Systematics

Revised by Louis Bevier

The nomenclatural history of Passerina cyanea involves the suppression of an earlier use of the species name cyanea . Linneaus described the Indigo Bunting as Tanagra cyanea in the 12th edition of Systema Naturae (1766). Earlier, in the 10th edition of Systema Naturae (1758), Linneaus described the South American Ultramarine Grosbeak, as Loxia cyanea, this being the first use of the specific name cyanea for the bunting-grosbeak complex. The international Commission on Zoological Nomenclature (ICZN 1979) suppressed the name Loxia cyanea and established the later name Fringilla brissonii for the South American bird. As applicant for the conservation of the name Tanagra cyanea Linneaus 1766, Eisenmann wrote, "The purpose of our application was to avoid the confusion that would have resulted from the transfer of the name Passerina cyanea (Linneaus, 1766) of the common North American Indigo Bunting, to the South American Blue Grosbeak, Cyanocompsa cyanea (Linneaus 1758) as a result of the generic merger proposed by Paynter" (IUCN 1979). Indeed, Indigo Buntings once were known as Cyanocompsa cyanea . In anticipation of ICZN action, Paynter (1968) combined the genus Cyanocompsa Cabanis 1861 into his more broadly conceived genus Passerina Vieillot 1816. Paynter, Eisenmann and Vaurie applied to ICZN to suppress the earlier cyanea Linneaus 1758, as without this ruling there would have been two species named " cyanea " in the same genus. On the IUCN committee, neotropical ornithologists who favored straight priority were outnumbered by the North American ornithologists and their northern European colleagues, as reflected by the split decision by IUCN. Earlier, the assignment of cyanea Linneaus 1766 as the type species of the genus Passerina Vieillot 1816 was a result of subsequent designation(G.R. Gray, 1840, List of the Genera of Birds) of Buffon's "Le Ministre" to refer to Indigo Bunting (Paynter 1968). Although the two bunting species are now again recognized in two different genera (Dickinson 2003), because of ICZN's formal suppression of Fringilla cyanea Linneaus 1758, the latter species cannot now be recognized by its earlier name and is now known as Cyanocompsa brissonii . Otherwise, Indigo Buntings might be known as Passerina cyanella (Sparrman, 1787).

No Geographic variation in size or plumage; no subspecies recognized.

Indigo Bunting and Lazuli Bunting differ in allozyme frequencies in areas of allopatry, but not in areas of sympatry and hybridization (Baker and Johnson 1998). In morphological features the Indigo Buntings are most similar to Lazuli Buntings in certain skeletal characters but not in others (Hellack 1976, Hellack and Schnell 1977, Tamplin et al. 1993 . In mtDNA estimates of species phylogeny in bunting genus Passerina, Indigo Bunting was thought to be basal to other species of buntings (including Blue Grosbeak P. caerulea) and possibly not the sister species of Lazuli Bunting P. amoena). That is, the other six buntings shared a common ancestry with P. cyanea, but each of the six had a more recent common ancestry with another species; specifically Lazuli Bunting P. amonea has a more recent common ancestry with Blue Grosbeak P. (Guiraca) caerulea than with P. cyanea (Klicka et al. 2001). According to this estimate, the two hybridizing bunting species are not each other's closest relatives. Successful hybridization in this case might indicate the behavioral and developmental characteristics which allow interbreeding were retained from a remote common ancestor, and not from traits that uniquely evolved from an immediate common ancestor of the two species of small blue buntings P. cyanea and P. amoena . However, the songs of these two small species are similar and are unlike the other buntings and the grosbeak (Thompson 1968, Ingold 1993), the females are nearly identical in morphology, and the two small blue buntings are very similar in behavior. In addition, mitochondrial genes are inherited differently and under different regimes of natural selection than are nuclear genes. Until the nucleotide sequences of nuclear genes are compared among the buntings, and unless phylogenetic estimate based on a number of nuclear genes turns out to parallel the tree of bunting species that was indicated by the mitochondrial gene, P. cyanea and P. amoena are best considered each other's closest relatives. Preliminary analyses of nucleotide sequence data from multiple nuclear genes suggest P. cyanea and P. amoena are more closely related to each other than either is to P. caerulea (M. Carling and R. Brumfield, pers comm.); however the phylogenetic placement of P. cyanea and P. amoena within the genus is still unclear. Bunting genus Passerina is thought to be closely related to Neotropical blue-plumaged bunting-and-grosbeak genera Cyanocompsa and Cyanoloxia, and the blue "seedeater" genus of Amaurospiza .

Interbreeds with Lazuli Bunting (P. amoena), hybridizing where their ranges overlap in Great Plains, Utah, Oregon, California (Sibley and Short 1959, Emlen et al. 1975, Kroodsma 1975, Garrett and Dunn 1981, Beedy and Granholm 1985, Baker and Johnson 1998, Marshall et al. 2003). In range of species overlap in e. North Dakota, no hybridization; further west where Lazuli Buntings are more common, some non-hybrid Indigo Buntings occur. In the western states, males breed with female Lazuli Buntings (Garrett and Dunn 1981, Robertson and Tenney 1993); the reciprocal cross may be under-reported because males are more distinctive in plumage than females.

Because these two buntings sometimes interbreed and hybridize, Phillips (Phillips et al. 1964), combined P. Cyanea and P. amoena into a single species, "Common Bunting," with two recognized subspecies of P. cyanea . Indigo buntings occasionally hybridize with Painted Buntings P. ciris (an adult male spring migrant in Florida, Taylor 1974; a wintering adult male in Cuba, Julie Craves, 2004, photograph and pers. comm.). These male cyanea x ciris have a blue head and breast, green back, and pale rump and belly with some red plumage. Molecular genetic confimation of hybridization of Indigo Buntings with Lazuli Buntings and Painted Buntings remains to be determined.