We studied Pycroft’s Petrel, Pterodroma pycrofti
, at a breeding colony in the Hen and Chickens Islands, New Zealand, during the prebreeding and breeding seasons of 1982/83. Breeding birds attended their burrows singly and in pairs, often for several days at a time, from at least 38 days before laying, and up to 19 days before laying.
No parents were in their burrows during the fortnight before laying, except a day or so before laying. In October/November the weights of breeders arriving at their burrows averaged about 150 g, but females arriving to lay weighed on average about 188 g, and males arriving to begin their first incubation stint averaged about 190 g – about 25% more than in October/November. The egg averaged 32 g. Daily weight loss over consecutive days in the burrow averaged 3.5-5 g/day, 2-3g of body weight. Laying took place from the second week in November to the end of the first week in December. Egg dimensions are given. Incubation lasted about 45 days and the fledging period was 77-84 days. Weights of fledglings ranged 156-188 g. From small samples, hatching success was 72% and fledging success 50%. Losses were caused by infertility, desertion and breakage resulting from fighting, probably with tuataras. One male failed breeder was at its burrow six weeks after its egg was broken. Non-breeders are defined as adults not known to breed in the current breeding season but may include some failed breeders. Some occurred in burrows in each observation period, and one was recorded throughout the study period, but more were at the colony in January than earlier. Non-breeders and breeders did not differ significantly in weight in October/November, but non-breeders did not usually achieve
the higher weights of breeders arriving to lay or incubate. Weight loss of non-breeders in burrows was similar to that of breeders. The state of the brood patch, egg formation and the history of previously ringed birds are briefly discussed.