Breeding phenology plasticity to climate revealed by automated monitoring of two Eudpytes penguin species across the sub-Antarctic.

The timing of breeding is a key component of a species' realised niche and has been shown to be flexible to environmental change. Using a network of time-lapse cameras around Patagonia and the sub-Antarctic, along with citizen science assisted AI data extraction, we demonstrate that semi-automated extraction of phenological parameters is feasible without extensive human presence at colonies. Our comparative analysis of two related penguin species across the Polar Front reveals shifts in breeding phenology linked to environmental variation. The Southern Rockhopper Penguin showed a potential contraction of its breeding season, with consistent arrival but earlier departures in recent years. In contrast, Macaroni Penguins arrived and departed earlier, suggesting a temporal shift rather than a shorter season. These phenological changes correlated with winter-integrated SST rather than late-winter conditions, while reduced SST during pre-moult periods extended pre-moult duration and delayed migration. Despite marked phenological shifts in one species, breeding success remained stable, suggesting that timing changes may reflect flexible responses to environmental shifts rather than hidden survival costs. Species-specific differences in baseline phenology and temporal trends suggest that while some seabirds show phenological plasticity to ocean warming, stable breeding success may mask subtle but consequential impacts on sub-Antarctic population dynamics.