Genetic, vocal, and body size divergence across the Northern Peruvian Low supports two species within the Masked Flowerpiercer (Diglossa cyanea)

Populations that become genetically isolated by geographical barriers may express phenotypic divergence more strongly in some traits than in others. Even when genetic differentiation among isolated populations accumulates at a rapid rate, this may not be reflected in phenotypic differentiation. This decoupling of trait divergence from genetic divergence has been found in multiple Andean bird lineages, which occupy latitudinally long, linear ranges that are fragmented by ecological and topographic barriers. One of these montane birds is Diglossa cyanea (Thraupidae; Masked Flowerpiercer); its distribution is interrupted by valleys and lowlands that often represent subspecies that differ only by subtle diagnostic phenotypic differences. We evaluated genetic and phenotypic divergence throughout its distribution and found support for two distinct lineages sharply separated by the Marañón River valley at the Northern Peruvian Low (NPL). Specifically, we found that the two populations from the opposite sides of the NPL show deep divergence in mtDNA (∼6.7% uncorrected p distance, n = 122), in song structure (exclusive final notes in southern populations, n = 88), and in wing length (longer wings in the southern population, n = 345). No genetic variation or song structure was observed within the large range of the southern group (from the NPL to central Bolivia) or within all northern populations (from the NPL to Venezuela). Moreover, these two lineages are possibly paraphyletic with respect to D. caerulescens (Bluish Flowerpiercer), which suggests a long history of isolation across the NPL between northern and southern populations of Masked Flowerpiercer. Our results suggest that the southern taxon, D. c. melanopis, should be recognized as a species-level taxon, distinct from a redefined D. cyanea. We highlight the need to continue amassing complementary suites of datasets from field observations and experiments, laboratory analyses, and collection-based assessments, to better characterize the evolutionary history and taxonomic diversity of birds.