We present detailed information on the breeding biology of a migratory population of the Tawny-bellied Seedeater (Sporophila hypoxantha), a typical member of the “capuchinos” group. Data were collected for 69 nests from 2007–2010 in areas of hilly dry grasslands in southern Brazil. Breeding begins in early November, a week after the seedeaters arrive, peaks in late November, and lasts until late February, when individuals migrate north. Females build small open-cup nests in small shrubs (mainly Vernonia chamaedrys and Eupatorium polystachyum), 41.9 ± 0.8 cm (range 27–60 cm, n = 38) above the ground. Clutch size is two (91%) or three eggs (n = 43), which measure 16.22 ± 0.51 mm by 11.93 ± 0.28 mm (n = 10), and incubation, which is performed by the female, lasts 12 days. Average nest attentiveness during incubation was 60 ± 5% (n = 5) with 20–31 min on-bouts. Males help feed the nestlings beginning on the fifth day after hatching. Provisioning visits/hr averaged 4.6 ± 0.7 (n = 6) and females brooded after feeding the young in 48% of the visits when nestlings were 1–4 days old. When nestlings were 6–9 days old, the average visits/hr increased (8.95 ± 1.8, n = 11; z = −2.5, P = 0.012), with brooding after feeding in 14% of the visits. Nestlings fledged after 9–10 days. The basic reproductive characteristics of the migratory population are very similar to those found in the resident population in the Formosa region, Argentina, but the breeding season, time spent building the nest, and length of visits to the nest during the nestling stage were shorter.