SUMMARYHulsman, K. 1977. Breeding success and mortality of terns at One Tree Island. Great Barrier Reef. Emu 77: 49–60.Defence of the nest by Black-naped, Roseate, Lesser Crested and Crested Terns and strategies of predation used by Silver Gulls were studied. Strategies used by Gulls differed as did the rate of success between individuals and only a few were responsible for most predation. Their behaviour was affected by the size of the defender of the eggs or chicks and by the size of an egg. Breeding success and causes of mortality of eggs and chicks of Black-naped, Roseate, Bridled and Crested Terns were determined. Mortality resulted mainly from predation (1.6 to 83.1%), flooding of nesting areas (6.6 to 50.6%) and starvation (<82.4%) caused by a cyclone preventing adults foraging for at least three days. However, the importance of these causes of mortality differed among colonies over three successive breeding seasons. Although the breeding success of Lesser Crested Terns was determined, the main causes of mortality were not. The generalizations that populations are limited by abundance of food or competition for it are discussed. Breeding success of Black-naped and Roseate Terns is too low to maintain the present size of their colonies at One Tree Island. In contrast, the colonies of the Bridled and Crested Terns can be maintained or possibly increased, depending on post-fledging mortality.