An epic year for Scytalopus!

I saw a Scytalopus tapaculo for the first time on 7 June 1977 in treeline cloud forest at Abra Málaga in Cusco, southern Peru. This tapaculo wasn’t much to look at, just a small, gray, wren-like bird with a whitish “eyebrow”, but then, that’s typical of members of this genus. Almost all of them are small and gray, variably mixed with brown, and with or without some paler markings on the face; truly distinctive Scytalopus are few and far between. I thought this bird was cute, and, as the first member of the family Rhinocryptidae that I had seen, I made a mental note of it; but at the time, I was more impressed by other novelties I saw that day, such as Pearled Treerunner and Plushcap.

My interest in tapaculos sharpened considerably the following year, while working along an elevational transect in the Cordillera de Colán in Amazonas, northern Peru. As we changed in elevation, so did the species of tapaculo. This in itself was not surprising, as there are many cases of elevational replacements within a genus from montane regions around the world. No, the surprise was my recognition that what at the time was thought to be individual variation within one species, the presence or absence of a white spot on the crown in one species, turned out to be correlated with differences in vocalizations, elevational distributions, and other features: these two types of Scytalopus turned out to be not one species, but two: White-crowned Tapaculo S. atratus and Rufous-vented Tapaculo S. femoralis (Krabbe and Schulenberg 1997). This discovery led me to wonder whether other species of tapaculos concealed similar secrets.

I thus joined the ranks of ornithologists with a consuming interest in Scytalopus, which to this day remains a small group. Niels Krabbe, the dean of scytalopologists, made the turn to tapaculos after Ted Parker told him “I hate tapaculos”: Niels decided that since Parker seemed to know all about every other South American bird, then the way to make a difference in Neotropical ornithology would be to figure out Scytalopus. Others, such as Marcos Bornschein, Daniel Cadena, Pete Hosner, Ken Rosenberg, and Bret Whitney, may have joined the club when they stumbled across what proved to be previously unknown species of tapaculo. By now, the basic story of tapaculo research is well-known: just as I had experienced with S. femoralis, field workers documented that careful attention to vocalizations and elevational distributions – sources of evidence that, in all fairness, were not available to earlier generations of museum-based ornithologists – led to the splitting of a small number of widespread polytypic species into a much larger number of range-restricted species, and also to the description of a great many newly recognized species. Indeed, of the 44 species of Scytalopus currently recognized by the Clements Checklist, no fewer than 14 were described to science only in the past 25 years.

Daniel Cadena’s initial encounter with an intriguing tapaculo inspired him to initiate what turned into a very long-term, collaborative project to resolve the phylogeny of the tapaculos: that is, to determine how the ever-expanding roster of tapaculos are related to each other. The results (Cadena et al. 2020) reveal that Scytalopus fall into three main groups of related species (clades, in the technical parlance): the Southern Andes clade (which actually includes species that occur from Colombia south to Tierra del Fuego, but represents species related to the southern Magellanic Tapaculo S. magellanicus); the Brazilian clade (occurring both in the interior cerrado region and in the coastal Atlantic Forest); and the Tropical Andes clade (which is distributed from Costa Rica south to Bolivia).

Also, as recounted by Ken Rosenberg, Cadena’s phylogeny provided a “road-map” for describing two species of Scytalopus that had been discovered more than 35 years earlier, but that never had been given formal names. Better late than never! – these species finally were described in a recent paper by Krabbe et al. (2020).  Of course, during that long period of gestation, both of these species had been seen by many birders and field ornithologists, and eBird has collected many records of what were referred to as Ampay Tapaculo (undescribed form) and Millpo Tapaculo (undescribed form), which now go by the names Ampay Tapaculo S. whitneyi and Jalca Tapaculo S. frankeae, respectively. The really big news is that the genetic survey by Cadena and colleagues also revealed that what we all thought was one species, Neblina Tapaculo S. altirostris, actually consisted of two, genetically unrelated species. In a process similar to that described recently by Guy Kirwan and Manuel Schweizer, this led to genetic sampling of the holotype (the “name-bearing” specimen) of Neblina Tapaculo, to determine which of the two lineages was “true” Neblina Tapaculo, and which one was “Tapaculo X”, the mystery bird. Armed with that knowledge, vocalizations and subtle plumage and habitat differences were sorted out, allowing for “Tapaculo X”, an unrecognized species that had been hiding in plain sight, to also be given a name, White-winged Tapaculo S. krabbei. By now we’ve gotten used to descriptions of new species of Scytalopus – but three new species described in a single publication is a rare event.

Finally, the phylogeny developed by Cadena et al. also allowed Cadena and Laura Céspedes to investigate how the world ended up with so many species of Scytalopus in the first place (Cadena and Céspedes 2020). Given that species of Scytalopus replace each other in elevation, often with four or five species along any given elevational gradient, the question was, do new species develop in situ along an elevational gradient, or do new species arise from the splitting of formerly widespread populations within the same elevational band? The answer turns out to be “B”: species found at different elevations but in the same region typically are not related to each other, instead being related to species found at similar elevations but in different cordilleras.

Issued in rapid-fire succession, each of these three publications elucidates a different aspect of the biology of Scytalopus tapaculos. It has been, in the words of Robb Brumfield, “An epic year for Scytalopus!”

Oh, and the bird I saw so long ago, in Cusco? I was told then that it was an Andean Tapaculo S. magellanicus – which now is considered to be a cluster of species (equivalent to Cadena’s Southern Andes clade). At the time, of course, I just thought of it as an interesting little gray bird, and I didn’t scrutinize it in detail. In view of the elevation and habitat, however, odds are very high that it was what now is called Diademed Tapaculo S. schulenbergi Whitney 1994. But in true Scytalopus fashion, there are some unresolved issues here, including suggestions that the population in Cusco is not Diademed Tapaculo after all, but may represent yet another undescribed species. Indeed, the phylogenetic paper by Cadena et al. suggests that quite a few species of Scytalopus contain two or more lineages that genetically are very different from one another, hinting at future splits yet to come (hints that already are inspiring a deeper look by committed scytalopologists). 2020 may have been an epic year for Scytalopus, but it certainly does not represent the end of the line for research on tapaculos.

References:

Cadena, C. D., and L. N. Céspedes (2020). Origin of elevational replacements in a clade of nearly flightless birds: most diversity in tropical mountains accumulates via secondary contact following allopatric speciation. Pages 635-659 in Neotropical diversification: patterns and processes (V. Rull and A. C. Carnaval, Editors). Springer International Publishing.

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: ukz077. https://doi.org/10.1093/auk/ukz077

Krabbe, N., and T. S. Schulenberg (1997). Species limits and natural history of Scytalopus tapaculos (Rhinocryptidae), with descriptions of the Ecuadorian taxa, including three new species. Pages 47-88 in Studies in Neotropical ornithology honoring Ted Parker (J. V. Remsen, Jr., Editor). Ornithological Monographs 48: 47-88. https://doi.org/10.2307/40157527

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 avian diversity in the High Andes: revision of the Scytalopus [magellanicus] complex (Rhinocryptidae) in Peru reveals three new species. Auk 137: ukaa003 https://doi.org/10.1093/auk/ukaa003

Whitney, B. M. (1994). A new Scytalopus tapaculo (Rhinocryptidae) from Bolivia, with notes on other Bolivian members of the genus and the magellanicus complex. Wilson Bulletin 106: 585-812.