Abstract

The recorded duration of stay for vagrant birds is a key metric in ornithology, influencing our understanding of vagrancy patterns and the assessment of record provenance. This study investigates the apparent stay lengths of five North American landbird species by comparing records from regions with differing levels of observer effort: Britain versus Iceland at a national scale, and the Isles of Scilly versus the Outer Hebrides at a local scale within Britain. Results indicate longer apparent stay lengths in more heavily birded regions compared to more sparsely watched ones at both the national and local level. While ecological factors may contribute to variations in actual residency, the study highlights the substantial impact of observer density and reporting networks on the probability of detection and resighting. The well-established birding community and information dissemination systems in Britain nationally, and the Isles of Scilly locally, likely lead to increased resighting rates and, consequently, longer apparent stay lengths. Conversely, lower observer effort in Iceland and the Outer Hebrides likely results in underestimation of true stay durations and potentially the frequency of vagrant occurrences. These findings underscore the crucial role of observer effort in shaping our understanding of avian vagrancy and emphasise the need to consider this bias in ecological interpretations and rarity assessments.