Mean flight dates moved 4.17 days earlier and date spread rose 0.46 days in eight butterflies at 77 UK transects, 1980–1999 to 2005–2024

We tested whether earlier butterfly flight dates came with more spread in those dates at the same local populations. We used the UK Butterfly Monitoring Scheme 2024 releases. The matched sample had 7,643 annual records for eight summer species at 77 transects, mainly in England. Each of 250 local populations had at least ten valid years in 1980–1999 and ten in 2005–2024. Each species had equal weight. Mean flight dates moved 4.17 days earlier. The mean standard deviation of flight dates rose from 10.42 to 10.87 days. The rise was 0.46 days, with a nominal 95% interval of 0.12 to 0.81 days from shared three-year calendar blocks. A check that also sampled geographic areas gave an interval of -0.01 to 0.88 days. Meadow Brown and Silver-washed Fritillary supplied almost all of the net mean rise; changes in the other six species nearly cancelled. Count, coverage and abundance checks gave small positive point changes. These selected count records show uneven changes in spread. They do not establish a general increase in flight-season length or a climate cause. The package contains unchanged source data, provenance, code, all results, figures, saved draws and independent arithmetic checks.

butterflies · phenology · flight date · seasonal spread · UKBMS · ecological change · reproducibility