Resolving disputed subspecies distribution limits, and revealing intraspecific intergradation, in the Common Redstart Phoenicurus phoenicurus using citizen science and museum data

Abstract Contact zones with hybridization are documented for several Palearctic bird taxa. However, their extent is still poorly understood in many cases, including the Common Redstart Phoenicurus phoenicurus. Nominate P. p. phoenicurus breeds across most of the species’ range, whereas P. p. samamisicus is restricted to the south-eastern part. Their precise distributions are disputed as is the existence and prevalence of hybridization between them within potential contact zones in the Balkans, Turkey and the Caucasus region. Adult males differ by the presence (samamisicus) or absence (phoenicurus) of a white wing patch; however, there is variation and white is lacking in most second-calendar-year males of both subspecies. Further, male samamisicus is thought to be darker on the mantle. We map the occurrence of pre-defined states in these two characters during the breeding season across the species’ range based upon photos in online citizen science databases, subsequently complemented by an examination of museum specimens to refute or verify the pattern inferred from photos. We demonstrate that the extent of white in the wing varies throughout the range, but nevertheless is geographically structured. Adult breeders with a large white wing patch and black mantle occurred only in Greece, southern Bulgaria, Turkey, Crimea, and the Caucasus to Iran, which should be considered the breeding range of samamisicus. Earlier reports of intermediates and/or phoenicurus breeding there perhaps reflect confusion with second-year male samamisicus. Broad areas in the Balkans and north of the Black Sea were revealed as zones of intraspecific intergradation. Employing citizen science data enabled us to acquire a much larger sample than by using museum material alone. Although photographs from citizen science databases are unlikely to ever completely replace museum specimens, our approach of combining both types of data could prove a model for similar future research.