The genus Calothorax contains two similar species of small hummingbirds. Both species have long black bills that are slightly decurved. The males have glittering gorgets and long, forked tails; females are buff or whitish below. Male Beautiful Hummingbird a long and deeply forked tail, and a glittering rose pink gorget. The upperparts are glittering green, with a white postocular stipe. The breast is whitish, but the flanks are dusky green. Females have a much shorter tail, with only a shallow fork. Female also have green upperparts, with dusky auriculars and a white postocular spot; their underparts are pale buffy cinnamon.
Beautiful Hummingbird is very similar to Lucifer Hummingbird (Calothorax lucifer). The breeding ranges of these two species do not overlap, but after breeding some Lucifers move south and overlap with Beautiful Hummingbird. These two species are "rarely separable" in the field (Howell and Webb 1995). Lucifer Hummingbird has a slightly shorter but more decurved bill. In the male, the tips to the outer rectrices of male Lucifer are more pointed.
The following description is based on Ridgway (1911) and Howell and Webb (1995):
Adult male: Upperparts metallic bronze green. Central two pairs of rectrices metallic bronze green, outer rectrices bronzy black. Remiges dull brownish gray or dusky. White postocular stripe. Chin and throat brilliant metallic magenta purple, becoming more violet and blue posteriorly; the more posterior feathers of the gorget also much elongated. Breast dull white or grayish white, becoming metallic bronze green on the flanks. Tibial tufts and undertail coverts white.
Adult female: Upperparts metallic bronze green, duller on the crown or forecrown. Central pair of rectrices bright bronze green; next pair of rectrices bright bronze green, blackish terminally or subterminally; the three outer pairs of rectrices basally light cinnamon rufous, distally black, and (on the two outer pairs) with broad white tips. Tail double rounded. Remiges dull brownish gray or dusky. Underparts pale dull grayish buffy, deepening on flanks into cinnamon buff. Tibial tufts white.
Immature male: Resembles female, but rectrices narrower, tail slightly longer and more deeply forked, and usually shows a few pink feathers on the throat.
Undescribed in Beautiful Hummingbird?
Iris: dark brown
Bare parts color data from Ridgway (1911).
Total length: 7.7-8.9 cm (Ridgway 1911), 8-9 cm (Howell and Webb 1995)
Linear measurements (from Ridgway 1911):
male (n = 8)
wing length: mean 36.7 mm (range 35-39 mm)
tail length (outer rectrices): mean 29.2 mm (range 28-31.5 mm)
tail length (central rectrices): mean 13.9 mm (range 12-16.5 mm)
bill length (exposed culmen): 17.9 mm (range 17-18.5 mm)
female (n = 3)
wing length: mean 38.5 mm (range 36.5-40.5 mm)
tail length: mean 23 mm (range 22-24 mm)
bill length (exposed culmen): 18.7 mm (range 18.5-19. mm)
Mass: mean 2.93 g (n = 3, sexes combined; Brown and Bowers 1985); male, 2.2 g (n = 1; Colwell 2000); female, 2.8 g (n =1; Colwell 2000)