Proposal (766) to South American Classification Committee

Clarify the taxonomic position of Zimmerius "chrysops" flavidifrons

Background: This proposal revisits ground previously covered by AOS-SACC in Proposal 173 (Treat Zimmerius chrysops as conspecific with Z. viridiflavus, submitted by Dan Lane 2005) and in Proposal 363 (Split Zimmerius chrysops into two or three species, submitted by Doug Stotz in 2008). These Zimmerius pose a complicated set of questions, given mismatches between morphology, vocalizations, and phylogenetic relationships. SACC has made progress in sorting this out, but one further action is necessary to clarify SACC's stance on the affinities on one taxon (flavidifrons) in this complex.

Please see the earlier proposals for a more detailed taxonomic history of the Zimmerius viridiflavus/chrysops complex. Briefly, Meyer de Schauensee (1966) and Traylor 1979 (Peters checklist) recognized a single species, viridifavus, including the taxa minimus, cumanensis, chrysops, albigularis, flavidifrons, and viridiflavus. Hellmayr (1927), Zimmer (1941), and Dickinson (2003), however, split viridiflavus of central Peru as a separate species, and this was the initial SACC baseline; viridiflavus is, in plumage, the most distinctive taxon among all these taxa. Although this was unknown to Hellmayr and Zimmer, viridiflavus also differs vocally from populations of nominate chrysops in Ecuador and northernmost Peru (primarily north of the Marañón). As discussed by Dan Lane in Proposal 173, however, there is a population in northern Peru south of the Marañón that seems to be morphologically identical to chrysops, but that has the distinctive call of viridiflavus. This vocally distinct population of "chrysops" in northern Peru apparently has no scientific name (indicative of its phenotypic similarity to true chrysops). This is the population referred to as "southern chrysops" or "Z. chrysops chrysops South" in Rheindt et al. (2008), and as "Zimmerius viridiflavus subsp. nov." or as "c. chrysops (San Martín)" in Rheindt et al. (2013). At some point this population will need a valid name, but that is not SACC's problem.

Finally, the taxa albigularis (southwestern Colombia and western Ecuador) and flavidifrons (southwestern Ecuador and extreme northwestern Peru) also differ vocally, from each other, from nominate chrysops, and from viridiflavus (although flavidifrons is somewhat similar vocally to both viridiflavus and "southern chrysops").

The phylogenetic relationships within Zimmerius were addressed by Rheindt et al. (2008, 2013); the first paper was the basis of Doug's Proposal 363. What Rheindt et al. uncovered (in both papers) is that albigularis is not a member of the chrysops group at all, on which basis SACC voted to recognize albigularis as a separate species. Furthermore, Rheindt et al. consistently find that viridiflavus and "southern chrysops" form a clade distinct from that of nominate chrysops, and that flavidifrons is basal to viridiflavus + "southern chrysops". See the diagram below, from Rheindt et al. (2008); the phylogeny in Rheindt et al. (2013) is similar.

One final point is that the distributions of viridiflavus and "southern chrysops" should be continuous, although no contact zone has been discovered between them (but this intervening area is difficult to access). On the other hand, flavidifrons is allopatric to viridiflavus and "southern chrysops", but, roughly speaking, the range of "southern chrysops" falls between the ranges of flavidifrons and of viridiflavus.

The problem: In Proposal 363, SACC voted on recognizing both albigularis and flavidifrons as separate species. SACC voted in favor of splitting albigularis, but not flavidifrons. Unfortunately, the structure of voting on Proposal 363 did not explicitly resolve the status of flavidifrons if it is not recognized as a species: retained as a subspecies of Zimmerius chrysops ("This would match morphology, but be in conflict with genetics and voice"), or transferred to Zimmerius viridiflavus ("the treatment recommended by Rheindt et al."). Doug was aware of this problem and implored committee members who voted against recognizing flavidifrons as a species to articulate their thoughts on where this taxon should be assigned. Most votes were to split flavidifrons or, if voting against the split, were silent on this issue, although at least one member who voted not to split flavidifrons was explicit that it should go to viridiflavus (thank you, Gary Stiles). As a result, the status of flavidifrons in SACC's taxonomy remains completely unclear. Implicitly, however, SACC continues to treat flavidifrons as a subspecies of Zimmerius chrysops, in that the Ecuador country list available from the SACC website does not list Zimmerius viridiflavus (which it would, if flavidifrons were transferred to that species), and the respective Peru country list identifies viridiflavus as a Peruvian endemic.

Elsewhere, however, most other taxonomic authorities have followed the implications of the phylogenetic data to the logical conclusion, and treat flavidifrons as a subspecies of Zimmerius viridiflavus, not of Zimmerius chrysops (del Hoyo and Collar 2016), or recognize flavidifrons as a separate species (IOC World Bird List). Dickinson and Christidis (2014), inexplicably in my view, retain flavidifrons in Zimmerius chrysops, but acknowledge that chrysops may contain more than one species, which presumably is a nod to the flavidifrons issue. None of these authorities, of course, acknowledge the undescribed "southern chrysops".

The solution: SACC can and should resolve this fundamental ambiguity over its treatment of flavidifrons. The options are:

A) Transfer flavidifrons from Zimmerius chrysops to Zimmerius viridiflavus.

B) Retain flavidifrons in Zimmerius chrysops.

C) Split flavidifrons as a species (as per Proposal 363).

My recommendation is Option A, which aligns directly with the available phylogenetic information, and is compatible with the vocal data. (Implicit in this decision, of course, is that "southern chrysops" also would be included in viridiflavus.) I'd also be happy with Option C; SACC rejected that proposal in 2008, but I include it here simply for the sake of completeness (and who knows, maybe SACC would see merit in it after a fresh look). I see little basis for supporting Option B (so if anyone votes for this option, please explain the merits of this approach).

English names: The English name of Zimmerius viridiflavus is Peruvian Tyrannulet. Rheindt et al. (2008) suggested that this English name would be inappropriate if flavidifrons, of southwestern Ecuador and immediately adjacent Peru, is included in viridiflavus. I don't see any problem with this English name, however, even if viridiflavus is expanded to include flavidifrons. Only a few of the species in SACC's checklist that carry the English name of "Peruvian xxxx" are endemic to Peru; the clear majority of such species occur in one, two, three, or even four (!) other countries (e.g. Peruvian Warbling-Antbird Hypocnemis peruviana, Peruvian Recurvebill Syndactyla ucayalae, and Peruvian Meadowlark Sturnella bellicosa, among many other examples).

If flavidifrons is recognized as a species, the name "Loja Tyrannulet" is widely adopted (Ridgely and Greenfield 2001, del Hoyo and Collar 2016, IOC World Bird List), and would be the appropriate English name.

Literature Cited:

Dickinson, E.C. (editor). 2003. The Howard & Moore complete checklist of the birds of the world. Third edition. Princeton University Press, Princeton, New Jersey.

Dickinson, E.C., and L. Christidis. 2014. The Howard & Moore complete checklist of the birds of the world. Fourth edition. Volume 2. Aves Press, Eastbourne, United Kingdom.

Hellmayr, C. E. 1927. Catalogue of the birds of the Americas and the adjacent islands. Part V. Field Museum of Natural History Zoological Series volume 13, part 5.

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.

Meyer de Schauensee, R. 1966. The species of birds of South America and their distribution. Livingston Publishing Company, Narberth, Pennsylvania.

Rheindt, F.E., J.A. Norman, and L. Christidis. 2008. DNA evidence shows vocalizations to be better indicator of taxonomic limits than plumage patterns in Zimmerius tyrant-flycatchers. Molecular Evolution and Phylogenetics 48:150-156.

Rheindt, F.E., A.M. Cuervo, and R.T. Brumfield. 2013. Rampant polyphyly indicates cryptic diversity in a clade of Neotropical flycatchers (Aves: Tyrannidae). Biological Journal of the Linnean Society 108: 889–900.

Ridgely, R. S., and P. J. Greenfield. 2001. The birds of Ecuador: status, distribution, and taxonomy. Cornell University Press, Ithaca, New York.

Traylor, M.A., Jr. 1979. Family Tyrannidae, tyrant flycatchers. Pages 3-229 in M.A. Traylor, Jr. (editor), Check-list of birds of the world. Volume VIII. Museum of Comparative Zoology, Cambridge, Massachusetts.

Zimmer, J.T. 1941. Studies of Peruvian birds. No. XXXVII. The genera Sublegatus, Phaeomyias, Camptostoma, Xanthomyias, Phyllomyias, and Tyranniscus. American Museum Novitates number 1109.

Tom Schulenberg, January 2018

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Comments from Robbins: "YES. Based on the points that Tom highlights in his proposal, at a minimum, flavidifrons should be transferred from Z. chrysops to Z. viridiflavus.  I also agree with Tom, that I would have no problem with recognizing flavidifrons as a species."

Comments from Stiles: "YES on B, although like Tom and Mark, I would not be averse to considering flavidifrons as a separate species.

Comments from Frank Rheindt: "Tom Schulenberg has done a great job in summarizing the limbo surrounding the taxonomic arrangement that SACC currently affords the taxon flavidifrons in the genus Zimmerius. However, he has perhaps omitted reference to the one paper that may have most bearing on the subject. Rheindt et al. (2014; Systematic Biology) used a mix of bioacoustics, morphometrics, colorimetrics and thousands of genome-wide markers to examine the mosaic population between northern chrysops and southern viridiflavus. This group of mosaic populations sounds very similar to viridiflavus but looks identical to chrysops. 

"Rheindt et al.’s (2014) molecular examination of these mosaic birds was restricted to unnamed populations from south of the Marañón that Tom referred to as a “…population [that] will need a valid name…”. Individuals of the taxon flavidifrons were not included in the molecular work for lack of specimen material. However, in our morphometric, colorimetric and bioacoustic analyses, Rheindt et al. (2014) did include multiple samples of flavidifrons as members of the mosaic population. This move was justified given that the flavidifrons samples emerged absolutely identical to the unnamed population (from south of the Marañón) in both biometrics and plumage color (while accounting for how colors are perceived by the birds, not by humans) (see Figure 4 in Rheindt et al. 2014). As for bioacoustics, the four flavidifrons samples were slightly offset from the cloud of viridiflavus and ‘Marañón population' birds in terms of a PCA accounting for 7 vocal parameters, but I would argue that vocal differences were minor (Figure 4a).

"In summary, I believe that Rheindt et al. (2014) produced a wealth of data that would argue for the inclusion of flavidifrons as a subspecies of viridiflavus. I do believe that the burden of proof rests squarely on the shoulders of those who might wish to propose species status for flavidifrons.

"Rheindt, F. E., Fujita, M. K., Wilton, P. R., and Edwards, S. V. 2014. Introgression and phenotypic assimilation in Zimmerius flycatchers (Tyrannidae): population genetic and phylogenetic inferences from genome-wide SNPs. Systematic Biology 63: 134-152."

Comments from Areta: "YES to A. Data in Rheindt et al. (2014) is enough for this move. I am less sure about the "mosaic" nature of flavidifrons and the undescribed San Martín population, but regardless of this, it is clear that vocal and genetic data show them to fit under viridiflavus and not under chrysops."

Comments from Claramunt: " YES to A, flavidifrons is clearly related to viridiflavus, not chrysops. Regarding option C, given its vocal, plumage (now that viridiflavus includes grayish birds) and genetic (low mtDNA divergence) similarity with viridiflavus, I think it is premature to suggest species status for flavidifrons, so NO to C."

Comments from Zimmer: “YES to A (Transfer flavidifrons from Z. chrysops to Z. viridiflavus.), as supported by Rheindt et al. (2014). The mosaic nature of population variation described in that paper and the status of the undescribed San Martín population seem complicated enough to make me reluctant to go any further in suggesting species status for flavidifrons, so NO on C.”

Comments from Stotz: “YES for Option A, transferring flavidifrons to viridiflavus.”

Comments from Jaramillo: “YES on C as in proposal 363.”

Comments from Pacheco: “YES to A. Transfer flavidifrons from Zimmerius chrysops to Z. viridiflavus.”