Daily cold nights added no information to the winter mean temperature for next-summer counts of winter-present birds on 4,571 North American routes in 1981-2025, but snow days and Winter Storm Uri did

Question: do daily winter extremes explain the next-summer decrease of bird species that stay at a route in winter better than the December-February (DJF) mean temperature, which four earlier campaign studies used? Data: North American Breeding Bird Survey 2026 release (88,406 route-years, 4,571 routes, 1981-2025), Daymet v4 daily minimum and maximum temperature and snow water equivalent at each route, and a GBIF winter-presence class per species and state. Outcome: mean change of log(1 + count) of winter-present species minus winter-absent species on the same survey; the absent species are the negative control. Plan written before the models. Results: alone, each extreme cold night (below the route 5th percentile) gave -0.30% (permutation z = -5.1); with the DJF mean in the model it gave -0.07% (SE 0.06; permutation p = 0.23), and the mean kept +0.98% per C (p = 0.001). Thus the pre-registered cold-night hypothesis was not supported. Snow-cover days (SWE >= 10 mm) gave -0.027% per day with mean and cold nights in the model (permutation p = 0.005; > 60 days -2.1%), negative in 6 of 6 region-period subsets, in a species-level model (-0.030% per day) and not in absent species; they reduced the mean term by 20%. Daily terms gave no out-of-sample prediction gain (skill 0.2-0.4% for all models). Winter Storm Uri (February 2021): on 233 southern routes with >= 5 extreme cold nights, the 2019-2021 contrast fell 10.9% (95% CI -15.5 to -6.2) more than on 258 mild southern routes, below all 40 placebo two-year periods (z = -2.5); the models predicted only -0.8% to -1.4%. Eastern Bluebird, Eastern Phoebe and Bewick's Wren decreased most; there was no recovery by 2023. Limitations: counts are an index without a detection model; Daymet is at the route start point; no ice data; Uri is one event with a two-year change across 2020.

Breeding Bird Survey · winter severity · extreme cold · snow cover · Winter Storm Uri · Daymet · resident birds · weather extremes · negative control · permutation test · out-of-sample test · population dynamics · climate change · North America · observation bias