Diet specialization in a colonial seabird studied using three complementary dietary techniques: effects of intrinsic and extrinsic factors

The breeding period has a critical influence on the trophic ecology of seabirds because of the energetic costs of egg production for females, the need to return regularly to the nest to provision chicks, the combined energetic demands of adults and chicks, and potential intraspecific competition if resources around the colony are scarce. The present study combined three dietary methods to investigate if and how these intrinsic and extrinsic factors influenced diet specialization in a colonially breeding seabird, the Cape gannet Morus capensis. The diet of this species was studied from November 2009 to October 2010 at the species’ largest colony at Bird Island, Algoa Bay (33°50′S, 026°17′E; South Africa). Potential prey species were sampled concurrently and dietary tracers (stable isotopes and fatty acids) were analysed. Stomach content and carbon and nitrogen stable isotope analyses indicated that adults relied heavily all year round on small pelagic fish (anchovy and sardine), with prey species composition and individual prey size changing with season, probably reflecting prey biology. Dietary tracers did not show any differences between adult and chick diets. Subtle differences were found between stable isotope values of adult males and females but these were not supported by a Bayesian mixing model. In contrast, differences between the sexes were highlighted in blood fatty acids. The combined results suggest that these were probably related to the cost of egg production rather than to inter-sex differences in diet. Individual diet specialization was observed using stable isotopes in adults. Altogether this dataset indicates the importance of combining complementary methods to understand multiple facets of seabirds’ trophic ecology, and highlights interactions with fisheries that require future monitoring.