Resource partitioning between incubating and chick-rearing brown boobies and red-tailed tropicbirds on Christmas Island

Background: In oligotrophic tropical marine environments, the main mechanism explaining the coexistence of sympatric seabirds is segregation by habitat or segregation by prey within the same habitat. Both types of segregation can play a role during the breeding season due to different constraints associated with different phases of the breeding cycle. By using stable isotope analyses, we investigated intra- and interspecific foraging segregation in two tropical seabird species, the red-tailed tropicbird Phaeton rubricauda and the brown booby Sula leucogaster, breeding sympatrically on Christmas Island, Indian Ocean. We compared isotopic values of delta C-13 and delta N-15 in blood from incubating and chick-rearing adults of both species. Results: The results showed small but significantly interspecific and intraspecific differences in delta C-13 and delta N-15 values. Differences in delta C-13 values suggest spatial segregation in the main foraging grounds between the two species during the breeding season as well as between incubating and chick-rearing brown boobies. In contrast, red-tailed tropicbirds probably exploited similar foraging habitats during both breeding stages. delta N-15 values did not indicate diet-related differences, neither within nor between species, suggesting a highly opportunistic feeding behavior to cope with the limited prey available in the oligotrophic marine environment. Conclusions: Competition for prey in breeding red-tailed tropicbirds and brown boobies seems to be reduced by spatial segregation enabling both species to successfully reproduce in sympatry in an oligotrophic tropical marine environment