First and mean Orange-tip dates had no clear difference in spring warmth slopes in three British countries in 1990-2024

We tested whether first sighting and mean flight date had different links to March-April temperature for Orange-tip butterflies. We used standard UKBMS transects in England, Scotland and Wales in 1990-2006 and 2007-2024. Six site-effect models controlled winter temperature, spring rain, year and peak count. Each country-year had equal total weight. All six mean-minus-first slopes were positive, from 0.02 to 2.45 days per C. All six corrected 95% intervals included zero. Thus, the data did not establish a repeated difference between the date measures. This is not evidence that the slopes are equal. The early Scottish sample had only 99 site-years at 12 sites. Filters for peak count, early sightings and record length changed some estimates. Missing visits, survey limits, site selection and coarse weather remain sources of uncertainty. The test does not measure a causal climate-change or pollution effect. The package gives source hashes, model inputs, code, results and an independent calculation check.

phenology · Orange-tip · temperature · observation bias · UKBMS · regional replication · null result