White-throated Mountain-gem has the typical shape and posture of all members of the genus Lampornis: it is a medium-sized hummingbird with a straight black bill, dark auriculars, and a long white postocular stripe. This species is sexual dimorphic. The male is green above with a white throat, green breast and a dark gray belly. On contrast to the male, the female has completely rufous or cinnamon rufous underparts.
White-throated Mountain-gem, especially the female, is very similar to Purple-throated Mountain-gem (Lampornis calolaemus), and these two formerly were considered to be conspecific. Separation of these two in the field is necessary because they could occur in the same places, specifically, in the highlands in the northern part of the Talamancas in Costa Rica, and in western Chiriquí highlands in Panama (Stiles and Skutch 1989, Angehr and Dean 2010, Garrigues and Dean 2014). The males are easy to distinguish by the color of the throat, but tail color is also helpful in Costa Rica, where White-throated (subspecies cinereicauda) has a gray tail (see Geographic Variation). The identification of females is more complicated, but the color and pattern of the tail are quite useful to separate both subspecies. In the Talamanca area, female White-throated have a gray tail and the outer two or three rectrices are broadly tipped pale gray, while female Purple-throated have a dark, duller metallic green or bronze-green tail, with a blackish subterminal band in the four outer rectrices, and the outer three pairs broadly tipped pale gray. Furthermore,there is some altitudinal segregation, with White-throated usually occurring at higher elevations than Purple-throated in areas where they are sympatric (Stiles and Skutch 1989). In western Chiriquí, in Panama, female White-throated (subspecies castaneoventris ) is somewhat paler than female Purple-throated. Also, in female Purple-throated the upperparts are more bluish green and the central rectrices are darker bronze green; the tail looks paler in White-throated compared to Purple-throated, as the white tipping on the outer rectrices is more extensive and the bluish black subterminal band is narrower in White-throated.
The following description is based primarily on Wetmore (1968), and refers to the nominate subspecies; see also Geographic Variation. Personal observations of birds in the hand are included for all plumages.
Adult male: Crown emerald-green; other upperparts, including wing coverts and uppertail coverts, dark bronzy green. Rectrices black to bluish black; Remiges dusky but in some light conditions might look purplish. Lores black. Feathers in the auricular region are blackish with dark green tips. White line extending back from eye, above auricular region. Chin and throat white. Sides of neck and upper breast bright green; lower breast and abdomen dark gray. Undertail coverts dark gray, feathers with paler with outer edges. Tibial tufts white. Axillars and underwing coverts pale bronze green.
Adult female: All upperparts bright green, but depending on light, some areas can appear bronzy green. Central rectrices dark metallic to bronze green, adjacent rectrices slightly paler. Remiges dusky. Lores black. Feathers in the auricular region are blackish. White line extending back from eye, above auricular region. Underparts from throat to belly dark rufous. Undertail coverts grayish, feathers bordered with white or buff. Tibial tufts white. Axillars and underwing coverts pale bronzy green.
Juvenile plumage has not been described yet for any sex.
No information available.
Bare parts are described from specimens at the Museo de Zoología of the Universidad de Costa Rica.
Tarsus and toes: dark brownish (soles of feet light flesh).
Total length: is 122 mm (Gould 1860).
Mass: Hartman and Brownell (1961) provides data for subspecies castaneoventris: five individuals (sex unknown), mean 5.66 ± 0.34 g. Strauch (1977) provides data for subspecies castaneoventris as follows: one male, 6.3 g; three females, mean 5.0 g (± 0.14). Juárez-Jovel (2016 unpublished data) provides mass data for subspecies cinereicauda as follow: three males, mean 5.9 g (± 0.03). All measurements are mean ± standard error.
Wetmore (1968) provides measurements (mm) for subspecies castaneoventris:
Male (n = 10 from western Chiriquí, Panama): wing length, mean 65.8 (range 64.2–67.0); tail length, mean 37.0 (range 36.0–38.0); bill length (culmen from base), mean 21.3 (20.2–22.0).
Female (n = 10 from western Chiriquí, Panama): wing length, mean 58.5 (range: 56.8–60.6); tail length, mean 32.9 (range 31.6–33.9); bill length (culmen from base), 22.3 (range 21.1–23.8).
Wetmore (1968) provides measurements for subspecies cinereicauda:
Male (n = 10 from Dota region, Costa Rica): wing length, mean 63.9 (range 62.9–64.6); tail length, mean 37.2 (range 36.0–38.5); bill length (culmen from base), mean 21.6 (range 20.6–22.5).
Female (n = 10 from Dota region, Costa Rica): wing length, mean 58.1 (range: 56.7–59.6); tail length, mean 32.5 (range 31.4–33.8); bill length (culmen from base), mean 22.5 (21.4–24.0) mm.