Taxonomy in flux: The story of three flatbills and a flammulated flycatcher

Genetic study clarified the phylogenetic relationships between the genera Ramphotrigon and Deltarhynchus.

Here is a fun ornithological quiz question: What is the largest family of birds? The answer is … the tyrant flycatchers (Tyrannidae). This family of songbirds contains over 400 different species. Within this species-rich assembly, you can find the genus Ramphotrigon which holds just three species: the large-headed flatbill (R. megacephalum), the dusky-tailed flatbill (R. fuscicauda) and the rufous-tailed flatbill (R. ruficauda). The exact phylogenetic position of this genus within the Tyrannidae family is still a matter of debate.

The closest living relative of the flatbills is probably the flammulated flycatcher (Deltarhynchus flammulatus), at least according to morphological and behavioral data. A recent study in the Journal of Avian Biology provided a genetic perspective on this evolutionary issue.

Geographic distributions of the three Ramphotrigon species and their relative Deltarhynchus flammulatus represented by different colors. From: Lavinia et al. (2020) Journal of Avian Biology

 

Taxonomic Shifts

The genetic analyses – based on three mitochondrial and one nuclear marker – uncovered some surprising relationships. The flammulated flycatcher falls right in the middle of the genus Ramphotrigon, clustering with dusky-tailed flatbill and rufous-tailed flatbill. In technical terms: the genus Ramphotrigon is paraphyletic (you can check this blog post for an overview of these phyletic terms).

The third Ramphotrigon species – the large-headed flatbill – is not only genetically quite distinct from the other species. Morphological measurements indicated that it is significantly smaller than the other three species: “it has a lower body weight and a beak that is narrower and shorter in length and height, as well as shorter wings and tail.”

These results call for a taxonomic revision of this small section with the Tyrannidae. There are two options: (1) moving flammulated flycatcher to the genus Ramphotrigon or (2) placing the large-headed flatbill to its own genus.

The phylogenetic analyses revealed that the genus Ramphotrigon is not monophyletic: the flammulated flycatcher (D. flammulatus) clusters with the dusky-tailed flatbill (R. fuscicauda) and the rufous-tailed flatbill (R. ruficauda). From: Lavinia et al. (2020) Journal of Avian Biology

 

Biogeographic Patterns

The findings in this study also pointed to an interesting biogeographic history. The three Ramphotrigon species are restricted to South America, while the flammulated flycatcher is endemic to the Pacific Coast of Mexico. What happened?

Our story begins about 11 million years ago when the large-headed flatbill split from the common ancestor of the other three species. This event could be associated with the uplift of the Andean mountains and the development of large lakes in western Amazonia. About 5 million years later, the ancestors of the flammulated flycatcher went their separate way. A population of birds might have become isolated in the northern Andes and subsequently migrated along the Isthmus of Panama to Mexico. The populations on the other side of the Andes would eventually give rise to the dusky-tailed flatbill and the rufous-tailed flatbill. Many details of this biogeographic scenario remain to be clarified, but the main story is already there.

 

References

Lavinia, P. D., Escalante, P., Tubaro, P. L., & Lijtmaer, D. A. (2020). Molecular phylogenetics and phenotypic reassessment of the Ramphotrigon flycatchers: deep paraphyly in the context of an intriguing biogeographic scenario. Journal of Avian Biology, 51(4).