Mean spring–autumn count correlation was higher for the same species in 22 Hanko migrants under fixed calendars, 1995–2022

We tested seasonal count agreement for 22 migrant bird species at Hanko, Finland, in 1995–2022. We used the same calendar days in each year, took log(1 + count), and removed separate linear year trends. Mean spring–autumn correlation was 0.262 for the same species and 0.089 for different species. The difference was 0.173, with a nominal 95% common three-year block interval of 0.064 to 0.219. The count-level contrast stayed positive in the tested calendar, rank, and species-panel checks. For consecutive annual changes, the difference was 0.036 (−0.038 to 0.103). That check did not establish a positive species signal. Counts combine observation methods and can include repeated birds. Calendar control does not correct hours of effort, weather, or detection. This result concerns agreement of recorded count indices at one site. It does not estimate true abundance, survival, or a climate effect. The package preserves the full source archive, provenance, code, figures, all results, and independent numerical checks.

bird migration · seasonal counts · Hanko · monitoring · species distributions · reproducibility