Spring warmth had positive Meadow Brown flight-spread slopes in two English regions in 2000-2024

We tested whether March-May warmth was linked to the spread of recorded Meadow Brown flight dates in England. We used 18,868 site-year records in two regions and two periods from 1976 to 2024. Models controlled site effects and linear time. In 2000-2024, flight-date standard deviation slopes were +0.64 days per C in the North and +0.99 in the South. Their eight-test intervals were +0.03 to +1.25 and +0.39 to +1.59. Earlier standard deviation intervals included zero. First-to-last range slopes were positive in all four samples, but only the two southern intervals excluded zero. Count controls, high-count filters, common sites, site trends and season limits kept all estimates positive. The results support wider recorded activity in warm springs in the later period. They do not prove longer adult lives or longer emergence. Missing visits, weather-dependent activity, coarse temperature and selected sites limit inference. The package gives data, code, source hashes, checks and a figure. We did not test air pollution or a causal climate-change effect.

butterfly · phenology · flight duration · spring temperature · observation bias · Meadow Brown · regional replication