Appearance

The Yellow-faced Grassquit is a small, greenish gray finch. As with other members of the genus Tiaris, this species has a conical, sharply pointed bill. The forecrown, sides of the face, and the upper breast of the male are black, contrasting with a yellow superciliary and throat. Females are duller and more uniformly colored; they primarily are olive green, with a paler superciliary and throat.

It would be difficult to confuse the male Yellow-faced Grassquit, with its conspicuous, contrasting facial pattern, for any other species. The duller female may be more confusing. The shape of bill - conical and pointed - helps to distinguish the grassquit from superficially similar species of Sporophila seedeaters, which have bills that are more rounded. The female grassquit also is more olive in color than most female seedeaters. The female has a pale superciliary and throat, which is a more subdued version of the facial pattern of the male; this pattern is not shared by any other species of small finch in its range.

The following description is based on Ridgway (1901), Wetmore et al. (1984), and, for immature plumages, Howell and Webb (1995):

Adult male: Forehead, sides of face and of throat, and breast black. Superciliary, chin and upper throat dark yellow. Crown, back, wing coverts, and tail olive green. Remiges dusky, edged with olive green. Belly and flanks grayish green. Bend of wing yellow.

Adult female: Upperparts olive green, with a faint buff-yellow superciliary. Remiges dusky, edged with olive green. Underparts dingy olive, with a buff-yellow chin; whiter on the center of the belly.

Juvenile (First Basic): Similar to the adult female., but duller, with a dull whitish superciliary and chin.

Formative ("First basic"): Similar to juvenile but the facial pattern is brighter, and the male has a small amount of black mottling on the face.

First alternate: The superciliary and the throat are yellowish, the male with more black mottling on the face and breast, "much like adult female."

Little information. The formative plumage is acquired "quickly" (Howell and Webb 1995) after the First Basic (juvenile) plumage.

Data from Wetmore et al. (1984):

Iris: dark brown.

Bill: Fuscous-black (male) or maxilla black, mandible mostly dull greenish gray (female).

Tarsi and toes: brownish neutral gray, dark neutral gray.

Total length: 10 cm (Stiles and Skutch 1989), 10.2 cm (Hilty and Brown 1986), 10-11 cm (Howell and Webb 1995)

Mass: mean 8.5 g (range 6–10 g; SD ± 0.6, n=318; Arendt et al. (1994).