Spring warmth gave no clear repeated change in the gap between first and mean Meadow Brown flight dates in Britain, 1980-2024

We tested whether spring warmth changed the gap between first sighting and weighted mean flight date for Meadow Brown. We used 15,789 site-year records at 942 British sites in 1980-2024. Four separate region and period models controlled site effects, winter temperature, time trend and peak count. Gap slopes were +2.19, +2.10, +1.47 and +0.68 days per C. All four primary intervals included zero after correction for four tests. Removing peak-count adjustment changed each estimate by less than 0.03 days per C. Each slope stayed positive when one year was removed. A model with year effects reversed the later northern sign. Thus the data did not resolve a common nonzero gap response under the fixed test. Missing visit histories prevent separation of real flight changes from detection changes. Selected sites, one species, coarse weather, shared regional conditions and short periods limit inference. We do not test pollution, daily extremes or causal climate-change effects. The package gives source hashes, code, paired date and weather evidence, all estimates, a figure and independent checks.

butterfly · phenology · Meadow Brown · temperature · observation bias · first sighting · weighted mean flight date · regional comparison · null result