Black-billed Seed-Finch

The black-billed seed finch is 15 to 16.5 cm (5.9 to 6.5 in) long; two males of subspecies S. a. gigantirostris weighed 23 and 29 g (0.81 and 1.0 oz). It has an "enormous" bill with a very deep base and a gently rounded culmen. The species is sexually dimorphic. Adult males of the nominate subspecies are almost entirely black with white underwing coverts and white bases on the primaries that show as a small patch on the closed wing. Adult females have a warm brown head and upperparts. Their tail and wings are a slightly darker brown with whitish underwing coverts; they lack the male's white primary patch. Their throat and underparts are a slightly paler brown than the upperparts. Both sexes of S. c. occidentalis have the same plumage as the nominate but are larger. Both sexes of both subspecies have a very dark iris, a black bill, and dusky or dark gray legs. Juveniles resemble adult females.

The black-billed seed finch has a disjunct distribution; the subspecies have entirely separate ranges. The nominate subspecies is the more northerly. It is found from extreme southern Colombia south intermittently through eastern Ecuador's lowlands into northern Peru's Huánuco and extreme northwestern Ucayali departments. Subspecies S. c. occidentalis is found from far southeastern Peru's Madre de Dios Department into northern Bolivia to Beni Department with a few records further east. The black-billed seed finch inhabits "damp grassy areas and clearings as well as shrubby lake margins", the last including oxbow lakes. In elevation it is mostly below 600 m (2,000 ft) in Ecuador and mostly below 1,050 m (3,400 ft) in Peru.

The black-billed seed finch's diet has not been studied but is assumed to be seeds. It forages singly or in pairs. Nothing else is known about its feeding habits.