Counts from separate years showed no clear median concentration change for 152 common landbirds on 812 BBS routes, 2000–2004 to 2015–2019

We tested whether common native landbird counts became more concentrated on fixed North American survey routes. We compared annual route shares from separate years. Under independent annual samples with constant route probabilities, this measure has no bias from annual sample size. We used 152 species on 812 complete BBS routes in 2000–2004 and 2015–2019. The median concentration change was −0.72%. The central 95% of spatial and year resamples was −3.31% to +2.90%. Smaller count totals were linked to greater concentration across species (Spearman correlation −0.51). A constant-share multinomial null with the observed annual totals did not reproduce this association. The selected subset with the same observer in all ten surveys gave a median increase of 5.20% for 103 species on 362 routes. Its resampling range was +0.08% to +6.82%. Thus the median result depends on the route panel. These counts do not correct detection changes or represent the whole continent. The package has source extracts, provenance, code, figures, simulations, and reproduction checks.

biodiversity · Breeding Bird Survey · spatial concentration · bird distribution · observer effects · sampling bias · reproducibility