Abstract: Knowledge of breeding success and its limiting factors is crucial in assessing species’ conservation needs. As cavity-nesters, parrots are particularly influenced by the availability of suitable cavities and low breeding output, whether due to natural processes or trapping. On the island of Sumba, Indonesia, the Critically Endangered Citron-crested Cockatoo (Cacatua citrinocristata) has the added problem of co-existing with an unusually rich hole-nesting bird community in a forested environment much constrained by habitat loss. We monitored 95 nesting cavities of cockatoos and their competitors and potential nest-predators, over one to four breeding seasons, using a combination of camera-traps, direct checks on nest contents, and observations from the ground. Competition for suitable cavities was intense among three large parrot species, two owls and a hornbill. Visitation rates by potential competitors were higher at unoccupied cavities than at those containing active nests, reflecting the guarding behaviour of the occupants. The Endangered Sumba Hornbill (Rhyticeros everetti) dominated observed direct confrontations and was the most frequent visitor to active parrot nests, suggesting a further role as a potential nest-predator. Cockatoos prospected many cavities but rarely then attempted to nest: instead the sites were usually occupied by other cavity-nesters, or by bees. At the few cavities where cockatoos did breed, predation pressure was likely low, and observed success rate high (10 successful of 15 nests), although the low number of nests found early in the breeding cycle suggests that some may have failed before detection. Intense competition for cavities suggests a shortage of suitable nest-sites, the need for preservation of old hole-bearing trees and a role for nestboxes. Accessible, known, safe artificial nest-sites would also provide opportunities to assess the scale of nest-site shortage, allow camera placements to study productivity, exclude some competitors and predators, and prevent illegal trapping. Especially given continued trapping pressure, the species would benefit from targeted local awareness-raising and law enforcement, with the whole endeavour backed up by longer-term forest restoration.