[1] The red-tailed comet was formally described in 1812 by the English naturalist George Shaw under the binomial name Trochilus sparganurus.

[6] The red-tailed comet is now the only species in genus Sappho that was introduced in 1849 by the German naturalist Ludwig Reichenbach.

[citation needed] The red-tailed comet has two subspecies, the nominate S. s. sparganurus (Shaw, 1812) and S. s. sapho (Lesson, R, 1828).

Their back and rump are reddish purple and their underparts mostly green with buffy undertail coverts.

Its upper surface looks reddish purple to green depending on the angle of the light striking it.

Juveniles are similar to adult females but have dull bronzy green upperparts with a slightly coppery rump and more white on their outer tail feathers.

The landscape is characterized by scattered trees, a bushy understorey, and dense tangles in ravines.

[8] The red-tailed comet feeds primarily on nectar and also includes small arthropods in its diet.

It collects nectar both while hovering and while perched; it captures arthropods in flight and sometimes by gleaning them from vegetation.

It is reported to span October to December in Argentina and seems to include April to June in Bolivia.

As is true of all hummingbirds, the female makes the nest, incubates the eggs, and cares for nestlings.

It also makes an "unmelodic tjrrrt" and "rather harsh tsha or zack notes, which may be monotonously repeated at more than 1 s intervals".