Autumn windows set by each cicada date gave negative slopes after date shuffling in four Japan samples, 1953-2020

We tested whether a weather window set by each observed event date can create a false temperature signal. We used 4,243 cicada first-song records at 71 Japan stations and NOAA monthly temperature in 1953-2020. Four separate region and period models used the 102 days from 322 to 221 days before each reported date. All 1,000 within-station date shuffles in each sample gave negative slopes. Their medians were -5.23 to -5.74 days per C. Windows fixed by July 15 gave shuffled slopes centered near zero. All four adjusted intervals for observed July 15 slopes included zero. A fixed seasonal climatology also gave negative moving-window slopes. An exact identity explains how a cooling season causes this association. The result supplies a method diagnostic, not a causal climate estimate or a direct reanalysis of earlier daily-weather models. Unknown detection effort, selected stations, monthly weather and wide fixed-window intervals limit biological inference. Code, source hashes, extracted data, figures and independent numerical checks are included.

phenology · cicada · temperature window · observation bias · seasonal cycle · date shuffling · climate attribution · reproducible method