Multiannual environmental forcing shapes breeding phenology and success in a sub-Antarctic seabird.

Climate-driven phenological mismatches threaten avian reproduction by disrupting food availability during critical breeding stages. In marine ecosystems, time lags between environmental changes and their effects on food webs are challenging to model, yet they can have a profound impact on reproduction at higher trophic levels. We disentangle how oceanic and climatic variability influence the breeding phenology and success of a major predator species in the Southern Ocean, drawing on 24 years of data from 17,000 marked king penguins. We document a 19-day advancement in breeding phenology, alongside increased breeding success (44% in 2000 and 62% in 2023). A sliding-window analysis reveals that sea temperature and primary production in key foraging zones predict both phenology and breeding success, with lags up to several weeks and 2 years, respectively. The amplitude of their recent response, as well as their dependence on multiyear environmental conditions, underscores a potential vulnerability in a context of decreasing environment predictability. However, so far, king penguins appear to be keeping pace with environmental changes and demonstrate a remarkable resilience and important flexibility in the face of variable oceanographic conditions.