Integrative taxonomy and geographic sampling underlie successful species delimitation

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.

Methods
This dataset was collected by compiling records of specimens and audio recordings of Artemisiospiza nevadensis and A. belli archived at the Museum of Vertebrate Zoology and used in various publications by the lead author (C. Cicero).

物种界定（species delimitation）需要基于多重证据对种群水平的变异开展广泛评估，这一过程被称为整合分类学（integrative taxonomy）。具体而言，有关物种界限的研究必须解决以下核心议题：是什么限制了种群的分布范围？性状如何随不同环境发生变异？所观测到的性状差异是否会通过生殖隔离引发物种形成？尽管基因组数据极大地革新了物种界定的工作流程，但这类数据应当与塑造个体在地理与环境空间中特征的表型、行为及生态性状结合分析。整合多类性状有助于维持分类学稳定性，应成为物种界定的核心指导原则。同等重要的是，需开展详尽的地理采样以充分表征种群水平的变异——涵盖异域分布种群与假定的物种接触带。我们结合物种概念与性状特征探讨了这两类因素的重要性，并列举了鸟类中用以阐明物种界定标准的多个实例。此外，我们梳理了过去十年间由美国鸟类学会北美分类委员会（American Ornithological Society’s North American Classification Committee）审议的物种级分类修订提案，并总结了判断物种拆分或合并的决策依据。最后，我们针对物种界定研究提出了建议，并讨论了其所面临的挑战（具体为许可审批、时间与资金问题）。如今正值鸟类物种界定研究的黄金时期：诸多物种层级的分类问题仍待解决，而方法学的进步与数据获取门槛的降低，为解决鸟类分类学中这一经典难题提供了全新路径。

方法
本数据集通过汇编存档于脊椎动物博物馆（Museum of Vertebrate Zoology）的艾草雀属（Artemisiospiza）物种——内华达艾草雀（Artemisiospiza nevadensis）与贝尔艾草雀（Artemisiospiza belli）的标本记录及音频录音构建而成，相关材料曾被第一作者C. Cicero用于多项已发表研究。