Tracing the origin of vagrant Siberian songbirds with stable isotopes : the case of Yellow-browed Warbler (Abrornis inornatus) in Fennoscandia

Vagrant birds are mesmerizing birdwatchers worldwide, but the nature of vagrancy and the true origin of the vagrants are poorly known. To Western Europe, the massive Siberian land mass delivers most of the vagrant songbirds, e.g. Yellow-browed Warbler (YBW) (Abrornis inornatus, formerly Phylloscopus inornatus). In this study we used stable hydrogen isotope ratios in tail feathers (delta H-2(f)) from two ringing stations in northern Fennoscandia in an attempt to link vagrant YBW to potential regions of origin. We could do this thanks to a collection of samples from nestling and breeding adult YBW in Central Siberia. Compared with the nestling samples, the Fennoscandian delta H-2(f) data indicated origins in the western and/or southern parts of the breeding range. The assignment map created in IsoMAP showed high probabilities of origins in the Komi Republic, N/NW of the Ural Mountains. Although our study rules out a large proportion of the YBW breeding range, our method could not pin-point a precise region of origin. The main reason for this is the similarity of environmental hydrogen isotope ratios across longitudes in Eurasia. For increased precision, we propose a multi-method approach (e.g. stable isotopes and genetics) based on significantly more data from across the vast and challenging Siberian territory. More international collaboration will be vital for this endeavour.