Proposal (342) to South American Classification Committee
Split Myiobius barbatus into multiple species
Effect on South American checklist: This would split Myiobius barbatus into two or three species.
SACC currently treats Myiobius barbatus as including sulphureipygius, whereas the North American committee treats sulphureipygius as a distinct species (AOU 1998). A form in southeastern Brazil (mastacalis) is treated as a group that could be specifically distinct in several references, and at least a couple I have seen say that it is sometimes treated as distinct, but I have not found any references that actually do treat it as a separate species (Actually Wikipedia lists mastacalis as a separate species, but when you click on the common name (Yellow-rumped Flycatcher) it takes you to an Asian species [more on that later]).
Hellmayr (1927) treated trans-Andean sulphureipygius as a distinct species from barbatus. Zimmer (1939) concluded that "It seems probable, therefore, that sulphureipygius and aureatus deserve inclusion in the barbatus group." His main basis for this was, in fact, the similarity between aureatus (the South American subspecies of the sulphureipygius group) and mastacalis (the SE Brazilian subspecies). He noted that aureatus and barbatus approached one another without signs of intergradation. However, they are across a range of Andes from one another. Since Zimmer there has been a diversity of treatments.
References treating sulphureipygius and barbatus as conspecific include Hilty and Brown 1986, Meyer de Schauensee 1966, 1970, Pinto 1944, Ridgely and Tudor 1994, Sibley and Monroe 1990, Traylor 1979, Zimmer 1939.
References splitting barbatus and sulphureipygius as separate species: Hellmayr and Cory 1927, Hilty 2003, Ridgely and Greenfield 2001, AOU 1983, 1998, Farnsworth and Lebbin 2004. Basically all of the Central American literature back at least until Blake (1953) treats the two as separate except for the 2nd edition of Birds of Panama (Ridgely and Gwynne 1989)
Analysis: There are really two current treatments that are widely applied. Most South American literature lumps everything into barbatus, whereas Central American literature splits out sulphureipygius. The almost 50/50 split in how these taxa are treated is a reflection of the fact that there is essentially no data that support either treatment. Myiobius sulphureipygius and M. barbatus are allopatric and show low levels of plumage and morphometric divergence. They are not very vocal and most references either don't mention voice or give a transcription that seems like it could be interpreted as indicating that they sound the same. Overall the morphological distinctiveness of trans-Andean sulphureipygius and eastern Brazilian mastacalis from Amazonian barbatus seems roughly equivalent, so it would seem like treatment as a single species or three species are the most logical option, although historically the splitting of mastacalis has not been a common treatment (or even a rare treatment?).
One weak piece of evidence that applies to the mastacalis/barbatus question is that Traylor (1979) indicated that birds from western Mato Grosso are intermediate between mastacalis and the subspecies insignis (part of the barbatus group) of southeastern Amazonia. He suggested that they might be intergrades between mastacalis and insignis. I assume that this refers to two specimens discussed by Zimmer (1939), one of which he assigns to insignis and the other, which he indicates "agrees better with mastacalis," although it is unusually large (for any Myiobius). Given that both these specimens are from Amazonian drainages and mastacalis as far as I can tell is not even in the Parana drainage, that these birds are really indicative of intergradation between insignis and mastacalis seems doubtful to me.
One weak piece of evidence that might suggest splitting the taxa within barbatus is the fact that barbatus and the very similar atricaudus are broadly sympatric in all three major forest realms. The Amazonian populations of these two species are very similar, such that specimens have been routinely misidentified. The species tend to replace one another in a patchwork; however, there are localities from which both species are known. If these taxa act as distinct species, it might indicate that the morphologically more divergent forms in Central America and eastern Brazil would act as distinct species - maybe.
Recommendation: Ridgely (1976) treats sulphureipygius as a distinct species. Later, Ridgely (Ridgely and Gwynne 1989 and Ridgely and Tudor 1994) argued against treating sulphureipygius as distinct from barbatus essentially because mastacalis is as distinctive as sulphureipygius. However, Ridgely and Greenfield (2001) followed AOU (1998) in splitting sulphureipygius. I find I agree with Ridgely--I really don't know what the best approach is. My weak recommendation is to maintain all the forms as a single species given the unimpressive morphological distinctiveness of all forms in the absence of vocal or genetic evidence suggesting otherwise.
Proposal A to split sulphureipygius from barbatus. I recommend a NO vote. If we maintain sulphureipygius as a subspecies of barbatus, I will write a proposal for the North American committee to change their treatment to a single species treatment, although I'm not sure I can see why they would necessarily change. Note that if split, the English name Sulphur-rumped Flycatcher would apply to sulphureipygius. Whiskered Flycatcher is usually used for barbatus.
Proposal B to split mastacalis from barbatus. I recommend a NO vote. If we split this, there is an English name issue. Meyer de Schauensee (1966) suggested, with the three-species treatment, barbatus as Whiskered Flycatcher, and Yellow-rumped Flycatcher for mastacalis. However, Yellow-rumped Flycatcher is used for the Asian species Ficedula zanthopygia. Its use for Myiobius mastacalis would seem like a bad idea. Sibley and Monroe (1990) use Bearded Flycatcher for the barbatus group and Whiskered for mastacalis. I would suggest that if we split this complex, that we follow the English names use by Sibley and Monroe.
AMERICAN ORNITHOLOGISTS' UNION. 1983. Check-list of North American birds, 6th ed. American Ornithologists' Union, Washington, D.C.
AMERICAN ORNITHOLOGISTS' UNION. 1998. Check-list of North American birds, 7th ed. American Ornithologists' Union, Washington, D.C.
BLAKE, E. R. 1953. Birds of Mexico: A guide for field identification. University of Chicago Press, Chicago, Illinois.
FARNSWORTH, A. AND D. J. LEBBIN. 2004. Sulphur-rumped Flycatcher and Whiskered Flycatcher accounts. Pp. 351-352 in "Handbook of the Birds of the World, Vol. 9. Cotingas to pipits and wagtails." (J. del Hoyo et al., eds.). Lynx Edicions, Barcelona.
HELLMAYR, C. E. 1927. Catalogue of birds of the Americas. Field Mus. Nat. Hist. Publ., Zool. Ser., vol. 13., pt. 5.
HILTY, S. L. 2003. Birds of Venezuela. Princeton University Press, Princeton, New Jersey.
HILTY, S. L., AND W. L. BROWN. 1986. A guide to the birds of Colombia. Princeton University Press, Princeton, New Jersey.
MEYER DE SCHAUENSEE, R. 1966. The species of birds of South America and their distribution. Livingston Publishing Co., Narberth, Pennsylvania.
MEYER DE SCHAUENSEE, R. 1970. A guide to the birds of South America. Livingston Publishing Co., Wynnewood, Pennsylvania.
PINTO, O. M. DE O. 1944. Catalago das aves do Brasil. Parte 2. Departamento de Zoologia da Agricultura, Industria e Comercio, Sčo Paulo, Brasil.
RIDGELY, R. S. 1976. A guide to the birds of Panama. Princeton Univ. Press, Princeton, New Jersey.
RIDGELY , R. S., AND P. J. GREENFIELD. 2001. The birds of Ecuador. Vol. I. Status, distribution, and taxonomy. Cornell University Press, Ithaca, New York.
RIDGELY R. S., AND J. A. GWYNNE. 1989. A guide to the birds of Panama, with Costa Rica, Nicaragua, and Honduras (2nd ed.). Princeton Univ. Press, Princeton, New Jersey.
RIDGELY, R. S., AND G. TUDOR. 1994. The birds of South America, vol. 2. Univ. Texas Press, Austin.
SIBLEY, C. G., AND B. L. MONROE, JR. 1990. Distribution and taxonomy of birds of the World. Yale University Press, New Haven, Connecticut.
TRAYLOR, M. A., JR. 1979a. Subfamily Elaeniinae. Pp. 3-112 in "Check-list of birds of the World, Vol. 8" (Traylor, M. A., Jr., ed.). Museum of Comparative Zoology, Cambridge, Massachusetts.
ZIMMER, J. 1939. Studies of Peruvian birds, No. 30. Notes on the genera Contopus, Empidonax, Terenotriccus, and Myiobius. American Museum Novitates 1042: 1-13.
Doug Stotz, April 2008
Comments from Remsen: "NO (both A and B). Until actual data are presented for one or the other treatment, any classification is largely arbitrary. As Doug recommends, I don't think we have a choice but to stick with to status quo until those data appear."
Comments from Nores: "NO (both A and B). Pienso que como en la propuesta 343, lo más apropiado será esperar por nuevas evidencias en vocalizaciones o análisis moleculares. Además, sulphureipygius es más similar a mastacalis que a barbatus (de acuerdo a los dibujos del HBW) así que de separa sulphureipygius debería también separar a mastacalis y ponerlos en una misma especie, que en este caso tendría prioridad mastacalis. Así que quedaría Myiobius mastacalis mastacalis y Myiobius mastacalis sulphureipygius."
Comments from Stiles: "YES. This is, as Doug notes, a tough one.. from my Central American-Colombian perspective, I would be tempted to go the other way and vote YES on both - given the often subtle differences in plumage among congeneric tyrannids, including the sympatric atricaudus and barbatus in Myiobius itself plus the considerably greater difference of sulphureipygius and the effective lack of a true "status quo", I'd place the three (barbatus, mastacalis and sulphureipygius) in a superspecies and hope that someone does the genetics and gets some good vocal data soon! As Doug notes, the evidence is pretty tenuous whichever way one goes, so my vote is decidedly sotto voce."
Comments from Pacheco: "NO to both A and B. Prefiro aguardar uma análise apropriada. As diferenćas morfológicas entre os táxons subordinados ao complexo M. barbatus sčo menos abruptas que aquelas observadas no complexo M. atricaudus. É sugestivo que J. T. Zimmer nčo tenha encontrado distinćčo entre um espécime do "Rio Roosevelt" (interflúvio Madeira-Tapajós) e aqueles da Mata AtlČntica (M. b. mastacalis)."