Diet of brown boobies at a globally significant breeding ground is influenced by sex, breeding, sub-colony and year

The causes of intraspecific variation in diet and isotopic niche width can provide important insights into the local food resource requirements for a population. This information is particularly important for highly philopatric colonially nesting species, where local competition for food resources may be high. We investigated the relative influence of environmental, temporal and spatial attributes on intraspecific variation in the diet of colonially nesting brown boobies Sula leucogaster using both regurgitant samples and stable isotope analysis of blood and feathers. Diet analyses revealed that Indian anchovy Stolephorus indicus was the predominant prey species in brown booby diet. Despite the predominance of Indian anchovy, there was significant population-level intraspecific variation in diet. Our results supported the intersexual competition hypothesis, with female diet not only exhibiting greater species richness, but non-breeding females likely to feed in a different habitat. The isotopic niche also varied according to life-history stage, with individuals utilising different food sources between the breeding and non-breeding seasons. Additionally, there were significant inter-annual differences in diet composition associated with warmer sea surface temperatures. Furthermore, we identified sub-colony differences in the non-breeding diet. The different patterns of food intake represent those typical of a central place forager: during the non-breeding season (when adults were not area-restricted due to breeding activity), the width of the isotopic niche of both sexes increased. This study has revealed multiple causes of intraspecific variation in diet and isotopic niche and highlights the need for comprehensive dietary analyses to manage seabird populations effectively within specific locations.