We studied nest-site selection and nesting success in the critically endangered Yellow-crested Cockatoo Cacatua sulphurea throughout a breeding season in Manupeu-Tanadaru National Park, Sumba. Within a 6 km2 study area, which supported about 60 birds, cockatoos displayed disproportionate interest in cavities in trees containing other active nests, and cavities already actually occupied by parrots or owls. Actual nesting attempts were made at only eight cavities, and a single chick fledged. Interspecific interactions were observed at every one of these nests. Breeding activity was negatively correlated with monthly rainfall, which was the heaviest for at least 10 years, and it is possible that adverse weather conditions disrupted cockatoo nesting this season. Whether this is the case or not, our study shows how low reproductive output can be in the species and this has serious implications for survival of the population on Sumba.