Studying processes acting on differentiated populations upon secondary contact, such as hybridization, is important to comprehensively understand how species are formed and maintained over time. However, avian speciation studies in the tropical Andes have largely focused on the role of topographic and ecological barriers promoting divergence in allopatry, seldom examining hybridization and introgression. We describe a hybrid zone involving 2 closely related Andean warblers (Parulidae), the Golden-fronted Redstart (Myioborus ornatus), and the Spectacled Redstart (Myioborus melanocephalus). Geographic ranges of these species abut near the Colombia-Ecuador border and many specimens from the region exhibit intermediate phenotypes, but a formal description of phenotypic variation in the contact zone was heretofore lacking. We collected specimens across a transect encompassing the area where ranges abut and areas where only “pure” parental phenotypes of M. ornatus chrysops and M. melanocephalus ruficoronatus occur. We described variation in plumage traits including patterns of head and ventral coloration and tail markings based on 321 specimens. To describe genetic variation in the contact zone and over a broader phylogeographic context, we used sequences of the mitochondrial ND2 gene for 219 individuals across the transect and the entire range of both species, including all subspecies, from Venezuela to Bolivia. We documented a hybrid zone ~200 km wide based on head coloration, where intermediate plumage phenotypes are most common and “pure” forms do not overlap geographically, consistent with extensive hybridization. Across the range of the M. ornatus–M. melanocephalus complex, mitochondrial genetic structure was shallow, with genetic breaks only coinciding clearly with one topographic feature. Such a low genetic structure is striking given the high diversity in plumage phenotypes and the current taxonomy of the group. Our phenotypic data suggest that barriers to hybridization are not strong, and allow us to postulate hypotheses to be tested using forthcoming genomic data.

中文翻译：

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两种 mtDNA 差异较浅的安第斯莺物种之间的广泛杂交

研究在二次接触时作用于分化种群的过程，例如杂交，对于全面了解物种如何随着时间的推移形成和维持非常重要。然而，热带安第斯山脉的鸟类物种形成研究主要集中在地形和生态障碍促进异源分化的作用，很少研究杂交和渐渗。我们描述了一个混合区，涉及 2 个密切相关的安第斯莺 (Parulidae)、金额红尾莺 (Myioborus ornatus) 和眼镜红尾莺 (Myioborus melanocephalus)。这些物种的地理范围靠近哥伦比亚-厄瓜多尔边界，该地区的许多标本表现出中间表型，但迄今为止缺乏对接触区表型变异的正式描述。我们在一个横断面上收集了标本，该横断面包括范围毗邻的区域和仅出现 M. ornatus chrysops 和 M. melanocephalus ruficoronatus 的“纯”亲本表型的区域。我们描述了羽毛特征的变化，包括基于 321 个标本的头部和腹部颜色模式和尾部标记。为了描述接触区和更广泛的系统地理环境中的遗传变异，我们使用了从委内瑞拉到玻利维亚的横断面和这两个物种的整个范围内的 219 个个体的线粒体 ND2 基因序列。我们根据头部颜色记录了一个约 200 公里宽的混合区，其中中间羽毛表型最常见，“纯”形式在地理上不重叠，与广泛杂交一致。在 M. ornatus–M. 的范围内。melanocephalus 复杂，线粒体遗传结构浅，遗传断裂仅与一个地形特征明显一致。鉴于羽毛表型的高度多样性和该群体的当前分类学，如此低的遗传结构是惊人的。我们的表型数据表明杂交的障碍并不强，并允许我们假设要使用即将到来的基因组数据进行测试的假设。