Introgression occurs https://www.authorea.com/users/447785/articles/546727-integrative-taxonomy-of-two-thrush-complexes-aves-turdidae-reveals-introgression-across-sister-speciesmore frequently among birds than in mammal species (Grant and Grant, 1992; Mallet, 2005). It has been reported that more than 9% of birds have undergone introgression, and the proportion of introgression among birds worldwide may be higher (Grant and Grant, 1992; Randler, 2002). These birds include Larus hyperboreus andL. argentatus in Iceland (Vigfusdottir et al., 2008),Aegithalos bonvaloti and A. fuliginosus in southwestern China (Wang et al., 2014), Ammodramus caudacutus and A. nelsoni on the northeastern coast of the United States (Walsh et al., 2016), and Strix varia and S. occidentalis in western North America (Hanna et al., 2018). Recently, some studies have shown that when reconstructing phylogenetic relationships among related species, introgression might exacerbate nuclear-mitochondrial discordance (Haines et al., 2017; Kinoshita et al., 2019; Lu et al., 2001; Walsh et al., 2016). There are several possible hypotheses to explain these phenomena among birds, such as sex bias (Dai et al., 2013; Toews and Brelsford, 2012), distribution changes (Seixas et al., 2018), and simple stochastic variation in coalescence times across loci (Phuong et al., 2017; Rosenberg, 2003).

Turdidae includes approximately 340 species, which are distributed worldwide. Among these species, 65 are in Turdus (Monroe B.L., 1993). The classification of several Turdus species, such as the Naumann’s and dusky thrush complexes and the red-throated and black-throated thrush complexes, is controversial (Kratter, 2014; Sibley C.G., 1990). One opinion is that they are two species and four subspeciee observation of intermediate specimens between Naumann’s and dusky thrushes (Dong et al., 2018) and red-throated and black-throated thrushes (Brazil, 2009). Therefore, an integrative taxonomy of these two thrush complexes is still necessary.

In the present study, we used morphological characteristics, including quantitative and qualitative features, to identify these two thrush complexes and define intermediate specimens. Using mitochondrial genes and microsatellite loci, we then analyzed the genetic structure of these two thrush complexes. It was revealed for the first time that they are sister species with extensive nuclear introgression.