Niche partitioning among and within sympatric tropical seabirds revealed by stable isotope analysis

The low productivity and unpredictable nature of resources in tropical waters would appear to make resource partitioning among predators difficult. Yet, stable isotope data from the present study suggest that substantial resource partitioning occurs among tropical seabird communities, both between and within species. In the present study, we compared delta C-13 and delta N-15 levels among 8 sympatric tropical seabird species. For a subset of these species, we also examined isotopic levels across years, breeding periods, and sexes. When breeding and non-breeding periods were both considered, we found that all species occupied distinct isotopic niches across at least one time period. Resource partitioning by age (within the same year) and year studied (for birds of the same age), within species was observed in all the species we examined. However, niche separation by sex appeared to be relatively uncommon even in sexually dimorphic species. As a group, seabirds were isotopically distinct from other marine predators. There was a strong correlation between both body mass and wing loading ratios and foraging area, as evaluated using delta C-13 levels. Cumulatively, the isotopic patterns observed are consistent with diet-based surveys of resource partitioning during the breeding period. Importantly, these data provide additional evidence to suggest that substantial niche partitioning among tropical seabird species persists even during non-breeding periods in open ocean environments, when direct diet-based surveys are difficult to conduct.