Eastern (Vireo gilvus gilvus) and western (V. g. swainsoni) forms of the Warbling Vireo have essentially allopatric breeding ranges across north-central North America, but come into contact in central Alberta, Canada. In 1986, Jon Barlow presented preliminary morphological and song evidence suggesting that the Warbling Vireo complex might comprise more than one valid species. However, to date, Barlow’s suggestion is supported by only limited DNA evidence, demonstration of molt and migration differences between the taxa, and anecdotal accounts of differences in song, morphology, plumage, and ecology. We analyzed variation in both mitochondrial and nuclear DNA in birds from Alberta and surrounding areas to determine the levels of genetic differentiation and hybridization occurring in the contact zone, and whether the two taxa warrant recognition as separate biological species. Our analyses reveal that Warbling Vireos in Alberta and the surrounding areas are separated into two well-defined, genetically differentiated, and monophyletic clades corresponding to previously recognized taxonomic groups. The two taxa come into contact in a narrow (~85 km) zone in Barrhead County, northwest of Edmonton, Alberta. They show evidence of limited hybridization. The distinct genetic differences are maintained in the contact zone, where individuals of the two taxa may occupy neighboring territories. Differences in spring arrival dates, molt schedules, and migration routes indicate that a migratory divide may play an important role in reproductive isolation. We suggest that the two taxa are distinct cryptic species: an eastern form, Vireo gilvus, and a western form, Vireo swainsoni.

中文翻译：

---

Warbling Vireo (Vireo gilvus) 中的神秘物种形成

东部（Vireo gilvus gilvus）和西部（V. g. swainsoni）形式的Warbling Vireo基本上在北美中北部具有异域繁殖范围，但在加拿大艾伯塔省中部接触。1986 年，乔恩·巴洛 (Jon Barlow) 提出了初步的形态学和歌曲证据，表明 Warbling Vireo 复合体可能包含不止一种有效物种。然而，迄今为止，巴洛的建议仅得到有限的 DNA 证据、分类群之间蜕皮和迁移差异的证明以及关于歌曲、形态、羽毛和生态差异的轶事说明的支持。我们分析了阿尔伯塔省和周边地区鸟类线粒体和核 DNA 的变异，以确定接触区发生的遗传分化和杂交水平，以及这两个分类群是否值得承认为单独的生物物种。我们的分析表明，阿尔伯塔省和周边地区的 Warbling Vireos 被分成两个定义明确的、遗传分化的和单系进化枝，对应于先前公认的分类群。这两个分类群在阿尔伯塔省埃德蒙顿西北部的巴尔黑德县的一个狭窄（约 85 公里）区域内接触。它们显示了有限杂交的证据。在接触区保持明显的遗传差异，两个分类群的个体可能占据相邻领土。春季到达日期、蜕皮时间表和迁徙路线的差异表明迁徙鸿沟可能在生殖隔离中发挥重要作用。我们认为这两个分类群是不同的神秘物种：东方形式，Vireo gilvus，