Site composition gave positive warmth slope estimates for Meadow Brown dates in four England and Wales samples, 1985-2024

We tested whether changes in the sites with records contributed to spring warmth slopes for Meadow Brown mean flight dates. We split each annual mean into a fitted site-composition term and a within-site term. Four England and Wales samples in 1985-2004 and 2005-2024 used 20,972 site-years. The composition slope estimates were +0.28, +0.03, +0.20 and +0.48 days per degree C. Two conditional four-test intervals excluded zero. Stricter record filters removed the early England result, and a simple site mean baseline gave an interval across zero in later Wales. All total and within-site corrected intervals included zero. A complete-site cohort gave a zero composition slope by construction. These results do not establish a common nonzero contribution across regions and periods or a causal climate effect. Fitted baselines, selected sites, missing visits, coarse weather and 20 annual exposures per fit limit inference. The package gives code, source hashes, aggregate results, a figure and independent matrix checks. External record data remain outside the publication.

phenology · Meadow Brown · site composition · observation bias · temperature · UKBMS · regional replication