Thorntails (Discosura) are very small hummingbirds with short bills (for a hummingbird) and a white band across the rump; in all these features, thorntails resemble coquettes (Lophornis). The tails of male thorntails, however, have outer rectrices that are narrow and greatly elongated; most species of thorntail also lack the crests or facial plumes of coquettes. Male Green Thorntail is mostly glittering green, with a narrow white rump band and a long, forked blue black tail. The female also is mostly green, but with a blackish throat, white stripe on the side of the face, white flanks and rump, and a short, notched blue black tail.
Green Thorntail is almost unmistakable within its geographic range; few other species of hummingbird in this region have the white rump band, and the long, wispy outer rectrices of the male also are distinctive. Black-crested Coquette (Lophornis helenae), which locally may overlap with the thorntail in Costa Rica, is small with a white rump, but lacks the elongated tail of male thornbill. The male coquette also has a crest and facial plumes, and the female coquette has buffy underparts.
The following description is based on Ridgway (1911) and on Wetmore (1968):
Adult male: Crown dark green, the feathers narrowly tipped with dusky. Back and wing coverts lighter, more metallic green. Rump and uppertail coverts coppery bronze, more or less mixed with bluish black, and with a white band across the rump. Rectrices bluish black with white shafts, becoming brownish gray distally; tail deeply forked, the outer rectrices much narrowed. Remiges and primary coverts dusky, glossed with purple. Throat and foreneck bright metallic green. Breast, belly, and undertail coverts darker green; center of breast more or less metallic greenish blue, with black bases to the feathers. Femoral and tibial feather tufts white.
Adult female: Upperparts like male, but rump darker. Tail notched, and not elongated. Four central pairs of rectrices bluish black, with dark green bases; outer pair of rectrices white basally, with a blue black subterminal band and a white tip. Chin and foreneck dull black, more or less spotted with white. A broad white stripe on the malar region. Breast and belly dull black, with a broad white spot on the flanks; flanks otherwise dull green.
Juvenile: Chin and upper throat grayish white.
Iris: dark brown
Bare parts color data from Ridgway (1911) and Züchner (1999).
male 9.5 cm (Ridgely and Greenfield 2001b), 10 cm (Stiles and Skutch 1999), 10.2 cm (Hilty and Brown 1986)
female 6.6 cm (Hilty and Brown 1986), 7 cm (Ridgely and Greenfield 2001b), 7.5 cm (Stiles and Skutch 1999)
Linear measurements (from Wetmore 1968; see also Ridgway 1911):
male (n = 10)
wing length, mean 41.6 mm (range 40.6-42.6 mm)
tail length, mean 56.7 mm (range 53.9-61.4 mm)
bill length (culmen from base), mean 13.8 mm (range 13.2-14.3 mm)
female (n = 9)
wing length, mean 39.4 mm (range 37.5-40.4 mm)
tail length, mean 22.0 mm (range 19.5-24.0 mm)
bill length (culmen from base), mean 14.1 mm (range 13.1-15.6 mm)
Mass: mean 3.00 g ± 0.40 g (n = 5, sex?; Brown and Bowers 1985)