Year-permutation tests show no robust increase in breeding-bird growth-rate synchrony in North American, UK, Swedish and Czech schemes, while temperature synchrony increased at NA and UK sites

Question: Koenig and Liebhold (2016) reported increasing spatial synchrony of North American wintering birds together with increasing temperature synchrony. Did the synchrony of year-to-year growth of breeding-bird populations increase, and can monitoring data detect the bird response that temperature predicts? Approach: We applied one method to four schemes: North American BBS 1966-2024 (263 species, 3,033 routes), UK BBS 1994-2024 (87 species, 2,765 squares), Swedish fixed routes 1996-2025 (74 species, 413 routes) and Czech JPSP 1982-2024 (42 species, 81 sites). We used within-observer growth rates, Pearson correlations between site pairs in 10-year windows, and within-pair trends at 0-250, 250-500 and 500-1000 km. All species share the same years, so we tested trends against a null in which we permuted the year order jointly for all sites and species. We added a known synchrony trend (+0.004 r per decade) to the real data as a positive control. We tested temperature synchrony at the same sites with CRU TS 4.10 and GHCN-CAMS. Results: Pooled bird-synchrony trends were between -0.0014 and +0.0055 r per decade in all 10 testable scheme-distance cells. No pooled p-value was below 0.048, and none stayed significant after correction for multiple tests. UK trends were positive but not significant (p = 0.06-0.29). A Swedish species-mean decrease (p = 0.005) was not confirmed by the pooled statistic. The 80%-power detection limits were about 0.003-0.005 r per decade in North America, 0.008-0.009 in the UK, 0.017-0.033 in Sweden and 0.032 in Czechia. In North America, the windowed correlation showed only 45-51% of the injected change. July-June temperature synchrony increased at North American routes (+0.009 to +0.045 r per decade, p <= 0.02), and May-June temperature synchrony increased at UK squares (+0.07 to +0.28, p <= 0.016). Both products agree. Bird synchrony is low (r = 0.01-0.04), but temperature synchrony is high (r = 0.67-0.98). Thus, if bird synchrony stays proportional to temperature synchrony, the expected bird trends are at most 0.0006 r per decade in North America and 0.001-0.004 in the UK, which are below the detection limits. Limitations: These are breeding-season counts only, so this study does not test the winter result. The North American null has 40 permutations. The European series are short, and the Czech test has very low power.

spatial synchrony · Moran effect · breeding bird survey · population dynamics · temperature synchrony · permutation test · positive control · statistical power · North American Breeding Bird Survey · UK Breeding Bird Survey · Swedish Bird Survey · Czech common bird monitoring · CRU TS · climate change