| Summary: | 15 pages International audience We examined the population dynamics of two Antarctic seabirds and the influence of environmental variability over a 40-year period by coupling the estimation of demographic parameters, based on capture–recapture data, and modeling, using Leslie matrix population models. We demonstrated that the demographic parameters showing the greatest contribution to the variance of population growth rate were adult survival for both species. Breeding success showed the same contribution as adult survival for Emperor Penguins, whereas the proportion of breeders had the next stronger contribution for Snow Petrels. The sensitivity of population growth rate to adult survival was very high and the adult survival variability was weak for both species. Snow Petrel males survived better than females, whereas Emperor Penguin males had lower survival than females. These differences may be explained by the different investment in breeding. Emperor Penguin adult survival was negatively affected by air temperature during summer and winter for both sexes; male survival was negatively affected by sea ice concentration during summer, autumn, and winter. On the other hand, there was no effect of environmental covariates on Snow Petrel adult survival. The Emperor Penguin population has declined by 50% because of a decrease in adult survival related to a warming event during a regime shift in the late 1970s, whereas Snow Petrels showed their lowest numbers in 1976, but were able to skip reproduction. Indeed, the retrospective analysis of projection population matrix entries indicated that breeding abstention played a critical role in the population dynamics of Snow Petrels but not Emperor Penguins. Snow Petrels did not breed either when air temperature decreased during spring (probably reducing nest attendance and laying) or when sea ice decreased during autumn (reducing food availability). Emperor Penguin and Snow Petrel breeding population sizes were positively influenced by sea ice through its effect on ...
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