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).
In Alberta, two subspecies of the Warbling Vireo (Vireo gilvus gilvus and V. g. swainsoni) come into contact. Genetic analyses detected several hybrids (Lovell et al., 2021).
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.
Lovell, S. F., Lein, M. R., & Rogers, S. M. (2021). Cryptic speciation in the Warbling Vireo (Vireo gilvus). The Auk, 138(1), ukaa071.