Four European bird schemes give 4–9 times less leverage than BBS routes for the species-and-site test of warm-range decline; the decline appears only with abundance-weighted niches

Question: do populations in the warm part of a species' thermal range decrease relative to cold-range populations of the same species in Europe, as an SCI study found on North American BBS routes in 1994–2023 (gamma = −0.92 %/year per SD of niche position, species and route effects)? Approach: we fitted within-observer trends for 172,828 species–site pairs (130 landbird species, 5,070 sites) in the UK Breeding Bird Survey (1994–2024), Swedish fixed routes (1996–2025), Norwegian TOV-E (2006–2025) and Czech JPSP (1994–2024). Niches came from the EBBA1 atlas and WorldClim May–July temperature. We used the same weighted species-and-site fixed-effects design. We also reanalysed the published BBS pair table. Results: with species and site effects, gamma is identified only by site temperature × 1/(niche SD), so the test needs a wide temperature gradient. In the BBS, gamma stayed when we added seven route-temperature × trait terms (niche mean, range size, latitude niche width, body mass, migration, habitat density, hand-wing index): −0.94 ± 0.17. In Europe, the SD of niche position left after the fixed effects was 0.032, against 0.296 on BBS routes. With presence-weighted niches (our primary choice), gamma was −1.10 ± 1.87, with about 7% power for the BBS value. With niches weighted by EBBA1 abundance class (the weighting of the BBS study), leverage was 2.5 times higher and gamma was −1.50 ± 0.72 (species-clustered p = 0.038; species-permutation p = 0.008). Binned trends decreased monotonically from cold to warm positions. This result stayed with latitude and abundance controls and strict site selection (−1.29 to −1.91). It was weaker with a range-size control (−1.17 ± 0.74), in 2006–2024 (−0.85 ± 0.77), in Czechia (0.00 ± 0.81) and in non-passerines. Limitations: national schemes cover 3–7 °C of site temperature; EBBA1 is from 1972–1995 with coarse abundance classes; many variants were tested; the analysis is observational.

thermal niche · range position · climate change · breeding birds · population trends · fixed effects · statistical power · EBBA1 · Breeding Bird Survey · replication