ANNUAL VARIATION IN RETURN RATE, MATE AND NEST-SITE FIDELITY IN BREEDING GENTOO AND MACARONI PENGUINS

Annual variation in return rate of breeding birds, mate and nest-site fidelity was studied in the resident Gentoo (Pygoscelis papua) and migratory Macaroni (Eudyptes chrysolophus) Penguin from 1986 to 1990 at Bird Island, South Georgia (54-degrees-S, 38-degrees-W). Return rates (proportion of birds breeding in year x which returned to breed in year x + 1) varied significantly between years: 20-79% and 35-73% in Gentoo and Macaroni Penguins, respectively. Lowest values in both species (in 1987) followed a severe, prolonged winter and were associated with delayed onset of breeding and decreased breeding population size. Decreased return rate was caused by (a) extensive non-breeding in adult birds: 25% of Gentoo and 14% of Macaroni Penguins which bred in 1986 were alive but failed to breed in 1987, and (b) increased adult mortality. Mate fidelity also varied between years in Gentoo Penguins (0-89%) with no birds retaining the same mate in 1987. In contrast, in Macaroni Penguins mate fidelity was high (71-79%) in all three years. This difference may be due to the shorter pre-breeding period, greater synchrony of arrival and pattern of continuous nest attendance in Macaroni Penguins. Failed breeders were more likely to change mate in Macaroni, but not Gentoo, Penguins but there was no difference in subsequent breeding success or mean laying date between new and established pairs in either species. Both species showed a high degree of nest-site fidelity in all years (Gentoo, 89-100%; Macaroni 69-87%). Females were more likely than males to change nest-site following mate change. Intra-season pair fidelity was high in Macaroni Penguins (74-78%) between breeding and the post-nuptial molt. However, the post-nuptial molt was also an important period for formation of new pair-bonds in this species. Despite marked differences in breeding chronology and life-history patterns in these two species, return rates, mate and site fidelity were only markedly dissimilar in one of the three years studied.