White-tailed Hillstar is a large, dark colored, long-billed hummingbird of the northern Andes. Both sexes are dark coppery green above, with a blue throat and breast and a gray belly. The tail also is extensively white, although the central pair of rectrices are dark; therefore, White-tailed Hillstar has more white in the tail than does Rufous-gaped Hillstar (Urochroa bougueri) of the west slope of the Andes, and lacks a rufous submoustachial stripe. The two species of Urochroa hillstars are not closely related to the Oreotrochilus hillstars, and also differ in habitat preferences: species of Oreotrochilus occupy puna and paramo grasslands above tree line, whereas White-tailed and Rufous-gaped hillstars occur in humid forest on the lower slopes of the Andes. White-tailed Hillstar is uncommon and not well known. White-tailed Hillstar forages on nectar and aerial insects in the under- and midstory of humid montane forest edge and forest clearings, often near water.