Sampling was conducted at five sites in Catalonia, north-eastern Spain, consisting of a mixture of grassland and Mediterranean oak forest and comprising different altitudes (0-1,400 m above sea level), from March to September 2019 (22 days) and from March to October 2020 (23 days). Data were collected between 10:00 and 18:00, except for some occasional records outside this interval; the necessary permits were obtained from the Generalitat de Catalunya (record Nº SF/0048/2019 and SF/0086/2020). For Great Britain, data were collected from five calcareous grassland and three upland grassland localities, in summer 2009 and 2018. For details see Bladon et al. (2020). No ethical approval was required for this study.

The same methods were used for collecting data in both regions to ensure comparability. Butterflies were searched for throughout the day at each site. When a butterfly was captured, its thoracic temperature (Tbody) was measured by touching a type-K thermocouple probe (TC direct, reference 406-477) attached to a handheld indicator (TC direct, reference 753-526) against the butterfly’s thorax. Measurements were taken within 30 seconds of capture, while holding the butterfly in the net without directly touching it and keeping it in the shade, to minimise the effect of temperature changes inside the net. All nets were of a similar colour (approximately AMT-4 green or Hex #645d40) to minimise possible warming differences. Next, air temperature (Tair) was measured at waist height in the shade and, if the butterfly had been on a substrate before capture, air temperature approximately 2 cm above the substrate (microclimate temperature or Tmicrocl) was also measured.

To estimate solar radiation experienced by the butterflies, we used the sum of beam normal irradiance (BNI) and diffuse horizontal irradiance (DHI) obtained from the Copernicus Atmosphere Monitoring Service (Schroedter-Homscheidt et al., 2022). BNI measures the radiation that arrives directly from the sun (i.e. without being scattered) at a surface perpendicular to the sunbeam. We chose BNI, instead of other measures of solar radiation, because butterflies can orient themselves to the sun to maximise the radiation they receive. DHI measures the radiation that arrives at a horizontal surface after being scattered by the atmosphere, which can be a considerable component of the radiation balance in some cases (Xie et al., 2021). These variables are interpolated from various input data at spatial resolutions from 3 to 5 km (CAMS; 2022). We downloaded data for each sampled site and month, measured as Wh/m2 over time windows of 1 minute.

To estimate average forewing length for each species in Catalonia, a minimum of eight specimens per species were captured and taken to the laboratory. The butterflies were photographed using a standardised setup, consisting of a Canon EOS Rebel T2i DSLR camera attached to a Kaiser RS 2 CP copy stand (product reference KA5304), surrounded by a protective case made from white cardboard to provide a shield from external lighting. ImageJ software (Schneider et al., 2012) was used to measure forewing length, by drawing the outline of the forewing and measuring the longest line contained within it. An average was taken for each species or, in the case of sexually dimorphic species, for each species and sex.