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 c rustacean 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 r econstituted mass) and the copepod Calanoides acutus (45 and 10%, respectiv ely), and that of P, urinatrix by the hyperiid Themisto gaudichaudii (61 an d 91%, respectively) and the copepod Paraeuchaeta antarctica (21 and 9%, re spectively). Chicks of P. georgicus were fed less frequently (on 81 vs 97 % of nights, respectively) with more digested prey items than chicks of P. u rinatrix. This, together with visual observations at sea and the known biog eography of their prey, shows a complete horizontal spatial segregation bet ween 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 of fshore waters. Maximum depth gauges indicate an incomplete vertical segrega tion in the water, P. georgicus reaching on average depths closer to the su rface than P. urinatrix (26 vs 33 m, respectively). The stable-carbon and - nitrogen isotopic composition of chick food and chick feathers differed bet ween the 2 species, thus emphasising the segregation at sea during the bree ding period when adult birds are central-place foragers. The stable isotopi c composition (delta C-13 and delta N-15) of adult feathers were, however, identical in both species, indicating no trophic segregation during the mou lting (inter-breeding) period when birds feed in offshore waters.