Peak-count slopes of scaled early flight tails were positive in eight England and Wales butterfly samples, 1985-2024

We tested whether first recorded flight dates were farther from mean flight dates when peak counts were larger. We used 32,999 site-species-years for Ringlet and Meadow Brown at 1,247 sites in England and Wales in 1985-2024. We divided each early date gap by its flight-date standard deviation. Eight models controlled site and calendar year. All peak-count slopes were positive. Six intervals adjusted for eight tests were above zero. The two uncertain intervals were for Ringlet in Wales. England slopes were 0.136-0.174 standard-deviation units per count doubling. Larger counts also had longer scaled late tails. Uniform multiplication of a fully observed count curve cannot change these scaled gaps. Sampling more of its tails is one possible explanation. Changes in flight shape, activity and visits are other explanations. The data do not identify detection bias or a climate effect. Few early Wales sites, selected detected populations, shared measurement errors and missing visit records limit inference. Code, source hashes, input extracts, model results, a figure and independent numerical checks are included.

phenology · butterflies · observation bias · peak count · flight period · regional comparison · ecological timing