Population declines and changes in community composition at the world's largest penguin colony.

The Antarctic Peninsula and Scotia Sea are experiencing the greatest pressures from climate change and fisheries in the Southern Ocean, with associated penguin population declines. However, surveys of the South Sandwich Islands in 2011 concluded that numbers there were stable. Zavodovski, the northernmost island in the archipelago, was then hailed as the largest penguin colony in the world. We show that, by 2025, rapid declines of maritime Antarctic chinstrap penguins mean this title has now been ceded to the Adélie penguin colony at Cape Adare, Ross Sea. Over the same period, the subantarctic macaroni penguins increased to become the most abundant species on Zavodovski, representing the largest community shift in Antarctic penguins that has been recorded at a single site. We infer that the most likely primary cause of the chinstrap decline was a reduction in krill availability associated with regional warming, as found across the rest of the Scotia Sea, rather than the volcanic eruption of Zavodovski in 2016 or competition from krill fisheries, macaroni penguins, or humpback whales. The declines of chinstrap penguins mean that reclassification of their global IUCN threat status, from Least Concern to Endangered, requires consideration.