 49.5 mm (range 47.4-51.3 mm)
tail length: mean 27.3 mm (range 26.0-28.9 mm)
bill length (culmen from base): mean 15.4 mm (range 14.5-16.5 mm)
female (n = 10)
wing length: mean 46.5 mm (range 45.6-47.6 mm)
tail length: mean 26.3 mm (range 25.3-27.4 mm)
bill length (culmen from base): mean 15.5 mm (range 14.4-16.9 mm)
Mass: 3.5 g (Stiles 1985); 3.4 g (Stiles and Skutch 1989); male, 3.0 g (n = 1; Brown and Bowers 1985)