A male black-faced grassquits is around 10 cm (3.9 in) long and weighs approximately 10 g (0.35 oz). It has a short conical black bill, a black head and breast with an olive green back. Females and immature birds have dull olive-grey upperparts and head, and paler grey underparts becoming whiter on the belly.
Males on the South American mainland have more extensively black underparts, shading to a grey belly.
The male has a display flight in which he flies for short distances, vibrating his wings and giving a buzzing dik-zeezeezee call.
The black-faced grassquit feeds mainly on seeds, especially of grasses and weeds. It is often found in small groups.
This is a common bird in long grass or scrub in open or semi-open areas, including roadsides and ricefields. It makes a domed grass nest, lined with finer grasses, and placed low in a bush or on a bank. The typical clutch is two or three whitish eggs blotched with reddish brown.
