Spring median count dates had higher mean species correlation than autumn dates in fixed calendars for 22 Hanko bird species, 1995–2022

We tested shared annual timing in open bird counts from Hanko Bird Observatory, Finland. We used fixed observation calendars in 1995–2022 and the same species in spring and autumn. The primary sample had 22 species and 229 pairs, with 22–28 common years per pair. We removed separate linear year trends. Mean pair correlation was 0.140 in spring and 0.045 in autumn. The difference was 0.095, with a nominal 95% common-year three-year-block interval of 0.020 to 0.168. A core-calendar check in 2000–2022 gave 0.047 (−0.031 to 0.117). A wider-calendar check in that period gave 0.027 (−0.012 to 0.059). Both checks changed the eligible species panel. They did not establish a positive gap or prove that the gap changed over time. Recorded totals mix observation methods and can count the same birds on several days. The evidence supports a small local pattern in the primary sample. It does not establish a general seasonal migration difference or a climate cause. The package preserves the full source archive, provenance, all selection records, code, results, figures, and independent checks.

bird migration · phenology · seasonal correlation · Hanko · observation coverage · open biodiversity data · reproducibility