March warmth gave no clear species difference in early butterfly records in England and Wales in 1985-2024

We tested whether March warmth had a different link to early records of Brimstone and Orange-tip butterflies. We matched 15,911 site-years in England and Wales in two periods, 1985-2004 and 2005-2024. The outcome was the Brimstone minus Orange-tip indicator for a first record by April 7. Site models controlled winter and April weather and a time trend. Each year had equal total weight. Difference slopes were -2.67 and +0.01 percentage points per C in England, and +11.23 and +4.08 in Wales. All four adjusted 95 percent intervals included zero. Count controls, a longer site history filter and short serial dependence checks did not give a clear difference. The early Welsh sample had only 41 site-years at three sites. Missing visits, selected records, coarse weather and 20 annual exposures per main fit limit inference. The test does not show equal responses or unbiased dates. Code, source hashes, aggregate estimates and numerical checks are included. External record data remain outside the publication.

phenology · butterflies · temperature · observation bias · survey boundary · paired species · null result