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Brown skuas (Catharacta skua lonnbergi) on rangatira (south east) and mangere islands in the chatham islands group were studied over a period of 14 years (1978/79 to 1992/93 breeding seasons) to determine factors promoting cooperative breeding in this population. The size and composition of the breeding population and overall breeding success were remarkably stable. No significant differences in breeding success, density of territories or periods of individual occupation in different places on these islands were demonstrated that would demarcate optimal and marginal breeding habitat. Neither could it be demonstrated that the islands were saturated by breeding birds as required for a habitat-forced origin of cooperative breeding.