Sharing menus and kids' specials: Inter- and intraspecific differences in stable isotope niches between sympatrically breeding storm-petrels

Species sharing resources are predicted to compete, but co-occurring species can avoid competition through niche partitioning. Here, we investigated the inter- and intra-specific differences using stable isotope analyses in the black-bellied storm-petrel (Fregetta tropica) and the Wilson's storm-petrel (Oceanites oceanicus), breeding sympatrically in maritime Antarctica. We analysed stable carbon, nitrogen and oxygen isotopes in samples representing different life stages; chick down (pre-laying females), chick feather (chick), and adult blood (chick-rearing adults). Pre-laying females had wider stable isotope niches than chicks or chick-rearing adults, due to pre-laying females being free roaming while chick-rearing adults were central-place-foragers. Chicks were fed at a higher trophic level than the adults (higher δ¹⁵N), likely to compensate for the high nutritional demands of the growing chicks. Wilson's storm-petrels showed substantial overlap in stable isotope niches between all life stages, while the black-bellied storm-petrel chicks showed very little overlap. Wilson's storm-petrel niches significantly overlapped with those of pre-laying and chick-rearing black-bellied storm-petrels, suggesting negligible niche partitioning. Chick growth rate was negatively correlated with chick δ¹⁵N values, suggesting nutritional stress resulting in the use endogenous instead of dietary amino acids in protein synthesis. The higher trophic level of the relatively larger black-bellied storm-petrel chicks may be due to their longer stay in the nest, and relatively larger body mass gain, despite chick growth rates being similar to the smaller Wilson's storm-petrel chicks. Despite breeding sympatrically, the studied storm-petrel species showed considerable overlap in isotopic niches, which may be explained by sharing the same main prey species, reducing the detectability of foraging niche partitioning through stable isotope analyses. We found dietary shifts in black-bellied storm-petrels that are absent in Wilson's, showing different chick provisioning strategies, and shows that the high productivity of the Antarctic marine ecosystem may facilitate foraging niche overlap of sympatrically living species.