A new contact zone in the Chiffchaff distribution range: Caucasian and European subspecies meet in the south of Russia

The study of contact zones between closely related taxa of animals sheds light on many important issues of evolution biology and taxonomy. Several such zones were described earlier within the huge distribution range of the chiffchaff species complex ( Phylloscopus collybita). ). We have documented for the first time the existence of a new contact zone between Caucasian ( Ph. c. caucasicus) ) and European ( Ph. c. abietinus) ) subspecies of the Chiffchaff in Ciscaucasia (southern Russia). We predicted the occurrence of hybridization between these subspecies belonging to the same group of "greenish" chiffchaff. In eastern Ciscaucasia, a population was found in which the abietinus and caucasicus haplotypes co-occur in a significant number. A mixed pair of Chiffchaff (male caucasicus and female abietinus) ) was also found here. The colour of the plumage of Chiffchaffs in Ciscaucasia varies more widely compared to allopatric populations. In many specimens bearing the abietinus haplotype in Ciscaucasia, the wing formula is identical to that typical for caucasicus and differs from that typical for abietinus in allopatric populations. The tret calls typical of caucasicus are included in the song of Chiffchaffs, which carry the abietinus haplotype and do not have notes characteristic of caucasicus in their song. Chiffchaffs from Ciscaucasia occupy an intermediate position between abietinus and caucasicus in the duration of individual song phrases and syllable time-frequency characteristics. During the captures, the males of caucasicus readily reacted to the broadcast of the abietinus song, and vice versa. Thus, all the results suggest hybridization between abietinus and caucasicus in Ciscaucasia.