Population modelling for one of Réunion Island’s endemic seabirds, the Barau’s Petrel (Pterodroma baraui), has highlighted its vulnerability to extinction. Conservation action demands information on its biology during different stages of its life cycle. Whilst most aspects of the species’ terrestrial ecology have been studied, at-sea information is scarce and frequently contradictory. In this context, we combine three complementary techniques to provide new information on the trophic ecology of the Barau’s Petrel and to augment recent telemetry data. Colonies were visited periodically through a single breeding season and samples gathered from adults, downy chicks, and fledglings, permitting intra-annual comparisons within and among ontogenetic stages. Stomach contents consisted mostly of accumulated cephalopod beaks, whereas structures from other molluscs, fishes, and arthropods were rare. Variations in carbon stable isotopes matched the patterns of adult foraging behaviour, as described using telemetry, and wide variation in nitrogen isotope values indicated dietary opportunism. Finally, the total fatty acid composition of blood differed greatly among adults, further suggesting opportunistic feeding; however, consistently low incidences of long-chain monounsaturated and n-3 fatty acids indicated little importance of fish. These results offer additional insight into the Barau’s Petrels’ trophic ecology are in concordance with recent telemetry data and will assist in preliminary assessment of the threats to the species whilst foraging. We also illustrate the value of an integrated approach to diet determination for endangered species, where hyper-ethical approaches need to be considered, and when isotope and fatty acid baseline data are not available and are not logistically attainable.