Proposal (440) to South American Classification Committee
Split Ramphastos swainsonii from R. ambiguus
Effect of Proposal: If it passes, this proposal would result in recognition of Ramphastos swainsonii (subspecies swainsonii and abbreviatus) (Chestnut-mandibled Toucan) as a species separate from Ramphastos ambiguus (Black-mandibled Toucan). This treatment is reflected in most modern literature. We recently reviewed the basis for split and lumped treatments in Donegan et al. (2010). By way of background, we moved the Colombian checklist to generally follow SACC treatments as from 2007, but did not accept this SACC lump as one of a handful of exceptions.
Discussion: In Donegan et al. (2010), we reviewed the literature on this species and studied a good sample of sound recordings from across the range of the greater ambiguus group. We stated as follows:
“Many authors, particularly in the field guide literature, treat Chestnut-mandibled Toucan Ramphastos swainsonii as separate from Black-mandibled Toucan R. ambiguus, (e.g. Meyer de Schauensee 1964, 1966, Hilty & Brown 1986, Fjeldså & Krabbe 1990, Ridgely & Gwynne 1989, Dunning 1987, Howell & Webb 1995, Ridgely & Greenfield 2001, Stotz et al. 1996, Dickinson 2003, Krabbe & Nilsson 2003, Hilty 2003, Restall et al. 2006, AOU 1998, 2019, Salaman et al.. 2000, 2007, 2008a, 2009; and Gill & Donkser 2010, the latter considering the split to be “accepted by all except SACC”). Despite the latter assertion, some other authorities treat them as lumped (e.g. Short & Horne 2001, 2002; Erize et al. 2006; Remsen et al. 2010; and post-2006 journal papers that require Remsen et al. 2010 to be followed, e.g. Donegan et al. 2007, Patané et al. 2010).
“R. swainsonii (subspecies: swainsonii and abbreviatus) occurs west of the Andes into the Magdalena valley, whereas R. ambiguus occurs on the east slope of the Andes. All three taxa were lumped by Haffer (1974) who noted overlaps in biometrics and plumage, based largely on studies of specimens. An inspection of specimens at BMNH gives some insights to this treatment. Once bare skin and bill coloration are lost, as occurs on specimens of a certain age, individuals are difficult to assign to one or other subspecies (except by collecting locality), because biometrics (bill, tail and wing length) overlap and plumage is essentially identical. Despite this, based on the literature review above, it is evident that Haffer (1974)’s lump has not been widely followed.
“The English names also correctly reflect the differences in bill coloration, with ambiguus being black-billed and abbreviatus/swainsonii being dark chestnut brown. As pointed out by Stiles et al. (1999), abbreviatus is a valid taxon and is closer to swainsonii in its morphology, habitat requirements, and range. The morphological differences between the swainsonii and ambiguus groups are rather striking when individuals are observed in the field.
“A recent molecular study showed R. ambiguus to be a monophyletic group based on the individuals sampled. There was 1.35% mtDNA variation between swainsonii and ambiguus, suggesting a Pleistocene divergence (Patané et al. 2010). Whilst this was a relatively high value for intraspecific mtDNA variation for a toucan, it amounts to only modest variation and was based on limited sampling (not including abbreviatus). On its own, this data forms no basis for either lumping or splitting a species and the authors suggested no such action.
“Turning to voice, Stiles et al. (1999) elucidated small differences between recordings from Colombia and a single recording then available of ambiguus from Peru. Recordings available today include good numbers and broad geographical sampling of all populations. An inspection of sonograms of available recordings was carried out (recordings inspected: all those in Krabbe & Nilsson (2003, Ecuador: 5 swainsonii, 2 ambiguus), Alvarez et al. 2007 (Colombia: 2 ambiguus), Jahn et al. 2001 (Ecuador: 1 swainsonii), Boesman 1999 (Venezuela: 2 x ambiguus) and www.xeno-canto.org as of 16 April 2010 (various countries: 13 swainsonii, 3 abbreviatus, 5 ambiguus); totals 19 swainsonii, 11 ambiguus, 3 abbreviatus. Both groups give two phrases in their songs, commonly transcribed as: “Dios te de” and “Dios te de te de”. Several recordings of ambiguus sound more hurried (hence, shorter) and higher than some recordings of the swainsonii group (as noted by Stiles et al. 1999). However, there is overlap, with slower ambiguus falling in the range of faster swainsonii (including if only “Dios te de” recordings are compared). It is possible that there are average differences in some acoustic variables (this was not tested statistically), but there would not appear to be diagnostic differences in note shape, song length, or acoustic frequency for any particular population. Ridgely & Greenfield (2001) previously noted that populations do not appear fully diagnosable on the basis of voice, a conclusion consistent with ours.
“A rationale for treating swainsonii as separate from ambiguus would be based on allopatric distributions, diagnostic differences in bare skin and bill coloration, moderate mtDNA variation, and average differences in biometrics and possibly song speed. R. swainsonii is clearly a phylogenetic species but is a questionable biological species. Despite these only moderate differences, votes on the Colombian checklist forum were strongly in favour of maintaining species rank for swainsonii, with 12-1 votes in favour, perhaps reflecting the status quo of treatments in the vast majority of leading texts.
“We see no strong reasons either to split or lump these taxa based on available studies and materials. Although the split is not strongly supported, the proposition that Remsen et al. (2010)’s lump treatment represents the “status quo” is also weakly supported. The SACC list is supposedly based on Meyer de Schauensee (1970) and Dickinson (2003), who both split this group, as does the AOU’s North American checklist committee (AOU 2010). For whatever reason, Haffer (1974)’s lump has not been widely followed. In accordance with the prevailing treatment in leading texts and the votes received on the checklist forum, we therefore tentatively maintain our current treatment but with little enthusiasm.”
Recommendation: Complete ambivalence. This is an old chestnut of allopatric populations that are easily diagnosable based on few characters. Different authorities have taken different views over the years on this point, although the split treatment prevails. SACC has had this issue down as requiring a proposal for many years. The lack of diagnosable vocal differences and moderate mtDNA variation do not mandate any split. On the other hand, the differences in bare skin and bill coloration between these populations are striking and might influence mate selection if populations were ever to occur together (which seems unlikely). For whatever reason, Haffer (1974)’s treatment has not been widely followed, and SACC’s current (lumped) position for these birds is contra most of the field guide literature and the North American AOU. With little enthusiasm for either treatment, we decided to maintain this split on the somewhat flimsy basis that it better reflects the status quo for Neotropical ornithology. This may be a rare instance where “YES” means “NO” and “NO” means “YES” (i.e. a “YES” vote would maintain a status quo treatment notwithstanding the SACC baseline). “YES” is to split; “NO” is to lump.
Donegan, T., Salaman, P., Caro, D. & McMullan, M. 2010. Revision of the status of bird species occurring in Colombia 2010. Conservación Colombiana 13: 25-54.
Other references are cited in this paper.
Thomas Donegan, May 2010
Comments from Robbins: “NO. I’m on the fence on this one. Given that there isn’t a strong argument to overturn our current treatment, I’ll vote “no” for now.”
Comments from Stotz: “YES. I have to admit that until this proposal I could not have told you that SACC lumped ambiguus and swainsoni. This isn’t quite a novel treatment, but it has only been followed by a few independent sources. It is not the treatment followed by Dickinson, which was largely the original base list for SACC. Unfortunately, the evidence does not strongly point to either treatment as the appropriate one. Vocal differences are clearly weak. The only significant differences are in soft part colors. There are conflicting treatments in the toucans regarding similar cases with weak to no vocal distinctions, disjunct distributions and soft part differences. Given that, in my view, this comes down to whether the committee feels strongly enough about the current treatment to remain at odds with most other treatments, in particular the North American committee. Based on this, recognizing a weak argument for any treatment, I favor splitting ambiguus and ambiguous to be consistent with most other literature (except for HBW by Short and Horne).
“If we don’t split swainsoni from ambiguus, I think we will need to change the common name of the broad ambiguus. Yellow-throated Toucan has been suggested. Doesn’t dazzle me, but I don’t have a better option, and it has been in the literature for a while.”