Recorded bird diversity rose from 2000–2009 to 2015–2025 on one Hubbard Brook plot, but a check for recording changes left the rise uncertain

We tested whether a reported bird count rise also increased the effective number of common species on one 10-hectare Hubbard Brook forest plot. We used a fixed list of 33 species and ten observed seasons in each period, 2000–2009 and 2015–2025. The 2020 census is missing. Inverse Simpson diversity rose from 7.87 to 9.49 effective species. The increase was 1.62, with a nominal 95% joint two-year block interval of 0.73 to 2.59. Total adult density rose by 28.2%, while the largest species share stayed near 25%. A check that removed four species with prior concerns about recording changes gave a smaller increase of 0.67, with an interval of −0.21 to +1.61. The increase was also uncertain when the recent period ended before the 2020 gap. Trace-value checks and block lengths one to three kept positive primary intervals. A subset of 19 species common in 1969–1978 gave no clear increase. The full recorded list supports a diversity gain under the stated time model, but recording and subset checks leave ecological recovery uncertain. This one-plot result does not show a regional change or a cause. The package has source bytes, provenance, code, estimates, resamples and figures. Independent calculations passed, and an offline repeat matched all numerical and figure files.

bird diversity · Hubbard Brook · long-term monitoring · inverse Simpson diversity · recording changes · reproducibility