Hylocharis are medium sized hummingbirds with straight bills that are bright red, with a black tip, and that are broad at the base. The sexes are similar in pattern, but females are duller in color. The crown and upperparts of male Blue-throated Goldentail are green, becoming coppery or golden on the uppertail coverts. The tail is glittering greenish gold. The throat is glittering bluish violet to violet; the belly is dirty buff, and the sides of the breast and the flanks are mottled with green. The bill of the female is similar to that of the male, but duller, and the maxilla may be blackish. The throat of the female is violet blue, mixed with grayish buff, and the belly is paler than in the male.
Blue-throated Goldentail is a distinctive species, and is not likely to be confused with other species. It is somewhat similar to Rufous-tailed Hummingbird (Amazilia tzacatl), but Rufous-tailed is larger, has a longer, rufous tail, green (not blue) throat and breast, and the red of the bill is not as deep. Bleu-throated Goldentail might contact Blue-headed Sapphire (Hylocharis grayi) in extreme eastern Panama and northwestern Colombia, although sympatry between these two species is not documented. Blue-headed Sapphire differs by its blue crown and sides of the head, green throat, and blue tail
The following description is based on Ridgway (1911), and refers to nominate eliciae; see also Geographic Variation:
Adult male: Crown, nape, back, scapulars, and wing coverts metallic green, becoming golden or bronzy on the rump and bright golden or coppery on the uppertail coverts. Rectrices brilliant golden bronze, tinged with golden green in some lights. Remiges dark brownish slate or dusky. Chin and extreme upper throat dull whitish, feathers tipped with metallic violet blue; rest of throat, and the upper breast, bright metallic violet blue. Lower breast and flanks metallic bronze green. Belly buffy grayish, slightly more buffy on lower buffy. Femoral tufts white. Undertail coverts grayish brown, broadly edged and tipped with cinnamon.
Adult female: Similar to the male, but underparts paler. The violet blue of the throat usually is much more restricted and is broken with dull whitish margins to the feathers.
Immature: Resembles the adult female, but the throat is grayish buff, sparsely spotted with bluish violet (Howell and Webb 1995). Dickey and van Rossem (1938: 270) suggest that iridescent feathers on the throat are present even in the juvenile plumage, and that "this condiiton must be comparatively rare in this family".
Little information. Dickey and van Rossem (1938) reported specimens in "postjuvenal" (preformative?) molt in February.
Iris: dark brown
Bill: male, coral pink with a black tip (the distal one fourth to one third); female, similar, but the maxilla is pink only basally (from the nares to the base)
Toes: dark brown
Bare parts color data from Dickey and van Rossem (1938).
Total length: 8-9 cm (Howell and Webb 1995), 9 cm (Stiles and Skutch 1989)
Linear measurements (from Wetmore 1968):
male (n = 17, earina)
wing length: mean 49.7 mm (range 48.2-51.8 mm)
tail length: mean 26.7 mm (range 25.6-28.1 mm)
bill length (culmen from base): mean 18.2 mm (range 17.0-19.6 mm)
female (n = 6, earina)
wing length: mean 47.7 mm (range 46.4-49.8 mm)
tail length: mean 25.9 mm (range 25.2-27.2 mm)
bill length (culmen from base): mean 19.4 mm (range 18.6-20.0 mm, n = 5)
Mass: 3.6 g (n = 13, sexes combined?; Brown and Bowers 1985)