The Coal-crested Finch (Charitospiza eucosma) is a rare, globally near threatened and poorly known species, endemic to the savannas of central Brazil and Bolivia. We investigated this species' breeding biology in the Brazilian savannas between 2008–2010. We found 44 nests of Coal-crested Finches during one and a half breeding seasons. Coal-crested Finches reproduced in two discrete peaks within the rainy season (i.e., bimodal breeding). Nests were cup-shaped and built on trees with a mean height of 2.18 ± 1.34 m (n = 42). Modal clutch size was 2 (range = 1–3). Eggs averaged 16.32 ± 1.16 mm (length; n = 46), 11.30 ± 1.06 mm (width; n = 46) and 1.40 ± 0.14 g (mass; n = 44). Incubation and nestling periods lasted on average 13.4 ± 0.42 days (n = 5) and 12.0 ± 2.63 days (n = 7), respectively. Young (0–4 days old), middle-aged (5–9 days old) and late nestlings (10–13 days old) weighed from 1.25–6.35 g (n = 10), from 5.30–12.65 g (n = 12) and from 9–11.90 g (n = 9), respectively. Estimated nesting success was 20.08% (n = 38). The Coal-crested Finch has similarities (e.g., nest material, clutch size, eggs, nestling period, nestlings and nesting success) and differences (e.g., nest structure, nest sites, incubation period, breeding season) with other Neotropical finches. We suggest that their bimodal breeding season may have evolved in response to two annual peaks of food resource abundance: arthropods followed by grass seeds.
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