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Abstract Secondary contact between previously allopatric lineages offers a test of reproductive isolating mechanisms that may have accrued in isolation. Such instances of contact can produce stable hybrid zones—where reproductive isolation can further develop via reinforcement or phenotypic displacement—or result in the lineages merging. Ongoing secondary contact is most visible in continental systems, where steady input from parental taxa can occur readily. In oceanic island systems, however, secondary contact between closely related species of birds is relatively rare. When observed on sufficiently small islands, relative to population size, secondary contact likely represents a recent phenomenon. Here, we examine the dynamics of a group of birds whose apparent widespread hybridization influenced Ernst Mayr’s foundational work on allopatric speciation: the whistlers of Fiji (Aves: Pachycephala). We demonstrate 2 clear instances of secondary contact within the Fijian archipelago, one resulting in a hybrid zone on a larger island, and the other resulting in a wholly admixed population on a smaller island. We leveraged low genome-wide divergence in the hybrid zone to pinpoint a single genomic region associated with observed phenotypic differences. We use genomic data to present a new hypothesis that emphasizes rapid plumage evolution and post-divergence gene flow. 

ABSTRACT The Ethiopian Highlands are divided by lowland biogeographic barriers, including the Blue Nile Valley (BNV) and Great Rift Valley (GRV). We show that the GRV is a more pronounced phylogeographic break than the BNV for 6 focal passerines. Previous research suggests that the BNV greatly shaped phylogeographic patterns in relatively sedentary montane taxa such as frogs and rodents, whereas the GRV shaped phylogeographic patterns in volant taxa such as birds. However, no previous research simultaneously compares the impact of each valley on phylogeographic patterns in birds, and as these barriers vary in geographic extent and topography, the relative extent of their effects on gene flow is unclear. Using whole-genome resequencing, we quantified genetic variation in 6 montane forest passerines in the Ethiopian Highlands and found that their phylogeographic patterns varied, with general trends distinct from those of taxa that were previously studied across the same barriers. Genetic variation was assessed by estimating genome-wide genetic diversity (HO), demographic history, phylogeographic structure, and phylogeographic concordance among taxa. Population pairs flanking the GRV showed higher FST and more distinct population clusters in principal component analysis than those separated by the BNV. HO was broadly consistent across populations, excluding noticeable reductions in 2 populations (1 population each in 2 separate species). The overall phylogenetic signature and concordance across study taxa supported populations separated by the BNV as sister and populations southeast of the GRV as most distinct.