Proposal (51) to South American Classification Committee
Split Trogon mesurus from T. melanurus
Effect on South American CL: This proposal would elevate a taxon to species rank that we currently treat, by implication, as a subspecies on our baseline list.
Background: For most of their recent history (e.g., Peters 1945, Meyer de Schauensee, Sibley & Monroe 1990, Collar 1996), the taxon mesurus has been treated as a subspecies of Trogon melanurus (Black-tailed Trogon). They are allopatric, highly disjunct taxa: mesurus is endemic to W Ecuador and NW Peru; the nearest populations of melanurus are in Amazonia or lowland N Colombia.
Mesurus is evidently "identical" to nominate melanurus except its iris color is pale, not dark (see plate in Collar (1996) Ridgely & Greenfield (2001). Interestingly, it shares this pale iris with parapatric T. comptus, to which it is also very similar in plumage -- comptus differs in having a red eyering, greener upperparts, and variable white chest-band.
New information: Ridgely & Greenfield (2001) considered mesurus a separate species from melanurus based largely on voice, but also plumage, iris color, and disjunct distribution. They described mesurus song as a "slow, short series of "cow" notes that often starts softly and builds in strength, e.g., "cuh-cuh-cuh-cuh-cow-cow-ców-ców-ców-ców.”
In contrast, Ridgely & Greenfield (2001) described Amazonian melanurus as: "an often long-continued series of up to 20-30 resonant "cow" notes that starts softly, e.g., "cuh-cuh-cuh-cuh-cow-cow-ców-ców-ców-ców." Thus, songs seem to differ primarily in length. Hilty (2003) described melanurus song from Venezuela as: "a long ser. of rather low-pitched, inflected whistles, waaoo, wahoo, ... (or cuuh, cuuh ...), delivered much more slowly than songs of most trogons." I don't know how to interpret the differences in these two descriptions in terms of real vs. artifact of descriptions.
I cannot find other published descriptions of mesurus.
I wondered whether mesurus may have been allocated to the wrong species, given its parapatry with T. comptus and T. massena, both clearly part of this group of trogons, and its sharing of iris color with comptus. However, the descriptions of their songs in Ridgely & Greenfield (2001) differ more from that of mesurus than mesurus does from geographically more distant melanurus (with all appropriate caveats on interpretation of qualitative descriptions).
Analysis: From the comparative standpoint, I think Bob has a good case on treating mesurus as a species. In coloration, it differs as much from melanurus as does T. comptus or T. massena. In song, it seems closer to melanurus, but whether that indicates sister status, retention of a song type that is basal in the group, or convergence cannot be answered without a phylogeny for the group. I suspect there is a fair chance, given the disjunct distribution with melanurus and the suspicious similarity and parapatric distribution with T. comptus, that mesurus and melanurus may not be sisters. For those reasons, I would favor treating mesurus as a species-level taxon. Another point is that massena and melanurus could be considered more similar in coloration than either is to mesurus, yet they overlap in E Panama and are clearly two species. However, is all of that sufficient evidence for a change from status quo, especially since the explicit rationale isn't really well explicated in Ridgely & Greenfield?
Recommendation: Although I personally think that there is sufficient published evidence for treating mesurus as a species, I very reluctantly vote "NO" on this proposal, only because the evidence isn't clearly presented in a coherent way in print that would allow us to cite it to defend our decision. All that is needed, in my opinion, is a very short publication that presents a few sonograms, briefly describes the coloration pattern, and makes the case that the "burden-of-proof" should be on those who want to treat mesurus as a subspecies of melanurus.
COLLAR, N. 2001. Family Trogonidae (trogons). Pp. 280-479 in "Handbook of the Birds of the World, Vol. 6. Mousebirds to hornbills." (J. del Hoyo et al., eds.). Lynx Edicions, Barcelona.
HILTY, S. L. 2003. Birds of Venezuela. Princeton University Press, Princeton, New Jersey.
RIDGELY , R. S., AND P. J. GREENFIELD. 2001. The birds of Ecuador. Vol. II. Field guide. Cornell University Press, Ithaca, New York.
Van Remsen, August 2003
P.S.: If the proposal does pass, then I'll work on another one on the English names of these two.
Comments from Zimmer: "I vote "yes" on splitting mesurus from T. melanurus. Although the published rationale for the split is a bit thin, I think that the combination of different eye color (a likely isolating mechanism) and real differences in voices, along with distributions that fit an established biogeographic pattern, all argue for a split. T. melanurus, in my experience, bucks the tendency (noted in my comments on Proposal #49) of South American trogons to exhibit greater vocal variation. To my ears, melanurus almost always has a measured "WOWP, WOWP, WOWP..." series of notes, and the only confusion comes from variation in pitch and pace of syntopic populations of T. viridis, some of which approach the sound of melanurus. Songs of mesurus differ in pace and pitch -- I'm guessing that sonograms would show noticeable differences in note shape."
Comments from Schulenberg: "My vote: No. It would not surprise me if this proposal were valid, but I'd prefer to wait until there is more of an analysis (even a simple minded one) for us to go on."
Comments from Stiles: "Trogon splits. NO to all, until all the evidence is in and published. Two or three will probably prove correct, but at this time we don't have enough solid evidence to accept them."
Comments from Jaramillo: "NO -- more reluctantly than for proposals 49 and 50. I think that this one is more convincing for a split with current data that is available. Nevertheless I agree that even a simple note with a couple of sonograms would be important to have in the literature for acceptance of this split."
Comments from Silva: "No. I agree that a detailed study including all populations of this taxon is needed before to propose any taxonomic change."
Comments from Nores: "NO. Idem a 49."