Proposal (493) to South American Classification Committee
Treat Basileuterus hypoleucus as conspecific with Basileuterus culicivorus
Effect on South American CL: This proposal would lump two species currently recognized as separate species on the baseline list.
Background: Although described as separate species on account of obvious plumage differences, doubt has frequently been cast about the specific validity of Basileuterus hypoleucus. Hellmayr (1935) commented that the ranges of the two overlapped considerably in Brazil and Paraguay and that the presence of intermediate birds "casts serious doubt on their specific distinctness". Mixed pairs of the two species have been reported from Brazil (Willis 1986) and Paraguay (Robbins et al 1999), and it is not uncommon for hypoleucus specimens to show some degree of yellow on their otherwise whitish underparts (Remsen & Traylor 1989, Robbins et al 1999, FAUNA Paraguay 2011). Robbins et al (1999) noted that all presumed hybrids are of the hypoleucus-type, being white with yellow patches, and that culicivorus-type yellow birds with white patches have never been reported. Sick (1993) considered the two species to be conspecific and remnants of a population that had undergone geographic separation as a result of ancient geoclimatic events but that were now coming into contact again. He considered the voices of the two species to be identical.
Contrary to other authors, Hayes (1995) stated that the vocalisations of B. hypoleucus are in fact closer to B. flaveolus than to B. culicivorus and mentioned undocumented observations of a mixed family of these two species in Dpto. Concepción, Paraguay. Additionally, he noted a difference in habitat preference, with B. culicivorus preferring more humid forest, citing this and widespread sympatry of range as evidence that they are two distinct but closely related species. Robbins et al (1999) later clarified that the vocalisations of B. hypoleucus are not similar to flaveolus and added that vocalisations of culicivorus and hypoleucus in Dpto Concepción, Paraguay are so similar that both species react strongly to playback of taped recordings of the others calls. Additionally, they clarified that the mixed family reported by Hayes did not refer to paired birds but to birds “intermingling together” (F. Hayes pers. comm.).
In a phylogenetic review of the Parulidae, Lovette et al (2010) provided data that confirmed a close relationship between the species and treated them as sister taxa. Focusing only on the B. culicivorus complex, Vilaça & Santos (2010) used molecular studies to demonstrate that B. hypoleucus did not form a monophyletic clade within the complex and was in fact related to B. culicivorus populations from Brazil and Paraguay. They stated:
"Although it is not currently possible to distinguish genetically between these two species, the restricted area of occurrence of B. hypoleucus could suggest either that this is an incipient species in the process of differentiation or the white color is a restricted polymorphism of a major taxon, B. culicivorus. Our results and the observation that these recognized species do not own [sic] differences in vocalization or morphometry (Silva 1992), might be a strong indication that these belong to a single species."
Using both mitochondrial and nuclear markers, and corroborated by the morphological study of Silva (1992), they concluded that the two taxa should be lumped into a single species.
Recommendation: I recommend a "YES" vote on this proposal because of the convincing molecular evidence presented by Vilaça & Santos (2010), the known hybridisation between the two "species" and the fact that they respond strongly to recordings of each other’s vocalisations. The name Basileuterus culicivorus (Lichtenstein) would be the correct name for the species. The status of the form hypoleucus (Bonaparte) is currently unresolved and further studies are required to determine whether it is in fact a valid subspecies or just a restricted colour morph.
FAUNA PARAGUAY. 2011. Basileuterus hypoleucus. Online images www.faunaparaguay.com/basileuterus_hypoleucus.html.
HAYES, F.E. 1995. Status, Distribution and Biogeography of the Birds of Paraguay. ABA Monographs in Field Ornithology 1. 230p.
HELLMAYR, C. E. 1935. Catalogue of birds of the Americas. Field Mus. Nat. Hist. Publ., Zool. Ser., vol. 13., pt. 8.
LOVETTE, I. J., J. PÉREZ-EMÁN, J. P. SULLIVAN, R. C. BANKS, I. FLORENTINO, S. CÓRDOBA-CÓRDOBA, M. ECHEVERRY-GALVIS, F. K. BARKER, K. J. BURNS, J. KLICKA, S. M. LANYON, AND E. BERMINGHAM. 2010. A comprehensive multilocus phylogeny for the wood-warblers and a revised classification of the Parulidae (Aves). Molecular Phylogenetics and Evolution 57: 753-770.
REMSEN, J.V. Jr., TRAYLOR M.A. 1989. An Annotated List of the Birds of Bolivia. Buteo Books, Vermillion, South Dakota. 79 pp.
ROBBINS, M.B., R.C. FAUCETT, AND N.H. RICE. 1999. Avifauna of a Paraguayan cerrado locality: Parque Nacional Serrania San Luis, Depto. Concepcion. Wilson Bulletin 11: 216-218.
SICK, H. 1993. Birds in Brazil. Princeton University Press, New Jersey. 703pp.
SILVA, W.R. 1992. Padrões ecológicos, bioacústicos, biogeográficos e filogenéticos do complexo Basileuterus culicivorus (Aves, Parulidae) e demais espécies brasileiras do gênero. Departamento de Ecologia. Universidade de Campinas, Campinas. p.132.
VILAÇA, S. T., AND F. R. SANTOS. 2010. Biogeographic history of the species complex Basileuterus culicivorus (Aves, Parulidae). Molecular Phylogenetics Evolution 57: 585-597.
WILLIS, E.O. 1986. Vireos, wood warblers and warblers as ant followers. Gerfaut 76: 177-186.
Paul Smith, August 2011
Comments from Remsen: “YES. With first-hand experience with both taxa in Bolivia, I became highly suspicious of the species rank of hypoleucus in 1984 – songs and calls sounded identical to me, and populations we sampled in Dpto. Santa Cruz showed signs of intergradation. Now, we have some actual data that show that hypoleucus is a pale-bellied form of southern culicivorus.”
Comments from Stiles: “YES – all the evidence seems to fit treating hypoleucus as a pale-bellied race of culicivorus rather than a species.”
Comments from Robbins: “YES, based on our Paraguay data (Robbins et al. 1995) coupled with the Vilca & Santos genetic data it would seem best to treat hypoleucus as conspecific with culicivorus.
Comments from Pacheco: “YES. In my personal experience, I agree that the vocalizations of both taxa involved are virtually indistinguishable. The simple calls can be something different but seem to fall within the range of complex B. culicivorus. I've found in eastern Minas Gerais pairs with individuals tending to each of the taxa. Given the results of Vilaça & Santos (2010) - in combination with those older Weber Silva (1992) – I vote yes.”
Comments from Stotz: “YES. This makes me “sad,” but this has always been out there. There is certainly a habitat difference between the two taxa with culicivorus more of a humid forest species and hypoleuca more in cerrado and gallery forest. But there are plenty of places where these forms come in contact with intergradation known. Vocally they are very similar if not identical.”
Comments from Pérez: “YES. Evidence available is against retention of B. hypoleucus as species. This is an interesting case showing that potentially independent evolutionary lineages might not persist in the face of their habitat dynamics and the lack of geographical or ecological barriers. It would be great to investigate the potential hybridization between these taxa and the patterns and mechanisms of plumage variation.”
Comments from Zimmer: “YES since there appears to be solid evidence of intergradation, the vocalizations are doubtfully distinct (and, in reciprocal playback experiments, the two taxa respond to one another’s voice), and molecular evidence for single-species treatment is convincing. Doug’s comments regarding the habitat distinctions are spot-on, but I’m afraid the crush of other evidence overwhelms this as a consideration.”