Buteo polyosoma and B. poecilochrous, the "Red-Backed Buzzards" of South America, Are Conspecific

Abstract

The Puna Buzzard (= Variable Buzzard or Gurney's Buzzard, Buteo poecilochrous) and the Red-backed Buzzard (B. polyosoma) are two morphologically similar and closely related South American raptors that have been and are currently separated as species mainly on the basis of wing length and Stresemann's wing formula. I reexamined wing formula, wing length, geographic variables, and plumage in adults and immatures, and adult vocalizations. For both adults and immatures, wing formula and wing length are not reliable taxonomic characters because of their continuous clinal relationship. In both age classes, wing tip contour changes allometrically with changes in wing length, which is itself a function of body size and mass. Thus, neither of the currently accepted morphometric characters examined here serve to separate taxon polyosoma from taxon poecilochrous. There are at least 27 distinct adult plumage morphs (formerly five) in this species group, perhaps more than any other in Falconiformes, with no relationship to morphometric variables and only minor geographic variation. Immature plumages are similarly ineffective in diagnosing taxa. Vocalizations are similar among adults from widely varying latitudes and altitudes. I therefore propose dropping poecilochrous (Gurney 1879) in favor of polyosoma (Quoy and Gaimard 1824) for all forms in this group, as the latter has priority. The English common name applied to this group should be Variable Buzzard, with the exception of the isolated population in the Juan Fernandez Archipelago which should for now remain the Juan Fernandez Buzzard (B. p. exsul), although it may well be elevated to species rank with further data.