Winter feeder-bird communities in North America shifted to warmer-adapted species in 2002-2024, but year-to-year and local winter temperature did not explain the shift

We tested the reported warm shift of winter bird communities at Project FeederWatch sites (Prince and Zuckerberg 2015, eastern North America, 1990-2012). We used 3.49 million public FeederWatch checklists (seasons 2001-2024) and NOAA GHCN_CAMS temperature. The analysis plan was written before the models were fit. The community temperature index (CTI) used species temperature indices from a separate half of the sites and only December-February checklists. With site fixed effects, CTI increased in the East by +0.09 C per decade in 2002-2012 (95% interval -0.02 to +0.19) and by +0.22 C per decade in 2013-2024 (+0.14 to +0.30). The extended early trend predicted the later seasons better than no change (skill +0.65). In the West, an independent region, CTI increased by +0.28 and +0.17 C per decade, but mean winter temperature at the sites did not change. Year-to-year CTI did not follow winter temperature anomalies: -0.006 C per C in the East (-0.022 to +0.010) and -0.037 in the West, against full-tracking slopes of 0.22 and 0.38. Exploratory tests found no lagged response, and 2-degree blocks that warmed more did not gain more CTI. Gains of a few warm-adapted species (for example Carolina Wren, Eastern Bluebird, Anna's Hummingbird, Eurasian Collared-Dove) and losses of irruptive northern finches caused most of the change. Limitations: feeder records mix distribution with feeder use and detection, the series starts in 2001, and we did not test feeder practices, land cover or food crops.

community temperature index · Project FeederWatch · winter birds · climate change · citizen science · observation bias · out-of-sample test · range shift · GHCN_CAMS · replication