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 Stripe-tailed Hummingbird is primarily emerald green. 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.
From southern Mexico south to Honduras, Stripe-tailed Hummingbird can be confused with Berylline Hummingbird (Amazilia beryllina), which also has a rufous wing patch, but Berylline Hummingbird has an entirely rufous tail, has a tawny (male) or white (female) belly, and the bill is partially red. In the northern part of its range, Stripe-tailed Hummingbird does not overlap geographically with any other species of Eupherusa. In Costa Rica and Panama, could be confused with Black-bellied Hummingbird (Eupherusa nigriventris). Male Black-bellied Hummingbird easily is distinguished by its black crown, sides of the head, and underparts. Female Black-bellied Hummingbird is more similar to female Stripe-tailed Hummingbird, but female Black-bellied is smaller and its rufous wing patch is less conspicuous.
The following description is based on Ridgway (1911), and refers to nominate eximia; see also Geographic Variation:
Adult male: Upperparts bright metallic grass green, becoming more bronzy posteriorly, especially on the uppertail coverts. The three central pairs of rectrices are dark bronze green; the two outermost pairs of rectrices have black outer webs, but the inner webs are pure white, broadly tipped with black. Primaries dusky, the innermost primaries basally cinnamon rufous; secondaries cinnamon rufous, tipped with dusky. Underparts bright metallic grass green; undertail coverts pure white.
Adult female: Upperparts similar to male, but secondaries not tipped with dusky. Central rectrices terminally black or dusky. Underparts and sides of face light brownish gray, the flanks spotted with metallic green.
Iris: dark brown
Bill: black, base sometimes dull vineaceous
Toes: dull dark brown
Bare parts color data from Ridgway (1911) and Wetmore (1968).
Total length: 8.0-9.3 cm (Ridgway 1911), 9.5 cm (Stiles and Skutch 1989), 9.5-10 cm (Howell and Webb 1995)
Linear measurements (from Ridgway 1911):
eximia, male (n = 11)
wing length, mean 58.7 mm (range 56.5-61 mm)
tail length, mean 33.5 mm (range 32.5-34.5 mm)
bill length, mean 17.6 mm (range 16-19 mm)
eximia, female (n = 8)
wing length, mean 53.1 mm (range 50.5-54.5 mm)
tail length, mean 29.9 mm (range 27-31 mm)
bill length, mean 17.6 mm (range 16-19.5 mm)
egregia, male (n = 18)
wing length, mean 60.1 mm (range 58-62 mm)
tail length, mean 34.9 mm (range 33-37 mm)
bill length, mean 18.6 mm (range 17-19.5 mm)
eximia, female (n = 18)
wing length, mean 54.9 mm (range 53.5-56 mm)
tail length, mean 31.9 mm (range 30-34 mm)
bill length, mean 18.9 mm (range 17.5-20 mm)
Mass: mean 4.27 g ± 0.3 g (n = 126, sexes combined?; Brown and Bowers 1985)