The male long-tailed sylphs are long including the outer tail feathers, and weigh . Females are long and weigh . All subspecies have a short black bill. Males of the nominate subspecies
A. k. kingii have a shining emerald green crown, metallic bronzy green upperparts, duller green underparts with an olive cast, and a blue or violet throat. The inner tail feathers are very short and the outer ones very long. (Despite the species' English name, its tail is not noticeably longer than those of the other sylphs.) The tail's upper surface is iridescent blue, green, and violet and the underside bluish black. Nominate females' upperparts are similar to the males'. Their tail is short and somewhat forked, with dark blue outer feathers broadly tipped with white. The head is metallic green with a white
malar stripe and a white to buffy throat with green speckles. The underparts are cinnamon. Males of subspecies
A. k. margarethae have paler green upperparts than the nominate; females are similar to the nominate.
A. k. caudatus does not have the nominate's blue throat and its females are also similar to the nominate.
A. k. emmae has a somewhat longer bill than the nominate. Males are a paler and duller green and have a bright green throat; females have more green spots on a white throat.
A. k. mocoas upperparts are a shinier green than the nominate and a sapphire blue to purple throat. The female is similar to the nominate.
A. k. smaragdinus has a shorter tail than the nominate and a greenish blue throat. The female has bronzy green upperparts, a more bluish green crown than the nominate, and a chestnut wash on the back. ==Distribution and habitat==