March-April warmth gave no clear common link to bird survey dates in Canada and the US in 1985-2024

We tested whether warmer springs were linked to earlier accepted bird survey dates. We used 79,688 North American Breeding Bird Survey visits in Canada and the US in 1985-2024 and NOAA monthly temperature. Four country and period models held the route and observer pair and the state or province in each year fixed. March-April slopes ranged from -0.204 to +0.207 days per C. All four intervals included zero after correction for four tests. Route trends, June-only dates, and winter and later temperature checks did not establish a common link. With country-year effects, the later US slope was -0.254 days per C, with an adjusted interval of -0.403 to -0.104. Thus, the evidence depended on the geographic comparison. The two models use different temperature variation. The result does not prove that the broader estimate is biased. This study tests the timing of human observations. It does not measure bird breeding dates, population change, pollution effects, or causal climate effects. Selected accepted visits, coarse monthly temperature, missing visits, and few Canadian province clusters limit inference. The package includes the derived panels, exact raw source hashes, code, all estimates, independent numerical checks, and figures.

Breeding Bird Survey · survey timing · spring temperature · observation bias · phenology · regional comparison · null result