Proposal (646) to South American Classification Committee
Recognize newly described Tolmomyias sucunduri
Effect on SACC: This would add a new species to the list.
Background: Our current SACC footnote describes the situation. I think that it has been widely understood that Tolmomyias assimilis consists of multiple species:
Ridgely & Greenfield (2001), followed by Hilty (2003), considered populations of Central America and trans-Andean South America to represent a separate species, T. flavotectus, from Tolmomyias assimilis; they restricted the name "Yellow-margined Flycatcher/Flatbill" to the latter and called the Amazonian species "Zimmer's Flatbill." Proposal needed. The latter is also likely to consist of more than one species (see Ridgely & Greenfield 2001). Fitzpatrick (2004) concluded that further research was needed before any changes are made to current species limits. Whitney et al. (2013) described a new species in the complex, Tolmomyias sucunduri, from south-central Amazonian Brazil. SACC proposal badly needed.
New information: Whitney et al. (2013; let me know if anyone needs a pdf) described T. sucunduri on the basis of 9 specimens from 7 localities collected W of the Tapajos and E of the Sucunduri in the lower part of that interfluvium (the upper regions are occupied by other members of the T. assimilis complex). An additional 6 localities documented by voice recordings provide critical delimitation of the range. Sonograms of T. sucunduri, including from one of the paratypes, illustrate how the song differs from that of T. a. assimilis (recording from type locality!) and T. a. calamae (also a recording from type locality!). Those sonograms show clearly the “widely modulated or ‘washboard’ quality” of the song notes that separate it from other members of the group. The specimens are diagnosably different from other members of the complex in having darker crowns (as is evident from a photograph of one of the paratypes). Some additional supporting material (mainly recordings) is available online at: http://ibc.lynxeds.com/content/supporting-information#tolmomyias-sucunduri.
These canopy-dwellers are hard to see, record, and collect, so the sampling represents a lot of intensive fieldwork.
Analysis and recommendation: At first this looks a straightforward case – the song of sucunduri looks (and sounds) distinctive, and thus the rank of species seems unquestionable by our standards, and the vocal differences match up with the plumage differences. However, what bothers me is that there is an area on the Tapajos at about 5.5 degrees S. Lat. where Whitney at al. found individuals with songs described as intermediate between sucunduri and T. a. assimilis/calamae: “At roughly this latitude, there appears to be a gradient of hybridization at least 30 km wide, as judged by the prevalence of hybrid-type vocalizations there (collected specimens represented by black triangles in Fig. 1), and we expect that genetic introgression will prove to be considerably more pervasive east of the Sucunduri.” In Fig. 1 these are “Black triangle = postulated hybrid or intergrade between T. sucunduri and T. a. assimilis/calamae, specimen voice-recorded before collection.” I do not think the plumage of these specimens is discussed in the text or SI, nor are sonograms included.
If there really is a wide zone of introgression between the taxa with no pure phenotypes, then sucunduri should be treated at the rank of subspecies, by definition. However, whether these songs and populations are actually intermediate is not documented, and the genetic samples of the complex have yet to be analyzed. Given the distinctiveness of the song of sucunduri, that it would intergrade with parapatric taxa punches a bothersome hole in my worldview of the importance of song in suboscines.
As I see it, here are the choices. Vote YES if you think that, given the distinctiveness of the voice of sucunduri, better documentation of the degree of interbreeding is required (or that this could be a case like that of Blue-winged/Golden-winged warbler in which one species is replacing the other through “aggressive” hybridization and displacement). Vote NO if you think that the mention of possible extensive hybridization is sufficient to put burden-of-proof on treatment as separate species or that the genetic data should be analyzed first to assess degree of gene flow. I am agnostic at this point and will be interested in the discussion.
English names: If the proposal passes, we will need a separate proposal on English names. Whitney et al. (2013) chose “Sucunduri Yellow-margined Flycatcher”, which would require a new modifier for the rest of “Yellow-margined Flycatcher”, which itself might be subject to further splitting. Retention of “Yellow-margined” would be helpful in labeling members of this complex; despite the dislike by many of compound names, I personally think that the usefulness of the compound names would outweigh the burden of longer names.
Van Remsen, September 2014
Comments from Robbins: “It is unclear whether Tolmomyias sucunduri should be recognized at the species level. Like Van, I interested in what others have to say about this.”
Comments from Stiles: “NO. This looks like a classical case of secondary contact with limited hybridization, which could either lead to the reinforcement or breakdown of isolating mechanisms – cases of both are known in the US Great Plains. Analysis of morphology and genetics of the hybrids vs. “pure” specimens of the two forms might help to clarify things, and until at least these have been studied, Ięll have to vote NO. Only continued monitoring of the hybrid zone (and establishing its true extent ) could give a definitive answer here.”
Comments from Stotz: “NO. We know that Tolmomyias is a mess, and that there are certainly additional species floating around within the currently recognized species. This is likely one of those additional species, but the alleged intermediate songs of some populations makes it hard to accept it currently. The lack of any info on morphology of these alleged intermediates and lack of sonograms is unfortunate. If this claim had come from somebody other than the original describers, I might be disinclined to pay attention to it, but it is hard to ignore coming from Bret. I think we should hold off until we have better info on the potentially intermediate population.”
Comments from Nores: “NO, for now. Differences in color and vocalizations seem to correspond to subspecies rather than to species.”
Comments from Zimmer: “NO. As pretty much everyone recognizes, there are multiple species nested within T. assimilis. I’m not sure what is going on in this particular case, given the reference to a potentially wide zone of introgression. I too, think that we need more data from the contact zone to adequately sort this out.”
Comments from Areta: “NO. I agree with comments on the problematic vocal intermediacy of some populations. Tolmomyias seems to me a special tyrant genus in which populations with vocally intermediate characters appear here and there in different species (somehow recalling the plumage intermediacy of several populations of Picumnus). Until a thorough quantified study has been carried out, I think a conservative decision is needed. My hunch is that once Tolmomyias is thoroughly studied, it will prove to be a mess with less clearly delimited units than hitherto believed.”
Comments from Cadena: “NO. I agree with others that more detailed study of the hybrid zone (extent of introgression, mating patterns, playback experiments) would be needed before treating this new taxon as a species. I realize, though, that this is a remote area and these birds are hard to study, so gathering the data would be rather difficult.”
Comments from Jaramillo: “YES, I will be the contrarian here. To me a 30km band of intermediacy, if in fact that is what is occurring there, is not wide at all. Having collected orioles with Jim Rising in Western Kansas back in the day, hybridization is not that bothersome to me, and that relatively narrow hybrid zone is much wider than 30 km. In this case the members on either side of the hybrid zone are well defined, and the hybrid zone is narrow. This does not appear to be similar to the Western Flycatcher case where that is more likely a ring species with hundreds of km of intermediate birds. I am more impressed by the vocal distinctiveness of the taxon, and the fact that it is visually diagnosable too, more so than the zone of possible intermediates. At this point I would rather separate this taxon, and work on the dynamics of the hybrid zone and genetics later.”
Comments from Remsen: “NO. Although I would say that the N is too small to be conclusive, all signs point to free interbreeding, i.e. 1 BSC species regardless of hybrid zone width. I would not be surprised that additional data will reverse this.”
“Alvaro’s comments inspire me to expound a little on hybridization. All modern BSC proponents allow plenty of hybridization between two taxa treated as species. However, it all depends on what one means by hybridization. Here’s my personal view; I wish someone with a strong background in hybrid zones would do an updated review paper on this that we could cite. Of course, trying to squeeze every example into a categorical scheme is dangerous, and any attempt to pigeon-hole all real-world situations into categories is doomed (in this case species vs. subspecies). Nonetheless, we will need to have a strong statement on this sort of thing in the eventual hard-copy version of our classification.”
“1. Occasional hybridization. Low levels of hybridization may be fairly good predictors of relationship (Trevor Price did a review paper on this) but do not indicate conspecificity. In fact, the lack of free interbreeding indicates separate species rank.”
“2. Frequent hybridization but still most mating is assortative. I can’t think of a good South American example, but Claramunt’s (2002) study of Cranioleuca obsoleta and C. pyrrhophia might fit. In North America, good examples would be Icterus galbula and I. bullockii and Passerina amoena and P. cyanea. I would have to check the latest synopsis of the Vermivora chrysoptera/V. cyanoptera situation, but I think the aggressive “displacement” hybrid zones likely fit this category.
“3. Non-assortative mating. In such cases free interbreeding produces a hybrid zone in which almost no pure parentals can be found. Regardless of width, what it says is that the two taxa do not discriminate between each other for mating … so the intuitive appeal is, ‘if they don’t think each other is different, why should we humans?” The problem with the move to treat “narrow” hybrid zones as indicating separate species rank is that there is no objective, meaningful way to define “narrow” – any definition is necessarily arbitrary as far as I can see. Some, including even Mayr himself, acknowledge that a hybrid zone with no sign of introgression outside that zone suggests a barrier to gene flow. The problems that I see with this view are (1) there may actually be substantial gene flow that would require strong genetic data to reveal, and (2) I can’t get past the intuitive appeal of “if they don’t discriminate between each other, then they aren’t species’.”
“4. Like #3 but with introgression outside hybrid zone. A no-brainer ‘1 species’ treatment under any definition of BSC.”
“All opinions welcomed. A proposal page might not be the best medium for getting such a discussion going, but … “