This passerine family occurs across Eurasia and contains two genera, Phylloscopus and Seicercus. Hybridization has only been documented in the former genus.
The Chiffchaff (P. collybita) complex is composed of several subspecies of which some are in secondary contact (Helbig et al., 1996). In Sweden, P. c. collybita is expanding north and engaging in hybridization with P. c. abientinus (Hansson, Bensch & Brannstrom, 2000) and in the Southern Urals, P. c. tristis and P. c. abientinus are interbreeding (Marova et al., 2009; Shipilina et al., 2017).
The most studied hybrid zone in this complex involves P. c. collybita and P. c. brehmii in the Pyrenees. It was first described based on morphological and vocal characteristics (Salomon, 1989; Salomon et al., 1997; Salomon & Hemim, 1992). Genetic analyses indicated extensive hybridization (Bensch et al., 2002b). However, gene flow was mostly restricted to nuclear alleles, possibly by means of hybrid males, while mitochondrial gene flow was inhibited by sterile hybrid females (Helbig et al., 2001). Both species also differ in flight-related morphology due to different migration patterns. Hybrids might be maladapted in terms of migration strategies (Perez-Tris, Ramirez & Telleria, 2003).
Subspecies of the Willow Warbler (P. trochilus) form a migratory divide in Sweden, confirmed by isotope analyses (Chamberlain et al., 2000) and experiments (Ilieva et al., 2012). The subspecies are clearly separated morphologically and behaviourally, but there is little genetic differentiation (Bensch, Akesson & Irwin, 2002a; Bensch, Andersson & Akesson, 1999). However, three genomic regions – on chromosomes 1, 3 and 5 – are highly differentiated and house putative ‘migration genes’ (Lundberg, et al. 2017). Possibly there is strong selection on hybrid migration strategies, because premating isolation is weak (Liedvogel et al., 2014). In addition, the circular distribution of this species around the Baltic Sea might make it an example of a ring (sub)species (Bensch et al., 2009).
Another likely example of a ring species is the Greenish Warbler (P. trochiloides), which extends around the Tibetan Plateau. The two terminal populations of this ring (viridamus and plumbeitarsus) meet, but do not interbreed (Irwin, Bensch & Price, 2001a; Irwin, Irwin & Price, 2001b). These populations sing different song and do not recognize each other’s song (Irwin, 2000; Irwin et al., 2001a). Genetic analysis using AFLPs demonstrated gene flow between the connected populations (Irwin et al., 2005). But recent studies are questioning the validity of the Greenish Warbler as a classical ring species. Morphology and song seem to be converging in sympatry (Kovylov, Marova & Ivanitskii, 2012), and a genomic analyses showed that there have been occasional geographic breaks during the development of the ring (Alcaide et al., 2014).
Hybridization between Western Bonelli’s Warbler (P. bonelli) and Wood Warbler (P. sibilatrix) has been confirmed genetically (Dietzen et al., 2007). Description of the Pallas’ Warbler (P. proregulus) complex indicates the existence of contact zones and possible hybridization between certain races (Martens et al., 2004). Alstrom et al. (2010) describe a new species, Limestone Leaf Warbler (P. calciatilis) in Vietnam, that might be interbreeding with Sulphur-breasted Warbler (P. richetti).
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