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According to the principles of community ecology, sympatric species may suffer a selective pressure to decrease their niche overlap through mechanisms of niche partitioning. However, there is still a gap in knowledge of the main drivers influencing niche differentiation, particularly in communities composed by small-sized and inconspicuous species. The coastal epipelagic community structure of mid-trophic level species in the Northeast Atlantic was examined using stable isotope analysis to (1) decipher intrinsic and extrinsic factors promoting niche partitioning and (2) examine species divergence in the use of resources. A total of 474 individuals from a guild of 11 species were sampled during three years, and the influence of potential drivers of isotopic divergence was assessed. The niche partitioning was mainly determined by the taxonomic group in accordance with the fundamental principles of niche theory, which was consistent for both δ13C (i.e. habitat) and δ15N (i.e. prey). Body size was highly important in driving niche partitioning within species. Our results show that the predominant species in the guild were the most generalist, when compared with more specialised species with which they coexist. This study reveals that the intrinsic factors are determinant to decrease inter- and intra-specific niche overlap at the community level.

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Morphological variation in biological structures may be driven by genetic and environmental factors, such as inter and intraspecific competition for resources. In seabirds, although the bill is also involved in vocalization, olfaction, sexual selection and defence, the main drivers of high morphological plasticity in bill size and shape appear to relate primarily to diet and thus to niche differentiation. Here, we combined geometric morphometrics and comparisons of linear measurements as a precise tool for measuring shape variation in anatomical features, to investigate the differences among species and populations (island groups) in bill shape of three abundant planktivorous petrels (Antarctic prion Pachyptila desolata, blue petrel Halobaena caerulea and thinbilled prion Pachyptila belcheri). Fieldwork was carried out in South Georgia (54°0′S, 38°3′W), Falkland Islands (51°42′S, 57°51′W), Diego Ramírez Islands (56°31′S, 68°44′W) and Kerguelen Islands (49°20′S, 69°20′E), across 2010-2021. As expected, the bills of Antarctic prions were more robust and shorter, appropriate for filtering large amounts of small prey. Blue petrels and thin-billed prions had narrower and longer bills, effective for catching and tearing large single prey. Also, Antarctic prions and blue petrels from Kerguelen had longer and narrower bills than conspecifics from other colonies, which could potentially be explained by geographic variation in diet. In conclusion, prey availability and diversity appear to be important factors influencing variation in bill morphology. This study highlights the utility of geometric morphometrics for investigating bill shape variation in seabirds. Nevertheless, further studies are needed to better understand selective pressures leading to morphological displacement of biological structures.