On the same BBS routes, landbird populations in the warmer part of their species' thermal range declined relative to colder-range populations in 1994–2023, but not in 1970–1993

Question: in North American landbirds, do populations in the warm part of a species' thermal range decrease relative to populations in the cold part of the same species' range? Approach: we used the 2024 North American Breeding Bird Survey release (standard runs, 1966–2023). We calculated a within-observer quasi-Poisson trend for each species on each route. For each pair we calculated the route's position z in the species' May–July temperature niche (abundance-weighted, focal route left out). We fitted trend = species effect + route effect + γz. The route effect removes all causes that change all species on a route by the same amount. Results: in 1994–2023 (286 species, 2,737 routes, 181,314 pairs), γ was −0.92 %/year per niche SD (species-clustered SE 0.16; species bootstrap 95% interval −1.41 to −0.65). The relation was approximately linear. The result stayed with controls for latitude, precipitation and longitude position and for local abundance, with five other data choices, and in eight regional and taxonomic subsets. A longitude-position placebo gave −0.04 ± 0.15. In 1970–1993, γ was −0.02 ± 0.30. On 1,193 routes and 232 species present in both periods, the difference between the periods was −0.86 ± 0.35 (p = 0.014); with a latitude control it was −0.73 ± 0.47. In sliding 15-year windows, γ was near zero until windows that ended about 2004, and then became more negative as the May–July anomaly increased. γ did not become more negative where routes warmed more: the interaction was +0.04 to +0.08 per SD of warming in GHCN-CAMS and CRU TS metrics. Main limitations: the design is observational; route-level warming is poorly measured (the two products agree with r = 0.25–0.45), so the local-warming test has limited power; the BBS area truncates the warm limit of many species. The pattern agrees with a time-dependent continental signal of thermal range shift, but it does not identify local warming as the mechanism.

Breeding Bird Survey · climate change · range shift · thermal niche · population trends · birds · North America · fixed effects · species distribution · climatic debt