Red-eyed Vireo (Vireo olivaceus) - HBW 15, p. 430

Taxonomy: Muscicapa olivacea Linnaeus, 1766, North America = (South) Carolina, USA.
Probably forms a superspecies with V. gracilirostris, V. flavoviridis, V. altiloquus and V. magister. Often treated as conspecific with V. gracilirostris, but latter appears distinct; sometimes regarded as conspecific with V. flavoviridis, but biochemical studies coupled with morphological and vocal differences strongly indicate that the two are distinct species. Neotropical races treated by some authors as representing a distinct species, under name of "V. chivi", although this apparently diverged from nominate only c. 370,000 years ago (whereas V. flavoviridis and present species diverged from each other c. 4-6 million years ago); it is hypothesized that the "chivi group" of races arose from wintering individuals of nominate race from North America that failed to migrate back N in spring, and in a situation where migration is largely unnecessary, and consequent gene-pool mixing of different populations is reduced, further subspeciation of Neotropical populations is likely. Additional work required on some resident South American races, which appear to have distinct vocalizations, before the matter can be resolved. One report of hybridization with V. philadelphicus in SE Canada (Quebec). Ten subspecies recognized.