Species limits in the pygmy antwren (Myrmotherula brachyura) complex (Aves: Passerif ormes: Thamnophilidae): 1. The taxonomic status of Myrmotherula brachyura ignota

•The taxonomic position of a trans-Andean avian population described as Myrmotherula brachyura ignota Griscom, 1929, has existed under a cloud of uncertainty for the past fifty years. Recent advances in the use of vocalizations in species-level taxonomy in suboscines, generally, and thamnophilids, specifically, provide a basis for a reexamination of the relationships of M. b. ignota to two closely related cis-Andean taxa, M. brachyura and M. obscura. Four independent vocal characters distinguish M. b. ignota from M. brachyura, whereas the vocalizations of M. b. ignota cannot be distinguished from those of M. obscura. Myrmotherula b. ignota and M. obscura also share plumage characters distinct from M. brachyura. Myrmotherula ignota is recognized as a distinct species, Moustached Antwren, of which M. i. obscura is a subspecies. Smallest of the typical antbirds (Thamnophilidae), Myrmotherula brachyura was also one of the first in its family to be described (Hermann 1783). Unlike most thamnophilid species with widespread distributions for which a plethora of subspecies were described in the following almost 150 years, M. brachyura remained a monotypic species as late as 1924 (Cory and Hellmayr 1924) with a geographic range extending throughout Amazonia, the Guianas, and the region immediately northwest of the Andes. In 1929, the trans-Andean population in Panama and Colombia (now known to extend to northwestern Ecuador) was described as M. b. ignota (Griscom 1929) based on the briefest of descriptions: "Similar to typical M. brachyura of Cayenne and Amazonia, but the light streaking on pileum, wings, and back greatly reduced, resulting in more solidly blackish areas; female with much paler rufous crown stripes, and no fulvous on throat." Zimmer (1932), after examining about 100 cis-Andean specimens of M. brachyura, noticed that some were "sharply distinguishable." He stated that if these specimens were found in allopatry with M. brachyura, he would have considered them conspecific, but because they occurred at the same locations as M. brachyura, he described them as a new species, Myrmotherula obscura. Short-billed Antwren. Zimmer's diagnosis and descriptions were much more complete than those of Griscom, but the salient points can be summarized as follows. Compared to M. brachyura, male upperparts and wings much darker (blacker), black mystacal stripe and black postocular stripe much broader, and length of bill reduced (ranges of measurements of exposed culmen abut but do not overlap); female with pale head markings narrower and more rufescent, back markings narrower and whiter, and mystacal streak, postocular area, and bill length differed like male. The geographic range of M. obscura has since been found to extend through southeastern Colombia, eastern Ecuador, northeastern Peru, 24 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON and extreme west-central and southwest Amazonian Brazil. In his description Zimmer (1932) did not compare specimens of M. obscura and M. b. ignota, apparently because he only had one male specimen of M. b. ignota at hand. Myrniotherula b. ignota is an uncommon bird in the field and in collections, and the paucity of specimens has hampered understanding of its taxonomic position. However, the similarity of plumage characters distinguishing M. b. ignota and M. obscura from M. brachyura is apparent from the descriptions. Subsequently, the similarity caused Bond (1950), who had obtained two additional female-plumaged specimens of M. b. ignota, to consult Hellmayr on the possibility that they could be more closely related to M. obscura than to M. brachyura. Their conclusions were inconclusive, confused by the finding that bill lengths of M. b. ignota fell into the range of those of M. brachyura, and Zimmer had emphasized bill length as an important character separating M. brachyura and M. obscura. Their considerations, however, led Meyer de Schauensee (1966) to write "It is possible that what is currently called M. brachyura ignota from Panama and northwest Colombia may prove to be specifically distinct from M. brachyura and conspecific with M. obscura . . ." Most recently, the seventh edition of the Check-list of North American Birds (American Ornithologists' Union 1998) noted the possibilities that M. b. ignota might be specifically distinct or more closely related to M. obscura than to M. brachyura. Today, bolstered by the growing understanding that vocalizations in nearly all suboscines (including the Thamnophilidae) are innate, vocal characters are increasingly employed in examining species level questions in these families (Baptista & Kroodsma 2001). In an effort to provide a point of reference for considering species limits in the Thamnophilidae using vocal characters, Isler et al. (1998) compared characteristics of vocalizations of eight pairs of closely related but reproductively isolated, syntopic antbird species. Among these were M. obscura and M. brachyura, whose vocalizations in the region of sympatry were analyzed. Three independent vocal characters were found to distinguish loudsongs (sensu Willis 1967) of the two species, and in addition, each species included a call in its repertoire not known to be delivered by the other. Given the diagnostic differences found between vocalizations of M. obscura and M. brachyura, an obvious question for empirical analysis is which of these species are the vocalizations of M. b. ignota most closely related? Earlier consideration of the question was hampered by the lack of vocal recordings of M. b. ignota, a situation similar to that facing earlier specimen comparisons. Ten years ago only one recording of M. b. ignota, a loudsong, was available. Its initial examination suggested that M. b. ignota was more closely related to M. obscura (M. & P. Isler, R. Ridgely in Ridgely & Tudor 1994), but the single example precluded analysis. Since then, a number of recordings of M. b. ignota have been obtained, and they allow us to address this fundamental question.