Proposal (391) to
Split Troglodytes cobbi from T. aedon
Effect on South American CL: this proposal would return the Falkland Islands form, cobbi to species status.
Background: The species Troglodytes cobbi was described and named by Chubb (1909) the year after the type specimen was collected by Arthur Cobb. Rollo Beck, who collected specimens for the AMNH between 6 November 1915 and 13 January 1916, remarked on the ‘quite noticeable’ absence of land birds and heard from the Colonial Secretary, who gave him a collecting permit, that the wren was ‘very rare’. The species was common on Kidney Island fifteen (sic) miles away but never seen about the town of Stanley (Beck, 1917).
In 1917, R.H. Wace, formerly a medical doctor for the Falkland Islands Company between about 1913 and 1917, typed his list of Falkland Islands birds. This was translated into Spanish by R. Dabbene and published in El Hornero in 1921. Wace listed this taxon as ‘Cobb’s House-wren, Tussac Island Wren, Troglodytes Cobbi Chubb’ in his typescript, but Dabbene modified the nomenclature of several species and listed this one as Troglodytes musculus Cobbi Chubb and stated that it nested in the islands. Why Dabbene did not accept this taxon as a species is not known.
(1921) reviewed the birds collected by Alcide D’Orbigny and between detailed
descriptions of other Troglodytes
taxa, he stated: ‘[On the Falkland Islands, a nearly related form, T. musculus cobbi Chubb is met
with. It is described as being similar
to T. ‘hornensis’ (viz. T. m. magellanicus), but larger, with
stronger feet and bill. This form I have
not seen.]’ His final sentence is
significant; he had not examined any Falkland specimens but decided, apparently
from the brief description available in Chubb (1909), that the Falkland taxon
was a geographic race rather than a distinct species.
Chapman & Griscom (1924) noted how different this wren is from mainland forms and stated, “It is rather surprising that it should have remained undescribed for so long.” Possibly they were referring to the century and a half between settlement by Europeans in the late 18th century and 1909. However, in our opinion this is unsurprising because the species has a preference for dense coastal Tussac grass Poa flabellata, which sheep had largely destroyed through overgrazing around the coasts of the larger islands by the end of the 19th century.
Chapman and Griscom examined the T. cobbi skins collected by Beck on Kidney and Sea Lion islands and commented (1924, page 284), ‘On the Falkland Islands long isolation has produced a distinct species of house wren (T. cobbi), a little-known bird, which is discussed beyond.’ On page 302 they describe the species, saying that it was ‘sharply distinct from any other house wren in the very slight colour contrast between upper and underparts’ and ‘with its large size and insular habitat’ was entitled to specific rank. In 1934, Chapman reviewed a few new taxa from Chile and the Falklands. He only commented that T. cobbi was, ‘A specifically distinct representative of the continental Troglodytes musculus.’
In 1934, Hellmayr included T. musculus cobbi in his Birds of the Americas, noted its similarity in colour to T. m. bonariae, but stated that it was much larger with a longer stronger bill. Hellmayr stated that he examined eight specimens from Kidney Island and two from Sea Lion Island, the same specimens that Chapman and Griscom had described, yet he did not mention their conclusions of 1924 nor Chapman’s reiteration of 1934.
The taxon cobbi is restricted to the Falkland
Islands and differs strikingly from all other forms of Troglodytes aedon, not just the southern taxa (magellanicus and chilensis). It is a large and bulky Troglodytes with a big bill and strong legs, as well as a
distinctly unicolored plumage. It is
almost as dark on the underparts as it is on the upperparts. Structurally cobbi is large headed and short tailed. Its head is slightly paler than the rest of
the body and lacks any dark eyeline or pale supercilium. Adult plumage fades during the summer due to
bleaching by sunlight to show an almost grey-brown head and paler brown
back. Juveniles in fresh plumage show
darker lores and ear-coverts, and have much richer chestnut brown on rump and
tail. This difference has, I believe
(RWW), led to confusion in some observers who cannot understand how such
variability in depth of coloration can occur in so small a species.
New information: Woods (1993) summarized much information on the appearance, distribution and ecology of Cobb’s Wren. He noted that this wren is tolerant of humans, but in contrast to House Wrens elsewhere, it is not found inhabiting human settlements. Its optimum habitat is dense, mature Tussac grass growing to the high-water mark behind extensive boulder beaches on which kelp is thrown by storms. The Falkland Islands Species Action Plan for Cobb’s Wren (2009) clearly shows that it cannot survive on islands that have been invaded and colonized by rats (Ship or Norway species), House mice, or domestic/feral cats. This wren is absent from farmed or heavily grazed areas, probably due to the presence of the above potential predators and lack of Tussac grass. Its nests are usually at or near ground level below thick Tussac or between rocks above high-water mark, not in cavities or houses as with House Wrens in other parts of the Americas. This wren commonly forages in the littoral zone, around or beneath large boulders, where invertebrates multiply. We know of no other population of House Wren, at least in South America that forages by preference in littoral marine habitats.
treated cobbi as a species. Some measurements in there bear noting. For musculus,
ranges given as follows: male wing (48-53); male tail (32-38.5); culmen
(12-14); and male tarsus (17-19.5). For cobbi, from Woods (1993): wing (52-63);
tail (41-42); culmen (13-20); tarsus (15.5-20).
Because the measurements in Woods include both the smaller females and
males, these are biased towards the smaller side in comparison to the male only
measurements for musculus. Unfortunately, mass measurements, in which
the real size difference would be apparent, are not available in quantity. Cobb’s Wren is actually a relatively
short-winged bird; the fact that the wings are so much longer than that of musculus gives an idea of how much
bigger this bird is.
Descriptions of the voice (such as in Woods 1988) sound distinctive: “call notes are harsher and buzzing, chiz, chiz-iz or a higher cheez. ….The song …consisting of a mixed phrase of quick trills and whistles with harsh notes, rapidly delivered and lasting about two seconds. …. Different males have distinctive songs, mostly similar in length but varying in pattern; slow trills repeated at ten second intervals or continuous warbling lasting 20 seconds have been noted.
My personal experience (AJ) with this wren was of utter amazement when I finally saw one. Not only was it a big, bulky brute of a wren, but also the coloration of the bird and its habitat in thick grass and boulders reminded me of one of the above tree line Scytalopus species…but a rather big one! Photos cannot convey how different and unusual this wren looks and “feels” in real life. Wrens in general, but Troglodytes in particular appear to be over lumped currently. They are not Scytalopus, but it is my opinion that there are many species level taxa in Troglodytes wrens that will come to light as more genetic work and vocalization work is done on them. Some of this hidden diversity will be difficult to uncover, but in some cases it is relatively clear. Cobb’s Wren is one of these cases. It doesn’t look like, act like or sound like the mainland House Wrens, and has marked ecological differences.
Photos of Cobb’s Wren are here:
English Names: Cobb’s Wren is the most common and appropriate and the name we suggest this species keeps. Falkland’s Wren might seem appropriate to some, but this is confusing as Cistothorus platensis is also present on the Falklands and the local English name for that species is ‘Falkland Grass Wren’ denoting its subspecific status.
Recommendation: We recommend a Yes vote to return cobbi to species status.
We believe we have shown that the logic for lumping such a highly distinct taxon in with the mainland House Wrens was unsoundly based by Hellmayr, who disregarded other more informed opinions. Woods’ (1993) summary of the biology of this species has led most writers on birds of the southern cone and on wrens to treat it as a separate species: Brewer (2001), Mazar Barnett & Pearman’s checklist to birds of Argentina (2001), Jaramillo (2003) Birds of Chile, and Kroodsma & Brewer (2005) in HBW. Although no new information is published, material for DNA assessment has been collected in the 2008-2009 season and RWW and S. Imberti respectively have substantial samples of song from the Falklands and Patagonia that are due to be compared in the near future.
Beck, R.H. 1917. Bird Photographing on the Falkland Islands. American Museum Journal, Vol. XVII, Number 7:429-460.
Brewer, D. 2001. Wrens, Dippers and Thrashers. Yale University Press, New Haven.
Chapman, F.M. & L. Griscom 1924. The House Wrens of the genus Troglodytes. Bull. Am. Mus. Nat. Hist. 50: 279-304.
Chapman, F.M. 1934 Descriptions of new birds from Mocha Island, Chile and the Falkland Islands, with comment on their bird life and that of the Juan Fernandez Islands and Chiloe Island, Chile. American Museum Novitates, Number 762:1-8
Chubb, C. 1909. (No title). Bull. B.O.C. 25: 15-16
Falklands Conservation and Falkland Islands Government, 2009. A Species Action Plan for Cobb’s Wren 2009-2019. (in press)
Hellmayr, C.E. 1921. Review of the birds collected by Alcide d’Orbigny in South America, Part II. Novit. Zool. 28: 230-276.
Hellmayr, C.E. 1934. Catalogue of Birds of the Americas and the adjacent islands. Part VII, pages 242-243. Field Museum of Natural History, Chicago.
Kroodsma, D.E. & Brewer, D. 2005. Family Troglodytidae (wrens). Pp 356-447 in del Hoyo J, Elliott A. and Christie DA. (Eds). 2005. Handbook of the Birds of the World. Vol.10, Cuckoo-shrikes to Thrushes. Lynx Edicions, Barcelona.
Wace, R.H. 1917. A List of the Wild Fowl of the Falkland Islands. Unpublished typescript.
Wace, R.H. 1921. Lista de Aves de las Islas Falkland [translated by R. Dabbene]. El Hornero, Vol. II:194-204.
Woods, R. W. 1988. Guide to Birds of the Falkland Islands. Anthony Nelson, Oswestry.
Woods, R.W. 1993. Cobb’s Wren Troglodytes (aedon) cobbi of the Falkland Islands. Bull. B.O.C. 113(4): 195- 207.
Robin Woods and Alvaro Jaramillo, March 2009
Comments from Niels Krabbe: “In both Patagonia and in the puna zone of Peru Troglodytes aedon can be found inhabiting tussock grassland with no nearby human habitation, although in Patagonia it also occurs near houses. I do not see anything in the proposal that directly addresses if they would interbreed if in contact. Distinctive vocal differences are mentioned, but as T. aedon is an oscine with numerous vocal dialects, it would seem in place to at least perform playback experiments before bringing the issue to a vote.”
Comments from Nores: “NO. Yo no veo en la propuesta diferencias muy notables como para que sea considerada especie, lo mismo que las fotografías que aparecen en la página citada. Habiendo sido tomada muestras de ADN y existiendo buena cantidad de grabaciones de canto, me parece más acertado esperar hasta tanto se publique algo con estos datos que indique que son especies diferentes.”
Comments from Zimmer: “NO. I’m torn on this one. I really do think that some insular forms in this complex are badly in need of splitting. My reaction upon seeing and hearing the “House Wrens” in the Lesser Antilles was much the same as Alvaro’s reaction to experiencing cobbi for the first time. I was utterly amazed that either of the island populations that I encountered could be considered conspecific with one another, let alone mainland populations. Contrarily, I’ve found myself consistently underwhelmed with differences between most mainland populations in voice (and that includes North American versus South American populations), especially given that they are oscine passerines. Also, it appears that the original lumping of cobbi with aedon was another of those unjustified “Peters-like” moves. I have little sympathy for maintaining a status quo based on such flimsy evidence. Robin and Alvaro have done a good job of showing why cobbi should probably be split. However, given our stated position that changes should be based on published analysis, and, given the statement in the proposal that tissue for “DNA assessment has been collected in the 2008-2009 season and that …substantial samples of song from the Falklands and Patagonia are due to be compared in the near future”, it seems as if we should just wait for the published analysis, which, I suspect, will confirm that cobbi deserves elevation to species status. So, with some reluctance, I vote NO for the moment, but look forward to reversing my vote once the data is published.”
Comments solicited from John Klicka, who is working on an extensive phylogeographic survey of House Wrens: " I have (although I have tried repeatedly) been unable to get my hands on any cobbi specimens so I can't tell you exactly how they stack up with other Troglodytes populations. We do know that the aedon group colonized S. America from the North within the last couple of million years. Given the latitudinal position of cobbi, I would guess that it is extremely recently derived and probably most closely related to one of the nearby mainland forms. Whether or not to elevate cobbi to species is an interesting question. From my (tree-based) perspective, I think that taxonomy should reflect evolutionary pattern and I don't find creating paraphyly a very attractive option (we already have the "biological species" tanneri and sissonii embedded within the aedon phylogeny). Also, I don't see how you could justify elevating cobbi while ignoring equally distinctive lineages such as the Central American group or the birds from Dominica.”
Comments from Cadena: “NO. The information does seem suggestive that cobbi might merit species rank (as might other members of the complex), but, as far as I can tell, the data have not been rigorously analyzed in a publication.”
Comments from Stiles: “NO, at least for now, on presently published evidence. When genetic data and playback experiments become available, I might well change my vote. (I am leery of using data on songs of oscines – or other groups in which song learning is important – given the really strikingly different dialects of some. The songs of Zonotrichia capensis are so different between the Eastern and Central Andes in Colombia that almost anyone would doubt that they are conspecific.) However, I think that with a bit of effort, a publication that shows the differences and analyzes them should not be too much work for the proponents, and I do agree with Van that we should rely on published, peer-reviewed evidence for such decisions.”
Comments from Pacheco: “YES. Voto sim para manter coerência com os meus votos precedentes, quando defendi que táxons diagnosticáveis, porém arbitrariamente reunidos no passado, sejam tratados no nível de espécie até que estudos demonstrem a incorreção do tratamento em separado.”