AbstractThe Yellow Cardinal,Gubernatrix cristata, has a small geographic range in Brazil, Uruguay, and Argentina. We studied the natural history of the only known Brazilian population of this “Endangered” species (∼50 individuals), which is socially monogamous and may breed cooperatively. During two breeding seasons (October to February 2013–2015), we monitored nests and described the main breeding traits. The breeding season started from the first week of October, with a peak of active nests in mid-November and lasted until mid-February. We found 32 nests, of which 30 were monitored. Female built the open-cup nests in six days (n= 1). All nests were built onProsopistrees, on average at 2.4 m from the ground. Modal clutch size was three eggs (n= 19), and female incubated for 13 days on average (n= 7). Hatching rate was 76% and nestlings fledged after 16 days (n= 6). Nestling survival rate was 67% with mean productivity of 1.6 fledglings/successful nest. Shiny Cowbird,Molothrus bonariensis, parasitized nests of the Yellow Cardinal, with a frequency of 67% and intensity of 1.9 egg per parasitized nest (1–4 eggs;n= 13). Parasitism in nestlings by botfly larvae (Philornissp.) occurred in 33% of nests with nestlings. Nest predation was the main cause of nest losses (73%) and we recorded a Geoffroy’s Cat,Leopardus geoffroyi, preying on a nest with eggs. The probability of success using the Program MARK was 13%. Fledgling survival rate during the first month outside the nest was 62% (8/13 fledglings). We found a longer breeding season, occurrence of second broods, and higher rates but fewer nest losses due to brood parasitism in comparison to the Argentinian population. We highlight the importance of natural history studies for the conservation of different populations of the same species.