Divergent responses of <i>Pygoscelis</i> penguins to unfavourable weather conditions in the South Shetland Islands

Abstract Pygoscelis penguins are valuable indicators of the effects of rapid warming in the Antarctic Peninsula. In the western Antarctic Peninsula, Adélie penguins show a declining population trend, whereas gentoo penguins are expanding. The notably low reproductive success of Adélie but not gentoo penguins at Ardley Island during the 2023/2024 breeding season provided an opportunity to explore the potential effects of weather conditions and food availability as possible determinants of reproductive output. We explore associations between reproductive output, air temperature, wind speed, wind chill temperature and accumulated rain and snow. As a proxy for food availability, we used data of penguins’ foraging trips, which reflect krill abundance. A late-winter storm at the end of October 2023 led to a record-low wind chill temperature and sustained snow cover, negatively affecting the number of eggs that hatched successfully and/or the number of chicks that survived the first days after hatching. The effects were similar for both species, yet for gentoo penguins chick survival in the late stage of the chick-rearing phase was remarkably higher, possibly due to high food availability and a longer nestling period. As previously suggested, the greater plasticity of gentoo penguins may allow them to mitigate the negative effects of environmental variability, potentially explaining this divergent breeding success despite unusually harsh meteorological conditions.