CONCLUSIONS
The taxonomy of Golden-winged and Blue-winged warblers has been debated for well over a century, since the initial description of Brewster’s and Lawrence’s warblers (Brewster 1874, Herrick 1874). The intensity of this debate has heightened recently, in light of genomic analyses (Toews et al. 2016) whereby genetic differences between Golden-winged and Blue-winged warblers are associated primarily with genes that control feather attributes. Indeed, the low level of genome-wide divergence has been cited as evidence that Golden-winged and Blue-winged warblers may be plumage morphs of a single-species complex (Kramer et al. 2018). Alternatively, our work suggests these genomic differences relate to plumage divergence that in turn lead to high levels of reproductive isolation. We demonstrate a strong, positive correlation between the frequency of pairing and plumage similarity (Fig. 3) so that hybridization occurred with only 1.2% of the phenotypically distinct Golden-winged and Blue-winged warblers. We document strong behavioral isolation and significant sexual selection against hybrids, which together provide a value of 0.966 for reproductive isolation. These attributes support the view that Golden-winged and Blue-winged warblers are distinct species under the biological species concept.
The taxonomic treatment of these lineages is especially important given the conservation challenges that Golden-winged and Blue-winged warblers face (Sauer et al. 2017). Extensive population decline in the Golden-winged Warbler has resulted in a petition for listing on the Endangered Species Act (Sewell 2009) and the decision regarding listing will be highly influenced by one’s interpretation of speciation in the Vermivora  spp. complex. Ecological and genetic interactions between Blue-winged and Golden-winged warblers as the former moves into sympatry with the latter appear to be a major cause of the decline of Golden-winged warblers (Gill 1980; Confer at al. 2011; Rohrbaugh et al. 2016; Rosenberg et al. 2016). The final genetic outcome of this expansion is unclear. On the one hand, cryptic introgression (Vallender et al. 2009; Wood et al. 2016) may prevent further divergence between lineages (Strasburg and Rieseberg 2008; Nosil et al. 2009; Sambatti et al. 2012; Karrenberg et al. 2019), while sexual selection against hybrids may enhance isolation. Despite the high levels of reproductive isolation between Golden-winged and Blue-winged warblers we document, we suggest that their continued existence may require conservation efforts that maintain or repair eco-geographic isolation (Roth et al. 2012).