**Abstract:**

According to niche theory, sympatrically breeding species with similar diets frequently show foraging niche partitioning to decrease inter-specific competition, e.g. through temporal or spatial separation, or foraging specialisation. In this study, using stable isotope analyses, we aimed to assess the extent of niche overlap, differences in niche width and the relationship between isotopic signatures and chick growth in two sympatric storm-petrel species, Fregetta tropica (BBSP) and Oceanites oceanicus (WSP) breeding in Maritime Antarctica. We analysed δ15N and δ13C in chick down (pre-laying maternal diet), chick feathers (under tail coverts, UTC; current diet) and adult blood (current diet), and δ18O in UTC. We found isotopic (N, C) niche overlap (proportion (± confidence interval) of the standard ellipse area intersection relative to BBSP and WSP areas) for blood (BBSP 0.39 ± 0.18; WSP 0.10 ± 0.05) and down (BBSP 0.57 ± 0.19; WSP 0.26 ± 0.11), but a lack thereof for UTC (BBSP 0.05 ± 0.13; WSP 0.01 ± 0.03). We found wider isotopic niches for WSP compared to BBSP in all three tissues, indicating differences in foraging flexibility between the species. In both species body mass growth was negatively correlated with δ15N (in UTC for BBSP and in down for WSP). Tarsus growth was positively correlated with δ18O in WSP. We found no correlations for δ13C. The observed niche overlap between BBSP and WSP adults during the pre-breeding (down) and chick-rearing (blood) phases can be explained by their reliance on superabundant Antarctic krill (Euphausia superba). Despite similar chick growth rates in both species, BBSP chicks were fed a more nutritious diet, as implied by higher δ15N values, and the more frequent occurrence of fish in BBSP regurgitations reported in previous studies. While the highly productive Antarctic environment might reduce inter-specific foraging competition, chick nutrition requirements might force parents to be more selective in chick prey selection.

**Authors:**

Anne Ausems¹, Grzegorz Skrzypek², Katarzyna Wojczulanis - Jakubas¹, Dariusz Jakubas¹

¹University of Gdansk, ²The University of Western Australia (M090)

Anne Ausems

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