Abstract: Continued warming of the Earth's atmosphere is having a range of ecological, behavioural and physiological impacts on seabirds. The effects of changes in sea surface temperatures on seabirds are well-documented, but the effects of high ambient temperatures on land are not. Like all penguins, moult in Little Penguins is 'catastrophic', with all feathers shed and replaced in ~18 days. As they do not have adequate insulation and waterproofing to forage at sea, birds are constrained to land in some of the hottest parts of the year. Long-term survival analysis suggests that adult mortality is greatest in years when high ambient temperatures are paired with low humidity during moult. Given current climate projections, successful management of Little Penguins requires a better understanding of the physiological processes and habitat features that influence mortality events. In this study we aimed to identify which birds are most at risk of heat stress during moult, and why. Using a novel approach to field respirometry, we investigated thermoregulatory costs of moulting birds in response to ambient temperature and humidity. Our results predicted that penguins in early stages of moult would be most affected by heat exposure, as they displayed a significant increase in metabolic rate at higher temperatures compared to mid and late moult birds. During a heat wave in 2019, the greatest mortality event of moulting adult Little Penguins on record occurred at Phillip Island in south-Eastern Australia. As predicted, all recorded deaths were birds in early stages of moult. We are currently quantifying the microclimates of burrows and vegetation types and investigating how birds are using structural habitat during extreme weather events. Results will help determine how Little Penguins are currently coping with extreme temperatures on land, and identify adaptation options most likely to reduce negative climate change impacts for the species across their distribution. Authors: Lauren Tworkowski��, Peter Dann��, Ursula Ellenberg��, Kylie Robert�� ��La Trobe University, ��Phillip Island Nature Parks