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Ageing refers to a decline in individual performance with increasing age that ultimately leads to a reduction in organismal fitness. Although the physiological causes of ageing are likely diverse, a decrease in mitochondrial function may play a central role, as mitochondria generate over 90% of the energy used by eukaryotic cells. Despite support from human and laboratory studies, the effect of age on mitochondrial function, and the extent to which it may differ between males and females, has largely remained unexplored in wild populations. As such, we investigated mitochondrial respiratory function in a long-lived seabird, the common tern (Sterna hirundo), sampling 161 breeding individuals of known sex and ranging in age from 2 to 24 years. We found that older birds showed reduced maximum electron transport system activity and proton leak, suggesting an age-related decline in mitochondrial capacity. Mitochondrial efficiency in ATP production, however, increased with age, suggesting it to be a potential marker of individual quality and survival. Mitochondrial efficiency, as well as spare respiratory capacity, also showed the highest level of individual repeatability between years, indicating consistent individual differences. Although sex differences in age-related trends were not evident, females exhibited significantly higher mitochondrial respiration, potentially due to the energetic demands of egg laying. Overall, these results contribute to our understanding of the role of mitochondrial respiratory function as one of the physiological mechanisms underlying individual performance and ageing in wild animals.