Origins of irruptive migrations by Common Crossbills Loxia curvirostra into northwestern Europe revealed by stable isotope analysis

We used analyses of stable hydrogen isotope (δ²H) measurements in Common Crossbill feathers (δ²H_f) to infer the region of origin of Crossbills collected from different irruptions into Britain, Iceland and the Faeroes, comparing these values with those from birds sampled in breeding areas in Britain and elsewhere in the western Palaearctic. No differences in δ²H_f values were found between different species or sexes of Crossbills that could be presumed to have grown their feathers in the same region, but juveniles had lower δ²H_f values than adults that had grown their feathers in the same region. On the basis mainly of museum skins, immigrant birds were sampled from 30 different irruption years, spanning the period 1866–2009, with annual samples varying from one to 29 individuals. The variation in δ²H_f values within irruptions was substantially less than the variation between irruptions, indicating that irruptions in different years originated in different parts of the western Palaearctic boreal zone. Birds with lower δ²H_f values tended to arrive later in the migration season, which was consistent with the idea that they had travelled further. In 17 of the irruption years, the birds had mean δ²H_f values more than ?120‰, suggesting that they had originated somewhere in the region extending from northern Scandinavia to northwestern Russia. In these years the birds arrived early, in June and July. In 10 of the irruption years, the mean δ²H_f values were between ?120 and ?130‰, suggesting origins further east, in northern Russia, east of Archangel (about 40°E). In only three of the 30 years (1898, 2002, 2009) the mean δ²H_f values were even lower (< 130‰), and these birds arrived in late July, August and September. Birds in these three irruptions had probably come from Siberia, east of the Ural Mountains. In at least three irruption years (1898, 1927, 1985) the observed range of δ²H_f values suggested that birds had come from more than one of these regions, including east of the Urals in 1898 and 1927.