Sparkling-tailed Hummingbird is a small hummingbird with a long, straight bill and, in the male, a very long, deeply forked tail; both sexes also have a large white patch on each side of the rump . The upperparts of the male are green, with a small white spot behind the eye. The tail of the male is mostly black, but the three outer rectrices are tipped with white. The male has a glittering violet blue gorget, bordered below with a white band across the breast; the lower underparts are dusky green. The tail of the female is much different: it is much shorter, and is only slightly notched. The underparts of the female are cinnamon.
The small size and prominent white rump patches should make Sparkling-tailed Hummingbird unmistakable within its range. Mexican Sheartail (Doricha eliza) and Beautiful Hummingbird (Calothorax pulcher) also have longer and more curved bills.
The following description is based on Ridgway (1911) and on Howell and Webb (1995):
Adult male: Upperparts metallic bronze or bronze green, with a large white patch on each side of the rump. Central two pairs of rectrices metallic bronze green; outer three pairs of rectrices purplish black, broadly tipped with white; the two outer pairs of rectrices also are crossed by two adjacent bands, basally chestnut or chestnut rufous and distally white, at about one third the distance from the tip. Remiges dull brownish slate or dusky. Gorget on chin and throat metallic violet blue. Breast grayish white. Belly and flanks dark metallic bronze green, some feathers narrowly tipped with pale grayish. Leg tufts white.
Adult female: Upperparts metallic bronze or bronze green, duller on the crown, and with a large patch of pale cinnamon buff or buffy white on each side the rump. Central pair of rectrices metallic bronze green, with black tips or a black subterminal band; remaining rectrices bronze green basally, with a broad black subterminal band and tipped with pale cinnamon rufous or white. Remiges dull brownish slate or dusky. Underparts buffy cinnamon or cinnamon rufous, deepest on flanks and on the lower belly.
Immature male: Some resemble the adult female, but with green mottling on the underparts, and the outer rectrices tipped with white. Other immature males (in First Basic plumate?) resemble the adult female, but with a longer tail, with a white median band, and with blue mottling on the throat and green mottling on the belly.
Very little information. In southern Mexico, molt primarily is June-July in Guerrero, and February-March in Chiapas (Wagner 1957).
Iris: dark brown
Bare parts color data from Ridgway (1911).
Total length: male 82-10.1 cm (Ridgway 1911), 9-10 cm (Howell and Webb 1995); female 5.8-6.7 cm (Ridgway 1911), 6.5-7.5 cm (Howell and Webb 1995)
Linear measurements (from Ridgway 1911):
male (n = 10)
wing length, mean 34 mm (range 33-35 mm)
tail length (outer rectrices), mean 43 mm (range 38-47 mm)
tail length (central rectrices), mean 10.5 mm (range 9-12 mm)
bill length (exposed culmen), mean 12.9 mm (range 12.5-13.5 mm)
female (n = 8)
wing length, mean 34.7 mm (range 33.5-36 mm)
tail length (outer rectrices), range 16.5-18 mm
tail length (central rectrices), mean 14.5 mm (range 13.5-16 mm)
bill length (exposed culmen), mean 13.6 mm (range 12-14.5 mm)
Mass: mean 2.23 g ± 0.15 g (n = 12, sexes combined?; Brown and Bowers 1985)