Chestnut-capped Blackbird

Male chestnut-capped blackbirds average 18.5 cm (7.3 in) long and females 17.1 cm (6.7 in). Males weigh an average of 41.3 g (1.5 oz) and females average 32.2 g (1.1 oz). The species is sexually dimorphic. Adult males of the nominate subspecies have a deep dark chestnut forecrown, midcrown, throat, and upper breast. The rest of their plumages is black with a faint bluish gloss. Adult females of the nominate subspecies have a mostly olivaceous brown head with a faint buffish supercilium. Their upperparts are also olivaceous brown with faint dusky streaks. Their throat and upper breast are pale buff to tawny and the rest of their underparts are olivaceous buff. Their throat and underparts have faint dark streaks. Adult males of subspecies C. r. frontalis have a ferruginous crown and breast that are paler than the nominate's. They also have a thin black band around the base of the bill. Adult females have a yellowish tinge on the head and a darker yellow tinge on the throat. Juveniles are similar to adult females but are paler buff-brown with warm brown edges on the upperwing coverts and heavier streaks on the underparts. Both sexes of both subspecies have a dark brown iris, a black bill, and black legs and feet.

Subspecies C. r. frontalis is the more northerly of the two. It is found in eastern Brazil from Amapá east to Rio Grande do Norte and from there south to São Paulo state with a western extent to Goiás. Several sources state that its range also includes French Guiana but the South American Classification Committee has no records there. The nominate subspecies is found in Brazil from Mato Grosso do Sul and São Paulo south and through Uruguay into Argentina as far as La Pampa and Buenos Aires provinces. From there its range extends west across Paraguay into eastern Bolivia's Santa Cruz, Chuquisaca, and Tarija departments. The chestnut-capped blackbird inhabits a variety of wet landscapes including marshes, rice fields, and wet grasslands and croplands. It occasionally roosts and nests in open woodlands near wetlands. Subspecies C. r. frontalis appears to be partial to irrigated fields and is common in barnyards and corrals.

The chestnut-capped blackbird feeds primarily on seeds and includes significant amounts of insects and spiders in its diet as well. It regularly feeds in rice fields; away from them it feeds on naturally-occurring grass seeds. In marshes it feeds mostly in emergent vegetation but also on dry ground. It typically forages in flocks of 1000 or more individuals and flocks of "several thousands" are known. The flocks sometimes include other icterid species.