Response to Hill and Powers: It is irrelevant that the mode and tempo of Cassia crossbill speciation is not typical for birds

Abstract

Species delineation has a long and contentious history, yet most agree that sympatric populations exhibiting high levels of reproductive isolation and evolving independently are species. In an opinion piece, Hill and Powers (2021; hereafter H&P) claim that several recognized species of crossbills (Loxia spp.) do not represent species because by no measure are they discrete, the vocalizations used to categorize crossbills are learned, modified and can switch to that of a different species, and reproductive isolation is incomplete and weak. We argue that the behavioral and genetic evidence indicate that Cassia crossbills L. sinesciuris, which we focus on because the data relevant to species status are more diverse and extensive, are genetically discrete; call modification rarely leads to crossbill misclassification and overwhelmingly results in call divergence and enhanced discrimination; and are nearly completely reproductively isolated with little evidence of introgression from sympatric red crossbills. The differences in our conclusions result in part from H&P mischaracterizing and misconstruing the ecology of Cassia crossbills, geographic context of their divergence, and evidence for reproductive isolation. H&P seemingly require that crossbills must adhere to the typical model of bird speciation–protracted divergence in allopatry, followed by a gradual increase in sympatry if reproductive isolation and ecological divergence allow–and require evidence such as initial long periods of allopatry, FST values > 0.2, divergent mtDNA and intrinsic postzygotic isolation. Although such evidence commonly distinguishes bird species, an increasing number of studies show that such criteria are not necessary to indicate sympatric, evolutionarily independent lineages.