Sylphs (Aglaiocercus) are short-billed, long-tailed hummingbirds of humid montane forests of the Andes. The Long-tailed Sylph is the most widespread member of the genus, and occurs from Venezuela south to Bolivia. This is the only species of sylph on the east slopes of the Andes. In Colombia and Ecuador, it also is found on the western slopes of the Andes; here it overlaps geographically with the Violet-tailed Sylph, but the Violet-tailed usually occurs at lower elevations. Although the outermost tail feathers of the male Long-tailed Sylph are extraordinarily long, they are not noticeably longer than are the rectrices of other species of sylph. The upper surface of the rectrices of the male are glittering blue green, and the male has a glittering green crown and throat. The female is very different: the tail is much shorter; the throat is white, speckled with green; and the belly is tawny buff. This species forages at all heights in the forest, and at forest edge. The nest of the Long-tailed Sylph is in a suspended ball or clump of moss, with a side entrance and tail of moss dangling below.