In NestWatch records of 19 North American birds (1960–2025), the spatial temperature slope of first-egg date was 1.5–1.8 times the year-to-year slope; a data-source change confounds long-term tests

Question: does the year-to-year temperature sensitivity of bird laying dates agree with the spatial (space-for-time) sensitivity and with the long-term change? Data: 98,797 first nesting attempts of 19 passerine species in the NestWatch Open Dataset v6 (USA and Canada, 1960–2025). The first-egg date comes from nest checks during laying. Temperature: GHCN-CAMS 0.5° monthly, and nClimGrid 4 km as a check. Method: b_t is the within-cell year-to-year slope of first-egg date on species-window temperature. b_s is the slope of cell mean dates, adjusted to the 1991–2020 normal, on the cell normal. We used a 2° block bootstrap and pooled species with random effects. Results: the median b_t was −1.30 and the median b_s −2.27 days/°C. The pooled b_s − b_t was −0.88 days/°C (95% CI −1.30 to −0.45). It was negative in all ten variants (−0.53 to −1.32), and the interval excluded zero in nine. No species had a significantly positive difference. With 4-km temperature, b_t did not become steeper and the difference became larger (−1.32). With latitude in the spatial model, the 0.5° difference became small and uncertain (−0.21, CI −0.76 to 0.34). Thus space-for-time models overpredict plastic advance per °C by about 1.5–1.8 times, and part of this goes with latitude. Long-term: from 1960–1994 to 2005–2025, dates in the same cells advanced more than early b_t and local warming predict (Eastern Bluebird −7.1 days excess, Tree Swallow −1.6 days). But the comparison crosses a change from digitized nest-record cards to NestWatch entries, with only 7 shared cell-years. The excess depends on cell size, and two species have abrupt steps at the change. Within each source, observed changes agreed with the prediction within about 2 days. Limitations: citizen-science sampling, monthly temperature, first-brood selection by mixture models, and no redistribution of the non-commercial NestWatch records. Code and all aggregate results are included.

breeding phenology · laying date · space-for-time substitution · phenotypic plasticity · climate change · NestWatch · nest record cards · citizen science · birds · temperature sensitivity · data-source bias