Emeralds (Chlorostilbon) are small hummingbirds. The males are primarily glittering green, with a blue tail that is notched or forked. Female emeralds are very different: they are green above and pale gray below, with dusky auriculars and a short whitish supercilium behind the eye. The tail in the female also is blue, but the outermost rectrices have grayish white tips. Males of all populations of Canivet's Emerald have a bluish black tail that is moderately long and is forked; the female is similar to females of most other members of the genus.
Species of emeralds (Chlorostilbon) are closely similar to one another; in many cases the females, in particular, are difficult to distinguish. Most species of emerald are allopatric, however, so in the field identification usually is not a problem. Canivet's Emerald apparently is allopatric from both Golden-crowned Emerald (Chlorostilbon auriceps), of the Pacific slope of western Mexico, and from Cozumel Emerald (Chlorostilbon forficatus), of Cozumel Island off the Yucatan Peninsula. Males of both species have a longer, more deeply forked tail than male Canivet's. Females of these species would be very difficult to distinguish in the field, although female Golden-crowned and Cozumel emeralds have slightly longer, more forked tails, with more extensive white on the bases of the outer rectrices. The range of Canivet's Emerald also approaches that of Garden Emerald (Chlorostilbon assimilis) in western Costa Rica. Male Garden Emerald has an entirely black bill, a less deeply forked tail, and a duller crown; female Garden Emerald is very similar to female Canivet's Emerald.
The following description is based on Ridgway (1911) and Howell (1993), and refers to nominate canivetii; see also Geographic Variation:
Adult male: Crown bright golden green or golden; rest of upperparts less brilliant golden green. Tail forked (see Measurements). Rectrices blue black or black glossed with blue, the four to six central rectrices tipped with deep brownish gray. Remiges dark brownish slate or dusky, faintly glossed with violaceous. Underparts brilliant golden green. Tibial tufts white.
Adult female: Upperparts bright metallic green or bronze green, usually duller or more bronzy on the crown. Central rectrices metallic green to bluish green; basal two thirds of the second, third and fourth rectrices metallic green or bluish green, with a black subterminal band and white tips. The outermost (fifth) pair with broader white or brownish gray tips and a band of light brownish gray or white between the black subterminal band and the dusky base. Remiges dusky, faintly glossed with purplish. Suborbital and auricular areas dusky, bordered above by a grayish white postocular streak. Underparts light gray, somewhat paler on the belly and vent; flanks glossed with metallic green or bronze green.
There is a complete molt after breeding; in El Salvador, this molt commences in early April, and may extend until the end of May (Dickey and von Rossem 1938).
Iris: brownish black
Bill: male, red with black tip; female, maxilla black, mandible red to pinkish with black tip
Toes: brownish black
Bare parts color data from Dickey and van Rossem (1938) and Howell and Webb (1995).
Total length: male 7.8-9.3 cm (Ridgway 1911), 9 cm (Howell and Webb 1995); female 7.5-8 cm (Howell and Webb 1995), 7.5-8.6 cm (Ridgway 1911);
Linear measurements (from Howell 1993, Howell and Webb 1995; see also Ridgway 1911):
male (n = 34), canivetii
wing length (chord), mean 46.2 mm (range 42.0-48.0 mm)
tail length, mean 34.7 mm (range 30.5-38.0 mm)
tail fork, mean 17.1 mm (range 16.0-20.0 mm)
bill length, mean 14.3 mm (range 13.0-15.0 mm)
female (n = 30), canivetii
wing length (chord), mean 45.0 mm (range 42.5-48.8 mm)
tail length, mean 29.7 mm (range 27-32.0 mm)
tail fork, mean 7.8 mm (range 6.5-9.0 mm)
bill length, mean 15.0 mm (range 13.9-16.0 mm)
Mass: male, mean 2.5 g (range 2.1-2.8 g, n = 7; Payntyer 1955); female, mean 2.3 g (range 2.1-2.6 g, n = 3; Paynter 1955)