The Black-thighed Puffleg is an Andean hummingbird with metallic green and bronze plumage. The sexes differ in their leg feathers and underparts, and detailed breeding records remain limited.
In Gould’s historical account, the male has golden-bronze upperparts and a shining golden-green underside. The female is lighter below; her leg feathers are black near the knee and grayish white closer to the feet.
Its breeding biology is poorly known. One nest found in northeastern Ecuador was attached to a fern frond in dense streamside vegetation and covered with moss. The observed female stayed at the nest to incubate.
An extinction-risk assessment has not been added yet.