Complete trophic segregation between South Georgian and common diving petrels during breeding at Iles Kerguelen

The food and feeding ecology of the 2 closely related and sympatric species of diving petrels Pelecanoides georgicus and P. urinatrix was investigated over 3 consecutive summers at lies Kerguelen, southern Indian Ocean, where they breed in large numbers. The 2 species fed consistently on different crustacean prey during the 3 yr, the diet of P. georgicus being dominated by the euphausiid Thysanoessa sp. (50% of the number of prey and 81% of the reconstituted mass) and the copepod Calanoides acutus (45 and 10%, respectively), and that of P, urinatrix by the hyperiid Themisto gaudichaudii (61 and 91%, respectively) and the copepod Paraeuchaeta antarctica (21 and 9%, respectively). Chicks of P. georgicus were fed less frequently (on 81 vs 97 % of nights, respectively) with more digested prey items than chicks of P. urinatrix. This, together with visual observations at sea and the known biogeography of their prey, shows a complete horizontal spatial segregation between the 2 species of diving petrels, with P. urinatrix foraging in coastal waters in the close vicinity of their colonies and P. georgicus in more offshore waters. Maximum depth gauges indicate an incomplete vertical segregation in the water, P. georgicus reaching on average depths closer to the surface than P. urinatrix (26 vs 33 m, respectively). The stable-carbon and -nitrogen isotopic composition of chick food and chick feathers differed between the 2 species, thus emphasising the segregation at sea during the breeding period when adult birds are central-place foragers. The stable isotopic composition (delta C-13 and delta N-15) of adult feathers were, however, identical in both species, indicating no trophic segregation during the moulting (inter-breeding) period when birds feed in offshore waters.