The Duke of Burgundy (Hamearis lucina) is a vulnerable species of conservation interest in the UK. Previous research has suggested that the butterfly is sensitive and vulnerable to temperature increases, with a strikingly limited ability to maintain a stable thoracic temperature across a range of air temperatures compared to other butterfly species. However, these calculations were limited by the narrow range of air temperatures in which thoracic temperatures were measured, as air temperatures above 20 °C are rare during the spring flight period of this butterfly in the UK. During the warmest spring in the UK since records began, we measured the body temperatures of six adult Duke of Burgundy butterflies at Totternhoe Nature Reserve, Bedfordshire, at air temperatures over 20 °C. Together with existing data, we used this information to recalculate a buffering ability for the Duke of Burgundy. We found that adult Duke of Burgundy shows a greatly improved ability to buffer its body temperature when including additional data points from air temperatures of 24.2-25.9 °C. Our findings indicate that the Duke of Burgundy is better at altering its thoracic temperature than previously thought, and hence may be less sensitive to increasing temperatures than predicted. However, our data also suggest that the Duke of Burgundy has narrow temperature requirements, which may contribute to vulnerability to climate change. Our research demonstrates the value of opportunistically gathering data in unusually hot weather and highlights the importance of research efforts exploring the thermal and habitat requirements of specialist butterflies.