Proposal (288) to South American Classification Committee
Split Icterus icterus into three species
There are just three viewpoints on troupial taxonomy: there are either one, two or three species (Hellmayr 1937, Short 1975, Jaramillo and Burke 1999). In this proposal, I summarize the arguments for the recognition of three species. Their respective names would be Venezuelan Troupial (Icterus icterus), Campo Troupial (Icterus jamacaii) and Orange-backed Troupial (I. croconotus). The first species is often called just "Troupial". Many important books and field guides have already recognized the three species (Jaramillo and Burke 1999, Ridgely and Greenfield 2001, Hilty 2002). Authors that recognize two species lump Campo and Orange-backed Troupials (e.g. Hellmayr 1937, Omland et al. 1999). Venezuelan Troupial is divisible into three subspecies, the Campo Troupial is monotypic, and the Orange-backed Troupial consists of two subspecies (Jaramillo and Burke 1999).
Troupials are not rainforest birds, and often occur in areas with less than 500 mm of yearly rainfall. Their main habitat is thorny scrub, open woodland and savannas, in arid and semiarid climates, or climates with a strong dry season. The main exception to the rule concerns Orange-backed Troupials of the nominate croconotus subspecies, which occurs in the várzea and other semi open riparian habitats in Amazonia.
The alleged intermediate subspecies metae.
One argument for lumping the three troupials is the existence of a population (Icterus icterus metae) apparently intermediate in plumage between the Venezuelan and the Orange- backed Troupials. The pattern of black and orange colors in the plumage of metae is basically that of the Venezuelan Troupial, but the nape is orange rather than black, and some individuals show an orange patch in the back. This partial resemblance of metae to Orange-backed troupials may be a case of convergence, as other characters of metae, particularly the divided white wing patch, are unique to this population, or are the same as in Venezuelan Troupials. The molecular phylogeny of orioles has revealed many cases of color convergence in plumage (e.g. Omland et al. 1999, Hoffman et al. 2006).
Subspecies metae has a limited range in eastern Colombia (Arauca) and western Venezuela (Apure). Populations of metae are separated by a huge distribution gap from the nearest populations of Orange-backed Troupials (Hilty and Brown 1986, Jaramillo and Burke 1999), an unusual fact for forms that supposedly hybridize or intergrade. Short (1975) suggested that metae is a product of a past hybridization event between icterus and croconotus, but his idea requires the existence of a former dry corridor across the humid forests of Eastern Colombia. I ´m not expert in the biogeography of Amazonian Colombia, but if such corridor existed, it should have affected other animals and plants, not just the troupials.
Troupials are well known by their rich melodious songs. Vocal characteristics shared by all troupials include the existence of female song (Skutch 1969, Sick 1990, pers. obs.), pair dueting (pers. obs.), and the ability to mimic other birds (at least in captivity) (Sick 1990, pers. obs.). A recent study mentions a few differences in song between two troupial species (Price et al. 2007). I have myself recorded songs from a few individuals of the three species (in the Paraguayan Chaco, Minas Gerais in Brazil, Cojedes in Venezuela) and noticed some differences. However, only a detailed analysis of many songs, for both sexes of (at least) all described subspecies, would be needed to clarify troupial song evolution.
There is almost no information on other troupial vocalizations.
Allopatry and sympatry
Other than the apparent intermediacy of metae, the main argument for lumping all troupials was basically their allopatric distribution. Their disjunct distribution mostly reflects basic disjunctions in climate and vegetation within South America. Under the biological species concept (BSC) all good species eventually evolve differences in behavior, size, bill morphology, etc. that would permit coexistence. Troupials have one extra behavioral character that would difficult coexistence. Most troupial populations (excepting I. i. ridgwayi) seem to breed only, or mostly, in other birds' nests (Pinto 1967, Skutch 1969, Pearson 1974, Lindell and Bosque 1999, Maugeri and Drozd 2006, and pers. obs. For jamacaii and croconotus). Aggressive nest piracy, with egg or chick removal, has been reported for the three species (sources above), and would probably decrease the nesting success of troupials breeding in sympatry.
Recently an area of sympatry between Campo and Orange-backed Troupials has been reported by Pacheco and Olmos (2006). It is located in the Brazilian state of Tocantins, and apparently the forms in contact behave as good biological species (at least no hybrids or mixed pairs were reported). Less relevantly, Campo and Orange-backed troupials had breeding feral populations around the city of Belem, Para, Brazil (Cardoso da Silva and Oren 1990), and likewise, no mixed pairs or intermediate specimens were reported.
I think that available evidence favors the recognition of three troupial species as good biological species, even if metae reflects a past episode of hybridization, because it would then involve a very restricted area. The three troupials were described as species as early as 1766, 1788 and 1829, at a time when avian taxonomy was based mostly on plumage characteristics of museum specimens. The forms are clearly diagnosable, as advocates of the phylogenetic species concept (PSC) should recognize. Lastly, the three forms were initially described as species on their own, and maintained as separate species well into the 1950s and 1960s (e.g. De Schauensee 1952, Pinto 1967). The official Brazilian checklist (CBRO 2006) recognizes the three species.
Cardoso da Silva, J. M., and D. C. Oren. 1990. Introduced and invading birds in Belém, Brazil. Wilson Bull. 102: 309-313.
CBRO. 2006. Lista das aves do Brasil. Versao 15/7/2006.
De Schauensee, R. M. 1952. The birds of the Republic of Colombia. Family Icteridae. Caldasia 5:983-1001.
Hellmayr, C. E. 1937. Catalogue of the birds of the Americas and the adjacent islands. Vol. 12. Field Museum of Natural History, Chicago, USA.
Hilty, S. L., and W. L. Brown. 1986. A guide to the birds of Colombia. Princeton, NJ: Princeton Univ. Press.
Hilty, S. L. 2002. Birds of Venezuela. Second edition. Princeton, NJ: Princeton Univ. Press.
Hoffman, C. M., T. W. Cronin, and K. E. Omland. 2006. Using spectral data to reconstruct evolutionary changes in coloration: carotenoid color evolution in New World orioles. Evolution 60:1680-1691.
Jaramillo, A., and P. Burke. 1999. New World blackbirds. The Icterids. London: A. & C. Black Publishers.
Lindell, C. & C. Bosque. 1999. Notes on the breeding and roosting biology of Troupials (Icterus icterus) in Venezuela. Ornit. Neotrop. 10:85-90.
Maugeri, F. G., and H. O. Drozd. 2006. Primer registro de nidificación del matico (Icterus croconotus strictifrons) para la Argentina. Ornitologia Neotropical 17: 301-305.
Omland, K. E., S. M. Lanyon and S. J. Fritz. 1999. A molecular phylogeny of the New World orioles (Icterus): the importance of dense taxon sampling. Molecular Phylogenetics and Evolution 12:224-239.
Pacheco, J. F., and F. Olmos. 2006. As aves do Tocantins, parte 1. Rev. Bras. Orn. 14:85-100.
Pearson, D. L. 1974. Use of abandoned cacique nests by nesting Troupials (Icterus icterus): precursor to parasitism? Wilson Bull 86:290-291.
Pinto, O. 1967. Do parasitismo provavel de Icterus jamacaii (Gmelin) em Pseudoseisura cristata (Gmelin). Hornero 10:447-449.
Price, J. J, N. R. Friedman, and K. E. Omland. 2007. Song and plumage evolution in the New World orioles (Icterus) show similar lability and convergence in patterns. Evolution 61: 850-863.
Ridgely and Greenfield 2001. The birds of Ecuador. Two volumes. Cornell Univ. Press.
Short, L. L. 1975. A zoogeographic analysis of the South American Chaco avifauna. Bull. Am. Mus. Nat. Hist. 154: 163-352.
Sick, H. 1993. Birds in Brazil. Princeton, NJ: Princeton Univ. Press.
Skutch, A. F. 1969. A study of the Rufous-fronted Thornbird and associated birds. Pt. 2: Birds which breed in thornbirds' nests. Wilson Bull 81:123-139.
Rosendo M. Fraga, June 2007
Comments from Stiles: "YES. I have long felt that croconotus and icterus were different birds from fairly extensive experience with both. I don't have any experience with jamacaii, but the fact that where it is sympatric in artificial situations (where one might expect interbreeding if small numbers of released/escaped birds were to found a feral population) very strongly suggests that reproductive isolation is alive and well. My limited experience with metae suggests that it is a bird of the borders of gallery forest (ecotone with the open llanos) and in general is much more like icterus in voice and behavior."
Comments from Cadena: "YES. The turning point here is the documented sympatry of jamacaii and croconotus with no apparent hybridization. The allopatric distribution of icterus implies that the case for splitting this taxon is not as strong, but I guess one could say that it differs from the two taxa that are reproductively isolated to a similar extent than these two differ from each other. Also, considering these taxa were described as good species and lumped without a strong rationale, and that Gary's experience indicates croconotus and icterus are different, I'll go along with the three-way split."
Comments from Remsen: "YES. The report of sympatry between jamacaii and croconotus refutes any previous treatment as conspecific, and for consistency, I. icterus should be ranked at the species level until further evidence indicates otherwise."
Comments from Robbins: "Without a molecular assessment, a tentative YES."
Comments from Zimmer: "YES. Sympatry of jamacaii and croconotus without apparent hybridization is the clincher. Morphological, vocal and ecological differences between each of those two forms and nominate icterus are on a par with consistent differences between croconotus and jamacaii, so treating icterus as a third species makes sense."
Comments from Nores: "YES. Como ha sido indicado en varias de las respuestas, la simpatría de jamacaii y croconotus sin hibridización es el mejor fundamento para separar las especies. Además, están las vocalizaciones, que aparentemente son algo diferentes. Sin embargo, Ridgely y Tudor mencionan que las voces son esencialmente similares. Como en el caso anterior sería muy importante un análisis molecular."
Comments from Jaramillo: "YES - For the record, as it has passed already. Also, the species differ in feather structure on the neck, icterus has sharp and pointed neck feathers (see Jaramillo and Burke 1999). This is unique to any species in the genus Icterus."
Comments from Pacheco: "YES. Creio que o tratamento de três espécies reflete melhor o conhecimento acumulado nos três táxons."