The green-tailed trainbearer (Lesbia nuna) is a species of hummingbird in the "coquettes", tribe Lesbiini of subfamily Lesbiinae.

The International Ornithological Committee (IOC) recognizes these six subspecies:[3] The Clements taxonomy and BirdLife International's Handbook of the Birds of the World (HBW) recognize a seventh, L. n. boliviana (Boucard, 1892) that the IOC treats as a synonym of the nominate subspecies.

The nominate female is similar but has white underparts with glittering green spots and a shorter tail.

[7] Males of subspecies L. n. gouldii are similar to the nominate but smaller, with a shorter bill and more green on the tail.

L. n. gracilis is also similar to the nominate but has a shorter and thicker bill and buffy undertail coverts with green spots.

[7][8] The IOC includes Venezuela with a question mark and the Clements taxonomy notes the one old record.

It has been recorded taking nectar from the flowers of Castilleja fissifolia, Cavendishia cordifolia, Rubus, Cuphea dipetala, and Palicourea angustifolia.

The green-tailed trainbearer's apparent song is " a repeated gravelly note 'drrrt...drrrt...' and similar variations."