Using age‐ratios to investigate the status of two Siberian <i>Phylloscopus</i> species in Europe

Distinguishing between vagrancy and regular migration has proved to be challenging as vagrants may, in theory, act as pioneers in the establishment of new migration routes. Because migratory songbirds tend to follow the same orientation and migration route throughout their lives, age assessment can help to distinguish individuals which have already used a route (i.e. adults) from those using it for the first time (i.e. juveniles). The presence of a significant proportion of adult individuals would suggest the presence of regular migrants in a vagrancy context. To decipher the status of two species of Siberian warblers in Europe, the Yellow‐browed Warbler Phylloscopus inornatus and the Siberian Chiffchaff P. tristis , we assessed the age‐ratios of (respectively) 324 and 117 individuals captured at different sites during the last decade. We compared whether the estimated age‐ratios differed from those estimated in Common Chiffchaff P. collybita , a common and widespread breeding and migrating species in Europe, used here as a control species. With 1.8% of birds being adult (one confirmed, five probable), the vast majority of Yellow‐browed Warblers occurring in Europe each autumn appear to be mostly, if not only, vagrants, implying that the species does not migrate regularly in Europe. These results are consistent with the very few spring records but still invite further research to unravel this remarkable phenomenon of large‐scale vagrancy. Conversely, we estimated that 9.4% and 29.7% of Siberian Chiffchaffs captured, respectively, during migration in Sweden and on regular wintering sites in southern France are adult birds, suggesting that the species could migrate towards southern Europe to winter, using a migration route in a markedly different direction from those known previously. These results urge more research to explore the origin, selection and causes of orientation variability in songbird species at the start of their first migration.