To split a mockingbird

By Steve N. G. Howell April 26, 2019
Tropical Mockingbird Mimus gilvus

This provisional note is not intended to be anything scientific, simply a note to alert eBird users and others in Central America of an interesting natural experiment that seems to be taking place, one that has seemingly flown under the radar.

In Mexico’s Yucatan Peninsula, Tropical Mockingbirds (Mimus gilvus, of the northern race gracilis) are common, conspicuous, and noisy, and a casual observer coming from North America might be forgiven for simply passing them off as ‘mockingbirds’—their songs and calls all sound much like Northern Mockingbird (Mimus polyglottos), and I’d be hard pressed to separate them by voice. They look different though, without the big white wing patches of a Northern, but with a broad white edging to the graduated tail, like a giant Lark Sparrow. Where the ranges of the two species meet, Northern and Tropical interbreed, at least on occasion; some authors have lumped them as a single species, but most treat them as separate species (Binford 1989:216). Indeed, a Tropical Mockingbird that showed up in Texas some years ago paired and bred with a Northern Mockingbird, producing young (Mark Lockwood, pers. comm.).

Photo 1. Typical Mayan Mockingbird, with face pattern much like Northern Mockingbird. Tabasco, Mexico, 3 March 2012. © Steve N. G. Howell.

Let’s jump now to South America, and the Colombian form of Tropical Mockingbird, subspecies tolimensis. This taxon is spreading south into Ecuador, where some years ago I encountered it for the first time. I was immediately struck that tolimensis looked and sounded different from the birds I was familiar with from Mexico. Not astonishingly different, but very much ‘not quite right.’ Still, I don’t keep lists and I didn’t really care enough to follow it up, so the ‘problem’ was put in the SEP file (SEP? ‘Somebody Else’s Problem’ for those not familiar with Douglas Adams’ classic book, The Hitchhiker’s Guide to the Galaxy). But SEPs have a habit of returning…

Photo 2. Well-marked tolimensis Tropical Mockingbird, with strong dark mask extending behind eye, and broad whitish eyebrow. Pichincha, Ecuador, 12 Feb 2011. © Steve N. G. Howell.

Back to today, and I have recently started writing a field guide to the birds of Mexico, co-authored and illustrated by Dale Dyer, who illustrated the excellent new Birds of Central America (Vallely & Dyer 2018). Part of the preliminary research for any field guide is to make a list of species to treat. Human knowledge, and with it avian taxonomy, have come a long way since the early 1990s, when I finished work on A Guide to the Birds of Mexico and Northern Central America (Howell & Webb 1995). In that book I split a fair number of bird species, for which some people criticized me at the time, although almost all of those splits have since been ‘officially’ adopted. This second time around, my recent research indicates that many more splits are overdue for Mexican birds, a conclusion aided by 25 years of continued fieldwork in Mexico and elsewhere in the Neotropics, combined with much easier access to sound recordings and myriad papers examining the genetics of this or that species, genus, or family.

Speaking of genetics, a recent paper examining the Mimidae found that Northern Mockingbird shares a common ancestor with Tropical Mockingbirds in Mexico, out on a different branch of the tree from Tropical Mockingbirds in Colombia—in technical terms, Tropical Mockingbird is paraphyletic (Lovette et al. 2012). Those authors recognized that this result was that of a gene tree, however, which is not necessarily the same as a species tree (don’t go there…) and so they didn’t recommend splitting or changing the status quo. In this case, however, morphology, voice, and biogeography all coincide with the molecular study, and treating Tropical Mockingbird as (at least) two species seems like a realistic course. While the name ‘Northern Tropical Mockingbird’ might appeal to some committees, I suggest the name Mayan Mockingbird for the northern taxon of ‘Tropical Mockingbird’ (Mimus [gilvus] gracilis), given that its range largely encompasses that of the Mayan civilization. The other populations of ‘Tropical Mockingbird’ remain an SEP, one worthy of study—by somebody else.

Photo 3. Typical Mayan Mockingbird—dark ‘mask’ is limited to lores, whitish eyebrow is narrow and diffuse, tail is extensively white below. Yucatan, Mexico, 1 December 2018. © Steve N. G. Howell.

This provisional mockingbird split, however, is of more than academic interest. The Mayan Mockingbird is native to northern Middle America and historically ranged south to Honduras (Howell & Webb 1995). As of around 1990, ‘Tropical Mockingbird’ was a vagrant to El Salvador, unknown from Nicaragua, and an occasional visitor to Costa Rica (these last presumed from escaped cage birds; Stiles & Skutch 1989). In southern Central America, Tropical Mockingbirds of the Colombian race tolimensis escaped or were released years ago in the Panama Canal Zone (Ridgely & Gwynne 1976). They seemed happy there for a while, but in recent years have spread north, into Costa Rica and perhaps beyond.

Photo 4. Less well-marked tolimensis Tropical Mockingbird than Photo 2, but dark mask still extends below and behind eye (creating ‘stern’ face relative to Mayan), and broad eyebrow relatively contrasting. Pichincha, Ecuador, 4 July 2016. © Steve N. G. Howell.

A quick look at the eBird map for 2018 shows Tropical Mockingbirds all over Costa Rica and El Salvador, as well as over much of western Nicaragua, with fewer in southeastern Nicaragua. For better or worse, the spread of mockingbirds in the region has coincided with the spread and increase of eBird users. Thus, maps every 5 years from 1900 through 1995 to 1900 through 2015, and a final map through 2018, show an increasing rate of eBird records from El Salvador to Costa Rica, with the biggest jumps since 2000—which could easily be interpreted as a sign of how the invasion has progressed. However, the pattern of steadily increasing records with a pulse since 2000 can also be seen in Mexico (in the known native range of Mayan Mockingbird), which can be attributed to more widespread use of eBird filling in the gaps, rather than a true range expansion. So, hard to say when and how quickly the real spread has occurred, at least on a quick perusal of the maps.

Maps from eBird of Tropical Mockingbird every 5 years from 1900 through 1995 to 1900 through 2015, and a final map through 2018.

Without citing a source, del Hoyo and Collar (2016; 598) state “population of tolimensis in Pacific lowlands of Central America (El Salvador south to Costa Rica) originates largely from escaped cagebirds imported from Colombia (emphasis mine).” (This odd wording implies that other tolimensis in this range might be natural vagrants, which seems unlikely.) Of interest, however, is that most Macaulay Library photos of ‘Tropical Mockingbirds’ from El Salvador look like Mayan Mockingbird, as do some from western Nicaragua, suggesting that species is also expanding its range. For better or worse, though, myriad photos of variable quality are no substitute for in-hand or specimen comparison, or for critical study in the field, and my online identification of most photos is provisional at best.

Photo 5. Even at longer range, Mayan Mockingbird shows fairly plain face, with dark ‘mask’ limited to lores. Quintana Roo, Mexico, 2 December 2007. © Steve N. G. Howell.

Neither of the new guides to the birds of Central America (Fagan & Komar 2016, Vallely & Dyer 2018) mention this issue, so what is going on in the middle of Central America? Have invading tolimensis Tropical Mockingbirds already met with their evolutionarily younger cousin the Mayan Mockingbird? Do they interbreed (readily or occasionally), compete and exclude one another, adapt to different niches and coexist, or what? It’s a potential natural laboratory just waiting for somebody to go and look, and listen.

Photo 6. Even at longer range, this tolimensis Tropical Mockingbird (with its extensive dark mask) looks distinctly different from Mayan Mockingbird. Pichincha, Ecuador, 2 February 2011. © Steve N. G. Howell.

How does one distinguish the two taxa? One way to distinguish them is by voice (which needs further study), but there are differences in face pattern and tail pattern (and perhaps other features—I’ve only just started to appreciate the issue; see Photos 1–6). Colombian has a more extensive dark mask (the dark ‘mask’ is restricted to the lores on Mayan, like Northern), giving it a ‘sterner’ face; and it averages less white in the tail (details of tail pattern variation require study). And if Mayan and the tolimensis Tropicals interbreed (perhaps they already have?), who knows what the hybrids may look (or sound) like?

An excellent way to appreciate the vocal differences is to go on line, where a good selection of sounds can be perused—a big thanks to the Macaulay Library at the Cornell Lab of Ornithology and to Xeno-Canto for making sound research so much easier! Basically, Mayan Mockingbirds sound like Northern Mockingbirds, including their song and their gruff chek! (often chek-chek!) and rasping sshhehh calls. The song of tolimensis ‘Tropical Mockingbird’ is distinctly richer and burrier, with a less hurried pace and less of the annoyingly tedious repetition of phrases that characterize most songs of Mayan (and Northern) Mockingbirds; calls of tolimensis sound rather similar to Mayan, but more study is needed on this.

In the meantime, Central American birders and field ornithologists should now be aware of this interesting situation and are in a position to monitor and study it.

References

Binford, L. C. 1989. A distributional survey of the birds of the Mexican state of Oaxaca. Ornithological Monographs no. 43.

del Hoyo, J., & N. G. Collar. 2016. Illustrated Checklist of the Birds of the World, vol. 2. Lynx Ediciones, Barcelona.

Fagan, J., & O. Komar. 2016. Peterson Field Guide to Birds of Northern Central America. Houghton Mifflin.

Howell, S. N. G., & S. Webb. 1995. A Guide to the Birds of Mexico and Northern Central America. Oxford Univ. Press.

Lovette, I. J., et al. 2012. Phylogenetic relationships of the mockingbirds and thrashers (Aves: Mimidae). Molecular Phylogenetics and Evolution 63:219-229.

Macaulay Library http://macaulaylibrary.org

Ridgely, R. S., & J. A. Gwynne, Jr. 1976. A Guide to the Birds of Panama. Princeton Univ. Press.

Stiles, F. G., & A. F. Skutch. 1989. A Guide to the Birds of Costa Rica. Cornell Univ. Press.

Vallely. A. C., & D. Dyer. 2018. Birds of Central America. Princeton Univ. Press.

Xento-Canto https://www.xeno-canto.org

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