The Vireos are a family of small passerine birds found in the New World and South East Asia. There are six genera, housing several species complexes.
The Red-eyed Vireo complex is currently divided into five species:
- Red-eyed Vireo ( olivaceus)
- Noronha Vireo ( gracilirostris)
- Yellow-green Vireo ( flavoviridis)
- Black-whiskered Vireo ( altiloquus)
- Yucatan Vireo ( magister)
Genetic data revealed that there are more species than meets the eye. The Red-eyed Vireo consists of Northern and Southern hemisphere populations, whereas the Yellow-green Vireo is structured into Eastern and Western populations. For the Red-eyed Vireo this distinction is quite obvious and there does not seem to be any gene flow between the populations. The situation for the Yellow-green Vireo, however, is less clear. It appears that this species is currently stuck in the “species/subspecies conundrum.” More data are warranted here.
Two species pairs show evidence for gene flow. The Northern population of the Red-eyed Vireo has exchanged genes with the Western population of the Yellow-green Vireo. And the Southern population of the Red-eyed Vireo has interbred with the Black-whiskered Vireo. The very low levels of introgression suggest that it probably concerns historic gene flow.
Interestingly, populations of Red-eyed Vireo on the island of Trinidad seem to be hybrids between Red-eyed Vireo and Black-whiskered Vireo. However, more extensive sampling is needed to check whether there is a hybrid zone on this island (Battey and Klicka 2017).
Battey, C. J. and J. Klicka (2017). “Cryptic speciation and gene flow in a migratory songbird Species Complex: Insights from the Red-Eyed Vireo (Vireo olivaceus).” Molecular Phylogenetic and Evolution 113: 67-75.