Spatial synchrony of 168 North American breeding bird species did not increase in 1970–2024 BBS data at 0–1000 km, although winter temperature synchrony on the same route pairs increased

Question: Koenig and Liebhold (2016) found that the spatial synchrony of wintering bird populations (Christmas Bird Count, 1960–2009) increased together with temperature synchrony. Did the synchrony of breeding populations also increase? Approach: We used the North American Breeding Bird Survey (2025 release) for 168 widespread species on 5,306 routes. We calculated within-observer annual growth rates and the Pearson correlation between route pairs in five 11-year windows (1970–2024). Then we fitted the change in correlation with pair fixed effects, so that changes in the surveyed routes cannot cause a trend. We used the same pairs and years for detrended May–June and December–February temperature from three gridded products. We also repeated the 5-year-window method of the original study. Main finding: Bird synchrony was low but positive (mean r 0.01–0.05). It did not change at 0–100, 100–250, 250–500 or 500–1000 km. At 0–250 km, the mean change was −0.000 in r per century (95% CI −0.019 to +0.020), and 76 of 168 species had a positive slope. The result was the same when we kept observer changes, used only high-count routes, or corrected for Poisson count noise. With the original 5-year-window method, only 38–45% of the species had increasing synchrony (original CBC study: 63–69%). On the same pairs, December–February temperature synchrony at 500–1000 km increased by 0.21–0.25 in r per century in all three temperature products. A year-resolved index showed synchronous resident decreases after severe winters, for example 1977. Limits: BBS counts have large observation error, which decreases the measured correlations. Species share weather, so the species-level tests overstate independence. The test is for the breeding season and does not show that the winter result is wrong.

spatial synchrony · Moran effect · Breeding Bird Survey · population dynamics · climate change · replication · temperature synchrony · North America · birds