THE BREEDING BIOLOGY OF THE GADFLY PETRELS PTERODROMA SPP OF THE PITCAIRN ISLANDS - CHARACTERISTICS, POPULATION SIZES AND CONTROLS

This paper reports the breeding biology and nesting seasons of the gad fly petrels which nest on the four islands of the Pitcairn group, Pitc airn, Henderson, Oeno and Ducie. The species currently breeding are Mu rphy's petrel Pterodroma ultima Kermadec petrel P. neglecta, Herald pe trel P. heraldica and Henderson petrel P. atrata. Of these, Murphy's p etrel is the most numerous; an estimated 250000 pairs bred on Ducie, w hich is probably the major breeding station of the species. Novel basi c breeding data for Murphy's petrel are presented. Incubation spells, averaging 19.3 days, are exceptionally long for a petrel. Phoenix petr el P. alba appears to have ceased to breed on the Pitcairn Islands sin ce the 1922 surveys of the Whitney Expedition. Nesting success was low on Henderson Island during the study. For all four breeding species, less than 20% of eggs laid yielded fledglings. Failure occurred at the early chick stage and observations indicated that it was due to preda tion by Pacific rats Rattus exulans. Although rats are present on Duci e, predation was apparently less severe there. The situation on Oeno m ay be intermediate. I consider how the populations of Henderson are ma intained in the face of this intense predation. The Murphy's petrel po pulation may be sustained by immigration from Ducie while the Herald a nd Henderson petrel populations could be undergoing a long-term declin e on Henderson. It is not clear how the Kermadec petrel population is maintained. The conservation implications of these findings are discus sed. (C) 1995 The Linnean Society of London