PROCEEDINGS of the BIOLOGICAL SOCIETY of WASHINGTON 116(L):23-28
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PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON 116(l):23-28. 2003. Species limits in the pygmy antwren {Myrmotherula brachyura) complex (Aves: Passeriformes: Thamnophilidae): 1. The taxonomic status of Myrmotherula brachyura ignota Morton L. Isler and Phyllis R. Isler Department of Systematic Biology-Birds, National Museum of Natural History, Washington, D.C. 20560-0116, U.S.A., e-mail: [email protected] Abstract. —The taxonomic position of a trans-Andean avian population de- scribed 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 tham- nophilids, 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 rec- ognized as a distinct species, Moustached Antwren, of which M. i. obscura is a subspecies. Smallest of the typical antbirds (Tham- ra, noticed that some were "sharply distin- nophilidae), Myrmotherula brachyura was guishable." He stated that if these speci- also one of the first in its family to be de- mens were found in allopatry with M. scribed (Hermann 1783). Unlike most brachyura, he would have considered them thamnophilid species with widespread dis- conspecific, but because they occurred at tributions for which a plethora of subspe- the same locations as M. brachyura, he de- cies were described in the following almost scribed them as a new species, Myrmoth- 150 years, M. brachyura remained a mono- erula obscura. Short-billed Antwren. Zim- typic species as late as (Cory 1924 and mer's diagnosis and descriptions were much Hellmayr 1924) with a geographic range more complete than those of Griscom, but extending throughout Amazonia, the the salient points can be summarized as fol- Guianas, and the region immediately north- lows. Compared to M. brachyura, male up- west of the Andes. In 1929, the trans-An- perparts and wings much darker (blacker), dean population in Panama and Colombia black mystacal stripe and black postocular (now known to extend to northwestern Ec- stripe much broader, and length of bill re- uador) was described as M. b. ignota (Gris- duced (ranges of measurements of exposed com 1929) based on the briefest of descrip- culmen abut but do not overlap); female tions: "Similar to typical M. brachyura of with pale Cayenne and Amazonia, but the light head markings narrower and more streaking on pileum, wings, and back great- rufescent, back markings narrower and ly reduced, resulting in more solidly black- whiter, and mystacal streak, postocular area, ish areas; female with much paler rufous and bill length differed like male. The geo- crown stripes, and no fulvous on throat." graphic range of M. obscura has since been Zimmer (1932), after examining about found to extend through southeastern Co- 100 cis-Andean specimens of M. brachyu- lombia, eastern Ecuador, northeastern Peru, 24 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON and extreme west-central and southwest lated but reproductively isolated, syntopic Amazonian Brazil. antbird species. Among these were M. ob- In his description Zimmer (1932) did not scura and M. brachyura, whose vocaliza- compare specimens of M. obscura and M. tions in the region of sympatry were ana- b. ignota, apparently because he only had lyzed. Three independent vocal characters one male specimen of M. b. ignota at hand. were found to distinguish loudsongs (sensu Myrmotherula b. ignota is an uncommon Willis 1967) of the two species, and in ad- bird in the field and in collections, and the dition, each species included a call in its paucity of specimens has hampered under- repertoire not known to be delivered by the standing of its taxonomic position. How- other. ever, the similarity of plumage characters Given the diagnostic differences found distinguishing M. b. ignota and M. obscura between vocalizations of M. obscura and from M. brachyura is apparent from the de- M. brachyura, an obvious question for em- scriptions. Subsequently, the similarity pirical analysis is which of these species are caused Bond (1950), who had obtained two the vocalizations of M. b. ignota most additional female-plumaged specimens of closely related? Earlier consideration of the M. b. ignota, to consult Hellmayr on the question was hampered by the lack of vocal possibility that they could be more closely recordings of M. b. ignota, a situation sim- related to M. obscura than to M. brachyura. ilar to that facing earlier specimen compar- Their conclusions were inconclusive, con- isons. Ten years ago only one recording of fused by the finding that bill lengths of M. M. b. ignota, a loudsong, was available. Its b. ignota fell into the range of those of M. initial examination suggested that M. b. ig- brachyura, and Zimmer had emphasized nota was more closely related to M. obscu- bill length as an important character sepa- ra (M. & P. Isler, R. Ridgely in Ridgely & rating M. brachyura and M. obscura. Their Tudor 1994), but the single example pre- considerations, however, led Meyer de cluded analysis. Since then, a number of re- Schauensee (1966) to write "It is possible cordings of M. b. ignota have been ob- that what is currently called M. brachyura tained, and they allow us to address this ignota from Panama and northwest Colom- fundamental question. bia may prove to be specifically distinct from M. brachyura and conspecific with M. Methods obscura ..." Most recently, the seventh edition of the Check-list of North American Because current species designations Birds (American Ornithologists' Union confuse rather than clarify the discussion, 1998) noted the possibilities that M. b. ig- henceforth in this paper we refer to the nota might be specifically distinct or more three taxa simply as brachyura, obscura, closely related to M. obscura than to M. and ignota. brachyura. Morphology. —Examination of speci- Today, bolstered by the growing under- mens available at the National Museum of standing that vocalizations in nearly all su- Natural History (USNM) indicated that de- boscines (including the Thamnophilidae) scriptions of earlier authors (cited in the in- are innate, vocal characters are increasingly troduction) accurately reflected plumage employed in examining species level ques- coloration of the three taxa. Consequently, tions in these families (Baptista & Kroods- no additional plumage data were obtained. ma 2001). In an effort to provide a point of Because of the importance of bill measure- reference for considering species limits in ments in the description of obscura, and be- the Thamnophilidae using vocal characters, cause the differences in bill length between Isler et al. (1998) compared characteristics ignota and obscura deterred earlier workers of vocalizations of eight pairs of closely re- from establishing a close relationship be- VOLUME 116, NUMBER 1 25 tween the taxa, measurements of bill length the earlier comparisons of brachyura and were obtained, using MAX-CAL electronic obscura. Number of notes, duration, and digital calipers, from specimens of the Lou- pace (notes/sec) were obtained for the entire isiana State University Museum of Natural vocalization. Then, loudsongs were divided Science, Baton Rouge, Museo de Historia into five sections (Isler et al. 1998) to obtain Natural de la Universidad Nacional Mayor the pace for each section and ratios of de San Marcos, Lima, Peru, and USNM. change of pace between sections. Measure- Vocalizations. —The sample of record- ments were also taken of the duration of the ings of brachyura and obscura used and second, middle, and next-to-last notes and referenced in an earlier study of vocal dif- the spaces following them. ferences (Isler et al. 1998) was employed in Finally, we determined whether vocal this analysis. The sample was obtained in characters differed diagnostically between the vicinity of the lower Rio Napo in north- taxa, employing guidelines developed ear- ern Peru. The reason for confining the sam- lier (Isler et al. 1998, 1999). As described ple to this area was to obviate the possibil- in more detail in the previous papers, such ity of introducing geographic variation in differences must be unambiguously distinct the vocalizations of the wide-ranging character states, or, in the case of continu- brachyura into the comparison. Analysis of ous variables, ranges may not overlap and geographic variation in the vocalizations of the means (x) and standard deviations {SD) brachyura was beyond the scope of this pa- of the population with the smaller set of per but will be addressed subsequently. measurements (a) and the population with Seven recordings of ignota from Panama the larger set of measurements (b) must and three from Ecuador were employed. meet the requirement: The recordings were not yet archived in an X, + t^SD, < Xb - t^SD^ (1) institutional collection. They were obtained where t^ = the r- score at the 97.5 percentile at the following locations (listed as country, of the t distribution for n — \ degrees of department, locality) which are followed by freedom (except for ratios where this statis- the name of recordist and number of indi- viduals recorded. PANAMA: Colon: Canal tical test is inappropriate). Zone (R. Ridgely 1, B. Whitney 3). San Results and Discussion Bias: Nusagandi (B. Whitney 1). Darien: Cerro Pirre (B. Whitney 1), Cana (B. Whit- Morphology. —Principal plumage char- ney 1). ECUADOR: Esmeraldas: 30 km E acters distinguishing the taxa are as follows. San Lorenzo (D. Lane 1). Pichincha: 20 km Males of ignota and obscura are similar to N Pedro Vicente Maldonaldo (D. Lane, 2). brachyura except mystacal streak is broader Recordings were analyzed using "Ca- and more distinct, black postocular streak nary 2.1.4" software (Charif et al.