Proposal (890) to South American Classification Committee
Treat Thamnophilus capistratus as a separate species from Thamnophilus doliatus
Background: The Barred Antshrike, Thamnophilus doliatus, is a highly polytypic species widespread in the Neotropics (Assis et al. 2007, Koloff & Mennill 2020). The population that occurs in northeastern Brazil, T. d. capistratus, was validated as a diagnosable taxon by Assis et al. (2007), but the authors have not demonstrated that this taxon is a species under the BSC.
Currently, the footnote of the Barred Antshrike on SACC is as follows: “
Assis et al. (2007) claimed that they have validated that the subspecies capistratus of eastern Brazil is a species, but all that they showed was that capistratus is a diagnosable taxon (i.e., treated as subspecies under Biological Species Concept); whether capistratus merits species rank under the criteria used in this classification seems unlikely in that Assis et al. (2007) found no significant difference in any loudsong characters between capistratus and doliatus from adjacent Brazilian Amazon.”
New information: Looking at some photos and sound recordings on Wikiaves (www.wikiaves.com.br), I found that doliatus and capistratus occur in parapatry in the state of Piauí, Brazil (Fig. 1). Perhaps they even occur in syntopy at a few localities, as suggested by the photos WA2622492 (www.wikiaves.com.br/2622492) and WA2622493 (www.wikiaves.com.br/2622493) — a female capistratus and a male doliatus recorded at the same date and locality (Fig 2). Parapatry without evidence for free gene flow is sufficient evidence for species rank under the BSC (Remsen 2015, 2016). Therefore, these two taxa must be treated as species.
Figure 1. Geographic distribution of Thamnophilus doliatus (green) and T. capistratus (red) based on records from Wikiaves. The area highlighted in yellow is enlarged in Figure 2.
Figure 2. Parapatry of Thamnophilus doliatus (green) and T. capistratus (red) in the state of Piauí, Brazil. The arrow indicates a point of syntopy highlighted in the text above.
Because of the evidence of parapatry/sympatry between the two taxa, no more information is needed to treat them as species. But just to add more details, it is worth mentioning that these taxa are easily diagnosable. Assis et al. (2007) demonstrated that they differ in plumage characters and also said that “label data also suggest that T. capistratus has distinctive orange irides compared to whitish, sometimes yellowish white, irides in T. doliatus.” Now, with thousands of photos available on the internet (e.g., see www.wikiaves.com.br/wiki/choca-barrada-do-nordeste), we can see that these two taxa really have quite distinct colored eyes. In addition, T. doliatus and T. capistratus also differ in their song, although Assis et al.’s (2007) analysis was done with a small sample size (n = 30). Now, hundreds of sound recordings are available (including more than 100 recordings of capistratus; see Wikiaves, Xeno-canto and Macaulay Library) in case anyone wants to make a more complete analysis of their songs. Finally, according to the only phylogeny that includes capistratus, it is also distinct and reciprocally monophyletic in relation to doliatus (Belmonte-Lopes 2013, Fig. 3).
As for the English name, I suggest “Caatinga Antshrike”, already adopted by the Brazilian Ornithological Records Committee. This name reflects well its distribution mostly in Caatinga — the seasonally dry tropical forest in northeastern Brazil; and it is also applied to other species that occur mostly in this region (e.g., Caatinga Antwren and Caatinga Cacholote).
Assis, C.P., Raposo, M.A., Stopiglia, R. & Parrini, R. (2007) Validation of Thamnophilus capistratus Lesson, 1840 (Passeriformes: Thamnophilidae). The Auk, 124 (2), 665–676.
Belmonte-Lopes, R. (2013) Investigando o isolamento esplêndido da américa do sul: filogenia e biogeografia histórica dos Thamnophilidae (Aves: Passeriformes: Tyranni). PhD thesis, Universidade Federal do Paraná.
Koloff, J. & Mennill, D.J. (2020) Barred Antshrike (Thamnophilus doliatus), version 1.0. In Birds of the World (T. S. Schulenberg, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.barant1.01
Remsen, J.V., Jr. (2015) [Review of] HBW and BirdLife International Illustrated Checklist of the Birds of the World Volume 1: Non-passerines. Journal of Field Ornithology, 86 (2), 182–187.
Remsen, J.V., Jr. (2016) A “rapid assessment program” for assigning species rank? Journal of Field Ornithology, 87 (1), 110–115.
Rafael D. Lima, November 2020
Comments from Peter Boesman: “Given voice has been so important in the taxonomy of Thamnophilidae, and this proposal is lacking such information, I think it is useful to point out I made a brief comparison of voice for capistratus vs northern and southern populations a couple of years ago. See: https://static.birdsoftheworld.org/on53_barred_antshrike.pdf
Comments from Dan Lane: “NO. As much as I would like to see solid evidence sufficient to split these two, this proposal simply doesn't cut it. The photo of the female "T. capistratus" is a single photo (that was uploaded as "T. doliatus" apparently), and Lima does not give any reasons why it is T. capistratus and not simply T. doliatus. I assume it is because of iris color, but there are photos of birds well embedded within the distribution of T. doliatus (http://www.wikiaves.com.br/3757156, http://www.wikiaves.com.br/657547#) that show darker, duller irides, suggesting that there is some variation in this character in that species, primarily age-related. Given the vagaries of light and this apparent variation in iris color within T. doliatus, I cannot accept this one photo as definitely being T. capistratus, much less evidence of T. capistratus co-occurring with T. doliatus. T. capistratus itself seems capable of having iris color variation, too (see http://www.wikiaves.com.br/3763745), and if iris color is the only field-useful character to separate the two in female plumage, then I think we are at a stalemate. Furthermore, even if the photos Lima has linked to are correctly identified as T. capistratus and T. doliatus, how do we know that the two do not interbreed at this site? Without someone confirming that the two co-occur in sympatry and treat one another as good species, the evidence presented by Lima is not compelling at all.”
Additional comments from Rafael Lima: “As Dan suggests, iris color is age-related. Juveniles T. capistratus may have lighter eyes (e.g., http://www.wikiaves.com.br/3514923). However, there is no overlap in adults — T. capistratus always has red/orange eyes whereas T. doliatus always has gray/yellowish-gray eyes. Even though we consider the case of syntopy to be invalid, there is parapatry. Consider that photo WA2622492 is doliatus and then the point indicated by the arrow in Figure 2 is now green. The red point below it has typical capistratus (http://www.wikiaves.com.br/3314602). With the evidence of parapatry, no further evidence is needed (see the reasoning in Remsen 2016: 112-113). Parapatry without evidence for free gene flow is sufficient evidence for species rank under the BSC. Dan also said that we don’t know if these two taxa treat one another as good species or interbreed in this area. However, even though if they hybridize in this area, hybridization is accepted under the BSC and occasional hybrids are irrelevant to species rank (Johnson et al. 1999, Proc. 22 Int. Ornithol. Congr.; Remsen 2016). If there is a hybrid zone between doliatus and capistratus, it is very narrow and both taxa continue to maintain their identity. In summary, there is a parapatry zone between doliatus and capistratus (which are proven diagnosable; Assis et al. 2007) and we have no evidence for free gene flow, as there are no known hybrids between these two taxa and their differences remain constant. Therefore, the treatment of these two taxa is justified under the BSC.”
Comments from Bret Whitney: “I agree that capistratus is a distinctive taxon. It has a wide range, and shows relatively little variation within it, from my personal field experience. It is parapatric with “Amazonian” doliatus as shown in the maps Rafael provided, and it would surprise me if they did not at least occasionally interbreed in that region. Beyond iris color, there are several plumage characters that typify the two taxa, including much coarser “spot-barring” in capistratus, especially in the underparts, and, on females, there is usually some fairly conspicuous dark scalloping/scaling on feather margins of underparts in capistratus. This is just to emphasize that the taxa are clearly diagnosable by plumage — except, possibly, for hybrids.
“I think voice deserves a thorough analysis, as there may well be diagnosable differences, especially in common calls. Furthermore, the characteristic “bowing” movements performed by singing males in the doliatus complex may also differ in these taxa. Making video of singing birds would allow an analysis of these characters. That brings me to the point: what I think is needed is more focused study of this very interesting biogeographic story — with an accompanying phylogeographic analysis — before accepting the assumption that parapatric or possibly syntopic occurrence translates directly to “they must be biological species”. Some straightforward fieldwork over the course of a couple of months would garner the data needed to resolve the present scenario, including the degree of genetic introgression between taxa, and probably shed light on how it happened, historically. It probably sounds like a “cop-out” to say “go do more study”, but that’s simply what has to happen in many of these cases where data are wanting.”
Comments from Areta: “NO. This seems to be one of this interesting study cases were recognizing taxa as different species does not immediately lead to a relevant understanding of what is going on in evolutionary terms. Because doliatus itself is a geographical mess, I think that we need more thorough and focused studies to make an informed decision. There seem to be several plumage features (plus eye color) that help separate the taxa, but I also see pictures (and specimens?) that do not fit the bill so easily, and the presumed lack of vocal differences is also a warning sign. I suspect that they will eventually turn out to be two different biological species, that may differ in vocalizations (once properly analyzed) and which may perhaps hybridize on a narrow zone of sympatry. However, I feel that there is not enough evidence to decide on this.”
Comments from Remsen: “NO. The data presented on sympatry and lack of hybridization are insufficient to establish either one. A situation like this requires a thorough study of the potential contact zone, with sampling to determine % pure pairs, mixed pairs, hybrids and so on. Hopefully, this interesting proposal will catalyze the study that will resolve the issue.”
Comments from Jaramillo: “NO – for the reasons stated by others. The proposal does not make its case unfortunately.”
Comments from Robbins: “NO, for now. I’m in the camp where more data are needed before accepting capistratus as a species. As Bret pointed out, it seems that this could be clarified with fieldwork over “a couple of months”.
Comments from Claramunt: “YES. After reading the proposal, Asis et al. (2007), and examining photos in wikiaves, I am persuaded by the strong pattern of concordance across multiple characters and the lack of any sign of introgression or admixed individuals near potential contact zones. Because these two taxa differ in multiple traits, intergrades or hybrids should be easy to detect; therefore, their absence is remarkable. This pattern is unlikely to be maintained without reproductive isolation, as there are no physical barriers separating the two forms.
Comments from Zimmer: ““NO. I’ve been keeping my powder dry on this one until I could dig up a bunch of field notes and tape logs from pertinent fieldwork that I did in part of the doliatus-capistratus contact zone in the state of Piauí in February of 2003. Let me start by agreeing with Bret in that I too, suspect that capistratus is distinct. As Bret noted, in addition to the distinctive red eyes, there are some notable plumage distinctions as well. I think of males of “pure” capistratus as being coarsely scaled with black on the underparts, as opposed to having obvious complete horizontal barring (extent of barring varying geographically) as do males of other subspecies of doliatus. Some individuals of capistratus appear almost heavily spotted, or with very interrupted, broken barring, particularly when compared to any examples of other taxa in the doliatus complex. And, as Bret noted, female capistratus typically shows a fainter pattern of dusky scaling on the ochraceous-buff underparts, and on parts of the dorsal plumage as well. I would also agree with Bret that I think that a vocal analysis that focuses more on calls might find character differences that song-based analyses to date have yet to reveal. All of that being said, I don’t think that the case for splitting has been made by either of the publications cited in the Proposal. In 2003, over the space of a week or so, Andy Whittaker and I encountered numerous individuals and pairs of Thamnophilus doliatus/capistratus during the course of survey work in Piauí. At Sete Cidades National Park, Andy and I found multiple pairs of Thamnophilus, most of which we tape-recorded, and some of which we videotaped. This population, which was inhabiting what I would call tropical deciduous forest and forest-edge, were phenotypically closer to capistratus, but all of them showed some signs of intergradation or intermediacy, both in iris color (pale or medium orange instead of strongly red or red-brown) and in pattern on the underparts (at least partially barred rather than coarsely scaled/spotted). Interestingly, when we traveled from Sete Cidades eastward to Ubajara NP on 13 Feb 2003, we stopped to census birds in a remnant stand of fairly pristine, xerophytic caatinga scrub (which included such indicator species as Sakesphorus cristatus and Megaxenops, neither of which we encountered at Sete Cidades), and the Thamnophilus we encountered there was phenotypically classic capistratus in iris color (deep red) and male and female plumage. Conversely, when we moved back west of Sete Cidades to check out some humid lowland evergreen/semi-deciduous forest IBAMA Florestal de Palmas Reserve (16 km E of the city of Teresina) on 16 Feb, we encountered pairs of phenotypically typical Thamnophilus doliatus difficilis, with yellow irides and either complete black horizontal barring (males) or without any dusky barring or scaling (females) on the underparts (we made audio and video recordings of these birds as well). So, my impression of the doliatus-capistratus contact zone, is that there is a definite hybrid/intergrade zone, but that it is both narrow and well-defined, and probably very much habitat based, with only phenotypically typical capistratus occupying the arid caatinga; only phenotypically typical difficilis occupying more typical (for “Barred Antshrikes” sensu lato) humid forest-edge; and predominantly intermediate phenotypes inhabiting the more mesic dry deciduous forest habitats in between. We did not find any mixed-phenotype pairs: from west to east, it was difficilis X difficilis; intermediated X intermediate; and capistratus X capistratus; without exception, according to habitat. So, my impression is that there is parapatry, and that it is maintained by habitat differences, with narrow contact zones at ecological ecotones, where only hybrids/intergrades were evident and no indication of pure individuals of either phenotype. But, again, this is based on less than 10 days of fieldwork, nearly 18 years ago, and although I have lots of audio recordings and a number of video recordings to back this up, I have never performed any vocal analysis, nor have I done any systematic examination of broad series of specimens to better define plumage and bare parts variation within capistratus. I would join with Bret, Dan, Mark and anyone else in calling for more fieldwork in the contact zone, as well as a more thorough vocal analysis that includes calls as well as loud songs, and perhaps, also some genetic sampling before splitting capistratus from doliatus (which I suspect will, eventually, prove to be the correct treatment).”
Additional comment from Rafael Lima: “ I agree that more solid evidence (fieldwork in the contact zone and a more thorough vocal analysis) can be obtained to make a better informed decision. However, if I thought that the current evidence is not enough to change the current treatment, I would not have made the proposal. I believe that no one will disagree that SACC has already accepted splits with less evidence than those of this proposal. For example, many papers used in past proposals presented only anecdotal evidence ("pers. comm./pers. obs." field observations) to establish cases of parapatry or sympatry. Therefore, I am surprised that this proposal did not passed. Also, this case is curious because it shows how ornithologists delimit species under the BSC when they have only evidence for parapatry. Remsen (2015: 185) expressed concern about Tobias et al.’s (2010) criteria for species delimitation because “parapatry is not treated as sufficient evidence of species rank” in their criteria. Remsen (2015: 185) said that “after seeing this, I had a difficult time taking the rest of the scheme seriously because parapatry without free gene flow is prima facie evidence for species rank – no further information is needed.” I agree with Remsen (2015) that parapatry without any sign of free gene flow is prima facie evidence for species rank, but apparently this doesn't work in SACC.:
Response from Remsen: “The problem is that the map shows that they are parapatric at a crude scale (just like many subspecies), but at only one locality at a fine-scale, the once that counts, i.e. direct contact. Until we have more data from the contact zone, we can’t be sure of what is really happening, e.g. potentially a narrow hybrid zone with nothing but intermediates. It should take only a small amount of additional fieldwork at the contact zone to characterize the nature of their interaction.?”
Comments from Bonaccorso: “NO. But reluctantly, I really would like to say yes. It is an interesting case for a more thorough study. According to the comments of other members, there is evidence of plumage differences and a narrow (?) contact zone; local adaptation to different habitats (according to Zimmer´s comment) sounds incredibly interesting. However, rigorous song analysis and enough genetic data should be gathered to make a more informed decision.”
Additional comments from Lane (from an internal email): “Just going over the WikiAves photos of T. capistratus, I am seeing a lot of variation in iris color, male breast patterning, and female barring on underparts and wings. There are birds that look a lot like "standard" T. doliatus (e.g., http://www.wikiaves.com/1632108&tm=f&t=s&s=11806&o=mp&o=mp, http://www.wikiaves.com/3763745&tm=f&t=s&s=11806&o=mp&o=mp, http://www.wikiaves.com/3759440&tm=f&t=s&s=11806&o=mp&o=mp, http://www.wikiaves.com/2120036&tm=f&t=s&s=11806&o=mp&o=mp) that indicate to me that one of the following scenarios must be operating: 1) the discrete characters that supposedly define T. capistratus are not as uniform as the proposal wants us to believe (which may or may not have bearing on the species status of capistratus, but at least indicate that these characters are not actually so discrete), 2) these odd individuals are indicative of pockets of true T. doliatus within the distribution of T. capistratus (which would bode well for species status for capistratus), or 3) that there is bleeding of the two in phenotypic characters (and thus genetics? Which would not bode well for species status for capistratus). Also, bear in mind, most of these records are strictly field observations, so conclusive confirmation of the identities of the birds involved would be needed (e.g., specimens). So, it certainly doesn't seem clear to me that the two can be said to be "maintaining diagnosability across a wide contact zone". I don't have an answer for what this pattern of mixed phenotypes within the distribution of T. capistratus means, so I would prefer a study that clarifies it more than the present proposal does before I vote to change the status of the taxon.
“The red eyes of the "classic" individuals of capistratus certainly stand out compared to other populations of doliatus, and it is very tempting to see this distinctive character as a reason to split the two at the species level... but does this character, or the plumage characters that usually accompany it, really mean the two see one another as different beasts? If there is yet no diagnosable difference in song, it seems doubtful. I think a genetic study, and perhaps a more careful study of vocal variation, would be necessary to answer these questions.”
Comments from Pacheco: “NO, not for now. I strongly assume that they will turn out to be two different species under the BSC, which can differ consistently in vocalizations (when properly analyzed) and which may perhaps hybridize in a narrow zone of sympatry.”