Body temperature and territory selection by males of the speckled wood butterfly (Pararge aegeria): what makes a forest sunlit patch a rendezvous site?

2. The present study focuses on the territorial perching behaviour in males of the speckled wood butterfly (Pararge aegeria L.). Recent studies suggested that the selection of territories (i.e. sunlit patches on the forest floor) is driven by structural characteristics of the site that affect male visual detection. However, given that adult butterflies are heliothermic organisms and that forests provide a diverse array of light environments, it seems likely that thermal aspects may also be used for territory selection. 3. We tested whether used and unused sunlit patches differed in thermal profile under field conditions in a Belgian woodland. We also used dummy butterflies to quantify variation in operative thoracic temperature and to calculate heating rates within (i.e. different vegetation structures) and between patches. 4. Sunlit patches occupied by a territorial male were larger, and were more frequently characterised by low vegetation structures compared with empty sunlit patches. It took longer to reach optimal thorax temperature (starting from a fixed suboptimal body temperature) in small patches compared with large patches. 5. We suggest that aspects of visual detection need to be combined with thermal aspects to fully understand territory selection in the speckled wood butterfly, as synergetic and/or trade-off effects of ambient temperature, solar radiation, and canopy/vegetation structure may be involved.