Identification and breeding biology of the diving petrels Pelecanoides georgicus and P. urinatrix exsul at South Georgia
Payne, MR.; Prince, PA.
New Zealand Journal of Zoology 62: 299-318
The biology of 2 spp. of diving petrel (P. georgicus and P. u. exsul) was studied at Bird Island, South Georgia. Existing criteria (bill shape and morphology, wing length) for distinguishing these species are reviewed, and several new characters are recognized. For adults bill depth, the color of the posterior part of the tarsus and vocalization are distinctive; chicks have down of different colors (pale grey in P. u. exsul, dark grey in P. georgicus). At South Georgia the species breed in different habitats and at different times; P. u. exsul nests in steep tussock slopes in very early summer, P. georgicus in fine scree slopes in midsummer. P. georgicus lays a proportionately larger egg and has an incubation period of 46 days (about 54 days in P. u. exsul) and a chick fledging period of 46 days. The fledging period of P. u. exsul is 54 days, very similar to recorded values for P. urinatrix urinatrix (53.5 days) and P. urinatrix chathamensis (56 days). Data on feeding frequency and feed size were derived from daily chick weighings and from twice-daily weighing during 30 days preceding fledging. In both species chicks are fed every night, often by both parents. In P. georgicus true mean chick feed size is about 37 g; in P. u. exsul it may be slightly less. Analysis of chick stomach contents suggests that P. u. exsul feeds extensively on copepods, while P. georgicus largely takes krill (Euphausia superba). P. u. exsul breeding adults commence molting before their chicks have fledged. P. georgicus molts exclusively in the non-breeding season. Ectoparasites were collected; the feather louse Pelmatocerandra setosa was restricted to P. u. exsul, and P. enderleini was found on P. georgicus. P. georgicus, which breeds later and whose chicks fledge faster, may be better adapted to the climatic and marine environmental conditions than P. u. exsul, which may be the more recent colonist of these high latitudes.