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 short whitish supercilium behind the eye. The tail in the female also is blue, but the outermost rectrices have grayish white tips. Male Golden-crowned Emerald has a notably long, deeply forked tail; the female is similar to 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. The long deeply forked tail of male Golden-crowned Emerald readily distinguishes this species from most others in the genus. Male Cozumel Emerald (Chlorostilbon forficatus) also has a long, deeply forked tail, but is restricted to Cozumel Island; Cozumel also "differs in its distinctly larger size (e.g. no overlap in wing chord) [and] broader and more rounded rectrices" (Howell 1993). Female Golden-crowned Emerald is similar to most other species of emerald, but does not overlap geographically with other Chlorostilbon.
The following description is based on Ridgway (1911):
Adult male: Crown brilliant golden green; rest of upperparts, including wing coverts, duller golden green or bronze green. Tail deeply forked (see Measurements). Rectrices black, glossed with violet bluish, the six central rectrices broadly tipped with brownish gray, the fourth rectrix (from the middle) also with the terminal quarter of the inner web broadly brownish gray. Remiges brownish slate or dusky, faintly glossed with violaceous. Underparts brilliant golden green, more strongly golden posteriorly, the chin and throat purer (less yellowish) green.
Adult female: Upperparts bright green or bronze green. Central rectrices bluish green to bronze green; basal two thirds of the second, third and fourth rectrices bluish green, with a black subterminal band and brownish gray tips, the outermost (fifth) pair with broader brownish gray tips and a large area of light brownish gray between the black subterminal band and the dusky (glossed with greenish blue) base. Remiges dusky, faintly glossed with purplish. Suborbital and auricular areas dusky, bordered above by a grayish white postocular streak. Underparts light gray, paler on the belly and vent.
Little information. Molt in adults extends from February to July (Wagner 1957).
Bill: male, red with black tip; female, maxilla black, mandible red to pinkish with black tip
Bare parts color data from Howell and Webb (1995).
Total length: male 8.8-9.35 cm (Ridgway 1911), 9.5 cm (Howell and Webb 1995); 7.5-8 cm (Howell and Webb 1995), female 8.3-8.5 cm (Ridgway 1911)
Linear measurements (from Howell 1993, Howell and Webb 1995; see also Ridgway 1911):
male (n = 34)
wing length (chord), mean 43.7 mm (range 41.9-45.8 mm)
tail length, mean 43.1 mm (range 40.6-46.5 mm)
tail fork, mean 28.7 mm (range 24.6-32.5 mm)
bill length, mean 13.0 mm (range 11.7-13.7 mm)
female (n = 23)
wing length (chord), mean 43.4 mm (range 41.9-44.5 mm)
tail length, mean 33.1 mm (range 29.5-35.6 mm)
tail fork, mean 14.9 mm (range 12.5-16.1 mm)
bill length, mean 13.9 mm (range 12.9-15.2 mm)
Mass: mean, 2.2 g (range 2.2-2.2 g, n = 2; Dunning 2008)