Proposal (45) to
Change Myrmotherula brachyura to Myrmotherula ignota
Background: Griscom (1929) described an antwren from eastern Panama as Myrmotherula brachyura ignota, considering it to be conspecific with the widespread Amazonian form Myrmotherula brachyura. Since then ignota has been found west of the Andes from Panama to northwestern Ecuador. Later, Zimmer (1932) concluded that within the Amazonian specimens called brachyura, there was another species hidden, which he called Myrmotherula obscura. It is broadly sympatric with brachyura across northwestern Amazonia. All three of these taxa are very similar, differing primarily in the relative proportion of black vs. white in the black-and-white streaking that dominates the plumage. It is routine that specimens of obscura and brachyura are confused in collections. Bond (1950) noted the similarity in plumage between ignota and obscura, suggesting that they were more similar to each other than either is to brachyura. Later authors, including AOU (1998), have retained ignota as conspecific with brachyura but noted that it might be a distinct species, or belong with obscura. Ridgely and Greenfield (2001) considered ignota a distinct species.
Previous Evidence: Ridgely and Greenfield (2001) in splitting ignota note vocal differences from brachyura, and comment that its song "closely resembles Short-billed Antwren's song."
New evidence: Isler and Isler (2003) analyzed both morphology and voice of ignota, brachyura and obscura. In both male and female plumage, all are very similar, with ignota more like obscura than brachyura, and in fact more distinctive than obscura is when compared to brachyura. M. obscura has a significantly shorter bill than brachyura (responsible for its English name Short-billed Antwren). The bill measurements of ignota are intermediate but closer to brachyura (although samples are small) . No other measurement data are provided.
Although plumage data suggest that ignota might be closer to obscura than to brachyura, the voice data are much more compelling. The loudsongs of ignota and obscura are very similar, and I daresay impossible to distinguish. Myrmotherula brachyura while grossly similar to the other two taxa, differs in the number of notes, in pace (faster), and in acceleration (the song speeds up in the last half of the song dramatically, almost becoming a roll, while the pace increases only slightly in the other two). Additionally, a common call note of brachyura differs substantially from the other two, which share a similar call.
Proposed change: This proposal suggests that ignota be removed from Myrmotherula brachyura and considered conspecific with Myrmotherula obscura. As ignota has priority, the species should be called Myrmotherula ignota. There are three possible English names for Myrmotherula ignota. The name Griscom's Antwren has been suggested for ignota (sensu stricto) by Meyer de Schauensee (and is used by Ridgely and Greenfield 2001), and Short-billed Antwren is well established for Myrmotherula obscura. The Islers suggest the name Moustached Antwren for the combined taxon in recognition of the stronger black malar mark that is characteristic of both ignota and obscura and is perhaps the best field plumage character.
Recommendation: I recommend following the Islers and treating Myrmotherula ignota and M. obscura as conspecific under the name Myrmotherula ignota. I also recommend following the Islers in the use of Moustached Antwren for Myrmotherula ignota. The other two names have always been applied to the component subspecies of ignota separately. Short-billed is much less appropriate when ignota is included with obscura. As for Griscom's Antwren, the Isler's rightly note it was Zimmer who really gave us the first understanding that there were multiple taxa in this group. Griscom did not recognize his bird as deserving of specific status, treating it as conspecific with brachyura and providing only a brief description. So despite my opposition in general to new English names, I think this lump merits a new one.
American Ornithologists' Union. 1998. Check-list of North American Birds. 7th edition. American Ornithologists' Union, Washington, DC.
Bond, J. 1950. Notes on Peruvian Formicariidae. Proc. Acad. Nat. Sci. Philadelphia 102: 1-26
Griscom, L. 1929. A collection of birds from Cana, Darién. Bull. Mus. Comp. Zool. 69: 149-190.
Isler, M. L. and P. R. Isler. 2003. Species limits in the pygmy antwren complex (Aves: Passeriformes: Thamnophilidae); 1. The taxonomic status of Myrmotherula brachyura ignota. Proc. Biol. Soc. Wash. 116: 23-28.
Ridgely, R. S. and P. J. Greenfield. 2001. The Birds of Ecuador. Vol. II. Field Guide. Cornell Univ. Press, Ithaca, NY.
Zimmer, J. T. 1932. Studies of Peruvian birds. III. The genus Myrmotherula in Peru, with notes on extralimital forms, part 1. Amer. Mus. Nov. 523: 1-19.
Doug Stotz, July 2003
Comments from Remsen: "if only we had this amount of information for all such decisions ... . In my opinion, the Islers have made a strong case for ignota and obscura being sister taxa and that obscura is best treated at the subspecies rank (which for me is a diagnosable taxon that has not diverged to the level associated with differences know to be associated with cessation of gene flow in related taxa that are parapatric or sympatric). I'll also go with 'Moustached' if, as I assume, that is the name to be used in forthcoming HBW vol 8."
Comments from Schulenberg: "Change Myrmotherula brachyura to Myrmotherula ignota [except that I think the vote is to change both Myrmotherula obscura and Myrmotherula brachyura ignota to Myrmotherula ignota]. My vote is "Yes": the Isler and Isler paper makes a strong case. "Moustached Antwren" will take some getting used to, but is acceptable. "
Comments from Zimmer: "I vote "yes" on the proposal to recognize Myrmotherula ignota as specifically distinct from M. brachyura, but as belonging with M. obscura. The rationale for this change is well-documented by the Islers, and it is what we have followed for the HBW treatment. I also favor the Islers' suggested English name of "Moustached Antwren". "Short-billed Antwren" makes no sense with the inclusion of ignota (especially since ignota has priority within the ignota/obscura complex), and "Griscom's Antwren" is tainted by published statements that it should be split from brachyura (without recognizing that it belonged with obscura). "Moustached Antwren" highlights one of the more noticeable field marks, so I favor this suggested name."
Comments from Robbins: " I vote "yes" to considering ignota and obscura as conspecific and applying the English name "Moustached" The Islers did an excellent job of documenting the rationale for these changes."
Comments from Silva: "No. In fact, my proposal is that the Isler's data are indicating that ignota, obscura and brachyura are distinctive species. If ignota is distinct in plumage from obscura but not in voice, it must have specific status as well because different characters have different rates of evolution and one cannot expect to find in nature biological species presenting differences in both plumage and voice."