Mexican Violetear is a medium-sized hummingbird, with a straight, black bill. The plumage primarily is iridescent green, with a patch of violet on the sides of the head. The rectrices are greenish blue, with a blackish band near the tip. See Similar Species.
No similar species occurs with Mexican Violetear.
The following description of nominate thalassinus is based on Ridgway (1911):
Adult, male: Upperparts generally metallic green or bronze-green, darker or duller on the crown. The tail is metallic bluish green or greenish blue, crossed by a broad subterminal blue-black band. Remiges dusky or dark brownish slate; edges and tips of secondaries more or less glossed with metallic green or bronze-green. Lores and suborbital and auriculars areas dark metallic violet-blue. Malars, chin and throat bright metallic green. Center of chest dark metallic blue or violet-blue, shading into metallic green on sides of chest; duller posteriorly. Undertail coverts metallic green or bronze-green, margined or edged with pale buffy brownish or grayish buff.
Adult, female: Similar to adult male, but slight duller in color. Blue on chest possibly less extensive than in males (Howell and Webb 1995).
Immature: Similar to adult female, but much duller. Uppertail coverts and feathers of rump (sometimes also of crown) narrowly margined with pale grayish buff. Green underparts duller and washed with grayish. Little or no blue on chest.
Little information. Molt (prebasic, also preformative?) reported in late February in El Salvador (Dickey and von Rossem 1938). Molt in Mexico in April (Wagner 1945).
Iris: dark brown
Toes: dusky grayish brown
Data from Ridgway (1911).
Total length: 9.7 cm (Hilty and Brown 1986), 9.9 cm (Hilty 2003), 10.5 cm (Stiles and Skutch 1989), 11-11.5 cm (Howell and Webb 1995)
Linear measurements (mm) of Colibri thalassinus (Wetmore 1968)
| || || wing|| tail|| culmen (from base)|
| male|| mean|| 65.7|| 40.2|| 23.6|
| || range (n)|| 63.4-68.8 (15)|| 38.1-43.1 (15)|| 21.5-25.5 (14)|
| female|| mean|| 60.1|| 36.7|| 22.6|
| || range (n)|| 58.2-63.5 (8)|| 35.5-38.3 (8)|| 20.2-24.3 (8)|
Mass: male, mean 5.3 g (n=2: 5.1, 5.47 g; Panama, Hartman 1954); female, 4.8 g (n=1; Panama, Hartman 1954); unsexed, 5 g (n=1; Peru, Weske 1972)