| Summary: | 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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