THE BREEDING BIOLOGY OF THE BLACK-BELLIED STORM-PETREL FREGETTA TROPICA

The breeding cycie and habits of the Black-bellied Storm-petrel are described from observations made over three seasons at Signy Island, South Orkney Islands. The species is strictly nocturnal on land and nests in stable scree slopes. With an estimated population of 100–200 pairs, Fregetta tropica is the rarest petrel breeding on the island. In general, the breeding cycle of F. tropica resembles that of Oceanites oceanicus. Birds usually arrive from mid-November onward and return to the same nest and mate in successive seasons. The female is absent from the nest for a week or more before the egg is laid, during which time the male continues to visit the site. From ten laying dates, egg laying appears normally to begin during the last week in December, but evidence is given that, in 1966-67, laying was delayed by heavy winter snow build-up coupled with a late melt. The egg comprises 26 % of the body weight of the female. Incubation is by both sexes in alternate spells of three days, the whole period lasting 38–44 days. The chick is left alone in the nest by the parents almost immediately after hatching. Chick growth is described briefly and the effects of drift snow on development are discussed. The fledging periods of two chicks were 65 and 71 days, departure from the nest taking place in mid-April. Measurements of 36 Signy Island birds show considerable variability but are similar to those from other breeding localities.