Some Factors Affecting the Laying Date, Incubation and Breeding Success of the Manx Shearwater, Puffinus puffinus

SUMMARY (1) The breeding success and timing of laying of manx shearwaters breeding on Skokholm Island, Wales, are discussed. (2) Newly formed pairs have a lower hatching success (66-2%) than established pairs (79'1%). This is due to the fact that new pairs include birds without prior breeding experience and not to the newly formed pair bond per se. (3) When forming new pairs experienced breeding birds tend to mate with other experienced breeding birds rather than new breeders. (4) Divorce and burrow movement are both more likely after breeding failure than after success. (5) The timing of laying and the size of eggs laid are very constant from year to year, both for the population as a whole and for individual females. (6) The relationship of laying date and egg size to female age is described. (7) It is suggested that birds do not lay earlier than they do because, early in the season, the food supply would be inadequate to enable the adults to maintain continuous incubation. (8) Circumstantial evidence is presented suggesting that conditions for incubation improve during the incubation period.