Species delimitation requires a broad assessment of population-level variation using multiple lines of evidence, a process known as integrative taxonomy. More specifically, studies of species limits must address underlying questions of what limits the distribution of populations, how traits vary in association with different environments, and whether the observed trait differences may lead to speciation through reproductive isolation. While genomic data have revolutionized the process of delimiting species, such data should be analyzed along with phenotypic, behavioral, and ecological traits that shape individuals across geographic and environmental space. The integration of multiple traits promotes taxonomic stability and should be a major guiding principle for species delimitation. Equally important, however, is thorough geographic sampling to adequately represent population-level variation—both in allopatry and across putative contact zones. We discuss the importance of both of these factors in the context of species concepts and traits and present different examples from birds that illustrate criteria for species delimitation. In addition, we review a decade of proposals for species-level taxonomic revisions considered by the American Ornithological Society’s North American Classification Committee, and summarize the basis for decisions on whether to split or lump species. Finally, we present recommendations and discuss challenges (specifically permits, time, and funding) for species delimitation studies. This is an exciting time to be studying species delimitation in birds: many species-level questions remain, and methodological advances along with increased access to data enable new approaches to studying age-old problems in avian taxonomy.

中文翻译：

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综合分类学和地理抽样是成功划分物种的基础

物种划界需要使用多条证据对种群水平的变化进行广泛评估，这一过程称为综合分类法。更具体地说，物种限制的研究必须解决以下基本问题：什么限制了种群的分布，性状如何与不同环境相关联，以及观察到的性状差异是否可能通过生殖隔离导致物种形成。虽然基因组数据已经彻底改变了物种划分的过程，但这些数据应该与在地理和环境空间中塑造个体的表型、行为和生态特征一起进行分析。多种性状的整合促进了分类学的稳定性，应该成为物种划分的主要指导原则。然而，同样重要的是，是彻底的地理抽样，以充分代表种群水平的变化——无论是在异源地还是跨假定的接触区。我们在物种概念和特征的背景下讨论了这两个因素的重要性，并提供了来自鸟类的不同例子，这些例子说明了物种划分的标准。此外，我们回顾了美国鸟类学会北美分类委员会考虑的物种级分类修订的十年建议，并总结了决定是否分割或集中物种的基础。最后，我们提出建议并讨论物种划界研究的挑战（特别是许可、时间和资金）。这是研究鸟类物种划分的激动人心的时刻：许多物种层面的问题仍然存在，