Colder winters were followed by lower next-summer counts of resident birds than of migratory birds on 4,097 North American survey routes in 1967-2024, with same-observer route-year controls

Question: does the temperature of the preceding winter change next-summer bird counts more for species that stay at the route in winter than for species that leave it? Approach: we used North American Breeding Bird Survey counts (2025 release) and GHCN-CAMS monthly gridded temperature. The response was the change in log count of each species on each route from one year to the next, with the same observer in both years (4,927,559 pairs, 361 species, 4,097 routes). AVONET migration classes separated resident, partially migratory and migratory species. A route-year fixed effect absorbed all survey conditions shared by species (observer, date, weather, effort). Migratory species, which are absent in winter, were the negative control. Results: after a winter 1 C warmer than the 1981-2010 route mean, resident counts changed by 0.94% more than migratory counts (95% CI 0.72 to 1.17); partial migrants by 0.83% more (0.61 to 1.05). The migratory class slope alone was 0.17% per C (-0.05 to 0.39). Cold winters had larger effects than warm winters: after winters 3 C or more below normal, resident counts fell 5.0% (3.8 to 6.2) relative to migrants. The contrast was positive with a CI above zero in all six region-by-period samples (West, Central, East; 1967-1995, 1996-2024) and in all three winter-climate terciles. A placebo with the winter after the survey gave no contrast (0.12%, -0.19 to 0.43). About 60% of a one-winter change reversed in the next year. The contrast did not clearly change between periods. Limitations: BBS counts are indices and class-specific detectability changes cannot be fully excluded; AVONET classes are coarse and contain some errors; monthly 0.5 degree temperature does not capture snow, ice or the duration of cold spells.

Breeding Bird Survey · winter severity · weather extremes · migration strategy · negative control · observer bias · population change · GHCN-CAMS · AVONET · climate change · birds · North America