Split Troglodytes cobbi from T. aedon
Effect on South American CL: this proposal would return the Falkland Islands form cobbi to species status. This proposal was declined previously, but new published data deems a re-submittal.
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.
Hellmayr (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; p. 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 it differs strikingly from all other forms of Troglodytes aedon, not only 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 and then shows 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.
Recent 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), the House Mouse, 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 sheds 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.
Brewer (2001) treated cobbi as a species. Some measurements in there bear noting. For musculus, ranges given for male wing (48-53); male tail (32-38.5); culmen (12-14); male tarsus (17-19.5). For cobbi, from Woods (1993): wing (52-63), tail (41-42), culmen (13-20), tarsus (15.5-20). Given that 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 are not available in quantity -- these would illustrate the real size difference. Cobb’s Wren is actually a relatively short-winged bird; that the wings are so much longer than those 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-treeline 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 in my opinion there are many species-level taxa in Troglodytes wrens that will come to light as more genetic work and vocalization studies are conducted. 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: http://www.falklandsconservation.com/wildlife/birds/cobbswren.html
****NEW INFORMATION 2012: Campagna et al. (2012) published a paper comparing molecular data from passerines of the Falkland Islands and those from the mainland. Mitochondrial DNA (cytochrome c oxidase I) sequences were compared between insular and continental populations of various species, including the House Wren. Most of the nine species did not show a sizeable genetic difference, three species showed a moderate difference, but the insular Cobb’s Wren and the mainland House Wren showed a distinctly larger genetic separation than the other passerines included in the paper. Troglodytes cobbi was found to have a single, unique haplotype separated by at least ten mutational steps from those of T. aedon. The Kimura two-parameter genetic distance calculated evaluating divergence in COI was 2.17%, and 2.86% for cyt b data for the Cobb’s and mainland House wrens. The divergence in COI observed between T. aedon/T. cobbi was confirmed using three additional genes: cyt b (mitochondrial), FGB, and CHD1 (both nuclear). The data show that Cobb’s Wren is the oldest lineage of landbird on the Falkland Islands and that the colonization leading to T. cobbi started about one million years ago. Note that they also suggest that there are at least three separate lineages of House Wrens on the mainland, and that as a whole the House Wren is paraphyletic when it includes cobbi. Their data also found no evidence of gene flow between Cobb’s and mainland House Wrens. In April 2012, Alan Henry photographed a vagrant mainland House Wren in a Stanley garden that was identified by shape, coloration, and calls. This sighting shows that mainland House Wrens can reach the Falklands and have probably done so on various occasions in the past since speciation occurred, so the lack of gene flow in the molecular data becomes all the more interesting given the confirmed occurrence of mainland House Wrens (admittedly rarely) on or near the Falklands.
This study also analyzed vocalizations of mainland House Wrens from Patagonia and from Cobb’s Wren. No playback experiments were performed. Their analysis found differences in several parameters measured, and they provided representative sonograms of both taxa for comparison. A discriminant function analysis based on seven measured song parameters found significant differences between songs of both taxa, and was able to reclassify 76.9% correctly, failing with two continental and four insular songs that were classified to the wrong group. AJ comments: To the ear the songs are significantly different; likely humans, and surely wrens, would be able to classify a higher percentage correctly than the DFA!
RW comments: Two historic specimens from Falkland waters with characteristics of a continental taxon were examined. The skin of one that flew aboard HMS Amethyst 225 km NW of the archipelago between 1873 and 1883 is at the Natural History Museum, Tring: bill 13.5 mm and wing 50 mm. The second came aboard a fisheries vessel en route to South Georgia in 2004 and was collected: bill 12.5 mm and wing 52 mm, measured at the Stanley Museum.
English Names: Cobb’s Wren is the most common and appropriate and the name we suggest this species keeps. Falklands Wren might seem appropriate to some, but this is confusing because 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 think 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.
The additional information from Campagna et al. (2012) clarifies that Cobb’s Wren is substantially divergent genetically from mainland birds, and that there is no evidence for gene flow. It is estimated that Cobb’s separated from mainland House Wrens during the Great Patagonian Glaciation in the early Pleistocene. This new data further supports that Cobb’s Wren is highly divergent from mainland House Wrens.
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.
Campagna, L., St. Clair, J. J. H., Lougheed, S. C., Woods, R. W., Imberti, S., Tubaro, P. L. 2012. Divergence between passerine populations from the Malvinas – Falkland Islands and their continental counterparts: a comparative phylogeographical study. Biological Journal of the Linnean Society (online): doi: 10.1111/j.1095-8312.2012.01898.x
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 762:1-8.
Chubb, C. (No title). Bull. Brit. Orn. Club 25: 15-16.
Falklands Conservation and Falkland Islands Government. 2009. A Species Action Plan for Cobb’s Wren 2009-2019.
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. Brit. Orn. Club 113(4): 195-207
Robin Woods and Alvaro Jaramillo, June 2012
Comments from Remsen: “YES. Published evidence now sufficient for this split.”
Comments from Stiles: “YES. The evidence given here clearly shifts the burden of proof onto those that would maintain cobbi as a subspecies of the mainland bird (whatever it finally turns out to be named).”
Comments from Pacheco: “YES. Em concordČncia com o tratamento proposto.”
Comments from Robbins: “YES. The recent genetic and song data support earlier characters for treating cobbi as a species.”
Comments from Cadena: “NO. I voted no on the earlier proposal on this same issue largely because no published analysis existed. Now, such an analysis is in the literature. Is it totally convincing? Well, not entirely. Marked genetic differentiation between island and mainland forms in birds of the same species with poor dispersal abilities like wrens is not unexpected (e.g. we have soon-to-be-published data showing comparable variation among a large number of populations of Henicorhina leucophrys). Vocal variation is also unsurprising considering that these are oscine birds in which dialects can easily arise with geographic isolation, and such variation need not be related to reproductive isolation. Further, although the authors of the proposal say that birds are probably better than a discriminant analysis in detecting differences, truth is that differentiation (as indicated by a test specifically design to find differences) between groups was not great and would hardly meet criteria for diagnosability. Looking at the sonograms in the paper, it strikes me how complex these songs are, and one has to wonder whether the sampling scheme was sufficient to fully characterize within-individual and within-population variation. I truly suspect that cobbi does merit species rank (as do other island populations such as those in the Caribbean), but I think we still need more data (e.g., in the way of playback experiments to show that such vocal variation -perhaps coupled with size and plumage differences, something one could address with dummies, e.g. Al Uy's work on Monarcha- does indeed matter). Add to this the fact that recognition of cobbi would lead to a paraphyletic T. aedon (see comments by John Klicka on the earlier proposal) and it all calls for caution. Note I am not opposed to the recognition of paraphyletic species if one adheres to the Biological Species Concept and one can determine that one of the lineages has indeed achieved reproductive isolation relative to members of the species within which it is nested, but I am afraid such a demonstration is still not quite complete in this case.”
Comments from Stotz: “NO. This is a hard one from my perspective. I suspect at some point we will split up Troglodytes aedon into multiple species. There are other insular groups that have been suggested as separate species (beani and martinicensis, and possibly taxa within the martinicensis complex), and of course musculus and aedon is an unresolved issue. So why pull cobbi out? Yes we have some vocal evidence and some genetic evidence, but neither just screams out that they have to be separate species. So T. cobbi is the most distinct of the Falkland landbirds. Given that I don’t think any of the others are considered endemic species, that it not very strong support for splitting cobbi. I’d really need for these genetic data to be placed in the context of aedon as a whole. Vocally, others have pointed out the issue of dialects in House Wrens. When I fly from Lima Peru to Iquitos and hear those House Wrens, they sound recognizably different. Somehow a weak discriminant function result and the statement that humans (and probably wrens) would be able to distinguish cobbi from mainland birds just seems to weak to establish this as a distinct species.”
comments from Remsen: “Although I recognize the validity of the reasoning presented by
Daniel and Doug, I think the issue of paraphyly and genetic distance is nearly
irrelevant. First, all we have is
a gene tree, one based on mtDNA.
As shown most recently and dramatically by Hailer et al. (2012), such gene trees
do not necessarily indicate paraphyly.
In fact, I have argued elsewhere (Remsen 2010) that terms
monophyly and paraphyly are not really appropriate at this level of evolution,
which is consistent with the views of Willi Hennig himself.
Second, I recommend taking a different, more holistic (?) view of this. Here we have a House Wren derivative that lives in tussock grass and feeds regularly in the intertidal zone! It cannot coexist well with rats. After a couple of hundred years of there being human habitations on the Falklands, cobbi ignores them. To me, this bird has differentiated to the point of no return – talk about an independent evolutionary trajectory! Its genome has clearly lost the adaptability that we associate with aedon, which does fine in remote aspen groves, treeless Andean villages, and Amazonian towns surrounded by forest, and seems to do very well in the presence of rats. I’m willing to bet the house that if aedon were released onto the islands, it would cling to the buildings, as it has almost everywhere on the continents, and interact minimally with cobbi. Maybe someday the introductees would start to probe the tussock grass niche, but would run into the hard wall of the cobbi genome, which is already ecologically, morphologically, and perhaps even physiologically adapted to that unusual zone, one approached in the Troglodytidae perhaps only by Cistothorus platensis.”
“Third, all those West Indian aedon derivatives should be ranked as species for similar reasons, and that they currently are classified as subspecies I think reflects only the absence of a coherent proposal to rank them as such. Their current ranking should not effect our verdict on cobbi, in my opinion.”
Comments from Zimmer: ““YES”. Here are my comments from the earlier proposal, which did not pass:
‘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.’
“We now have the published analysis that was lacking when I voted “NO” the first time around. Okay, so the genetic data, and particularly the vocal data are not a slam-dunk for species-status, and both Daniel and Doug make some valid points regarding the potential for learned song dialects in oscine passerines. However, like Van, I have a hard time getting past the ecological distinctions between cobbi and any population of aedon. To me, the vast majority of mainland aedon populations, local song dialects or minor plumage distinctions aside, are simply House Wrens. They basically look the same, sound the same, and behave the same, regardless of whether they are in the Amazon, the Andes, or North America. But cobbi really seems to be a totally different beast, and I just cannot reconcile that with any template of aedon. I would also second Van’s comments about not using the West Indian populations (still treated as subspecies of aedon) as a yardstick for evaluating cobbi. Some of those populations are pretty clearly deserving of recognition as distinct species. Maintaining them as subspecies of aedon is setting the comparative bar for what constitutes a species in this complex unrealistically high.“
Comments from Pérez-Emán: “YES. I think the evidence is in favor of recognizing T. cobbi as a different species. Alhough vocal analyses are not strongly convincing (basically as a result of the complexity of the song and the difficulty of interpreting this variation in the context of oscine songs), both mainland and island (cobbi) populations seem to be recognized by songs. Molecular divergence, as pointed out by Daniel, might not be surprising in Troglodytidae. However, this divergence might be informative if congruent with divergence in morphology and ecology, as I can see in this case. Finally, I think that once Klicka’s study is out, a careful evaluation on House Wren’s taxonomy will need to be done.”