Proposal (1086) to South American Classification Committee

Modify species limits in Ramphotrigon megacephalum

Background: The current SACC footnote below sums up the slim history, and this proposal is to treat Ramphotrigon megacephalum as two species as proposed by Lavinia et al. (2020).

142c. Ridgely & Tudor (1994) noted differences in vocalizations between nominate megacephalum from the Atlantic forest and populations from western Amazonia. Lavinia et al. (2019, 2020) found evidence that suggests that Ramphotrigon megacephalum consists of at least two species. SACC proposal badly needed.

Ridgely & Tudor stated: “In Peru and Ecuador this [call] is an even, fairly fast ‘whoo-whoo’, whereas in se. Brazil it is a somewhat burrier (especially first note) ‘wheeu … whoo’, typically with a long interval, often several seconds or more, between notes.”

I cannot find any previous explicit suggestion that more than one species is involved in this polytypic species. The most recent classifications (e.g., Dickinson and Christidis 2014) recognize four subspecies:

R. m. pectorale Zimmer and Phelps, 1947 (north of the Amazon from se. Colombia, e. Ecuador, and ne. Peru to sw. Venezuela and nw. Brazil)

R. m. venezuelense Phelps and Gilliard, 1941 (north of the Amazon from se. Venezuela and central Brazil east through the Guianas)

R. m. bolivianum Zimmer, 1939 (mostly south of the Amazon in Peru and Bolivia east to the R. Madeira in sw. Brazil)

R. m. megacephalum (se. Brazil, ne. Argentina, extreme e. Paraguay)

The four subspecies differ primarily in differences in color intensity on back, breast, and belly (HBW). Tidbit: when Zimmer described bolivianum in 1939, he had only three specimens, from one locality in Bolivia; evidently, no specimens were known (or recognized?) from Peru, Ecuador, Colombia, or Venezuela at that time.

R. m. pectorale from Huila by Hernán Arias:

R. m. bolivianum from Pando by Rini and Jacob Wijpkema

R. m. megacephalum from São Paulo by Tomás Grim:

New information:

VOCALIZATIONS

Lavinia et al. (2019) quantified 12 parameters from the primary vocalizations using recordings of 22 bolivianum and 17 nominate subspecies (SI Table s6), and found statistically significant differences in all 12 parameters, but more importantly, little or no overlap in several parameters, e.g. bandwidth of the song (the “burriness” referred to by Ridgely & Tudor above), and minimum and maximum frequencies of the first note. The stylized spectrogram they presented (see below) shows some of these differences as well as the longer inter-note interval mentioned by Ridgely & Tudor for Atlantic forest birds (although not the same degree of note separation). Note the frequency difference between the two is not particularly evident until you compare axes:

Sample recordings:

• R. m. bolivianum:

https://xeno-canto.org/37962 (from Mato Grosso by Andrew Spencer)

https://xeno-canto.org/1068542 (from Madre de Dios by Frank Lambert)

and a nice video with song by Tomaz Melo from Acre:

https://media.ebird.org/catalog?birdOnly=true&taxonCode=lahfla2&sort=rating_rank_desc

• R. m. megacephalum:

https://xeno-canto.org/415583 (from Paraná by Jayrson Araujo de Oliveira)

https://xeno-canto.org/334798 (from São Paulo by Jerome Fischer)

No playback trials were performed.

I also did a little digging on my own based on the mention that nominate megacephalum might not have a longer dawn song, as was known for birds from western Amazonia and found that it has what I am interpreting as a dawn song [see correction from Andrew Spencer in Comments]; more importantly, the presumed dawn songs [see correction from Andrew Spencer in Comments] of the two also differ substantially:

• R. m. bolivianum:

https://xeno-canto.org/47935 (from Mato Grosso by Andrew Spencer)

• R. m. megacephalum:

https://xeno-canto.org/82383 (from São Paulo by Jeremy Minns)

GENETICS

Lavinia et al. (2019) sampled 35 specimens, all of nominate megacephalum or bolivianum, to generate sequence data for three mitochondrial genes (cyt-b, COI, ND2) and one nuclear intron (FIB5); the results of the latter are shown below.

Fig. 2 from Lavinia et al. 2019. Bayesian majority rule consensus mt DNA gene trees of Ramphotrigon megacephalum

In their study of the relationships among Ramphotrigon and Deltarhynchus, Lavinia et al. (2020) sampled some of the same individuals of the two subspecies of R. megacephalum and unsurprisingly found strong divergence between the two populations and estimated the divergence time at about 3 MYA.

Harvey et al. (2020) included single individuals from western Amazonian (Ucayali) and the Atlantic Forest (Santa Catarina). Just eyeballing the tree, the depth of the node that joins them is comparable to species-level node depths for several related tyrannids in that clade.

TAXONOMY

Although Lavinia et al. (2019, 2020) treated bolivianum as a subspecies, they recommended it be treated a species.

Discussion and recommendation:

This one seems to me to be a straightforward split even if based only on voice, and so I recommend a YES to a two-way species split.

The genetics are consistent with a two-species treatment, and the depth of the node that joins nominate and bolivianum is reasonably similar to that associated with species differences, although I would not use that as the sole basis for species rank.

The main question would be what to do with genetically unsampled pectorale and venezuelense.

Based on checking a few recordings (with all appropriate caveats), I think it is safe to include venezuelense as a subspecies of bolivianum based on primary call/song, which to my ear sound “identical). Here are some samples:

• venezuelense

https://xeno-canto.org/272184 (from Suriname by Peter Boesman)

https://xeno-canto.org/209926 (from Suriname by Peter Boesman)

I’m not so sure about the intervening subspecies pectorale from eastern Ecuador and Colombia). Here are some examples, which differ to my ear in the second note being the same of higher pitched than the first, i.e. opposite of bolivianum and venezuelense:

• pectorale:

https://xeno-canto.org/1073681 (from Napo by Juan Freile)

https://xeno-canto.org/688357 (from Caquetá by Mauricio Cuellar Ramirez)

https://xeno-canto.org/258812 (from Napo by John V. Moore)

Suspicious! Perhaps just a quirk of small N or something real? Ridgely & Greenfield Birds of Ecuador described the voice as the having the second note lower in pitch, i.e., just like bolivianum, so something is weird. Something to be investigated, at least. Regardless, I think the only option is to include pectorale in bolivianum based on the general tone of the vocalization.

English names:

I suggest we defer this to a separate proposal once we see how the taxonomic proposal fares, but please be mulling over the possibilities. Amazon Large-headed and Atlantic Large-headed are two obvious choices. Western and Eastern could also be used. Neither pair is inspired, but I don’t see a lot of colorful possibilities out there for two species that do not differ much in plumage, and whose vocal differences are not easily translated to simple English words. I think “High Large-headed Flatbill” might make a Jimmy Fallon monologue, however.

References:

HARVEY, M. G., G. A. BRAVO, S. CLARAMUNT, A. M CUERVO, G. E. DERRYBERRY, J. BATTILANA, G. F. SEEHOLZER, J. S. MCKAY, B. C. O’MEARA, B. G. FAIRCLOTH, S. V. EDWARDS, J. PÉREZ-EMÁN, R. G. MOYLE, F. H. SHEDLON, A. ALEIXO, B. T. SMITH, R. T. CHESSER, L. F. SILVEIRA, J. CRACRAFT, R. T. BRUMFIELD, AND E. P. DERRYBERRY. 2020. The evolution of a tropical biodiversity hotspot. Science 370: 1343-1348.

LAVINIA, P. D., A. S. BARREIRA, L. CAMPAGNA, P. L. TUBARO, AND D. A. LIJTMAER. 2019. Contrasting evolutionary histories in Neotropical birds: divergence across an environmental barrier in South America. Molecular Ecology 28: 1730–1747.

LAVINIA, P. D., P. ESCALANTE, P. L. TUBARO, , AND D. J. LIJTMAER. 2020. Molecular phylogenetics and phenotypic reassessment of the Ramphotrigon flycatchers: deep paraphyly in the context of an intriguing biogeographic scenario. Journal of Avian Biology 51(4).

Van Remsen, July 2026

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Vote tracking chart:

https://www.museum.lsu.edu/~Remsen/SACCPropChart1044+.htm

Comments from Andrew Spencer: “I was very happy to see this proposal come up, since it reminded me of vocal variation that I had noticed in the past, but am embarrassed to admit I had completely forgotten about. I'm much more familiar with the voices of the Amazonian taxa than I am with the Atlantic one, but the differences between them do seem consistent and (in my opinion), species-level. After reading this proposal, I went into a bit of a Ramphotrigon rabbit hole, and believe the vocalizations can be divided into a few different categories. 

The most important of these is what I think of as the "day song", the common, two note phrase that I first learned as "bam-boo", and is forever cemented in my mind for the species. Both Amazonian and Atlantic taxa have a two note day song, but it's when you dig further that the interesting differences come to light. The most striking of these is what Van summarized above, the pitch and spacing differences between the first and second note. 

“But as I looked through a number of recordings on the Macaulay Library, I realized it's not quite that simple. It appears that day songs are made up of particular notes, for which I'll borrow terminology mostly used when discussing Empidonax vocalizations: an "A" phrase, and a "B" phrase. By far the most commonly delivered variation is these two notes given together, in an AB...AB...AB... etc pattern, hence the "bam-boo...bam-boo..." etc song. However, it is also not uncommon for just the A phrase to be given by both forms (e.g. in https://macaulaylibrary.org/asset/623416870 for Amazonian, and https://macaulaylibrary.org/asset/657136419 for Atlantic), or for a mixed delivery (e.g. in https://macaulaylibrary.org/asset/147945 for Amazonian, and https://macaulaylibrary.org/asset/658454475 for Atlantic).

“Where this becomes interesting is that it appears that Atlantic birds, and so far as I can tell, only \ Atlantic birds, also have a "C" phrase. This is also regularly given in isolation (e.g. https://macaulaylibrary.org/asset/656864316), and in series of just A and C phrases (e.g. https://macaulaylibrary.org/asset/609169290), and in bouts where it or the A phrase are combined with the B phrase, with delivery something like CB...CB...CB...AB...CB...A...CB...AB...etc (see https://macaulaylibrary.org/asset/22176 for that specific example).

“It does appear that recordings with all three phrases like this are a distinct minority on ML, but they are common enough, and geographically widespread enough in the broader range of nominate megacephalum, that I am comfortable saying it's a feature of the subspecies. For the Amazonian taxa, I did notice there is quite a bit of minor variation in the A phrase, and also in the spacing, but that this variation seems to be on a very local level and possibly just individual variation. From Mato Grosso for example, there are some songs with noticeably kinked A phrases (e.g. https://macaulaylibrary.org/asset/110006), but also those with what I consider more normal A phrases (e.g. https://macaulaylibrary.org/asset/309963). I think this type of variation accounts for what Van was seeing in recordings from Ecuador when compared to those from elsewhere in the Amazon.

“I've made a quick and dirty graphic of day songs from both Amazonian and Atlantic birds, roughly ordered from north to south, shown below. Amazonian is on the left, Atlantic on the right; for Atlantic recordings where there were both AB and CB phrases in the same recording, I included both on the same line, divided by a blue bar. For the Amazonian taxa, the locations are: northern Venezuela, Meta (Colombia), Ecuador, Madre de Dios (Peru, 2x), Pando (Bolivia), Mato Grosso (BR), Para (BR), Guyana, and Suriname. For the Atlantic side, the ordering is Espiritu Santo, Rio de Janeiro, Sao Paolo (x2), Santa Catarina (x2, AB from one individual, CB from another), Parana, and Misiones (Argentina). The examples, especially from the Amazon, were selected to highlight some of the individual variation in A phrases.

“Finally, my impression as I was going through recordings was that the Atlantic forms give series of just A or C phrases more commonly than Amazonian ones given those of just A phrases, but I didn't keep count so that may not be accurate.

“The other main sound heard from these birds, at least Amazonian ones, is dawn song. This has long been one of my favorite vocalizations not because it's spectacular, but because on some of my earliest outings in the Amazon I would hear an unknown song from bamboo patches while it was still almost pitch black. At first I thought it was some weird nightjar that I could never match to recordings of any expected species, and could never find no matter how hard I tried with spotlights or playback. And I can still remember well the eureka moment when I figured out what was making that sound. In the Amazonian forms there are a number of recordings available (https://macaulaylibrary.org/asset/135174 or https://macaulaylibrary.org/asset/35141 for example), and the vocalization appears relatively stereotyped in every recording I can find.

“For the Atlantic forms there is far less to go on; in ML the only one tagged as dawn song is this one by Ted Parker (https://macaulaylibrary.org/asset/30144). It's much simpler than recordings from the Amazon, but does have unique notes and phrasing as compared to both day songs and the dawn song of the Amazonian birds. I haven't gone through every single recording in ML looking for another example, but I have at least glanced over most of them and have not seen another cut of it. It is either much more rarely given than the dawn song in the Amazon, or (perhaps more likely?), recordists just haven't been in place during the brief window the song is delivered.

“I've made another comparison illustration of dawn songs, shown below, with bolivianum on left, nominate megacephalum on right..

“Finally, I can't resist a chance to get a quick word on common names in. I've always disliked the name "Large-headed Flatbill" - the head really isn't that large when compared to other Ramphotrigon. I'd advocate for using Ridgely's "Bamboo Flatbill", maybe with "Amazonian" and "Atlantic" as modifiers. I know that R. fuscicauda is also a bamboo specialist, although maybe not so much as megacephalum. And I know that megacephalum is not strictly a bamboo species absolutely everywhere it occurs. But as has been said ad nauseum, common names don't need to be perfect. They can just be better than "Large-headed"!