The mean flight-date gap showed no clear change among nine butterflies at 84 UK sites between 1980–1999 and 2005–2024

We tested whether summer butterfly species flew closer together in time at the same sites. We used the UK Butterfly Monitoring Scheme 2024 release. We compared 1980–1999 with 2005–2024. The sample had 16,524 annual paired records from 525 site-pair units, 25 species pairs, nine species and 84 sites. Each unit had at least ten shared years in each period. Each species had a peak count of at least five in the selected years. We gave each species pair equal weight. The mean absolute gap between recorded mean flight dates rose from 14.51 to 15.26 days. The change was +0.75 days, with a 95% resampling interval of −0.44 to +1.88 days. The mean date of each pair moved 4.57 days earlier, with an interval of −7.18 to −2.15 days. The uncertainty calculation kept common calendar-year variation across sites. An areas-only calculation gave a narrower gap interval that excluded zero. Checks with larger counts and more years did not show a clear mean gap change. This sample mainly represents persistent local pairs in England. It does not measure full flight overlap or the cause of timing changes. The package has exact source data, code, selection records, saved draws, figures and independent arithmetic checks.

butterfly · phenology · seasonal timing · paired comparison · shared year variation · open biodiversity data