Knowledge of the ontogeny of the sex ratio is required for proper framing of evolutionary hypotheses regarding adaptive parental sex allocation, competition for mates, evolution of mating systems, and endangered species management. We traced the development of a male-biased adult sex ratio in Nazca Boobies (Sula granti) by determination of the sex ratio at hatching, fledging, and at returning to join the adult population. The sex ratio at hatching and fledging did not differ from unity, while the return sex ratio was significantly male-biased. Therefore, the observed adult sex ratio bias arises after the period of parental care ends, during the subadult/juvenile stage, with important implications for mate competition.