Proposal (1067) to South American Classification Committee

Treat Thamnophilus cearensis as a separate species from Thamnophilus caerulescens

Background:

The Variable Antshrike (Thamnophilus caerulescens) currently includes eight subspecies distributed from Peru to eastern Brazil. These taxa have traditionally been treated as conspecific due to general plumage similarity and apparent clinal variation among some populations (Zimmer and Isler 2003).

Previous studies focusing on the western subspecies aspersiventer, paraguayensis, and dinellii indicated weak reproductive isolation (Brumfield 2005, Isler et al. 2005, Marcondes et al. 2020). A range-wide genetic analysis, however, found that the geographically isolated cearensis, from northeastern Brazil, is deeply divergent from all other taxa (Bolívar-Leguizamón et al. 2020). Nonetheless, both this and other large-scale studies (e.g., Marcondes et al. 2020 for plumage) suffered from sparse geographic sampling, preventing a thorough taxonomic assessment. Moreover, vocal variation has never been comprehensively studied across the species’ range, despite the widely perceived importance of vocalizations in antbird taxonomy.

New Information:

Lima et al. (2025) conducted an integrative analysis of genome-wide nuclear data, plumage, morphology, vocalizations, and behavioral responses to song playbacks across the full range of the Variable Antshrike, using much denser geographic sampling than any previous study.

They identified nine phenotypically distinct populations, corresponding to seven of the eight currently recognized subspecies plus two newly described ones. The following figure illustrates the geographic ranges of the nine taxa recognized by Lima et al. (2025):

Crucially, Lima et al. (2025) confirmed that cearensis is noticeably divergent genetically from all other taxa (F_ST on the order of 0.24 to 0.38 between cearensis and other taxa) and also that it has a remarkably distinct song, similar only to that of the geographically distant melanchrous. In field playback experiments, cearensis individuals responded to song recordings of its own taxon and melanchrous while ignoring others.

Recommendation:

The combination of distinct song, discrimination in playback experiments, and deep genomic divergence indicates that cearensis has likely evolved substantial reproductive isolation from all other populations of the T. caerulescens complex. While song-based behavioral isolation between cearensis and melanchrous appears weak, the two show the highest pairwise F_ST values, suggesting that postzygotic genetic incompatibilities may be strongest between them.

Although direct tests of postzygotic isolation are unavailable, genetic divergence provides a reasonable proxy. The number of Dobzhansky–Muller incompatibilities is expected to increase exponentially with genetic distance between two populations (Orr and Turelli 2001, reviewed in Coyne and Orr 2004). Consistent with this expectation, Lima et al. (2025) found that parapatric subspecies with lower genetic differentiation tend to form broader hybrid zones than those with deeper genetic divergence (see their Fig. 12). Thus, the high genetic differentiation between cearensis and melanchrous (F_ST = 0.38) likely indicates meaningful postzygotic barriers. Inferring reproductive isolation from this pattern is no more speculative than inferring behavioral isolation from territorial responses in playback experiments.

Based on this evidence, I recommend a YES vote to treat Thamnophilus cearensis as a separate species from Thamnophilus caerulescens.

References:

Bolivar-Leguizamon, S. D., Silveira, L. F., Derryberry, E. P., Brumfield, R. T., & Bravo, G. A. (2020). Phylogeography of the Variable Antshrike (Thamnophilus caerulescens), a South American passerine distributed along multiple environmental gradients. Molecular Phylogenetics and Evolution, 148, 106810.

Brumfield, R. T. (2005). Mitochondrial variation in Bolivian populations of the Variable Antshrike (Thamnophilus caerulescens). The Auk, 122(2), 414–432.

Coyne, J. A., & Orr, H. A. (2004). Speciation. Sinauer Associates.

Isler, M. L., Isler, P. R., & Brumfield, R. T. (2005). Clinal variation in vocalizations of an antbird (Thamnophilidae) and implications for defining species limits. The Auk, 122(2), 433–444.

Lima, R. D., Bolívar-Leguizamón, S. D., Bocalini, F., Marcondes, R. S., Brumfield, R. T., & Silveira, L. F. (2025). Geographic variation, population genetic structure, and taxonomy of the Variable Antshrike (Thamnophilus caerulescens). Zoological Journal of the Linnean Society, 205(2), zlaf129.

Marcondes, R. S., Stryjewski, K. F., & Brumfield, R. T. (2020). Testing the simple and complex versions of Gloger’s rule in the Variable Antshrike (Thamnophilus caerulescens, Thamnophilidae). The Auk, 137(3), ukaa026.

Orr, H. A., & Turelli, M. (2001). The evolution of postzygotic isolation: accumulating Dobzhansky‐Muller incompatibilities. Evolution, 55(6), 1085-1094.

Zimmer, K. J. & Isler, M. L. (2003). Family Thamnophilidae (Typical Antbirds). In: del J Hoyo, A Elliot, DA Christie (eds), Handbook of the Birds of the World. Barcelona: Lynx Edicions, 448–681. 

Rafael D. Lima, November 2025

Note from Remsen on English names: Cory & Hellmayr (1924) called this taxon “Ceará Ant Shrike”. I suggest we simply adopt that sensible name (as Ceara Antshrike); if anybody has any better ideas, speak out – and do a separate proposal for the English name.

Vote tracking chart:

https://www.museum.lsu.edu/~Remsen/SACCPropChart1044+.htm

Comments from Remsen: “YES. Vocal differences with playback experiments make this one an easy decision, in my opinion.  That cearensis is the most isolated taxon, in the complex, geographically, also makes sense.”