The breeding biology of the gentoo penguin, Pygoscelis papua, was studied over a three-year period (1986-1988) at Bird Island, South Georgia, with particular reference to birds of known age or breeding experience. Laying date varied significantly between all three years, being three weeks later in 1987, when the breeding population decreased markedly. Factors involved in the timing of breeding are discussed. Within years egg-laying was highly synchronous: 95% of clutches were initiated in 14.5 days or less. The incubation period was 35 days and the laying interval, between the two eggs, 3.3-3.4 days. Chicks creched when 25-30 days old. and this varied between years. possibly related to food supply and chick growth. Chicks left the colony for the first time between 75 and 85 days of age. The breeding population at Bird Island decreased by 20% and increased by 84% in successive years during the study period. Breeding success (chicks fledged per egg laid) varied between 0.33 and 0.65 within colonies, but for the whole island was very consistent over the three years: 0.45, 0.51 and 0.47. Overall. colony differences were not correlated between years. Disturbance from Antarctic fur seals, Arctocephalus gazella, is suggested as the cause of consistently lower breeding success at one colony. Mean egg weight varied annually, and with age of the breeding bird, nest location and, in one year, with laying date. Young, first-time breeders laid smaller eggs and had lower breeding success compared to older, experienced birds, similar to other seabirds. However, they differed from other species in laying on average earlier than older birds. The relationship between age, egg weight, laying date and breeding success is discussed in relation to predation and seasonal food supply.
