Proposal (636) to South American Classification Committee

 

Split Cercomacra fuscicauda from Cercomacra nigrescens

 

Effect on South American CL:  If passed, this proposal would result in the elevation of the subspecies Cercomacra nigrescens fuscicauda of the Blackish Antbird to that of a distinct species, and in the process, add a species to our list.

 

Background:  Over the past two decades, various workers have remarked on the vocal distinctiveness of the western Amazonian subspecies fuscicauda of the Blackish Antbird (Cercomacra nigrescens), relative to other named subspecies in the nigrescens-complex (e.g. Mayer 1996, P. Coopmans in Ridgely and Greenfield 2001, Zimmer and Isler 2003, Schulenberg et al. 2007), something noted by T. A. Parker as early as the 1980s.  In recent years, speculation that more than one species was involved has turned into the “conventional wisdom” among those familiar with the various taxa nested within C. nigrescens, but in the absence of any published analysis, there have been no changes to the taxonomic status quo. 

 

Enter Mayer et al. (2014), who, for the first time, presented a vocal analysis of the six named subspecies comprising C. nigrescens.  The authors analyzed audio recordings of all six subspecies of nigrescens, plus recordings of possible intergrades between the subspecies aequatorialis/notata from San Martín and Amazonas, Peru.  Their analysis revealed that male loudsongs of fuscicauda differed significantly in the number of notes and in mean pace from those of all other populations.  The ranges of those measurements were non-overlapping, and distributions met tests of the likelihood that they would not overlap with larger samples.  Therefore, two independent characters of the male loudsong of fuscicauda were considered diagnostic.  Furthermore, no intermediate song types between fuscicauda and adjacent forms were found, despite the fact that recording were obtained from potential contact zones in regions of parapatry between fuscicauda and aequatorialis, fuscicauda and notata, and fuscicauda and approximans – all recordings from these areas were clearly assignable to one song type or the other, in spite of the absence of any apparent habitat barriers between subspecies.  The lack of evidence for intermediate song types near the contact zones is evidence that either the various subspecies maintain their integrity despite limited hybridization, or, that they do not interbreed at all.  Songs of fuscicauda also differed, on average, but with overlap, in a longer introductory note, higher maximum pitch of the rattle, and in decelerating rather than accelerating pace.

 

Female song in all six subspecies was found to be very similar, as also indicated by Zimmer and Isler (2003).  The various subspecies of C. nigrescens also give various calls, but Mayer et al (2014) found these varied widely within populations as well as between populations.  Loudsongs of the other five subspecies (minus fuscicauda) were not found to vary diagnosably from one another.

 

Interspecific variation in plumage within the genus Cercomacra is conservative, so it is not unexpected that males of all six subspecies of C. nigrescens are very similar to one another in plumage characters, showing subtle variation in overall degree of color saturation of the mostly gray plumage, as well as variation in the size of the white interscapular and shoulder patches, and the white tips to the wing coverts.  Males of fuscicauda most resemble males of nominate nigrescens (Guianan region) in being darker gray overall, with smaller white interscapular and shoulder patches and reduced white-tipping to the wing coverts relative to the paler neighboring subspecies of the Andes and central Brazil.  As is often the case with thamnophilid antbirds, females within the nigrescens complex are more readily identified to subspecies group than are the males.  Females of fuscicauda differ from females of the other 5 subspecies of nigrescens in having a blackish tail (versus fuscous in nominate and other Brazilian forms and pale brown in the Andes), and in being paler and duller ochraceous than the rest, with the crown washed tawny, and showing a markedly less contrasting facial pattern.

 

Mayer et al. (2014) concluded from their analysis that fuscicauda is diagnosable in plumage characters (at least as regards females) and in two characters of the male loudsong from the other five subspecies of nigrescens.  They also take the lack of intermediate song types in areas of parapatry as evidence that fuscicauda does not interbreed with geographically adjacent forms, and conclude that it should be ranked as a biological species.

 

Because fuscicauda is, throughout most of its range, confined to seasonally flooded forest or stands of Gynerium cane and adjacent thickets on sandbars and riverbanks, and appears always to be associated with riparian habitats, Mayer et al. (2014) followed Krabbe and Nilsson (2003) in applying the English name of “Riparian Antbird.”

 

Analysis & Recommendation:  Vocal differences are far more important in the field recognition of most species of Cercomacra than are the relatively subtle plumage distinctions, and I would bet that is even truer for the birds themselves than it is for the field ornithologists.  The loudsongs of fuscicauda are more different from those of all other subspecies of C. nigrescens than are the songs of C. nigrescens (minus fuscicauda) from C. tyrannina.  For anyone not familiar with the vocalizations of these taxa, I would urge you to check out examples of the loudsongs of each on Xeno-canto or the Cornell website.  It is one thing to read that the analysis identified 2 diagnostic vocal characters in the loudsong of fuscicauda, but it is another thing entirely to listen to just how different those songs sound relative to the songs of other members of C. nigrescens.  I have zero doubt that the authors’ analysis and conclusions are correct, and that fuscicauda is vocally and morphologically differentiated from all other populations of C. nigrescens, and that it should be treated as a distinct species under any species concept.  The English name advocated by Krabbe and Nilsson (2003) and by Mayer et al. (2014) seems very appropriate, given that fuscicauda is pretty much restricted to riparian habitats, whereas other taxa within C. nigrescens occur in other habitats as well, some exclusively so.  Ornithologists have already pretty much exhausted any descriptive possibilities for English names in Cercomacra, all of which attempt to capture some shade of color between gray and black (e.g. Gray, Dusky, Blackish, Black, Jet), and neither the distinguishing characters of the female plumage, nor the geographic distribution of fuscicauda lend themselves well to English names.

 

In conclusion, I strongly recommend a YES vote on splitting fuscicauda from the rest of Cercomacra nigrescens, and treating it as a distinct species, Cercomacra fuscicauda, Riparian Antbird.

 

Literature Cited:

 

Krabbe, N. and J. Nilsson.  2003.  Birds of Ecuador:  Sounds and photographs.  DVD-ROM.  Bird Songs International, Westernieland.

 

Mayer, S.  1996.  Birds of Bolivia:  Sounds and photographs.  CD-ROM.  Bird Songs International, Westernieland.

 

Mayer, S., P. Coopmans, N. Krabbe, and M. L. Isler.  2014.  Vocal evidence for species rank to Cercomacra nigrescens fuscicauda J. T. Zimmer.  Bulletin British Ornithologists’ Club 134: 145-154.

 

Ridgely, R. S., and P. J. Greenfield.  2001. The birds of Ecuador. Vol. I. Status, distribution, and taxonomy. Cornell University Press, Ithaca, New York.

 

Schulenberg, T. S., D. F. Stotz, D. F. Lane, J. P. O'Neill, and T. A. Parker III.  2007. Birds of Peru. Princeton University Press, Princeton, New Jersey.

 

Zimmer, K. J. and M. L. Isler. 2003. Family Thamnophilidae (typical antbirds). Pages 448-681 in J. del Hoyo, A. Elliot, and D. A. Christie, editors. Handbook of the Birds of the World. Vol. 8. Broadbills to Tapaculos. Lynx Edicions, Barcelona.

 

Kevin J. Zimmer, July 2014

 

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Comments from Robbins: “YES, the vocalizations of fuscicauda are quite distinct from those of other members of nigrescens.”

 

Comments from Stiles: “YES, the vocal evidence and the good geographical coverage of recordings clearly indicate that two species are involved.  The plumage differences, while slight, are on a par with differences between other species of Cercomacra.”

 

Comments from Remsen: “YES.  Vocal differences are convincing and consistent with taxa ranked as species in this group, among which differences are often slight, e.g. C. parkeri.”

 

Comments from Jaramillo: “YES – A well-crafted proposal, but what sold me was listening to the songs on xeno-canto. Good to have the published analysis as backup for this decision.”

 

Comments from Pacheco: “YES. The data indicate this taxon should be treated as a distinct species under any species concept.”