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A study of the Sooty Shearwater (Puffinus griseus) was made at the Snares Islands during four summers. The work is based partly on the activities of 31-45 marked pairs in burrows and partly on more general observations. These Iarge, powerful shearwaters (weight 819 g) return in late September and, after a ‘scratching-out’ period of about a month, part of the population leaves on a prelaying exodus which lasts about two weeks. Laying reaches a peak from 20 to 24 November and incubation takes 53 days, most eggs hatching from 11 to 16 January. No precise nestling periods were obtained, but adults depart on migration from the end of March and most leave by the third week of April. Most chicks leave during the last weeks of April and the first week of May.  Surface-laid eggs are plentiful. They tend to be smalIer and narrower than burrow-laid ones and, on average, are laid three days later than those underground. During incubation the male takes the first stint on the egg after the female’s brief initial one. Thereafter the stints are of similar length, averaging about 9.4 days for both sexes. Samples of chick weights for their first 40 days are given as well as weights of 500 chicks at the time of banding just before their first flights. Flooding of burrows is the chief overt cause of chick mortality. Differences in timing of breeding at Whero Island and The Snares are discussed. In general, the timing seems to be similar throughout the birds’ range from Australasia to the Falklands and Tierra del Fuego,