Proposal (615) to South American Classification Committee
Recognize newly described Sporophila beltoni
Repenning and Fontana (2013) described a new species of Sporophila (Sporophila beltoni) that breeds in s. Brazil (Rio Grande Do Sul Santa Catarina, and Paraná) and winters mainly in Goiás and Minas Gerais. The existence of these birds has been noted since at least Hellmayr (1938) but had been treated as individual variation in bill color in Sporophila plumbea.
Repenning and Fontana (2013) found that these yellow-billed individuals actually represent an undescribed species restricted to an odd ecotone between Araucaria forests and grasslands, and that migration into areas also inhabited by the very similar, dark-billed Sporophila plumbea has led to the treatment of these yellow-billed individuals as a variant of that species. Their paper is a fascinating detective story. The abstract summarizes the evidence for species rank for this form (and see the paper for the details):
Abstract.—An analysis of morphology, vocalization, habitat preference, and distribution revealed that the southern yellow- billed population ascribed to the Plumbeous Seedeater (Sporophila plumbea) is actually an undescribed species. Individuals of this new species are distinguishable from other Sporophila, particularly S. plumbea, by a combination of diagnostic characters: (1) the adult male has a robust, bright yellow bill with an arched culmen, distinguishing it from S. plumbea and other gray seedeaters; (2) the adult male is larger and heavier than S. plumbea; (3) the adult plumage of males is bluish gray (not plumbeous gray, as in S. plumbea); (4) vocalizations include song that is structurally similar to that of S. plumbea but with clear (whistled) introductory syllables and unique call notes. The new seedeater is segregated from other gray seedeaters in its breeding habitat and breeding distribution. It is endemic to Brazil, where it breeds in upland shrub-grasslands associated with Araucaria forests in southern Brazil and migrates northward to the Cerrado to winter.
The authors documented a narrow contact zone between S. beltoni and S. plumbea, where they segregate abruptly by habitat. No mixed pairs were found; although two males with somewhat intermediate bill color were noted, they showed no other signs of being intermediate. Therefore, they pass the ultimate test of species rank: parapatry without significant gene flow. The authors did a great job of evaluating and dismissing alternative hypotheses to species rank for S. beltoni.
An intriguing dimension of this study is that the authors documented the ontogeny of plumage variation in this species and how it differs from Sporophila plumbea by color-banding nestlings of known parentage! If only we had these kind of data for all Sporophila! (In general, this paper is in my opinion exceptionally good and could be used as a model for similar situations).
I see no reason to treat this new taxon as anything as a valid species-level taxon and recommend a YES on this one. It is presumably the sister species to S. plumbea and thus should follow it in the linear sequence of species.
The proposed English name, Tropeiro Seedeater, is creative and appropriate, and is explained as follows:
“The common name refers to the breeding distribution and migratory pattern of the species, which is congruent with the Rota dos Tropeiros (Herders’ Trail). This inland trail was used for general travel and to drive herds (cattle, mules, and horses) and transport charque (beef jerky) from the south to markets in southeastern Brazil. This cycle began in the early 18th century and continued until 1930.”
Van Remsen, December 2013
Comments from Stiles: “YES. A very well-documented split, clearly beltoni deserves species rank.”
Comments from Pacheco: “YES. Putting a spotlight on the case. A job well done in many respects.”
Comments from Cadena: “YES - Very nice work.”
Comments from Zimmer: “YES. The authors have done an excellent job of documenting this interesting case.”
Comments from Robbins: “YES. Indeed, the authors have done a thorough job in documenting that this yellow-billed Sporophila deserves species status.”