Joint positive May–June NAO and NPO phases did not precede pine siskin irruptions or low conifer seed crops, and a fitted ridge index had no out-of-sample skill (1989–2026)

Question: Bai et al. (2024, ERL) state that pine siskin North-South irruptions occur after a May–June with NAO and NPO both above their upper terciles, and that a fitted North American Ridging (NAR) index correlates with the irruption index at r > 0.63 (1989–2021, two irruptions). Approach: we rebuilt the irruption index from Project FeederWatch (1989–2024; 36% of cell variance, irruptions only in 1989/90 and 2008/09, as published). We tested the two statements with NCEP R1 (1948–2025) and ERA5 (1959–2021) May–June circulation, four NAO/NPO index pairs and two lags. We used a shuffle null, leave-one-season-out validation and forecasts. We added an iNaturalist index for winters 2024/25–2025/26 (r = 0.95 with FeederWatch), MASTREE+ conifer seed series (1962–2018) and nine other irruptive species. Results: the joint trigger came before neither irruption (0 of 2 for all 8 index-lag combinations). Trigger summers in 2024 and 2025 came before low-siskin winters. NAR in-sample r was 0.24–0.39, equal to the mean in-sample r of the same fit to shuffled years (0.36–0.38). Cross-validated r was −0.24 to 0.22, and a map fitted on 1989–2012 forecast 2013–2024 with r = −0.25 to −0.62. Low conifer seed crops did precede irruptions (Spearman rho = −0.57, p = 0.0008), but trigger summers did not reliably precede low seed years (one-sided p = 0.08–0.54), and NAR did not correlate with seed (r = 0.04, −0.05). In ten species, a trigger preceded 1 of 18 irruption seasons. Limitation: we could not reproduce the published r > 0.63 because their exact index boxes, 0.25-degree ERA5 and FeederWatch gridding were not available. Two events permit only weak tests, and the 2025–2026 values come from a proxy index.

pine siskin · irruption · Project FeederWatch · North Atlantic Oscillation · North Pacific Oscillation · mast seeding · MASTREE+ · cross-validation · teleconnection · replication · boreal birds · iNaturalist · ERA5 · out-of-sample test