Violet-bellied Hummingbird resembles a small woodnymph (Thalurania) with a rounded (not notched) tail. The male is largely iridescent green, with a contrasting bright violet breast and belly, while the female is green above but has pale gray underparts.
Violet-bellied Hummingbird is superficially similar to Crowned Woodnymph (Thalurania colombica), but is smaller than the woodnymph with a reddish or pinkish mandible. Male Violet-bellied Hummingbird further differs from the male woodnymph by its rounded (not notched) tail, and its green (not violet) upper back. Populations of Crowned Woodnymph in northern Colombia (subspecies colombica) also have a glittering violet forecrown. Male Sapphire-throated Hummingbird (Lepidopyga coeruleogularis) has similar colors to male Violet-bellied Hummingbird, but has a bluish throat and a green belly (not a green throat and a violet belly), and a notched tail.
The following description is based on Wetmore (1968); see also Geographic Variation:
Adult male: Upperparts metallic green, crown more glittering green in some subspecies; rump more or less bronze green. Rectrices blue black. Wings dusky, with a purple sheen. Throat and lower sides of the head brilliant metallic green; some individuals have a bluish cast to the gorget. Breast and flanks metallic violet blue. Tibial tufts white. Belly and undertail coverts dull black, with a blue or dull green sheen; sides of the belly adjacent to the tibial tufts partly white.
Adult female: Upperparts metallic bronze green, bronzier on the rump and uppertail coverts. Rectrices blue black to dull bluish green; two outer rectrices tipped with pale gray. Wings dusky with a purple sheen. Underparts pale gray, whiter on the breast; on some individuals the sides of the throat are spotted with metallic green, especially in subspecies panamensis. Undertail coverts dull brownish gray.
Immature male: Similar to female, or with foreneck spotted with shining bluish green.
Bill: maxilla black; mandible pinkish or yellow brown with a black tip
Bare parts color data from Wiedenfeld et al. (1985) and from specimens in the Louisiana State University Museum of Natural Science.
Total length: male, 7.5 cm (Ridgely and Greenfield 2001b), 8.7-9.2 cm (Wetmore 1968); female 7 cm (Ridgely and Greenfield 2001b), 8.0-8.4 cm (Wetmore 1968)
Linear measurements (from Wetmore 1968):
male (panamensis, n = 10)
wing length: mean 43.8 mm (range 42.8-45.0 mm)
tail length: mean 30.3 mm (range 28.6-31.4 mm)
bill length (culmen from base): mean 15.0 (range 14.2-15.7 mm)
female (panamensis, n = 10)
wing length: mean 42.9 mm (range 41.2-43.8 mm)
tail length: mean 26.5 mm (range 25.2-27.5 mm)
bill length (culmen from base): mean 16.5 (range 15.6-17.3 mm)
Mass: male, mean 3.35 ± 0.08 g (n = 11, panamensis; Hartman 1954); female, mean 3.03 ± 0.08 g (n = 5, panamensis; Hartman 1954)
male, mean 3.4 g (n = 4, feliciana; Wiedenfeld et al. 1985); female, mean 3.0 g (n = 3, feliciana; Wiedenfeld 1985)