January-February warmth had no clear common link to the gap between butterfly and swallow sightings at 69 Japan stations in 1953-2020

We tested whether January-February warmth changed the gap between first butterfly and first barn swallow sightings in Japan. We used 4,077 paired station-years at 69 fixed stations in 1953-2020. Four separate North and South, early and late models removed station-by-location-code and year effects. Slopes ranged from -5.78 to +0.51 days per C. All four adjusted uncertainty intervals included zero. The early southern marginal interval was negative, but the planned repeated-association rule failed. Station trends, adjacent-year differences, and January-only checks did not give a common link. Hypothetical independent report delays of 0.17-1.28 days permit zero slopes in the fixed-record models. These are possibility bounds, not estimates of real delay. Missing pairs, abundance and detection, coarse weather, current coordinates, and small residual weather contrasts limit inference. We do not estimate feeding overlap, air pollution, daily extremes, or causal climate-change effects. The package includes source event tables, an audited panel, weather extracts, source hashes, code, all estimates, figures, and independent numerical checks.

phenology · butterfly · barn swallow · Japan · temperature · first sighting · observation bias · regional replication · date uncertainty · null result