Warmer springs were linked to wider recorded Meadow Brown flight periods in two English regions in 2000-2024

We tested whether spring warmth was linked to the spread of recorded butterfly flight dates. We used 29,886 site-species-years at 988 English sites for Meadow Brown and Ringlet in two regions and two periods from 1976 to 2024. Eight models controlled site effects and linear time. Meadow Brown flight-date standard deviation had slopes of 0.71 and 1.01 days per C in the North and South in 2000-2024. Their eight-test adjusted intervals were 0.06 to 1.36 and 0.37 to 1.65. All six other adjusted intervals included zero. Count controls, high-count filters, common sites, site trends and season limits kept both later Meadow Brown slopes positive. These checks do not separate biological spread from adult activity or missed visits. Shared weather, selected detected populations, coarse temperature and few early northern sites limit inference. This is a weather association, not a causal climate or pollution effect. Code, model inputs, raw source hashes, all estimates, a figure and independent numerical checks are included.

phenology · butterfly · flight period · temperature · observation bias · regional replication