Appearance

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 Cozumel 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. Canivet's Emerald (Chlorostilbon canivetii) occurs on the mainland adjacent to Cozumel, but is smaller. Male Canivet's Emerald has a much shorter, less deeply forked tail. The tail of female Canivet's also is shorter and less deeply forked compared to female Cozumel, but the differences are less striking in the male; additionally, the outer rectrices of female Canivet's have less white on the outer webs, especially basally.

Cozumel Emerald is most similar to Golden-crowned Emerald (Chlorostilbon auriceps) of western Mexico, which also has a long, deeply forked tail; Golden-crowned Emerald is slightly smaller and has narrower, more pointed rectrices. Further distinctions are that the tips of the inner rectrices of male Golden-crowned Emerald are purer (less brownish) gray, and that the inner rectrices of female Golden-crowned Emerald are more golden (less bluish) green.

The following description is based on Ridgway (1911) and Howell (1993):

Adult male: Crown bright golden green or golden; rest of upperparts bright golden green (but slightly duller than the crown). Tail long and deeply 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 metallic golden green. Tibial tufts white

Adult female: Upperparts bright metallic green or bronze green. Postocular stripe white. Auriculars blackish. Central pair of rectrices metallic green to blue green; the adjacent pair of rectrices similar, but with a blue black or black subterminal band and narrow white tips; the two outermost pairs of rectrices white basally on the outer web, with a broad black subterminal band and white tips. Underparts light gray.

Undescribed?

Iris: dark brown

Bill: male, red with black tip; female, maxilla black, mandible red to pinkish with black tip

Toes: dusky

Bare parts color data from Ridgway (1911) and Howell and Webb (1995).

Total length: male 9-9.5 cm (Howell and Webb 1995), 9.5-10.5 cm (Ridgway 1911); female 8-9 cm (Howell and Webb 1995), 8.3-9.1 cm (Ridgway 1911).

Linear measurements (from Howell 1993, Howell and Webb 1995; see also Ridgway 1911):

male (n = 18)

wing length (chord), mean 48.4 mm (range 47.0-50.5 mm)

tail length, mean 43.1 mm (range 39.0-45.8 mm)

tail fork, mean 26.3 mm (range 23.0-30.0 mm)

bill length, mean 14.8 mm (range 13.9-15.5 mm)

female (n = 11)

wing length (chord), mean 47.1 mm (range 46.0-49.0 mm)

tail length, mean 32.2 mm (range 30.5-35.0 mm)

tail fork, mean 10.3 mm (range 9.5-11.5 mm)

bill length, mean 15.5 mm (range 14.6-16.5 mm)

Mass: male, mean 2.5 g (range 2.4-2.6 g, n = 2; Paynter 1955); female, 2.5 g (n = 1; Paynter 1955)