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Understanding resource partitioning in sympatric seabirds living in tropical marine environments

ABSTRACT Two closely-related species living in sympatry can only coexist if a shift towards a novel ecological niche occurs. For seabird species breeding in tropical regions, competitive pressures intensify due to high population densities in colonies and sporadic prey resources. Here, we used a stable isotope approach to infer inter- and intraspecific variation in foraging behaviour in two congeneric seabird species: Brown Booby (Sula leucogaster, N = 31) and Red-footed Booby (S. sula, N = 30) breeding on Raine Island in the Great Barrier Reef, Australia. Blood (red blood cells and plasma components) and feather samples were collected from breeding birds for δ13C and δ15N analysis. We found significant δ13C interspecific differences in feather and red blood cell tissue, but not in plasma, whereas δ15N values did not differ significantly between species in any of the tissues. Although interspecific differences were small, higher δ13C values found in Brown Boobies suggest differences in foraging strategy between the two species. Inter-sexual differences, either in size or ecology, did not translate into dietary niche segregation. This study suggests that niche-partitioning processes are flexible in the temporal scale, and can be constrained by reproductive behaviour and availability of prey resources.