Spatial synchrony of 183 breeding bird species did not increase on North American BBS routes in 1967–2024, while summer temperature synchrony at the same routes increased

Question: Koenig and Liebhold (2016) reported that the spatial synchrony of wintering bird populations (Christmas Bird Count) increased in 1960–2009 together with summer temperature synchrony. Does this increase also occur in breeding populations? Approach: We used the North American Breeding Bird Survey 1966–2024 (183 widespread species, 3,031 routes). We measured route-pair synchrony at 0–1000 km in moving windows with five metrics: the original design (raw counts, 5-year windows), within-observer growth rates, change within fixed route pairs, and a split-route covariance ratio that is robust to sampling noise. We measured temperature synchrony at the grid cells under the same routes with four gridded products. Results: Bird synchrony did not increase. With the original design, 43%, 46% and 43% of species had positive trends at 0–250, 250–500 and 500–1000 km (95% CIs 35–54%). The CBC study reported 69%, 63% and 65%. The noise-robust ratio at 0–250 km changed by −0.9% per decade (95% CI −5.8% to +4.0%). It decreased at 500–1000 km. Summer Tmax synchrony in CRU TS 4.10 increased in 1961–2009 by +0.02 to +0.06 r per decade, similar to the original report. The increase became smaller after 2009. Bird growth rates had almost no correlation with local May–July temperature change (median |r| ≈ 0.02). Thus the temperature trend predicts only about 0.0002 r per decade of change in bird synchrony. Limitations: This study tests breeding-season roadside counts only and does not reanalyse CBC data. Species share years and places, so the species-level confidence intervals can be too narrow. Gridded temperature products differ in the size of the change.

spatial synchrony · Moran effect · North American Breeding Bird Survey · climate change · temperature synchrony · population dynamics · replication · CRU TS · observer effects · metapopulation