The Chiffchaff superspecies complex is a menagerie of numerous species and subspecies. Two subspecies of the Common Chiffchaff (Phylloscopus collybita) – the ‘Siberian’ Chiffchaff (P.c. tristis) and the ‘European’ Chiffchaff (P.c. abietinus) – meet in a hybrid zone that runs alongside the Ural Mountains. Outside this hybrid zone both subspecies are clearly different, but inside the zone intermediates reign supreme.
The hybrid zone between Siberian and European Chiffchaff was already described in 1879 by Suskhin in his report about “Birds of the Ufa province” in the Proceedings of the knowledge about the flora and fauna of the Russian Empire. The zone is characterized by substantial variation in body size, plumage coloration and song types (Marova et al. 2009). This lush morphological and acoustic diversity has prompted some to propose a third subspecies in this region: P.c. fulvescens (Dean & Svensson 2005).
Now, Shipilina et al. (2017) revisit this hybrid zone, combining genetic, phenotypic and acoustic data. They show that the subspecies are clearly differentiated in allopatry (i.e. outside the hybrid zone). Where they co-occur, however, there are many intermediate phenotypic characters as well as mixed singers, interweaving notes from abietinus and tristis songs. The genetic analyses, based on mtDNA and whole-genome sequence data, indicates high levels of gene exchange between the subspecies. All in all, your typical avian hybrid zone.
The analysis of this hybrid zone also shows that the putative third subspecies (P.c. fulvescens) does probably not exist. It is merely a mirage in a desert of hybridization.
Common Chiffchaff (left) and Siberian Chiffchaff. Picture by Vincent van der Spek (http://www.birdingnetherlands.com/blog/identification-of-siberian-chiffchaff)
References
Dean, A. R. & Svensson, L. (2005). Siberian Chiffchaff’revisited. British Birds 98, 396.
Marova, I., Fedorov, V., Shipilina, D. & Alekseev, V. (2009). Genetic and vocal differentiation in hybrid zones of passerine birds: Siberian and European chiffchaffs (Phylloscopus [collybita] tristis and Ph.[c.] abietinus) in the southern Urals. In Doklady Biological Sciences, vol. 427, pp. 384-386. Springer.
Shipilina, D., Serbyn, M., Ivanitskii, V., Marova, I. & Backström, N. (2017). Patterns of genetic, phenotypic, and acoustic variation across a chiffchaff (Phylloscopus collybita abietinus/tristis) hybrid zone. Ecology and Evolution.
Avian Hybrids 
[…] Based on the mitochondrial ND2-gene, all these subspecies form distinct clades. However, this was not the case for the other genetic marker. Genetic variation is shared between the subspecies abientus and collybita, which are known to hybridize in Sweden (see for example Hansson et al. 2000). The data did not reveal hybridization between abientus and tristis, altough recently a hybrid zone was discovered in the Urals (see for example Shipilina et al. 2017 and this blog post). […]