Cold winters decreased next-summer counts of resident birds but not of long-distance migrants on the same North American survey routes in 1968-2019

Question: does a cold winter decrease bird counts in the next breeding season, and only in the species that were present in that winter? Approach: we used 86,809 same-observer pairs of consecutive-year North American Breeding Bird Survey counts on 4,508 routes (winters 1968-2019) and the December-February temperature anomaly of the GHCN_CAMS 0.5 degree cell of each route. We compared the change in a resident-species index (130 species) with the change in a long-distance-migrant index (114 species) on the same survey. The migrants were absent in winter, so they control for observer, date, weather and route effects. Models had route and year fixed effects, a spring-anomaly covariate and standard errors clustered by year. Result: each +1 C of winter anomaly increased the resident-minus-migrant change by 0.0133 log units (95% CI 0.0097 to 0.0169). Residents responded (0.0128) and migrants did not (-0.0004, CI -0.0027 to 0.0019). The response was asymmetric: winters more than 3 C colder than normal were followed by a resident index approximately 7% lower (CI -9.0% to -4.5%), with no migrant change. A placebo exposure (the winter after the survey) had no effect. The slope replicated in eastern and western, northern and southern routes, in 1968-1993 and 1994-2019, and with four alternative guild definitions. Canadian routes alone gave an uncertain slope. Of 83 resident species, 77% had positive slopes (12 clearly positive, 0 clearly negative); migrant species were centred on zero. Body mass did not predict sensitivity. An out-of-sample test on Winter Storm Uri (February 2021) was inconclusive: on 110 severe southern routes the 2019-2021 contrast fell by 0.110 relative to milder routes, but mainly because migrants increased, and similar contrasts occurred in 2 of 20 placebo year pairs. Limitations: monthly gridded temperature does not resolve short extremes; resident-specific detection changes after cold winters cannot be fully excluded; guild lists use expert knowledge of winter ranges because raw AVONET migration codes misclassify many North American breeders.

winter severity · weather extremes · Breeding Bird Survey · resident birds · long-distance migrants · negative control · observation bias · Winter Storm Uri · GHCN_CAMS · AVONET · population dynamics · climate change