Splitting the Black-throated Trogon into five species

An integrative approach to taxonomy leads to the description of a new species.

When people hear that I work on speciation and hybridization in birds, I often get the question: are you a lumper or a splitter? For readers unfamiliar with these terms: “lumpers” prefer to reduce the number of taxonomic groups by merging them, while “splitters” tend to break up larger groups into smaller ones based on small differences. My own position – to answer the question posed at the start – is difficult to determine. I prefer to take an integrative approach where multiple lines of evidence are combined into a taxonomic decision (see this blog post for the details). Based on the strength of the evidence, I will be a lumper or a splitter. Hence, it depends on the situation (a popular phrase among biologists). Just as it can be difficult to pigeonhole variation in nature, it is challenging to classify biologists into lumpers and splitters. A recent study in the Zoological Journal of the Linnean Society provides a nice example of my own approach to taxonomy. The researchers present convincing evidence for splitting one species into five.

Six subspecies

The taxonomy of the Black-throated Trogon (Trogon rufus) has been relatively stable since 1943 when W.E. Clyde Todd proposed six subspecies:

  • T. r. rufus in the Guiana Shield
  • T. r. sulphureus in upper Amazonia
  • T. r. chrysochlorosin the Atlantic forest
  • T. r. tenellus throughout Central America
  • T. r. cupreicauda in the Choc√≥-Magdalena region
  • T. r. amazonicus in lower Amazonia

Over the years, several authors have proposed a species status for some of these subspecies. To provide some clarity in this taxonomic turmoil, Jeremy Kenneth Dickens and his colleagues amassed an impressive set of data. They examined the morphology and plumage of 906 museum specimens of the Black-throated Trogon, carefully analyzed 273 song recording and sequenced the mitochondrial DNA of 29 birds.

Barred Patterns

Morphologically, almost all male specimens were assigned to the correct taxon. Only one individual from the subspecies amazonicus was confused with rufus (but more on these taxa later). The different barring patterns on the undertail were one of the most defining features. This finding is not that surprising because this trait is known to play a role in species recognition (see for example this study). Interestingly, the iridescent colors on the head and body were not informative to classify these birds. In fact, an experimental study showed that feather coloration can change rapidly depending on the local humidity level. Not the most reliable feature to use in taxonomy. The morphological patterns were corroborated by the song analyses where 88% of the recordings could be assigned to the correct taxon. We already have two convincing lines of evidence for splitting the Black-throated Trogon into several species. What about genetics?

Linear discriminant analyses of several morphological and plumage traits revealed clear clusters corresponding to the currently recognized subspecies. The patterns were most obvious for males (left figure). From: Dickens et al. (2021).

Genetics

Phylogenetic analyses of two mitochondrial genes (ND2 and cytb) recovered several well-supported clades that correspond to the taxa tenellus, cupreicauda and chrysochloros. Moreover, one population from Alagoas was genetically distinct from the other samples. The three taxa that encircle the Amazon (rufus, sulphureus and amazonicus) clustered together but could not be separated into monophyletic clades. The genetic overlap between these taxa was also reflected in the morphological patterns. Across their distribution, there are zones of intergradation between rufus and amazonicus, and between amazonicus and sulphureus. It is likely that some hybridization is occurring in these regions, but more sampling is needed to confirm this suspicion.

Genetic analyses of 29 specimens were in line with the morphological analyses, leading to the decision to elevate several subspecies to the species level. From: Dickens et al. (2021).

A New Classification

Based on the congruent patterns across the different data sets, the authors propose to elevate four subspecies to species rank, namely T. rufus, T. tenellus, T. cupreicauda and T. chrysochloros. The population in Alagoas is recognized as a new species: T. muriciensis. You can check the original paper for detailed descriptions of these species, but I will leave you with these beautiful drawings by Eduardo Brettas. Whether you agree with the splitting of the Black-throated Trogon or not, I am sure that you will admit that these are stunning birds.

Illustrations of the Trogon species described in the blog post. From: Dickens et al. (2021).

References

Dickens, J. K., Bitton, P. P., Bravo, G. A., & Silveira, L. F. (2021). Species limits, patterns of secondary contact and a new species in the Trogon rufus complex (Aves: Trogonidae). Zoological Journal of the Linnean Society193(2), 499-540.

Featured image: Black-throated Trogon (Trogon rufus) © Charlie Jackson | Wikimedia Commons

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