Rufous-rumped Seedeater

The rufous-rumped seedeater is about 10 cm (3.9 in) long; one young male weighed 9.7 g (0.34 oz). It is one of the smallest of its genus. It has a smallish bill with a rounded culmen. The species is sexually dimorphic. Adult males have a grayish crown and nape that give a "helmeted" appearance; the rest of their face is chestnut. Their mantle and upper back are grayish and their lower back and rump are chestnut. Their tail is mostly blackish with gray edges on the outer feathers. Their wings are blackish with white bases on the primaries that show as a patch on the folded wing. Their tertials are black with gray edges. Their throat and underparts are chestnut. Females have a sandy brown head with often a paler buff forehead and supercilium. Their upperparts are also warm brown and their tail is darker brown. Their wings are darker brown with sandy edges on the tertials and a small whitish primary patch like the male's. Their underparts are warm buff that is palest in the middle of the throat and on the belly. Both sexes have a dark brown iris. Males have a dark gray to black bill and legs; females have a dark horn-colored to blackish bill and dusky legs. Juveniles resemble adult females.

The rufous-rumped seedeater is found from northern Buenos Aires Province in Argentina north in a narrow strip to eastern Chaco and eastern Formosa provinces; the strip includes far western Uruguay. From there its range expands into eastern Paraguay and then northwest, north, and northeast to a line from western Beni Department in northern Bolivia east to southern Minas Gerais in Brazil. However, within that overall range it appears to have a disjunct distribution with centers in eastern Bolivia and northeastern Argentina. The rufous-rumped seedeater inhabits tall grasslands near marshes and other open wetlands. In elevation it ranges from sea level to 1,100 m (3,600 ft).

The rufous-rumped seedeater feeds on grass seeds. It typically feeds while clinging to stems. It forages in small groups and sometimes in larger ones that include other seed-eating species.