Orange-tip flight-spread warmth slopes had no clear period change in two English regions in 1980-2024

We tested whether April-May temperature slopes for Orange-tip flight-date spread changed between 1980-2001 and 2002-2024. We used a verified published panel with 16,461 site-year records in northern and southern England. Models allowed separate site intercepts, temperature slopes and time trends in each period. Late-minus-early slopes were -1.01 and -0.55 days per degree C. Two-test 95% intervals were -2.69 to +0.67 and -2.00 to +0.90. All planned count, survey-boundary, same-site, year-effect and short serial-dependence checks also included zero. These tests do not establish a period change or equal slopes. Selected sites, changing survey coverage, missing visit effort, few annual error groups and coarse weather limit inference. This is an ecological timing test, not a causal climate or pollution test. The package includes code, aggregate results, source hashes and an external-input reproduction script.

phenology · butterflies · temperature · period contrast · observation bias · null result