Yellowhammer (Emberiza citrinella) - HBW 16, p. 510

Taxonomy: Emberiza citrinella Linnaeus, 1758, Sweden.
Most closely related to, and perhaps forming a superspecies with, E. leucocephalos; sometimes considered conspecific, but the two differ clearly in plumage, as well as in behaviour and osteology, supporting treatment as separate species. Closest species to this pair are E. stewarti and E. cirlus. Present species and E. leucocephalos hybridize extensively in W & C Siberia (across c. 3000 km in taiga and forest-steppe zone from R Ural E to L Baikal), and intensity of interbreeding is increasing, since each species is expanding its breeding range across/towards that of the other; this type of interbreeding has been described as intermediate between occasional hybridization (not influencing gene pools of parent species) and introgressive hybridization (resulting sometimes in local fusion of species). An independent recent analysis of phylogeny supports recognition of two independent species, although cytochrome b sequences are very similar, resulting in a strong indication that they are sister-species; sharing of haplotypes even in very distant locations seems to be result of introgression of mitochondrial DNA. An independent alternative study also argues that both species are older than their closest relatives and that mtDNA has recently introgressed between them, most likely as a result of selective sweep. Has been suggested that race erythrogenys is only the result of hybridization with E. leucocephalos. Nominate race intergrades with erythrogenys in a broad zone extending from Baltic Republics, W European Russia and W Belarus S to E Hungary, E & S Balkans and Greece. Three subspecies recognized.