Proposal (993) to South American Classification Committee

Treat Thamnophilus ruficapillus as conspecific with T. torquatus

A recent proposal in this committee (SACC Proposal 968) suggested maintaining the traditional treatment of the Thamnophilus ruficapillus complex into two species taxa (T. ruficapillus and T. torquatus), as opposed to adopting the split of T. ruficapillus into two species proposed by Del Hoyo & Collar (2016), or any other possible additional splits. A possibility that has not been considered is to treat the T. ruficapillus complex as a single biological species (i.e., treating ruficapillus as conspecific with torquatus), which is proposed below.

The taxonomic history of the complex and other relevant pieces of information were summarized in Proposal 968. The argument here for treating the complex as a single biological species is twofold:

1. Divergence in vocalizations are currently assumed to be the key phenotypic proxy for biological species-level divergence in antbirds, and no vocal differences are known between ruficapillus and torquatus. The current two-species treatment of this species complex is based on early analyses of plumage differences (Cory and Hellmayr 1924, Peters 1951), and this treatment has remained until now just because no one has reassessed it in light of the realization that vocalizations, not plumage differences, are the main phenotypic proxy for species-level divergence in antbirds. In other words, the ornithological literature currently treats ruficapillus and torquatus as separate biological species only due to inertia.

2. Recent genetic work indicates that the current taxonomic treatment of the T. ruficapillus complex is incorrect. Phylogenetic work based on genome-wide genetic markers showed that T. ruficapillus and T. torquatus as currently defined are not reciprocally monophyletic (Harvey et al. 2020, Bolívar‐Leguizamón et al. 2024). Although paraphyly is not a problem in species delimited under the BSC, treating ruficapillus and torquatus as a single species eliminates paraphyly. Most importantly, a recently published analysis of population genetic structure of the complex found range-wide signatures of genomic introgression between ruficapillus and torquatus (Bolívar-Leguizamón et al. 2024). This finding is particularly remarkable when considering that they only had a few samples from the contact zone between the two taxa, thus suggesting that introgression is likely to be pervasive. In Proposal 968, Dan Lane provided several photographs of apparently phenotypically intermediate birds from the contact zone between ruficapillus and torquatus. Therefore, there is both genetic and phenotypic evidence indicating that ruficapillus and torquatus are weakly reproductively isolated, and so the current treatment of these two taxa as two biological species is incorrect.

Similar proposals (e.g., 173, 174, 977) have not passed because several people prefer to wait for more comprehensive analyses to avoid changing the taxonomy multiple times. I understand that point of view, but, personally, I prefer to see species limits as hypotheses formulated within the context of currently available evidence, and current evidence indicates that the classification of ruficapillus and torquatus as two separate biological species is incorrect. Even though this complex may potentially include multiple biological species, I think that recognizing a broadly defined species taxon for now, until a thorough analysis is published, is better than maintaining species taxa that are plainly improperly defined. In addition, I think it’s more likely that future taxonomic work will address the taxonomy of the complex as a whole if it’s treated as a single species for now. In fact, a single-species treatment will not only eliminate the possibility that future taxonomic revisions address only part of the complex (e.g., as in the BirdLife split), but also it may stimulate further taxonomic study of the complex by showing more clearly that this system is an interesting one whose taxonomy is poorly resolved.

References:

Bolívar‐Leguizamón, S. D., F. Bocalini, L. F. Silveira, and G. A. Bravo (2024). The role of biogeographical barriers on the historical dynamics of passerine birds with a circum‐Amazonian distribution. Ecology and Evolution 14:e10860.

Cory, C. B., and C. E. Hellmayr (1924). Catalogue of birds of the Americas and the adjacent islands in Field Museum of Natural History. Volume XIII, Part III. Field Museum Press, Chicago.

Harvey, M. G., et al. (2020). The evolution of a tropical biodiversity hotspot. Science 370:1343–1348.

del Hoyo, J., and N. J. Collar (2016). HBW and BirdLife International Illustrated Checklist of the Birds of the World. Volume 2. Lynx Edicions, Barcelona, Spain.

Peters, J. L. (1951). Check-list of the birds of the world. Museum of Comparative Zoology, Cambridge.

Rafael D. Lima, March 2024

_________________________________________________________________________________________