Resource partitioning by species but not sex in sympatric boobies in the central Pacific Ocean

Sympatric species with similar ecological requirements and differences in body size would be expected to partition resources to facilitate coexistence. For sexually dimorphic species, we may expect resource partitioning by gender as well as by species identity. However, it is difficult to document species and sexual resource partitioning in marine ecosystems, given the intractability of these systems and the vagility of many marine animals. Here, we examined differences in the foraging behavior and prey selection of 2 congeneric and coexisting seabird species — masked boobies Sula dactylatra and red-footed boobies Sula sula — on Palmyra Atoll in the equatorial Pacific Ocean. These seabirds exhibit substantial size dimorphism between species and also have reverse sexual dimorphism within species. Pronounced interspecific differences in foraging patterns were observed with high-resolution GPS tracking devices, where smaller red-footed boobies conducted longer foraging trips (in both time and distance). Red-footed boobies also had more frequent landings on the sea surface, slower mean and maximum travel speeds, and more westerly trip azimuths. Stable isotope and diet analyses produced data that complemented returns from electronic tagging. Stable isotope ratios of carbon indicated that masked boobies foraged on prey with a less pelagic δ13C signature compared to red-footed boobies. In contrast, no gender differences were identified in any foraging parameters. The mechanisms for maintenance of these pronounced differences in niche partitioning across species were not explicitly examined, yet the data suggest physiological variation across species may be a more plausible mechanism than competitive interactions.