Proposal (820) to South American Classification Committee
Treat North American Cistothorus stellaris as a separate species from Cistothorus platensis
Using the mitochondrial ND2 gene and two nuclear introns, Robbins & Nyari (2014) found that Cistothorus platensis, as currently recognized, is not monophyletic because C. meridae and C. apolinari are more closely related to the Neotropical members of C. platensis than to the migratory C. p. stellaris from northern North America, which was found in a more basal position. This suggests that that C. p. stellaris is a separate lineage.
Under the BSC, the key question is whether there is evidence of reproductive compatibility or isolation between stellaris and platensis. Levels of genetic differentiation are usually not good predictors of reproductive isolation in birds. However, the song of stellaris is very different from the songs of Neotropical platensis, indicating a mechanism for prezygotic isolation. Boesman (2016) compared the songs of stellaris and Neotropical platensis and found numerous differences. Most notably, the song of stellaris is relatively simple and uniform across its range (“a few "tsik" notes followed by a rattled series of notes”), whereas the songs of Neotropical platensis all differ from the song of stellaris, and are also more complex and variable. This observation can be corroborated in online sound databases (https://www.xeno-canto.org/species/Cistothorus-stellaris,https://www.xeno-canto.org/species/Cistothorus-platensis). The migratory behavior of C. stellaris is another intrinsic characteristic that makes interbreeding with the sedentary C. platensis unlikely. HBW/BirdLife and IOC already treat C. stellaris as a separate species from C. platensis.
Given that C. stellaris is a separate lineage with a different song and migratory behavior, I recommend splitting it from the Neotropical C. platensis.
Boesman, P. 2016. Notes on the vocalizations of Sedge Wren (Cistothorus platensis). HBW Alive Ornithological Note 285. In: Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from http://www.hbw.com/node/1251727 on 12 October 2016).
Robbins, M. B., & Á. S. Nyári 2014. Canada to Tierra del Fuego: species limits and historical biogeography of the Sedge Wren (Cistothorus platensis). The Wilson Journal of Ornithology 126(4):649-662.
Taylor, A. M., Jr. 1988. Geographic variation and evolution in South American Cistothorus platensis (Aves: Troglodytidae). Fieldiana Zoology New Series 48:1–35. [https://www.biodiversitylibrary.org/item/21223#page/24/mode/1up]
Comments from Stiles: “YES. Splitting stellaris from platensis (really a decision for NACC) is an obvious YES. However, this dodges the issue for SACC, because the genetic data clearly justify several splits for the South American taxa. although some degree of conservatism might be justified where vocal data are scarce or lacking, at least three splits seem perfectly justified. Over to you, Mark!
Comments from Pacheco: “YES. The data available justifies the decision.”
Comments from Zimmer: “YES”. This one is long overdue, as suggested by vocal, morphological and behavioral (migratory versus sedentary) differences, and, as confirmed by the genetic data, which establishes that C. platensis, as currently recognized, is not monophyletic. We still need to deal with species-limits within the South American “Grass/Sedge” Wrens – a stickier and more complicated proposition.”
Comments from Jaramillo: “YES – this is a given, now on to the more complex issues within South America. By the way, I recall reading in one of the popular books that D. Kroodsma wrote he noted that stellaris was a “random song generator” in that within the very basic pattern that they use, that each song is slightly different, and created on the spot seemingly. Different from other wrens in this respect. I do not think this is published anywhere else, and I could not find where I had read it. So, take it as hearsay, or poor recollection if I am wrong.”
Comments from Remsen: “YES, but somewhat reluctantly because no matter how obvious this split seems to everyone, the published evidence for it is weak. A rather small sample of genes indicate that platensis as traditionally recognized is paraphyletic with respect to two South American taxa traditionally treated as species, but at the species level, peripheral speciation predicts paraphyletic species. As for voice, the published data are strongly suggestive but fall far short of a rigorous analysis. As for migratory vs. sedentary, that stellaris is migratory is in itself not a species-level marker, because individuals in the same population of some species can be migratory or sedentary, some individuals of some species can be migratory in some years, not in others, and within many monotypic species, the higher-latitude population can be highly migratory and the lower-latitude population sedentary (a familiar North American example being Mimus polyglottos).”
Comments from Areta: “A hesitant YES. This one is, to me, one of this thought-to-be obvious cases in which no one has taken the time to do a proper and careful comparative analysis and publish it. The “work” by Boesman has few samples of stellaris and cannot be considered by any means a thorough vocal analysis, and I believe that it cannot be used as a justification of range-wide vocal consistency or even as a good description of vocal features of stellaris. Kroodsma´s papers provide much better insights, even though they were not in general focusing on continent-wide geographic variation in vocalizations and their importance in taxonomy. I echo all of Van´s concerns on the other points, and so my hesitation.”
Comments from Bonaccorso: “YES. Well-supported reciprocal monophyly of Cistothorus stellaris and C. platensis (mt DNA), two nuclear introns recovering the same topology (according to Robbins and Nyari 2014, not shown), differences in song, and migratory behavior in C. stellaris, suggest reproductive isolation.”
Comments from Ribas: “YES. I agree with the proposal that C. stellaris should be treated as distinct from C. platensis. Even considering that the genetic evidence comes almost only from ND2, the combined geographical, vocal and behavioral evidence end up being strong in my opinion for considering them distinct species. In addition, support for the (C. meridae, C. apolinari) clade as sister to all other C. platensis except C. stellaris is high, and this would hardly change with additional genetic sampling.”
Comments from Stotz: “NO. I think that stellaris is a distinct species from platensis (although published data are on the weak side). I am voting NO because I think this is a decision that should be made first by NACC since stellaris is extralimital to South America, and whether these are split or lumped, South American birds would still be C. platensis.”