First and mean Orange-tip dates had no clear temperature slope difference in four English samples in 1980-2024

We compared first appearance and count-weighted mean flight dates for Orange-tip butterflies in northern and southern England in 1980-2001 and 2002-2024. The study used 16,461 site-year records and local February-March temperature. Models controlled site effects and a linear year trend. Both dates had negative temperature slopes in all four samples. First-minus-mean slopes ranged from -0.54 to -0.03 days per degree C. All four adjusted intervals included zero. Peak count control and a short serial-dependence check did not give a clear repeated difference. Year effects changed the slopes and increased uncertainty. Removing records near the April survey boundary changed some gap signs. These data do not show equal slopes or absence of observation bias. Missing visit records, survey limits, selected sites and common weather limit the result. The package gives a derived panel, source hashes, code, a figure and checks.

phenology · butterflies · temperature · observation bias · Orange-tip · paired outcomes