Extreme weather events are becoming more frequent and severe, leading to an increase in direct, adverse thermoregulatory impacts to wildlife. Here we document an unprecedented, single day, heat-related mortality event of Magellanic penguins (Spheniscus magellanicus) at Punta Tombo, Chubut Province, Argentina, one of the largest breeding colonies for this species. We found 264 dead adults and 90 dead chicks in the breeding colony and along the beaches after recording the highest temperature in the shade (44°C on 19 January 2019) since the study started in December 1982. We found dead adults and chicks in postures used to release heat, i.e., lying prone with flippers and feet extended away from the body and/or bills open. We found no evidence for other causes of mortality other than heat (e.g., disease, toxic algae, starvation). Adults likely died of dehydration, because dead adults were in significantly worse body condition than adults that survived. Dead adults had either empty stomachs or < 50g of food and 27% of the dead adults died traveling between the nesting area and the water. More males died than females (83% male, 17% female, n=94). In one section of the colony, ~5% of 1153 adults died in the heat. Mortality rates of adults were unevenly distributed across the colony, suggesting that the presence of microclimates or easier beach access were important factors to penguin survival. The body condition indices of dead and live chicks were similar. Chicks that died from heat had food in their stomachs (mean = 405 ± 128g, n=14), suggesting digestion inhibited their ability to thermoregulate. Documenting the effects of extreme weather events on populations is crucial to predicting how they will respond to climate change because these events, although rare, are expected to become more frequent and could have severe impacts on populations.

All data were collected in accordance with the University of Washington IACUC (Protocol #2213-02). The Offices of Turismo and Flora and Fauna in the Province of Chubut, Argentina provided permits.

Author contributions:

1. Conceived the idea, design, experiment (supervised research, formulated question or hypothesis): KAH, PDB

2. Performed the experiments (collected data, conducted the research): KAH, PDB

3. Wrote the paper (or substantially edited the paper): KAH, PDB

4. Developed or designed methods: KAH, PDB

5. Analyzed the data: KAH