February-March warmth gave no clear common link to paired butterfly sighting gaps in Japan in 1953-2020

We tested whether February-March warmth changed the first-sighting date of Papilio machaon relative to Pieris rapae. Four separate region and period models used 2,945 paired station-years at 58 Japan stations in 1953-2020. Models controlled station and report-location pairs, year, and January temperature. Slopes were -1.99, +2.57, +0.72 and +0.85 days per C. All four intervals included zero after correction for four tests. Southern slopes were +2.50 and +2.79 with a linear year trend, with corrected intervals above zero. Year effects reduced those estimates and precision. Thus, a clear common local response did not repeat. This does not show equal biological responses. Unknown search effort and abundance, missing dates, changing report locations, small local weather contrasts, and coarse temperature data limit inference. Monthly weather windows extend past some sightings. The study does not test pollution, daily extremes, harmful mismatch, or causal climate-change effects. The package preserves source hashes, the paired panel, code, all planned models, figures, an independent regression check and reproduction instructions.

phenology · butterflies · Papilio machaon · Pieris rapae · seasonal temperature · observation bias · paired records · regional replication · null result