Mountain Avocetbill is a small but heavy bodied, plain colored hummingbird, and is the only species in the genus Opisthoprora. Its most notable feature is its short black bill, which is sharply recurved at the tip, somewhat recalling the form of the bill of an avocet (Recurvirostris). Otherwise the avocetbill is green above, with a white postocular spot. The underparts are white, heavily streaked with dark green. The tail is notched, and the outer rectrices have narrow white tips. There is no brilliant throat patch or gorget, and the sexes of Mountain Avocetbill are similar.
The shape of the tip of the bill of Mountain Avocetbill is unique among Andean hummingbirds, but can be difficult to see sometimes under field conditions. Otherwise this species is distinguished from similarly sized species (such as metaltails Metallura) by its streaked (not spotted) underparts.
The following description is based on Fjeldså and Krabbe (1990) and on Schuchmann (1999):
Adult: Sexes similar. Upperparts bronzy green, with a prominent white postocular spot. Central pair of rectrices bronzy green; outer rectrices dark blue with narrow whitish tips. Tail notched. Underparts white, with rufous flanks and belly, and throughout streaked with dark green.
Toes: pinkish brown
Bare part color data from specimens in the Louisiana State University Museum of Natural Science.
Total length: 10 cm (Fjeldså and Krabbe 1990, Ridgely and Greenfield 2001b), 10.2 cm (Hilty and Brown 1986), 11 cm (Schulenberg et al. 2010)
bill length: 1 cm (Fjeldså and Krabbe 1990), 1.3 cm (Hilty and Brown 1986, Ridgely and Greenfield 2001b, Schulenberg et al. 2010)
Mass: male, 6.4 g, 6.6 g; female, 5.8 g, 6.2 g (data from specimens in the Louisiana State University Museum of Natural Science)