Scytalopus Tapaculos, a Continental Non-Adaptive Radiation

By Carlos Daniel Cadena

Linked paper: Systematics, biogeography, and diversification of Scytalopus tapaculos (Rhinocryptidae), an enigmatic radiation of Neotropical montane birds by C.D. Cadena, 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, The Auk: Ornithological Advances.

Birds in the tapaculo genus Scytalopus are a perennial source of headaches for birdwatchers seeking to increase their life lists and for professional ornithologists interested in species limits. These small, poorly flighted birds behave and look like mice, are often impossible to tell apart from each other based on their drab plumage, and are difficult to observe and collect. Therefore, the true diversity of Scytalopus has been a matter of guesswork, relationships among species have been obscure, and the potential of the group as a model for evolutionary studies has remained largely untapped.

Our newly published paper presents analyses of the phylogeny, biogeography, and diversification of Scytalopus that answer various questions about the relationships among species and clades. In addition, our work shows that evolutionary diversification in Scytalopus was remarkably rapid, with speciation rates during the Late Miocene and Pliocene rivaling those of iconic avian radiations driven by natural or sexual selection. Because tapaculos diversified swiftly but diverged little in morphology, they represent what one might call a continental non-adaptive radiation. Rapid speciation in tapaculos was likely spurred by the interaction between their limited dispersal abilities and their occurrence in rugged landscapes in the Andes and other mountains where populations readily diverged in geographic isolation.

My interest in the systematics of Scytalopus tapaculos goes back to early 1999. While doing fieldwork for my undergraduate thesis project in the coffee-growing region of the Central Andes of Colombia, I recorded the calls of a small bird I was unable to see in the dense forest understory. It took me a while to get that recording into a computer (digital recorders were a dream back then), but after consulting with Niels Krabbe, one of the few experts on Scytalopus, I learned that the calls most likely were of an undescribed species in the genus which had already been found by conservation biologist Luis Miguel Renjifo.

When I first got into ornithology, some of my favorite readings were papers describing birds new to science. The stories of places, people, and birds these articles told were fascinating. Field and museum ornithologists like George Lowery, John O’Neill, Ted Parker, Gary Graves, and Gary Stiles became familiar characters, and I often dreamed about being involved in a story of discovery like the ones these giants had experienced. I thus vividly remember my excitement at the prospect of participating in the formal description of a new species of bird. However, at the turn of the millennium no museum specimens of the mystery Colombian tapaculo I had recorded existed (not to mention I had zero taxonomic expertise), and so my hopes of describing it had to be postponed.

Things changed quickly when my friend Andrés Cuervo explored montane forests in his home region of Antioquia in the early 2000s, where he found the same undescribed species and managed to collect a few specimens with tissue samples for genetic analysis. By then I was a graduate student and had started to learn about molecular systematics, so Andrés and I decided I should try to sequence DNA from the new species to see how different it was from others and what species was it related to. We eventually completed the analyses and, together with Niels and Luis Miguel, described the new species honoring our academic hero Dr. Gary Stiles. The description of Scytalopus stilesi was the first paper I published in The Auk and marked the beginnings of the study that we are now thrilled to publish in the same journal 15 years later. Having expanded our data set to include all known species as well as several undescribed taxa in our phylogeny of Scytalopus, it feels like we have come full circle since that first publication.

There are various reasons why it took us more than 15 years to finalize and publish our study. One is simply that it was hard to stop collecting data; given the never-ending potential for discovering something cool in such enigmatic birds, any time we heard of newly collected specimens we wanted to study them! Here I should give a shout-out to multiple field collectors and natural history museums that gave us access to their hard-won specimens. Collections are essential, and without them studies like ours would simply be impossible.

Another more recent cause for delay in the completion of our study was our decision to complement our extensive mitochondrial DNA data set with data for thousands of nuclear markers to produce a much more robust phylogenetic framework. With the inclusion of the genomic data set, several colleagues came on board as coauthors, joining a team that already included Andrés, Niels, and tapaculo experts Tom Schulenberg and Jon Fjeldså. We were also joined by my former student Laura Céspedes, who was instrumental in helping us finalize analyses, produce nice figures, and organize messy files and notes accumulated over 15+ years. Although the first four authors of our paper are Colombian, coauthors are from various institutions in the U.S., Denmark, and Brazil; work on such a widespread and complicated group of birds truly required extensive international collaboration.

Ornithologists have learned a lot about Scytalopus diversity lately. While only some 10 species were recognized when John T. Zimmer revised the genus in the 1930s, there are now close to 50 known species in the group. Such a dramatic increase in diversity partly resulted from field work in unexplored areas leading to discovery of new forms, but mostly reflects greater knowledge about vocalizations and a better understanding of the ecological and geographical distributions of species, as exemplified by the now classic ⁠— yet modern⁠ — Krabbe and Schulenberg monograph. We hope that the molecular data linked to voucher museum specimens we have made available will be of use to multiple researchers interested in contributing to completing the puzzle of our understanding of the true diversity of such fascinating birds.