Abstract. 1. Understanding which factors influence the ability of individuals to respond to changing temperatures is fundamental to species conservation under climate change. 2. We investigated how a community of butterflies responded to fine-scale changes in air temperature, and whether species-specific responses were predicted by ecological or morphological traits. 3. Using data collected across a UK reserve network, we investigated the ability of 29 butterfly species to buffer thoracic temperature against changes in air temperature. First, we tested whether differences were attributable to taxonomic family, morphology or habitat association. We then investigated the relative importance of two buffering mechanisms: behavioural thermoregulation versus fine-scale microclimate selection. Finally, we tested whether species’ responses to changing temperatures predicted their population trends from a UK-wide dataset. 4. We found significant interspecific variation in buffering ability, which varied between families and increased with wing length. We also found interspecific differences in the relative importance of the two buffering mechanisms, with species relying on microclimate selection suffering larger population declines over the last 40 years than those that could alter their temperature behaviourally. 5. Our results highlight the importance of understanding how different species respond to fine-scale temperature variation, and the value of taking microclimate into account in conservation management to ensure favourable conditions are maintained for temperature-sensitive species. $$ \ $$ Methods. Data were collected in the UK, on four calcareous grassland reserves in Bedfordshire owned and managed by The Wildlife Trust for Bedfordshire, Cambridgeshire & Northamptonshire (Totternhoe Quarry [-0.56836, 51.89199], Totternhoe Knolls [-0.58039, 51.88989], Pegsdon Hills [-0.37020, 51.95354] and Blows Downs [-0.49580, 51.88321]); at two sites in Cumbria (Irton Fell [-3.34000, 54.40672] and Haweswater [-2.84598, 54.50756]); at one site in Scotland owned and managed by the National Trust for Scotland (Ben Lawers [-4.27326, 56.53287]); and at a calcareous grassland reserve in Wiltshire owned and managed by the Royal Society for the Protection of Birds (Winterbourne Downs [-1.68500, 51.14963]). Between April and September 2009 in Bedfordshire, and between May and September 2018 at all sites, adult butterflies were surveyed at each site. After recording the behaviour of each butterfly when first encountered, we caught as many individuals as possible using a butterfly net. We did not chase individuals, to ensure that the temperature recorded reflected the activity of the butterfly prior to capture. Immediately after capture, we used a fine (0.25 mm) mineral-insulated type K thermocouple and hand-held indicator (Tecpel Thermometer 305B) to record external thoracic temperatures (Tbody). Only three individual devices were used for data collection, and were calibrated to the same readings prior to use. The thermocouple was pressed gently onto an exposed area of each butterfly’s thorax, while the butterfly was held securely in the net, away from the hands of the recorder to avoid artificially elevating the recording or causing any damage to the butterfly. Butterflies were then released. A second temperature recording was then taken at the same location in free air, in the shade, at waist height (Tair). In 2018, for butterflies perching on vegetation at the time of capture, a third temperature reading was taken by holding the thermocouple a centimetre above the perch location (Tperch). Information on habitat specialisation was added from Asher et al. (2001), mean wingspan was taken from Eeles (2020), and colour category was determined by assigning each species a score from 1 (white) to 6 (black) on the basis of how pale/dark wing colours appeared by eye (see Bladon et al. 2020 for details). $$ \ $$ References. Asher J, Warren MS, Fox R, Harding P, Jeffcoate G & Jeffcoate S (2001) The Millennium Atlas of Butterflies in Britain and Ireland. Oxford: Oxford University Press. Bladon AJ, Lewis M, Bladon EK, Buckton S, Corbett S, Ewing SR, Hayes MP, Hitchcock GE, Knock RI, Lucas CBH, McVeigh A, Menéndez R, Walker J, Fayle T & Turner EC (2020) How butterflies keep their cool: physical and ecological traits influence thermoregulatory ability and population trends. Journal of Animal Ecology. Eeles P (2020) UK Butterflies. Retrieved from UK Butterflies website: https://ukbutterflies.co.uk/index.php