Understanding how populations and communities will respond to global changes is a major focus of modern ecology, and demographic studies are crucial for understanding the dynamic of wild populations and their responses to change. Here, I first conducted an analytically robust literature to demonstrate that seabird mortality in global fisheries (bycatch) tends to be biased by sex and age, mainly related to differential at-sea distributions, underpinning the population level effect bycatch on the most threatened group of birds. Next I performed a comprehensive longitudinal study, to address effects of fisheries, environmental variability and climate oscillation on population dynamics of northern (NGP, Macronectes halli) and southern giant petrel (SGP, M. giganteus) at Bird Island, South Georgia. I showed that annual survival and breeding success of NGP and SGP was influenced mainly by climatic oscillation and oceanographic conditions, including fisheries, and that the responses varied by sex and age. Giant petrels survived and reproduced better in warmer years, contrasting with the negative effects of warmer conditions on a range of marine land-based vertebrates in the same ecosystem. Differential accessibility to food resources during chick-rearing due to allochrony have contributed for the NGP breeding success and delayed reproductive senescence compared to SGP, supporting empirical evidence for role of allochrony on their divergent population trajectories. Finally, the findings in this thesis sheds a new light on how phenological mismatch can influence demographic process and on the role of environmental conditions on reproductive senescence, which are among the poorest understood processes in population ecology.