Proposal (944) to South American Classification Committee

Recognize Agelasticus atroolivaceus as a separate species from Agelasticus cyanopus

Effect on SACC classification: This would add a species by virtue of two-way split of an existing species, Agelasticus cyanopus (Unicolored Blackbird).

Background: Agelaius cyanopus is treated in most classifications as a polytypic species with four subspecies (Jaramillo & Burke 1999, Fraga 2011, Dickinson & Christidis 2014), following Parkes’s (1966) revision, which was based on a small number of specimens. This species occurs in marshy habitats throughout most of tropical and subtropical South America south of the Amazon with isolated populations also at the eastern periphery of the Amazon. Male plumages are virtually identical; the subspecies designations are based on rather strong differences in female plumage. Here is Peter Burke’s plate from Jaramillo and Burke (1999); see also extensive photos of specimens in Lopes (2017).

57a = adult male plumage.

57b = female cyanopus

57d = female “xenicus” (= unicolor; see Lopes paper)

57e = female atroolivaceus

New information: Lopes (2017) undertook a detailed, rigorous examination of geographic variation in the species based on analysis of 288 specimens (likely most of the specimens that exist), including type specimens. A detailed history of the taxonomy and. nomenclature of these taxa is also included. In fact, this is one of the most thorough specimen-based analyses I’ve read in quite a while, and if one were looking for a model example on how to conduct such a study, this would be the paper I would use. The results are loaded with interesting details as well as lessons on the importance of re-examining current taxonomic treatments. (By the way, Lopes credits Alvaro’s species account in his book on the Icteridae for catalyzing the study.)

But for the purposes of SACC and species-level classification, Lopes found that two populations are apparently broadly parapatric or nearly so in south-central Brazil with no signs of intergradation. Below is a screen shot of Lopes’ Fig. 12, but I added a pair of orange lines to show the rough boundary between the two. To the west of that line is cyanopus (with beniensis, but Lopes concluded that this was not a diagnosable taxon), and to the east is unicolor + atroolivaceus, which intergrade in southeastern Brazil.

Lopes’ interpretation of his morphometric and plumage (Munsell color terminology) analysis is that there is no sign of introgression between nominate cyanopus and unicolor despite a seeming absence of barriers between them. However, there was no quantitative analysis of within-taxon variation that might have revealed variation in the direction of another taxon as one approaches a potential contact zone, but Lopes’ careful attention to plumage details and interpretation of plumage character variation as revealing introgression in two cases suggests that nothing obvious was overlooked in the other cases, particularly the cyanopus-unicolor potential contact zone.

Note, however, that true parapatry has yet to be demonstrated because the sampling gap between the nearest localities between the two is at least 200 km. Note also that Alvaro stated in the species account in Jaramillo & Burke (1999): “A flock of 20 birds seen in Belo Horizonte, Minas Gerais, is geographically intermediate between atroolivaceus and cyanopus, their subspecific identify is unknown, and it is not known if this is a record of a vagrant flock or a new population.”

Discussion and Recommendation: Without an analysis of potential vocal differences or stronger evidence for true parapatry, the case for species rank is open to debate. On the one hand, the results fulfill the “necessary” conditions for species rank, i.e. no sign of gene flow between the two groups, but is the evidence “sufficient”? On the one hand, sampling in the gap might reveal populations with intermediate characters; on the other hand, intermediate populations were detected in two other cases in which the between-taxon geographic sampling gaps were as large or larger, and the disjunct, distant populations (“xenicus”) of unicolor maintain their phenotypic integrity. It seems unlikely (to me), that the difference in female plumage itself is a barrier to gene flow because I predict that male Agelasticus aren’t particularly picky about the plumage of a receptive female, but that is entirely conjectural; it seems more likely that the difference in female plumages is a consequence of some unknown barrier to gene flow, e.g. differences in male display and vocalizations. The differences between plumages of the females seem beyond what I would expect for intraspecific variation, at least in the Icteridae, but that opinion needs quantification before taken seriously.

I’m not familiar with the potential contact zone, but I don’t see any obvious geographic or ecological barriers that would prevent direct contact between cyanopus and unicolor, especially given that marsh birds in general are great dispersers.

So, in my view this one boils down to one’s standards for a taxonomic change. One could vote NO on this because the evidence clearly does not meet “beyond reasonable doubt” criteria, and wait for additional sampling in the contact zone or at least data on vocalizations and display. On the other hand, one could vote YES because all the evidence so far is consistent with an absence of gene flow between cyanopus and unicolor (vs. the opposite for unicolor-atroolivaceus and cyanopus-beniensis) and that this represents the best taxonomic interpretation with respect to what we know so far. I don’t have a strong recommendation, but I lean towards YES, pending comments of others.

English names: I’m not going to spend time on this unless the proposal passes. My only comments are (1) this is a true parent-daughter split, with both daughters having large ranges, so coining two new names would be the way to go in my opinion; it’s a good example in which a major taxonomic change “requires” destabilization of the English name in my view; and (2) “Unicolored Blackbird” isn’t a particularly useful name and is blatantly gender-biased, given that regardless of species limits, the standout feature of the species is the multi-colored and geographically variable female plumage, embarrassingly masked by the English name; I for one would be happy to see it disappear. Anyway, be thinking about appropriate English names.

Van Remsen, June 2022

_______________________________________________________________________________

Comments from Bonaccorso: “NO. I would be cautious and wait until the potential contact zone is well sampled and there is genetic evidence of no introgression between both populations (or even that introgression exists, but it is negligible). And, of course, song evidence would be highly valuable to advocate for reproductive isolation. I think it is risky to give species status now and then go back to considering them conspecific.”

Comments from Stiles: “NO. Although the specimen evidence is quite suggestive, I agree that genetic and specimen samples from the 200-km wide zone of possible parapatry would be important. Also, I feel more comfortable for such splits where two independent sources of data can be applied to support them. Here, because the birds of the unicolor group are considered to be common birds, at least locally, there ought to be some sound recordings available that could be brought to bear on this case!”

Comments from Claramunt: “NO. I think it is premature to split this species with the evidence at hand. The evidence is essentially a single trait (female plumage). This trait could be produced by a single gene in an otherwise uniform genome. There is no true parapatry either so the lack of introgression should not be overinterpreted.”

Comments from Robbins: “NO. more information (vocal, genetic) is needed from the potential area of contact before making this change.”

Comments from Areta: “NO. The plumage evidence alone is inconclusive. This thorough study provides a framework on which other sources of evidence can be put in perspective, but falls short to justify a species-level split. Is the gap between both groups real or is it a sampling artifact? Striking geographic variation in female plumages in other Icterid taxa (e.g., Molothrus bonariensis) is not considered to provide evidence for species status. I would like to see more sources of evidence (vocal, behavioral, genetic) brought together in an integrative study instead of rushing a split.”

Comments from Pacheco: “YES. Because the samples studied by Lopes (2017) are consistent with an absence of gene flow between A. cyanopus and A. unicolor vs. the opposite for A. unicolor-A. atroolivaceus and A. cyanopus-A. beniensis. Nominate cyanopus is mainly associated with the Pantanal and the drainage of the Paraguay River. The taxon unicolor is associated with the headwaters of the Paraná River and, disjunctly, to the upper Araguaia River and points in eastern Amazonia. This represents the best taxonomic interpretation with respect to what we know so far.”

Comments from Jaramillo: “YES – If this is amongst the best specimen based studies anywhere, I think it is valid to listen to what the author is saying here. The outcome is that there is little evidence of gene flow between the more eastern and western groups. The female plumage differences are substantial here.

“Also of importance is that the Pale-eyed Blackbird (A. xanthophthalmus) is part of this group. If it did not have a pale eye, it would likely have been described as a subspecies of cyanopus. But we have chosen one feature, a very obvious feature, to convince ourselves of species status. Females of xanthophthalmus are brownish, some younger females with yellow washed bellies and streaks below. Not quite like either group of cyanopus, but with features that make the relationship clear. Songs are similar in all three forms, although they are complex. Some repeated vocalizations are similar, but whistled repetitive parts perhaps different? More work needed on song. The point though is that the three are part of a closely related set of taxa.”

My thought here is that if we are recognizing xanthophthalmus as separate, it makes sense to separate the two forms of cyanopus as species to be consistent. The differences are not that much greater for xanthophthalmus, and it is widely allopatric, so a test of gene flow is not possible. Yet the cyanopus pair, seems to show a very close approach, and no evidence of gene flow based on morphology. This is important. I think the most consistent thing to do based on the various data here, are to threat this group as three closely related species.”