Emerald-chinned Hummingbird is a small hummingbird with a relatively short, straight bill and a short, broad tail. The upperparts of the male are green, with a prominent white postocular spot. The chin and upper throat are glittering green; the lower throat is blackish. The underparts of the male otherwise are dusky green. The female is green above, and also has a prominent white postocular spot; the underparts are mostly white, with green mottling on the flanks.
Emerald-chinned Hummingbird usually is readily identifiable due to its small size, short bill, and prominent white postocular spot. It may overlap in eastern Honduras with Violet-headed Hummingbird (Klais guimeti), which is similar in size and shape, and also has a bold postocular spot. Male Violet-headed Hummingbird has a violet blue crown and throatl female Violet-headed Hummingbird has a light blue crown.
The following description is based on Ridgway (1911):
Adult male: Upperparts metallic bronze green or greenish bronze. Central pair of rectrices bronze green, remaining rectrices black, faintly glossed with bluish or bronzy; rectrices tipped with brownish gray, the pale tips broadest on the outermost rectrices, and with basal half of outer web dull greenish brown or bronze green. Remiges dusky. Bold white postocular spot. Chin and upper throat brilliant metallic emerald green or yellowish green. Lower throat velvety black to dusky metallic bronze green; remaining underparts deep brownish gray, more or less glossed with metallic bronze green, except on the belly. Undertail coverts metallic bronze green, broadly margined with deep brownish gray.
Adult female: Similar to the adult male, but the outer rectrices more distinctly tipped with pale brownish gray. Underparts pale gray, spotted in sides with metallic green or bronze green.
Immature male: Similar to adult female, but with some emerald green spots on the throat (Howell and Webb 1995).
There is little information on molts of Emerald-chinned Hummingbird. The prebasicmolt of adults in southern Mexico (Chiapas) primarily occurs from mid April to mid June (Wagner 1957).
Iris: dark brown
Bill: dull black
Bare parts color descriptions from Ridgway (1911).
Total length: 7-7.5 cm (Howell and Webb 1995), 7.1-8.6 cm (Ridgway 1911)
Linear measurements (from Ridgway 1911):
male (n = 16)
wing length: mean 47.8 mm (range 44-50.5 mm)
tail length: mean 30.3 mm (range 27.5-33 mm)
bill length (culmen): mean 10.6 mm (range 10-11.5 mm)
female (n = 3)
wing length: mean 43.8 mm (range 41.5-46 mm)
tail length: mean 26.8 mm (range 26.5-27 mm)
bill length (culmen): 11.5 mm (range 11-12 mm)
Mass: male, 3.0 g (n = 1, immature; Paytner 1957); female, 3.5 g (n = 1; Paynter 1957); 2.7 g (n = 3, sex unreported; Brown and Bowers 1985)