Seasonal models gave positive spring warmth slopes for the Large Skipper minus Orange-tip flight-date gap in England, 1985-2024

We tested March-April warmth and the Large Skipper minus Orange-tip mean flight-date gap at matched English sites. Four site-effect models used 3,143 site-years in North and South England in 1985-2004 and 2005-2024. Models controlled winter temperature, later spring temperature and a time trend. Slopes were +4.84, +2.53, +6.67, +2.68 days per C in North early, North late, South early and South late order. All four corrected intervals are above zero. Under the seasonal model, warmer March-April conditions are linked to a larger Large Skipper minus Orange-tip gap in each partition. This supports a repeated conditional association. It does not establish a causal temperature effect or a difference caused by overwintering stage. Missing visits, selected populations, count-weighted dates, shared regional weather and coarse temperature limit inference. All year-effect intervals include zero, and one point estimate is negative. Regional weather correlations exceed 0.96. The package gives source hashes, the paired panel, code, figures, sensitivity tests and independent numerical checks. No causal climate, pollution, extreme-weather or ecological harm claim is made.

phenology · butterflies · matched species · observation bias · temperature · regional replication