With the same observer, date shifts had a small estimated link to temporary changes in recorded bird communities on 419 Swedish routes, 1996–2025

We tested whether a temporary shift in survey date was linked to a temporary change in the recorded bird community. We used 2,397 sequences of three consecutive survey years on 419 Swedish routes in 1996-2025. Each sequence had the same observer. The two outer dates were within three days. The response was mean adjacent Jaccard dissimilarity minus outer dissimilarity. The model included route, year and date controls. The mean slope for earlier and later date departures was 0.0094 per seven days. The route-cluster 95% interval was 0.0010 to 0.0177. A stricter match of outer dates gave 0.0030, with an interval of -0.0129 to 0.0188. Intervals for three secondary measures also included zero. The main link was small and was not established across all checks. This result does not identify a causal date effect or true colonisation and extinction. The package includes a return-date diagnostic, the source archive, code, all sequence rows, figures and independent checks.

biodiversity monitoring · survey timing · bird communities · Jaccard dissimilarity · detection · Sweden · reproducibility