| Summary: | International audience The food and feeding ecology of the 2 closely related species of prions Pachyptila belcheri and P. desolata was investigated over 3 consecutive chick-rearing periods at Iles Kerguelen, the only place where they nest sympatrically in large numbers. In all years, the 2 prion species fed on crustaceans, with a small proportion of mesopelagic fish and squid. The hyperiid amphipod Themisto gaudichaudii was consistently the dominant prey item, accounting for 76 and 70% by number, and 57 and 57% by reconstituted mass of the diet of P. belcheri and P. desolata, respectively. Prions, however, were segregated by feeding on different euphausiids, P. belcheri on Thysanoessa sp. (18% by number and 16% by mass) and P. desolata on Euphausia vallentini (9% by number and 15% by mass). P. desolata also caught more small prey such as copepods (9 vs <1% by number) and cypris larvae of Lepas australis (8 vs 3% by number) than P. belcheri, which can be related to the beak filtering apparatus present only in the former species. Biogeography of the prey and their state of digestion indicate that prions foraged in a wide variety of marine habitats, including the kelp belt, kelp rafts, and coastal, neritic and oceanic waters. Noticeable is the occurrence of E. superba in a significant number of food samples (15 and 10% for P. belcheri and P. desolata, respectively), suggesting feeding in distant foraging grounds in southern Antarctic waters, >1000 km from the breeding colonies, during the chick-rearing period. The stable-carbon and -nitrogen isotopic compositions of chick feathers were identical in both species, indicating no important trophic segregation during the breeding period, when adult birds are central-place foragers. The ratios were, however, different in adult feathers, suggesting moulting in Antarctic waters for P. belcheri and in subtropical waters for P. desolata, i.e. in distinct foraging areas when birds are not constrained to return to the colonies.
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