Elvira emeralds are small hummingbirds with a relatively short bill, and a short, notched tail with mostly white outer rectrices. The body of Coppery-headed Emerald is mostly green, but the crown, rump and uppertail coverts, and the central pair of rectrices all are coppery bronze. The bill of Coppery-headed Emerald is mostly black, and is slightly decurved. The sexes are similar, but the female is white below, speckled with green.
The small size, mostly white tail, and coppery rump of Coppery-headed Emerald render this species difficult to mistake for other species of hummingbird. There is no geographic overlap with White-tailed Emerald (Elvira chionura), which has a straighter bill and green crown and rump. Eupherusa hummingbirds (Stripe-bellied Hummingbird Eupherusa eximia and Black-bellied Hummingbird Eupherusa nigriventris) are larger, with straight bills, have rufous secondaries, and have green crowns and rumps. Male Snowcap (Microchera albocoronata) is mostly purple, with a white crown; female Snowcap is white below.
Note that the bill of Coppery-headed Emerald is slightly but distinctly decurved. The following description is based on Ridgway (1911); see also Stiles and Skutch (1989) and Stiles (1999):
Adult male: Crown dull coppery bronze. Nape, back, scapulars, and rump bronze green. Uppertail coverts bright coppery bronze. Central pair of rectrices bronze (much less coppery than the uppertail coverts); four outer pairs of rectrices white; the three outer pairs are tipped with pale gray, and the outermost pair is tipped with black. Wing coverts bronze green. Remiges mostly dusky purplish brown, but the inner secondaries are bronze green. Underparts bright yellowish metallic green; undertail coverts pure white.
Adult female: Upperparts metallic green (less bronzy than the adult male). Uppertail coverts and central pair of rectrices bright bronze (more coppery on the uppertail coverts). Four outer pairs of rectrices white, with a V shaped subterminal bar of gray or dusky, the outerpair also tipped with black or gray, and edged with blackish along the terminal half. Remiges as in the male. Underparts dull white or grayish white, becoming pure white posteriorly; the flanks are more or less spotted with metallic green.
Immature male: Similar to adult male, but coloration slightly duller, especially on the underparts, and the four outer pairs of rectrices are tipped with black or grayish black.
Molts of Coppery-headed Emerald are undescribed.
Bill: black, with pinkish base to mandible (Stiles 1999)
Total length: 7.5 mm (Stiles and Skutch 1989)
wing length, male: mean 48.4 mm (range 46-51 mm, n = 10; Ridgway 1911)
wing length, female: mean 44.6 mm (range 41-46 mm, n = 10; Ridgway 1911)
wing length (sexes combined?): mean 46.58 mm (n = 26; Brown and Bowers 1985)
tail length, male: mean 25.4 mm (range 24-27 mm, n = 10; Ridgway 1911)
tail length, female: mean 23.1 mm (range 22.5-25.5 mm, n = 10; Ridgway 1911)
bill length, male: mean 14.1 mm (range 13.5-15 mm, n = 10; Ridgway 1911)
bill length, female: mean 14.6 mm (range 13.5-15.5 mm, n = 10; Ridgway 1911)
bill length (sexes combined?): mean 15.12 mm (n = 26; Brown and Bowers 1985)
Mass: mean 3.00 g (sexes combined?, n = 13; Brown and Bowers 1985); mean 3.3 g (n = 52?; Stiles 1985)