Spatial synchrony of breeding bird population growth on 1,299 North American BBS routes did not increase from 1969–1996 to 1997–2024, while annual air temperature synchrony increased

Koenig and Liebhold (2016) reported that the spatial synchrony of many North American wintering bird populations (Christmas Bird Count) increased from 1960 to 2009, together with temperature synchrony. We tested this claim with the North American Breeding Bird Survey (BBS 2025 release). For 198 species on a fixed set of 1,299 routes, we calculated the mean Pearson correlation of same-observer annual log growth rates between route pairs in four distance classes, for 1969–1996 and 1997–2024. A year bootstrap used the same resampled years for every species and for temperature. Bird synchrony did not increase. The change was −0.0016 to +0.0007 at 0–500 km and −0.0040 [95% CI −0.0076, +0.0002] at 500–1,000 km, and 42–46% of species increased (upper 95% limits 0.51–0.55, below the 0.63–0.69 reported for the CBC). The result was the same with all observer transitions, with a fixed-variance covariance, and with growth rates aggregated to 2°×2° cells (151 species; −0.0116 [−0.0208, −0.0014] at 500–1,000 km, 36% of species increased). The change did not differ between migration classes. A replication of the 5-year window design of Koenig and Liebhold on raw counts gave fewer than half of species with increasing synchrony below 2,500 km and 59–64% at 2,500–5,000 km. Observer turnover decreased from 35% to 15–20% of route-windows in the same period. At the same routes, the synchrony of detrended July–June air temperature increased in CRU TS 4.10, GHCN-CAMS and Berkeley Earth (+0.09 to +0.11 at 500–1,000 km; +0.21 to +0.22 at 1,000–2,500 km). The summer (June–August) increase was smaller and not significant beyond 500 km. Thus, an increase in temperature synchrony did not cause a measurable increase in breeding population synchrony. Limitations: route-level correlations are small (0.002–0.03) because of count noise; the bootstrap treats years as exchangeable; the BBS samples breeding populations, so the result does not refute the winter CBC result.

spatial synchrony · Moran effect · Breeding Bird Survey · population dynamics · climate change · observer effects · replication · temperature synchrony