Proposal (852) to South American Classification Committee
Recognize Scytalopus krabbei
Effect on AOU SACC classification: The addition of a new, Peruvian endemic, member of the Scytalopus magellanicus complex.
Background: The tapaculo genus Scytalopus has become the poster child for unrecognized diversity in the Neotropics. In the early 20th century (e.g., Cory and Hellmayr 1924) only 16 species were recognized, this was reduced to only 9 (10, counting S. indigoticus, which is now in genus Eleoscytalopus) by Zimmer (1939) and Peters (1951). Now, nearly a century after Cory and Hellmayr, SACC recognizes 43 spp., and new species are being described nearly annually. The tropical Andes seem to be home of the highest species richness within the genus, with strongly elevational stratification among species—as many as seven species can share a slope from about 1000 m to treeline (usually around 3500 m); along a slope, seldom do two closely related species occur, rather, each elevational replacement is sister to an allopatric form, usually separated by an intermontane valley. This pattern of allopatric replacements is, as one might predict, most pronounced among the highest-elevation species, the members of the S. magellanicus complex. This complex stretches from Colombia to Tierra del Fuego, and comprises at least 16 well-defined genetic clades (and probably more, with more sampling). In addition to genetic differentiation, these clades are defined by vocal differentiation, something that aligns with the general application among birds of the Biological Species Concept (perhaps the most conservative of present-day favored species concepts), as has been applied by recent suboscine work (e.g., Isler et al. 1998). Thus, specialists studying the family have been churning out descriptions of new taxa with regularity. The most recent publication to tackle the group is Krabbe et al. (2020), which described three new species, all endemic to Peru.
Analysis: Of the three new taxa described in Krabbe et al. (2020), one was actually a bit of a surprise to those who had encountered it. It had been collected at first on the Cordillera Colán in Amazonas dept., Peru, then again at Unchog in Huánuco, Peru, and finally on Cerro Patricia on the border of Amazonas and San Martín depts., Peru. In each case, the specimens are easily distinguished by the presence of white feathers on the greater wing coverts. Nevertheless, in each case, the specimens were cataloged as other species (mostly S. altirostris), or as unidentified “Scytalopus sp.?” with the white plumage regarded as a recurring plumage mutation. Cadena et al. (2020) discovered that two specimens, both identified as S. altirostris, were not closely related to each other; advance knowledge of this surprising find focused attention on the nature of variation in all records attributed to that taxon. When vocalizations of “S. altirostris” were surveyed across its range for the work finally published in Krabbe et al. (2020), it was clear that there were two distinct voice types. The accompanying phylogeny placed the white-winged birds on a branch sister to the rather disjunct S. affinis (of the Cordillera Blanca and nearby C. Huayuash in Ancash and Lima depts., respectively), whereas S. altirostris (including the holotype of the taxon) was in a clade containing the S. magellanicus complex otherwise comprising the taxa from southern Peru to northern Argentina, and sister to S. frankeae.
Using ancient DNA sequencing of toe pad samples, all specimens of this new species showed the characteristic white on the greater wing coverts, meaning that this character is actually a valid species specific one! This, combined with the distinctive voice and phylogenetic placement make the species status of this form unquestionable. The species was named S. krabbei by a subset of the authors of the main paper to honor Niels Krabbe, who has been one of the leading authorities of Andean Scytalopus taxonomy. The English name, White-winged Tapaculo, draws attention to its most obvious field character.
Recommendation: I believe Krabbe et al. (2020) have made a strong case for the recognition of S. krabbei, and recommend that SACC include it in its list of Scytalopus. The suggested English name White-winged Tapaculo highlights an relatively easily-seen field character, something rare within the genus. I suggest that it also be adopted by SACC.
Cadena, C.D., A.M. Cuervo, L.N. Céspedes, G.A. Bravo, N. Krabbe, T.S. Schulenberg, G.E. Derryberry, L.F. Silveira, E.P. Derryberry, R.T. Brumfield, and J. Fjeldså. 2020. Systematics, biogeography, and diversification of Scytalopus tapaculos (Rhinocryptidae), an enigmatic radiation of Neotropical montane birds. Auk 137: xxx. https://doi.org/10.1093/auk/ukz077
Cory, C. B., and C. E. Hellmayr. 1924. Catalogue of birds of the Americas., Part III. Field Museum of Natural History Zoological Series, Volume XIII, Part 3.
Fjeldså, J., and N. Krabbe. 1990. Birds of the high Andes. Apollo Books, Svendborg, Denmark.
Isler, M. L., P. R. Isler, and B. M. Whitney. 1998. Use of vocalizations to establish species limits in antbirds (Passeriformes: Thamnophilidae). Auk 115: 577-59.
Krabbe, N.K., T.S. Schulenberg, P.A. Hosner, K.V. Rosenberg, T.J. Davis, G.H. Rosenberg, D.F. LANE, M.J. Andersen, M.B. Robbins, C.D. Cadena, T. Valqui, J.F. Salter, A.J. Spencer, F. Angulo, and J. Fjeldså. 2020. Untangling cryptic diversity in the High Andes: Revision of the Scytalopus [magellanicus] complex (Rhinocryptidae) in Peru reveals three new species. Auk 137: xxx. https://doi.org/10.1093/auk/ukaa003
Peters, J. L. 1951. Check-list of birds of the world. Volume VII. Museum of Comparative Zoology, Cambridge, Massachusetts.
Zimmer, J. T. 1939. Studies of Peruvian birds. No. XXXII. The genus Scytalopus. American Museum Novitates number 1044.
Daniel Lane, April 2020
Comments from Remsen: “A. YES. All data are consistent with recognizing this as a new species. B. YES – perfect name.”
Comments from Areta: “A. YES. I recorded this bird last year and had trouble in assigning it to any know species until the eponymous Niels told me what was going on there. Amazing! B. Good name, innocuous but accurate.”
Comments from Robbins: “A. YES, the combination of that unique white in the wing and voice make this straightforward.”
Comments from Bonaccorso: “A very enthusiastic YES. Thanks, Krabbe et al! At last a Scytalopus that anybody can tell apart!”
Comments from Jaramillo: “A. YES – clear-cut case. B. YES – to White-winged Tapaculo for the English Name.”
Comments from Zimmer: “ A. YES, all data sets support the recognition of krabbei as a distinct species. B. YES to White-winged Tapaculo as the proposed English name – so rare to have an appropriately descriptive name for a Scytalopus!
“As an aside, I was struck by the overview of the S. magellanica complex in the Background Section of this proposal, specifically, with the observation that “along a slope, seldom do two closely related species occur, rather, each elevational replacement is sister to an allopatric form, usually separated by an intermontane valley.” This appears to be the opposite of the situation found in species diversification among white-eyes (genus Zosterops) in East Africa. Zosterops, like Scytalopus, is a speciose genus containing both highland and lowland representatives, whose true species diversity was long underestimated by plumage-based taxonomies. Among East Africa Zosterops, the most egregious failure of a plumage-based taxonomy to reflect the species-level relationships and phylogeny, involved six isolated montane populations that were long treated as subspecies of one polytypic montane species, Z. poliogaster. The incorporation of vocal and ecological data sets with recent genetic investigations has revealed that not only are several of these montane populations worthy of full-species status, but, that most of them are more closely related to geographically proximate taxa with differing habitat and elevation preferences than they are to other “sky island” montane isolates, even neighboring ones. They are, therefore, thought to have arisen independently as a result of niche divergence, as opposed to through vicariant events resulting in the fragmentation of an ancestral montane population.”
Comments from Schulenberg: “YES. I don't think it's a great name - this bird is not white-winged in the sense of Xipholena atropurpurea - but I don't have anything better to offer, and I gather no one else does either.”
Comments from Pacheco: “A. YES. The treatment of S. krabbei at the species rank seems to me justified for multiple reasons.”