Sampling depths of 25 and 100 birds gave no clear difference in weather slopes in four North American survey samples, 1985-2024

We tested whether bird survey weather slopes changed with the number of counted birds used to estimate taxon richness. We held survey rows fixed and compared expected richness at 25 and 100 birds in 31,620 surveys from Canada and the US in 1985-2004 and 2005-2024. The four differences in log-richness ratio slopes had adjusted intervals that included zero. A common-sample check at 200 birds, fair-weather filters and route trends test sensitivity. This result does not establish equal slopes or unbiased observation. Count thresholds select surveys, and rarefaction cannot recover undetected birds. The countries have separate routes, but periods share routes. We reuse verified published count histograms and do not claim an independent raw-data replication, a climate-change effect or a pollution effect. The package includes code, input hashes, all model results and a figure. The later Canadian 200-bird check was not identifiable, and the later US route-trend check did not converge.

birds · rarefaction · observation bias · sampling depth · survey weather · null result