Abstract: The Duke of Burgundy butterfly (Hamearis lucina) has undergone severe declines over the last four decades. However, in recent years the UK population appears to have begun expanding again. This is likely to be due to beneficial management, although a warming regional climate may also have contributed to the resurgence of this spring-flying species. In this study, we investigated the effect of air temperature on the flight behaviour of adult male Duke of Burgundy butterflies. We also looked at the ability of adult males to behaviourally thermoregulate their body temperature and assessed their tendency to remain within small established territories. Increasing air temperature lead to a marked increase in the number and duration of flights associated with territorial behaviour but had no significant effect on other flights. This suggests that high temperatures are particularly important for sustaining energetically-demanding flights involved in territory defence and mate interception, which could impact the reproductive potential of Duke of Burgundy populations. We also found that butterflies had only a limited ability to regulate their body temperature behaviourally and may, therefore, be especially dependent on suitable environmental conditions to maintain the right temperatures for these flights. During observations, most males also remained confined to a few square meters within their territories, which could further restrict butterfly ability to thermoregulate by limiting relocation to other habitat types. However, we did find more males to leave the confines of their territories than expected from reports in previous studies. Our findings highlight the key role that warm, sheltered locations on reserves have in supporting the Duke of Burgundy. If this traditionally poor disperser is to take advantage of a warmer climate and extend its range North, a close network of such areas, appropriately managed, may be critical.