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.
Literature Cited:
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."