At North American bird feeders in 1989–2024, the winter community temperature index did not follow temperature anomalies over days to years, but it increased 0.18 °C per decade

Question: at which timescales does the winter bird community at a fixed site change its thermal composition when temperature changes? We used Project FeederWatch counts (3.93 million checklists at 104,450 sites in the USA and Canada, winters 1989–2024) and NOAA CPC daily gridded temperature. For each checklist we calculated a community temperature index (CTI) from species temperature indices of 184 species. Across sites, CTI increased 0.36 °C per 1 °C of December–February normal temperature. We used this slope as full tracking. Within winters (site x winter and calendar-date fixed effects, 2.37 million checklists), the response to a 7-day anomaly was −0.6% of full tracking (95% CI −1.2% to 0.0%), and no lag window up to 56 days was above 2.2%. Between years (site and winter fixed effects, 272,521 site-winters), the response was −1.5% (−6.6% to +3.6%), and the previous winter had no effect. In 782 half-degree cells, decadal CTI trends did not increase with local warming at the full-tracking rate (slope 0.07, CI −0.14 to 0.25, against 0.36). But the national CTI index increased 0.183 °C per decade (CI 0.147 to 0.201), and a linear trend fitted to 1989–2012 predicted 2013–2024 (RMSE 0.15 °C against 0.25 °C for no change). Weather changed the reporting rate of 99 of 184 species, but these responses did not depend on species thermal affinity (r = −0.07 and 0.08 for 61 common species). Most species came to feeders less in warm spells. The results were the same with presence weights, an STI from 1989–2000 data, and without 10 irruptive species. Conclusion: feeder communities do not follow weather or year-to-year temperature on the thermal axis, and their steady CTI increase is a slow change that is spread over many species and is not proportional to local warming. Limitations: feeder counts include feeder attendance; the STI is truncated at the USA–Mexico border; local warming trends are noisy; the cause of the slow increase stays unknown.

community temperature index · Project FeederWatch · winter birds · climate tracking · citizen science · timescale · temperature anomaly · range shift · irruption · North America