First-date warmth slopes were more negative than mean-date slopes for Meadow Brown at 120 England sites in both 1980-2001 and 2002-2024

We compared the spring temperature slopes of first recorded dates and count-weighted mean flight dates for Meadow Brown. We used 4,458 site-years at 120 England sites and three Wales sites in two periods. Models controlled site, linear year and spring rain. In England, first-minus-mean slopes were -2.50 and -2.58 days per C. Four-test adjusted intervals were below zero. Wales gave -4.86 and +2.77 days per C, so the pattern did not repeat across both countries and periods. Peak-count adjustment and site trends retained the England pattern. These are weather associations in selected long-record sites. Missing visit records, local weather error, count changes and flight shape can affect both dates. The contrast cannot separate biology from observation bias. The package gives source hashes, a site-year panel, code, all estimates, a figure and checks.

phenology · butterflies · temperature · observation bias · Meadow Brown · regional replication