Warmer April–May conditions were linked to wider gaps between mean flight dates in 55 butterfly pairs at 441 British transects, 1976–2024

Species can move to earlier flight dates at different rates. We tested whether spring warmth was linked to the time gap between mean adult flight dates at the same site. We used the 2024 UK Butterfly Monitoring Scheme releases and NOAA monthly temperature data. The sample had 123,715 paired records for 55 species pairs from 12 summer species on 441 Great Britain transects in 1976–2024. Each site and species pair had its own baseline and linear time trend. We estimated a temperature slope for each species pair, then gave each pair equal weight. A 1 °C warmer April–May was linked to a 1.16-day larger absolute gap. The nominal 95% interval from shared five-year calendar blocks was 0.66 to 1.69 days. Of the 55 pair estimates, 51 were positive. Higher abundance thresholds, peak dates, fixed date order and an inland climate check gave the same sign. A separate time-trend estimate was 0.14 days per decade, with an interval of −0.61 to 1.06. Thus, the warm-year link did not establish long-term widening. These observational adult counts do not measure causal temperature effects, full flight-period overlap or population fitness. The package contains the source ZIPs, a climate subset, provenance, all results, figures and code for a full offline repeat.

butterflies · phenology · temperature · temporal spacing · UKBMS · reproducibility