Disentangling drivers of Magellanic penguin demography: climate variability and terrestrial predation at Southern Patagonia

Penguin populations are shaped by environmental variability and local ecological pressures, yet the demographic pathways linking climate, predation, and population dynamics remain difficult to disentangle. We analyzed breeding success and population abundance of Magellanic penguins (Spheniscus magellanicus) at Monte León National Park, Argentina, using 15 years of reproductive data (2010–2025) and 10 years of population abundance. Breeding success was modeled as an ordinal response (0, 1, or 2 chicks fledged per nest) and related to large-scale atmospheric variability (Southern Annular Mode, SAM), sea surface temperature (SST), and puma (Puma concolor) predation during the chick-rearing period, while population abundance was evaluated in relation to lagged predation and the same environmental variables, incorporating winter conditions to account for potential delayed effects on recruitment and survival. Breeding success varied markedly among years and was primarily driven by shifts between complete failure (0 chicks) and full success (2 chicks), with intermediate outcomes remaining comparatively stable. Reproductive output showed non-linear responses to SAM and SST, with optimal success at intermediate environmental conditions, and declined with increasing predation on both adults and chicks. In contrast, population abundance remained stable, showing no clear association with predation and only limited evidence for an effect of SST variability. These results reveal a decoupling between processes affecting reproductive performance and those shaping population size, and suggest that bottom-up climatic variability plays a dominant role in driving interannual breeding dynamics at this colony.