The breeding biology and social system of the eclectus parrot Eclectus roratus, a species with a unique form of sexual dichromatism (red and blue females, green males) was examined. Our 4-year study at Iron Range National Park on Cape York Peninsula, Australia, showed that females guarded their nest hollows in emergent rainforest trees for up to 9 months each year, often starting well before they laid their first clutches early in the dry season. During this time they rarely left the nest and relied on males for food, both for themselves and their nestlings. Intrasexual competition for scarce hollows and the threat of egg destruction by other eclectus parrots may explain this extreme guarding behaviour. Reproductive success was low. The combined effects of egg and chick loss to conspecifics and predators, brood reduction and flooding of hollows from heavy rain meant that only 18% of eggs and 27% of clutches produced a fledgling. Over 4 years, reproductive success amongst females was strongly skewed: 39% of females were never successful, and only 29% produced more than one fledgling per year. Eclectus parrots were found to breed co-operatively with as many as five males feeding a single female at the nest. Multiple mating, involving one female and four males, was observed once. Despite an even sex ratio at fledging, the adult sex ratio was strongly male-biased, suggesting higher mortality of females. Our data shows that reproductive variance for females is high, and suggests that a shortage of tree hollows and eligible females may enforce co-operative breeding amongst males.