The Warbling Vireo comprises of two cryptic species

Genetic analyses confirm behavioral differences between two main groups.

There is more to avian diversity than meets the eye. Some species might look very similar although they are genetically and behaviorally distinct. An example that most European birders will be familiar with concerns the Chiffchaff (Phylloscopus collybita) and the Willow Warbler (Phylloscopus trochilus). Morphologically, these passerines are difficult to tell apart. But when they start singing, it is easy to discriminate between the two-syllable song of the Chiffchaff and the string of notes produced by the Willow Warbler. Recently, a study in the journal Ornithology (previously The Auk) described a similar case in North America where the Warbling Vireo (Vireo gilvus) might actually comprise two cryptic species.

Genetic Markers

Currently, the Warbling Vireo is classified into four subspecies that can be assigned to two main groups. The gilvus group houses only one subspecies (gilvus) and can be observed in deciduous forests across eastern North America. The swainsoni group holds three subspecies (swainsoni, victoriae, and leucopolius) that occur in the deciduous and mixed forests of western North America. Both groups meet along a contact zone in central Alberta (Canada). Some ornithologists suggested that these two groups might represent distinct species, but the evidence supporting this taxonomic arrangement is limited. Hence, Scott Lovell and his colleagues decided to unravel the genetics of this species complex by sequencing some mitochondrial and nuclear DNA.

The mitochondrial gene cytochrome b (cytb) supported the two groups described above, separated by 35 mutations. The genetic divergence between these mitochondrial clades amounted to 4% which translates into a divergence time between 1.9 and 2.5 million years ago (depending on the chosen mutation rate). The nuclear markers corroborated this pattern, clearly dividing most individuals into the gilvus and swainsoni groups. I write “most individuals”, because the genetic analyses uncovered some hybrids. Nine out of 145 individuals showed signs of mixed ancestry. These hybrids were detected within and near the contact zone, suggesting limited introgression outside of this area.

The mitochondrial DNA clearly separates the two groups. From: Lovell et al. (2021) The Auk.

Migratory Divide

Genetic data is just one line of evidence to support a taxonomic split. The two groups do show some minor differences in morphology and song, but the main difference probably relates to their migratory behavior. Individuals from the gilvus and swainsoni groups follow distinct migration routes: gilvus individuals fly through the mid-western and eastern USA whereas swainsoni individuals follow a route across the western USA and Mexico. Moreover, the two taxa arrive on the breeding grounds at different times: swainsoni individuals settle about two weeks earlier. This mismatch in timing reduces the possibility of mixed couples, allowing these taxa to diverge genetically. Hence, the authors argue to recognize these two groups as distinct species. In addition, they write that “future work may determine that additional cryptic species occur within the swainsoni group.” Who know what taxonomic discoveries lie ahead?

Analyses of the nuclear DNA discriminated between the two groups, but also uncovered some hybrids (green squares). Yellow triangles represent individuals with mismatches between nuclear and mitochondrial assignment. From: Lovell et al. (2021) The Auk.

References

Lovell, S. F., Lein, M. R., & Rogers, S. M. (2021). Cryptic speciation in the Warbling Vireo (Vireo gilvus). The Auk138(1), ukaa071.

Featured image: Warbling Vireo (Vireo gilvus) © Francesco Veronesi | Wikimedia Commons