Proposal (#362) to South American Classification Committee

Split Zimmerius gracilipes into two species

Effect on South American checklist: This would split Zimmerius acer from Z. gracilipes.

Background: The genus Zimmerius was erected by Traylor (1977) for a set of small tyrannulets placed in the genus Tyranniscus, which lacked true wingbars among other characters. There have been a number of taxonomic issues at the species-level within the genus. In this case, the Guianan taxon Zimmerius acer has essentially always been treated as one of three subspecies, within Zimmerius gracilipes, along with the nominate subspecies of western Amazonia and gilvus of southern Amazonia.

Ridgely and Tudor (1994), while not questioning the taxonomy of Zimmerius gracilipes, noted significant vocal variation in the species. Hilty (2003), citing Mark Robbins, indicated that birds in the highlands of Guyana are different from birds in the lowlands.
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Analysis and new information: Rheindt et al. (2008) examined pieces of mitochondrial (NADH) and nuclear DNA (Fibrinogen intron 5) for the genus Zimmerius. They found that Zimmerius gracilipes was polyphyletic. Specifically they found that the Guianan subspecies, acer, was basal to the rest of Zimmerius (a not uncommon pattern in lowland forest bird groups), whereas the rest of gracilipes was sister to Z. bolivianus, with the rest of Zimmerius interposed between acer and gracilipes on the tree. Based on this, plus evidence of a vocal difference between Guianan birds and the rest of gracilipes, they recommended treating acer as a distinct species.

Recommendation:
I recommend a YES vote to separate Zimmerius acer from Zimmerius gracilipes. The genetic work is clear. These are not closely related within Zimmerius. Further, as noted by Mark Robbins in the discussion around Proposal 173, the voice of Guyanan birds (acer) is distinctly different from Bolivian birds (gilvus). This is not a completely clear situation, however, as there is some question whether the birds that are currently called acer (ranging over all of northeastern Amazonia) all belong to the taxon with the "different" voice and distinct molecules. Rheindt et al (2008) suggested that acer may be a Tepui species, and that NE lowland Amazonian birds may fit in with true gracilipes The type of acer comes from Bartica Grove, which is in the Guyanan lowlands along the Essequibo River, but the acer tissues samples used by Rheindt et al come from "Guyana" and Iwokrama Reserve (in the lowlands along the Essequibo River). It appears that the molecules at least do refer to acer proper. Whether these lowland Guyanan bird have the same voice is the highland birds is uncertain. I personally don't think this admittedly major uncertainty interferes with us recognizing acer, but it is clear that further work needs to be done.

References:

HILTY, S. L. 2003. Birds of Venezuela, 2nd ed. Princeton University Press, Princeton, NJ.
RHEINDT, F. E., J. A. NORMAN, AND L. CHRISTIDIS. 2008. DNA evidence shows vocalizations to be better indicator of taxonomic limits than plumage patterns in Zimmerius tyrant-flycatchers. Molecular Evolution and Phylogenetics 48:150-156.

RIDGELY, R. S., AND G. TUDOR. 1994. The birds of South America, vol. 2. Univ. Texas Press, Austin.

TRAYLOR, M. A., JR. 1977. A classification of the tyrant-flycatchers (Tyrannidae). Bulletin, Museum of Comparative Zoology 148:128-184.

Doug Stotz, July 2008

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Comments from Stiles: "YES. Clearly two species-level taxa are involved, whatever the exact geographic limits turn out to be."

Comments solicited from Mario Cohn-Haft: "It's been pretty clear to me ever since i crossed the rio negro for the first time about 20 years ago, that acer and gracilipes must be good species. since I’ve never been to the type locality of either, i never ventured to make the point in print. however, what i can say is that there is an abrupt vocal shift on opposite banks of the lower rio negro (below the mouth of the Branco). interestingly, the songs on opposite banks sound very similar to me, if not identical. the calls, though are strikingly different. (this is a fairly frequent phenomenon, as mentioned i believe in the recent Hypocnemis cantator revision.) eastern birds (presumably acer) give a dry "chip-CHUP" call (rather frequently throughout the day). on the west bank of the lower negro and both sides of upper negro (and throughout western Amazonian brazil, wherever I’ve been--presumably gracilipes), the apparently analogous call is a liquid "uip". the exact location of the division between these vocal types above the lower rio negro is probably the rio Branco, although i can't now remember offhand how close to the west bank of the Branco I’ve heard the "gracilipes" vocal type, it wouldn't be hard to recover from my tapes; anyway, this is one of luciano's (Naka) focal groups, and i'm sure he's now got a good handle on where they split between the upper negro and Branco.

"The split in southern Amazonia is probably equally easy to trace, i just can't remember with certainty off the top of my head whether it's the Madeira or Tapajós or one of the tribs in between. i'm pretty sure the east bank of the Tapajós has birds with calls more like "acer" than like "gracilipes", and am quite sure that west of the Madeira the calls are "gracilipes" types. the issue of type localities has to be taken seriously, of course, and checked out carefully. however, it's no great leap of faith to treat the Manaus vocal type as being typical of a form that (according to traylor's [in peters] description of the distributional limits of the acer morphotype) is found throughout the Guianas and northeastern Amazonian brazil and further east. again, this is a common pattern, so without more detailed data it seems perfectly reasonable to assume that the vocal difference I’ve just described refers indeed to the taxa acer and gracilipes, respectively. likewise, use of the name gracilipes for the west Brazilian Amazon vocal type represents a conservative interpretation of what appears to be a common pattern, given no evidence (to my knowledge) of any vocal variation anywhere within the area west of the negro and (at least) Madeira rivers in brazil.

"So the short answer is, i'm very much in favor of recognizing gracilipes and acer as species-level taxa. but, to continue the discussion of which one is where, i repeat that it's important to have vocal and genetic samples from (at least near) the type localities and to publish samples from throughout the ranges of both.

"This leads to an interesting further issue. assuming (reasonably i think) that acer is the name for all birds from the "Guianan area of endemism" (the entire region east of the rio Branco and lower negro and north of the Amazon and in extreme eastern Venezuela), the question is: what's in the rest of acer's presumed range in southeastern Amazonian brazil and northeast non-Amazonian brazil? these birds are presumably similar enough morphologically to be treated as acer by traylor. vocally, in my experience, they have a call more like that of Guianan area birds than of western gracilipes. but it's not at all identical. I’ve never heard the 2-syllable "chip-CHUP" in southeastern Amazonia or in northeast brazil. what i hear in those places is a single chip, like one note only of the usual "acer" call. area-area "acer" will also sometimes give 1-syllable calls, so it's not a diagnostic difference. rather, i believe it's a repertoire difference: i don't think the southern and eastern "acer" ever give the 2-syll call that northern (true) acer gives all the time. this to me suggests a real, but more recent differentiation within what we're currently calling acer. i'm willing to bet there will be clear genetic evidence to back this up, but nevertheless that northern and southeastern "acer" will be sister taxa.

"In other words, i believe it's safe and correct to split acer and gracilipes, but that it's very likely that acer itself contain more than one diagnosable taxon."

Comments from Zimmer: "YES. Mario Cohn-Haft's comments regarding vocal differences between populations match precisely with my observations. As he suggests, the songs of all populations sound pretty much the same, and the differences are primarily in the calls. North bank acer from east of the lower Rio Negro routinely give a two-noted "whit-SUCK" call, whereas acer from the south bank east of the Madeira (including the populations from northeastern Brazil) give a single note call that resembles the one note of the north bank population. I'm not sure that I've ever heard the south bank birds giving the 2-note call, although I have heard north bank birds occasionally do 1-note calls. So, as Mario suggests, it may be less of a diagnosable difference between north and south bank acer, and more of a case of repertoire frequency differences [In a seemingly analogous situation, Lophotriccus galeatus populations from the Guianan region sit around all day giving metallic "pik" calls, and only rarely seem to give the song, which is an angry sounding trill. Conversely, south bank populations of galeatus from east of the Madeira routinely sing the trill, while only sporadically giving the "pik" notes.]. So, there may be species-level differences within acer, and gilvus too may prove to be different. But that can all be worked out in the future. The pertinent point is that acer and gracilipes are distinctly different in voice, and, it turns out, are not that closely related to one another. So, in spite of the fact that the situation is more complex than a simple two-way split, I would say splitting acer from gracilipes is an excellent first step."

Comments from Robbins: "YES. I fully support this proposal as there is no question that the acer (as Doug pointed out the holotype is from coastal Guyana) is a separate species from gracilipes. As Doug and Mario point out, birds from eastern Amazonia may involve further unrecognized species.

"To clarify what was stated in Hilty (2003) about birds from the Tepui highlands of Guyana being different from lowland birds, birds throughout the lowlands in Guyana are referable to acer, whereas the birds on the slopes of Mt. Roraima (probably other Tepuis as well) are gracilipes. In fact, the call notes and song (to my ear the song is reminiscent of some Pachyramphus-like calls) from Mt. Roraima are very similar to birds as far away as Bolivia. The dramatic differences in voice between acer and gracilipes can be heard by consulting either the Macaulay Library of Natural Sounds or Xeno-canto Americas on-line references. For example, you can compare a couple of my digitized Guyana recordings of acer with those of gracilipes from a number of localities. Note that not all recordings listed on the Macaulay site have been digitized, so those cannot yet be consulted on-line. Both of these on-line resources are invaluable for making vocal comparisons and we should be using these to illustrate vocal references in our proposals."

Comments from Nores: "YES. El análisis molecular (si está bien hecho) muestra claramente que son dos especies diferentes. Además, está lo de las vocalizaciones marcado por Robbins y Cohn-Haft."

Comments from Schulenberg: "YES. I think it's pretty clear that the acer sampled by Rheindt et al. is very different from gracilipes. It also sounds as if there is work yet to be within acer, but getting to the bottom of that will have to wait."

Comments from Jaramillo: "YES - We have clear morphological, vocal and genetic data to make this change. There may be refinements in the future, but the available data is there to divide acer from gracilipes right now. The question I have is what the English names are going to be?"