Survey-date slope changes were uncertain in Canada and US bird records in 1985-2024

We tested whether the relation between survey date and recorded bird richness changed with year. We used 64,682 accepted Breeding Bird Survey visits from Canada and the US. We fit four country-period models with route-observer and state-year effects and survey weather controls. All four intervals for the date by year term included zero after correction for four tests. Estimates ranged from -0.614 to +0.104 taxa per 10 days later date per decade. Route trends, local date slopes, June-only visits, and log richness did not establish a common change. This result does not show that biological timing was stable. Selected roadside visits, changing local communities, single annual visits, a fixed quadratic date shape, and approximate cluster intervals limit inference. The records do not separate bird availability, detection, and presence. The package preserves code, a raw-data rebuild check, source hashes, the derived panel, all model results, and a figure.

bird richness · survey timing · observation bias · phenology · Breeding Bird Survey · temporal replication