Annual bird community dissimilarity showed no clear rise between 1996–2010 and 2011–2025 on 103 Swedish routes with the same observers

We tested whether the annual difference between recorded bird communities increased between 1996–2010 and 2011–2025. We used Swedish fixed-route bird counts and kept the same observer on each route in both periods. Survey dates within a pair had to be within seven days. Each route and observer combination needed at least three annual pairs in each period. The sample had 1,558 pairs on 103 routes, with 83 observer IDs. With equal route weights, mean Jaccard dissimilarity was 0.3548 early and 0.3580 late. The late-minus-early difference was 0.0032. Its approximate 95% interval was −0.0047 to 0.0110, with errors grouped by spatial block and calendar year. Stricter date and sample rules, date and time controls, two secondary measures, and an added observer dependence check also gave small, uncertain mean differences. The evidence does not show a clear rise in annual recorded dissimilarity in this selected, mainly southern cohort. It does not prove equal ecological turnover. Detection can change even with the same observer, and annual dissimilarity can stay constant while communities change over decades. The package has the source archive, provenance, code, tables, figures and a checked reproduction run.

bird monitoring · temporal beta diversity · annual dissimilarity · observer effects · Sweden · reproducibility