A team of researchers led by Paul Dufour of the University of Gothenburg in Sweden have reviewed evidence on whether Yellow-browed Warbler is still solely a vagrant to Europe, or whether the species is demonstrating the evolution of a genetically determined new migration route.
Yellow-browed Warbler breeds in Siberia and almost all individuals spend the winter in southern Asia, but the last 30 years have been an exponential increase in the frequency of the species in Europe. Most years now see thousands reported in northern and western Europe, from Fennoscandia to the Iberian Peninsula. The question gripping scientists and birders alike has been whether this upsurge is the ongoing result of vagrancy in a successful and expanding species, or whether a proportion of Yellow-browed Warblers have been successfully and repeatedly migrating through Europe as a new migration route evolves.
Yellow-browed Warbler has become a staple of autumn birding in much of Europe over the last few decades (Brian Small).
If the birds we see each autumn are simply vagrants, we are bearing witness to thousands of doomed birds exiting the gene pool every year. However, if at least some are returning to breed in Siberia and passing on the urge to head south-west instead of south-east to their offspring, then Yellow-browed Warbler would join Eurasian Blackcap and Richard's Pipit as one of the few species in which the recent evolution of an alternative migration route has been documented.
The argument for the yearly flood of Yellow-broweds involving vagrants – individual cases of birds going in the wrong direction, with no real hope of passing on their DNA – is sound. The authors pointed out that practically all birds detected in north-west Europe in autumn are birds hatched that year, suggesting few or none survive to go back the other way. Between 2010 and 2021, every single Yellow-browed Warbler trapped at the bird observatory at Ottenby in Sweden was considered a first-year.
Furthermore, passage through countries like Norway and Britain is hardly the most direct route to the potential wintering grounds in Iberia and north-west Africa, and with birds routinely making it to extreme locations like Shetland and Iceland, it may be that most end up in the sea. Although spring records have seen an increase over the years, the number of birds in spring only represents 1-2 % of the figures logged in north-west Europe during autumn passage.
Isotope analysis, tracking studies and ringing may shed light on the evolutionary significance of the thousands of Yellow-browed Warblers that appear in Europe each year (Graeham Mounteney).
However, it might be that surviving birds take a more direct route through under-watched parts of Europe on their way back to Siberia. A similiar scenaro was revealed in Richard's Pipit through tracking data, with positions logged in central Europe but the birds evading the attention of any observers in the field. The authors cite the increase in winter records in the Iberian Peninsula and north-west Africa as an indication that at least some Yellow-browed Warblers may be migrating there through Europe to reach new wintering grounds.
One ringed bird was seen for two winters running at a site in Andalucia in Spain, supporting this idea, while a bird ringed on the German island of Heligoland in autumn 2013 and retrapped in Lanzarote the following January also lends support to the migration hypothesis. Indeed, the overall timing of Yellow-broweds flowing through Europe over the course of the season, and a number of other ringing recoveries, fits with a consistent south-westerly migration from Siberia.
Some recoveries have pointed towards the birds being able to reorient their course to the south-east upon hitting western France, so it might be that they don't all just keep going on an erroneous course and fall into the sea after all, as vagrants might reasonably be expected to.
To be considered true migrants, rather than vagrants, it would need to be proven that Yellow-browed Warblers survive the western route, return to Siberia and pass on the genetically determined heading to their offspring, which would go on to do the same. Proving this presents no easy task, but Dufour and his team put forward a set of proposals for a wide-scale effort to solve the mystery.
The implications of breaking ground in the knowledge of the mechanisms behind the movements of Yellow-browed Warbler could have profound implications for our understanding of how migration orientation evolves.
The authors suggested putting greater effort into accurately ageing Yellow-browed Warblers in the hand and in the field, alongside the use of geolocators on birds reaching Europe to start to build an evidence base. Undertaking isotope analyses from feathers dislodged from trapped birds would give a good idea of the origin of the birds too. To learn more, this information could be followed up with fieldwork at these sites on the breeding grounds.
Orientation experiments on trapped birds at a range of locations across Europe could build a picture too, with birds orienting south in Shetland and western France compatible with migration, but a western heading from these locations pointing to vagrancy.
With coordinated work between ringers, birders and scientists across Europe and beyond, the answer to this fascinating question may be within reach.
Dufour, P, Åkesson, S, Hellström, M. et al. The Yellow-browed Warbler (Phylloscopus inornatus) as a model to understand vagrancy and its potential for the evolution of new migration routes. Movement Ecology 10, 59 (2022). DOI: https://doi.org/10.1186/s40462-022-00345-2