Proposal (537) to South American Classification Committee


Elevate Pauxi unicornis koepckeae to species status



Effect of Proposal: If it passes, this proposal would result in treatment of Pauxi unicornis koepckeae as a separate species from P. u. unicornis (Horned Curassow).


Background:  Current classifications typically treat Pauxi unicornis as consisting of two subspecies, with disjunct ranges: nominate unicornis in the Andean foothills of Bolivia, and koepckeae, known only from the isolated Sira range in Peru, ~1000 km north (and ~60 km east from the nearest Andean slopes over 1000 m, Terborgh & Weske 1971).  The two subspecies are differ in morphology (Weske & Terborgh 1971, Gastañaga et al. 2011), vocalizations, and behavior (Gastañaga et al. 2011).


New information:  Gastañaga et al. (2011) studied vocal, behavioral, ecological, and morphological differences between the two populations of P. unicornis, as follows:


Vocal differences – koepckeae has a low booming vocalization that lacks the final emphatic far carrying note that is the most distinctive feature of the song of unicornis.  The booming sequence of koepckeae is a series of up to four notes followed by a pause of approximately 2 sec, repeated every 4-5 sec.  In contrast the unicornis booming sequence comprises four phrases totaling eight notes that lasts 8-10 sec and is repeated every 15 sec, and also has a very distinctive and very loud final sharp ‘BMM!’ note that koepckeae lacks (Figure and Table below). These vocal differences are based on N = 97 encounters with singing individuals (N = 25 koepckeae and N = 72 unicornis) between 1999 and 2008. 





Sonograms of koepckeae (upper) and unicornis (lower). The song of koepckeae has a phrase of four notes (indicated by the black horizontal lines) with the first note somewhat louder than the following 3 notes.  In the sonogram the third note was so quiet that it could not be separated from the background noise of the recording although it was audible to the observers in the field.  The song of unicornis has eight notes (indicated by the black horizontal lines), forming the four phrases shown in the sonogram.  The song of unicornis is three times longer, with the final distinctive and far-carrying note much louder and this is completely absent from the song of koepckeae.


Variation in vocal signatures of P. u. koepckeae and P. u. unicornis.





loud note

Phrase duration

Pause duration


of phrases


of notes

Tail gesture during alarm calls




3 sec

2 sec



horizontal fanning




8-10 sec

5-7 sec



vertical pumping




The song of the only other curassow found in the Sira Mountains, Mitu tuberosum, has a similar number of notes and length of song to unicornis, and its song is much more similar to unicornis than koepckeae is to unicornis.

 The ‘Ksop!’ alarm call was accompanied by a horizontal fanning of the tail feathers concordant with each call in koepckeae; unicornis accompanies each alarm call by pumping the tail up and down concordant with each call.


Habitat and altitudinal differences – koepckeae is resident in cloud forest (1100-1600 m), whereas unicornis occurs in humid forest on the edge of tropical lowlands and lower montane forest (400-1100 m).

The Sira Mountains are isolated from the Andes, characterized by habitat zones that are at lower altitudes than the Andes; the cloud forest to montane forest transition is at 1050 m, which is several hundred meters lower than this transition at comparable locations in the main Andes (low of 1380 m, Terborgh & Weske 1975).  Therefore the difference in habitats occupied by the two taxa is even greater than is suggested by a simple comparison of altitudinal ranges. 


Morphological differences – As noted by (Weske & Terborgh 1971) there are striking differences in casque form, with a flattened shield-like ellipsoid in koepckeae and an erect cone-shaped horn in unicornis (see Figure below), as well as koepckeae having less white at the tip of the tail than either unicornis or pauxi.  We have also confirmed the casque shape of koepckeae (N = 14 observations) differs consistently from unicornis (N > 30 observations) in the field, with all observation recording the upright horn in unicornis and the flat shield in koepckeae and no intermediate casque forms recorded in either population.


Pauxi unicornis SC ZooPauxi in Peru 5 headPauxi unicornis koepckeae 2006g head

P. koepckeae (Peru)                      P. unicornis (Bolivia)


         Although Weske & Terborgh (1971) offer measurements of casque diameter and culmen tip to top of casque for koepckeae, they were not compared to other congeners, which we offer in the Table below.  We note non-overlapping measurements between koepckeae and its two congeners for culmen tip to top of casque, maximum diameter of casque, and casque length. For culmen tip to top of casque and casque length the differences between koepckeae and unicornis are greater than the differences between pauxi and unicornis. We also note the measurements are not consistent with a latitudinal cline across the 3 species as the koepckeae measurements are not intermediate between those of unicornis in Bolivia and pauxi in Colombia/Venezuela.


Variation in casque morphology among separate forms of Pauxi.




Culmen tip to top of casque

Max diameter

of casque

Casque length



























Discussion:  As noted above, the vocalizations of koepckeae differ strongly from those of unicornis, and in many ways the vocalizations of unicornis are more similar to those of Mitu tuberosum than koepckeae.  For ranking allopatric taxa under the Biological Species Concept, differences of this magnitude in voice are typically considered strong evidence for ranking the taxa as species.  The differences in casque size and shape might also be associated with divergence to the point of being a barrier to gene flow if these structures are associated with mate selection.  Although differences in habitat and elevation are not species-level characters per se, they are at least consistent with species-level differences.


Recommendation: A “YES” vote.  Although assessment of species limits among allopatric taxa is problematic, we think that the degree of differentiation between these widely separated populations is consistent with the level of differences associated with taxa ranked as species in the Cracidae.  For example, these differences are arguably greater than those between taxa treated as separate species in Penelope, Aburria/Pipile, and Ortalis.


English names: We recommend “Sira Curassow” for koepckeae and retaining “Horned Curassow” for unicornis.  Although there are advantages to creating two new names for the split species, we prefer the simpler names to something like Northern Horned Curassow and Southern Horned Curassow.



Gastañaga, M. 2006. Peruvian Horned Curassow (Pauxi unicornis koepckeae) rediscovered in the Sira Mountains, Peru. Bulletin of the Cracid Specialist Group 22: 10-23.

Gastañaga, M., A. B. Hennessey, & R. MacLeod. 2007. Rediscovery of Southern Horned Curassow Pauxi unicornis koepckeae in Cerros del Sira, Peru. Cotinga 28: 63-66.

Gastañaga-C., M., R. MacLeod, D.M. Brooks and B. Hennessey. 2011. Distinctive morphology, ecology, and first vocal descriptions of Sira Curassow (Pauxi [unicornis] koepckeae): evidence for species rank.  Ornitol. Neotrop. 22: 267-279.

Graham, J.G. 2009. A new specimen of Southern Horned Curassow Pauxi unicornis from Peru. Cotinga 31: 73.

Gúzman, E., D. M. Brooks, & G. Sedaghatkish. 1999. Notas sobre la historia natural de los crácidos albergados en el Museo “Noel Kempff Mercado”, Santa Cruz, Bolivia, con notas sobre la taxonomía de las pavas del genero Pipile. Bulletin of the Cracid Specialist Group 8: 20-28.

Herzog, S. K., & M. Kessler. 1998. In search of the last Horned Curassows Pauxi unicornis in Bolivia. Cotinga 10: 46-48.

Maillard-Z., O. 2006. Reciente espécimen de la Pava Copete de Piedra (Pauxi unicornis) para Bolivia. Kempffiana 2: 95-98.

Weske, J.S. and J.W. Terborgh. 1971. A new subspecies of curassow of the genus Pauxi from Peru. Auk 88: 233-238.



Daniel Brooks, Ross MacLeod, and Melvin Gastañaga-C.

August 2012



Comments from Remsen:  “YES.  I think the proposal clearly places burden-of-proof on treatment of koepckeae at the subspecies rank.  In fact, I think that species rank is better-justified than it is for a number of allopatric species in the Cracidae.”


Comments from Jacob Socolar: “I want to offer three brief comments.  I do not expect any of these comments to substantially influence the committee members' decisions, but perhaps it could somehow be useful to mention these points:


“--On August 7, 2010, I observed a Pauxi [unicornis] koepckeae at great length at 1650 masl on Terborgh and Weske's original transect of the Sira.  This individual clearly had white tipping to all rectrices, and I do not believe that the extent of white in the tail is a consistent difference between koepckeae and nominate unicornis.


“--The casque morphology is somewhat variable in koepckeae.  In particular, I have in my possession a very poor photo of a P. [u.] koepckeae (which I can send to anyone who wants to see it) that shows an individual of koepckeae with a casque that appears to approach Pauxi pauxi in shape.  The casque morphology in Pauxi pauxi is also variable.  Among the specimens at the Smithsonian, the only gender/subspecies combination with multiple specimens involves three female P. p. gilliardi, and their casques are tremendously variable in size and shape, apparently due to damage and wear during the birds' lifetimes.


“--Whether koepckeae is truly sister to P. unicornis or P. pauxi is not entirely clear to me.


“While I think that additional data (likely including genetic work and also a further attempt to follow up on Ted Parker's possible record) are required to thoroughly understand the relationships between the various taxa in the genus Pauxi, I agree with the recommendation for a 'yes' vote on this proposal.”


Additional comments from Brooks and MacLeod: “Jacob, thanks for your comments, here are some brief responses:


“The individual you saw at Terborgh and Weske's site was probably in molt and simply shed the central rectrix.  Mel and Ross found two shed rectrices in the Sira in mid July 2007, which is close to your date of 7 August.  The Holotype at AMNH deposited by Weske and Terborgh from the same site where you saw the bird in question also lacks a white tip centrally.


-           “We're not surprised about the casque variation in the small series of female P. p. gilliardi, as variation increases with size of a given character, and P. pauxi has a large casque.  More importantly there is some ontogenetic variation in casque size of different age classes since Pauxi reaches adult body size within a year, but the casque doesn't attain full size for 2-3 years.  Despite the potential for size variation due to growth we've seen at least 14 casques of koepckeae, and they were all similar in size and flat shield-like appearance, which contrasts with the invariably upright horn of unicornis.  In captive individuals of unicornis at the Santa Cruz Zoo (Bolivia) the upright horn shape was present from a young age; although the size of the horn increased with age it didn't change shape or ever resemble the casque of koepckeae.”


Additional comments from Jacob Socolar: “I am currently in the field without my notes to say with certainty how many rectrices I observed with white tips on the Terborgh-Weske transect.  It is indeed possible that the bird only showed 10 rectrices, although that is not my recollection.  In any case, a recent brown-morph specimen from the east slope of the Sira (details forthcoming) shows cream (rather than white) tips to the rectrices, and clearly shows such tips on all twelve rectrices (save the outermost, whose tips are worn off entirely).


“Furthermore, Weske and Terborgh (1971) note the absence of white tips on four central rectrices.  I am certain that the bird I observed was not missing four central tail feathers, and so there is definitely variability in the number of white-tipped rectrices in P. koepckeae.


“Again, I suspect that this information won't substantially affect the admirably documented case for a split, but I just don't think that the white tipping on the tail should go down as an "official" diagnostic character to distinguish between the forms (absence of white seems to be diagnostic for koepckeae, but presence of white is not diagnostic for nominate unicornis).”


Comments from Zimmer:  “YES”.  I think that koepckeae and unicornis could be justifiably split on the vocal differences alone, but the rather extreme differences in casque shape, even if proven to vary individually in size, provide clinching evidence.  These differences are not only consistent with the levels of differentiation between other species pairs of cracids, but they exceed those differences for many pairs of taxa already treated as specifically distinct.  Should this proposal pass, I would also recommend adoption of the English names suggested by the authors for the two resulting species.”


Comments from Stiles: “YES. Differences in voice and casque shape, probably both important in mate choice, suggest to me that this split is biologically meaningful.”


Comments from Pacheco: “YES.  A partir do exposto, eu sou favorável ao tratamento sugerido.”


Comments from Pérez-Emán: “YES. Vocal and morphological differences argue in favor of this split. It would be interesting to pursue Jacob’s question on phylogenetic relationships within the genus Pauxi because koepckeae might not be closer to unicornis (compared to pauxi).”


Comments from Jaramillo: “YES.  In particular, the vocal data is what swayed me the most. The further morphological differences are icing on the cake.”


Comments from Nores: “YES. The vocalizations and the casque form are different enough to treat Pauxi (unicornis) koepckeae as a species. No other differences are so important. I agree with Socolar that genetic data are required to thoroughly understand the relationships between the various taxa in the genus Pauxi.”