Accurately determining avian species limits has been a challenge and a work in progress for most of a century. It is a fascinating but difficult problem. Under the biological species concept, only lineages that remain essentially independent when they are in sympatry are clearly species. Otherwise, there is no clear line yet found that marks when a pair of diverging lineages (e.g., in allopatry) become different enough to warrant full biological species status. Also, with more data, species limits often require reevaluation. The process of divergence and speciation is itself very complex and is the focus of intense research. Translating what we understand of that process into taxonomic names can be challenging. A series of issues are important. Single-locus criteria are unlikely to be convincing. Genetic independence is not a species limits requirement, but the degree of independence (gene flow) needs to be considered when there is opportunity for gene flow and independence is not complete. Time-based species (limits determined by time of separation) are unsatisfactory, though integrating time more effectively into our datasets is warranted. We need to disentangle data signal due to neutral processes vs. selection and prioritize the latter as the main driver of speciation. Assortative mating is also not likely to be an adequate criterion for determining species limits. Hybridization and gene flow are more important than ever, and there is a condition not being treated evenly in taxonomy: evolutionary trysts of 2 or more lineages stuck together through gene flow just short of speciation over long periods. Comparative methods that use what occurs between good species in contact to infer species limits among allopatric forms remain the gold standard, but they can be inaccurate and controversial. Species-level taxonomy in birds is likely to remain unsettled for some time. While the study of avian speciation has never been more exciting and dynamic, there is no silver bullet for species delimitation, nor is it likely that there will ever be one. Careful work using integrative taxonomy in a comparative framework is the most promising way forward.

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鸟类物种形成和物种限制概述

在一个世纪的大部分时间里，准确确定鸟类物种限制一直是一项挑战和一项正在进行的工作。这是一个有趣但困难的问题。在生物物种概念下，只有当它们处于同源状态时基本上保持独立的谱系才是明显的物种。否则，当一对不同的谱系（例如异源性）变得足够不同以保证完整的生物物种状态时，还没有发现明确的界限。此外，随着数据的增多，物种限制通常需要重新评估。分化和物种形成的过程本身就非常复杂，是深入研究的重点。将我们对该过程的理解转化为分类名称可能具有挑战性。一系列问题很重要。单基因座标准不太可能令人信服。遗传独立性不是物种限制要求，但当有基因流动机会且独立性不完全时，需要考虑独立程度（基因流动）。基于时间的物种（由分离时间确定的限制）不能令人满意，尽管需要更有效地将时间整合到我们的数据集中。由于中性过程与选择，我们需要解开数据信号，并将后者作为物种形成的主要驱动因素。分类交配也不太可能成为确定物种限制的充分标准。杂交和基因流动比以往任何时候都更加重要，并且在分类学中有一个条件没有得到公平对待：两个或更多谱系的进化幽会通过基因流动粘在一起，只是在很长一段时间内没有物种形成。使用接触的良好物种之间发生的情况来推断异域形式之间的物种限制的比较方法仍然是黄金标准，但它们可能不准确且存在争议。鸟类的物种级分类学可能会在一段时间内保持不稳定。虽然鸟类物种形成的研究从未像现在这样令人兴奋和充满活力，但物种划界没有灵丹妙药，也不可能永远有灵丹妙药。在比较框架中使用综合分类法进行仔细工作是最有希望的前进方式。物种划界没有灵丹妙药，也不可能永远有灵丹妙药。在比较框架中使用综合分类法进行仔细工作是最有希望的前进方式。物种划界没有灵丹妙药，也不可能永远有灵丹妙药。在比较框架中使用综合分类法进行仔细工作是最有希望的前进方式。