Unmasking hidden genetic, vocal, and size variation in the Masked Flowerpiercer along the Andes supports two species separated by Northern Peruvian Low

Genetic divergence among isolated populations is not always reflected in phenotypic differentiation. We investigated the genetic and phenotypic differentiation in Diglossa cyanea (Thraupidae; Masked Flowerpiercer), a widely distributed species in the tropical Andes. We found strong evidence for 2 main lineages separated by the Marañón River valley in the Northern Peruvian Low (NPL). These 2 lineages show a deep sequence divergence in mitochondrial DNA (mtDNA; ∼6.7% uncorrected p-distance, n = 122), spectral frequency and song structure (with exclusive final whistles in southern populations, n = 88), and wing length (the northern populations are smaller, n = 364). The 2 divergent D. cyanea mitochondrial lineages were not sister to each other, suggesting a possible paraphyly with respect to D. caerulescens (Bluish Flowerpiercer) that remains to be tested with nuclear genomic data. No genetic variation, size difference, or song structure was observed within the extensive range of the southern group (from the NPL to central Bolivia) or within all sampled northern populations (from the NPL to Venezuela). These vocal differences appear to have consequences for song discrimination, and species recognition, according to a previously published playback experiment study. We propose that the southern taxon be elevated to species rank as D. melanopis, a monotypic species (with the proposed name Whistling Masked-Flowerpiercer). In turn, we provide a redefinition of D. cyanea (Warbling Masked-Flowerpiercer), which is now restricted to the northern half of the tropical Andes as a polytypic species with 3 subspecies (tovarensis, obscura, and cyanea). Based on our results, the subspecies dispar should be treated as a junior synonym of cyanea. Our study highlights the need to continue amassing complementary data sets from field observations, experiments, and collection-based assessments to better characterize the evolutionary history, biogeography, bioacoustics, and taxonomy of Neotropical montane birds. We analyzed variation in mitochondrial DNA, songs, and body size among populations of a bird species of the Andean mountains (Masked Flowerpiercer) that shows relatively little plumage variation across its 3,800-km-long distribution from northern Venezuela to central Bolivia. We found that the Masked Flowerpiercer consists of two divergent populations separated by a deep geographic depression known as the Northern Peruvian Low (NPL). The degree of mitochondrial sequence div