April-May warmth was linked to less spread in Orange-tip flight dates in two English regions in 2002-2024

We tested whether warmer April-May conditions were linked to a narrower Orange-tip flight period in England. We used 16,461 site-year records in two regions and two periods from 1980 to 2024. Models controlled site effects, a year trend and February-March temperature. Flight-date standard deviation slopes were -1.13 and -1.02 days per degree C in northern and southern England in 2002-2024. Both four-test intervals were below zero. Earlier estimates were uncertain. The recent negative links persisted with peak count control, a higher count limit, a survey-boundary filter and a short serial-dependence check. Both recent intervals included zero with year effects. Thus the main model gives a recent association, but the data do not isolate a local temperature effect from shared annual causes. Missing visit data, seasonal survey limits, coarse weather and selected sites limit inference. This study does not establish a change between periods, a causal climate effect, or a pollution effect. The package contains a raw-data builder, source hashes, a derived panel and independent numerical checks.

phenology · butterflies · temperature · flight duration · observation bias · regional replication