rnal introgression, such as mtDNA introgression (Haldane, 1922). Thus, we could suppose that the gene flow between T. naumanni and T. ruficollis was mainly driven by backcrossing with male hybrids.
Alternatively, nuclear-mitochondrial discordance might also be related to the sex-biased dispersal pattern of birds. In general, the dispersal pattern of birds is female biased, and female individuals are more frequently dispersed to areas where males of the same species do not exist, which promotes hybridization between different species (Clarke et al., 1997; Greenwood, 1980; Wang et al., 2014). However, a small number of birds have opposite dispersal patterns (Dallimer et al., 2002; Li and Merila, 2010; Williams and Rabenold, 2005). T. naumanni andT. ruficollis might also experience male-biased dispersal, and we need to further evaluate this possibility using more effective methods.
In parallel, increasing global warming might cause breeding areas to further overlap, which would increase the possibility of interspecific hybridization and eventually lead to species amalgamation (Rieseberg and Carney, 1998). In addition, considering the high level of gene flow between T. naumanni and T. ruficollis , we speculate that as the introgression between them continues, genetic information will be exchanged, which might affect their morphological and genetic integrity and decrease their interspecific differences.