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.
LAY SUMMARY
Standardized taxonomy and nomenclature are fundamental to biodiversity research and conservation, with wide-reaching impacts for diverse stakeholders.
Rigorous approaches to species delimitation require an integrative assessment of population-level variation using multiple lines of evidence and robust geographic sampling.
Sampling should reflect phenotypic, ecological, and behavioral trait variation and include contact zones between divergent populations if relevant.
We review basic concepts and criteria in species delimitation, drawing from examples across different avian lineages.
We summarize the outcomes of a decade of species-level proposals considered by the American Ornithological Society's North American Classification Committee.
We provide recommendations and discuss challenges for researchers interested in pursuing species-level taxonomic studies.
Many species-level questions remain in ornithology, and this is an exciting time to be studying integrative species delimitation in birds.