Despite its abundance and wide geographic range, the Philadelphia Vireo remains one of North America's most obscure birds. At least one reason for its obscurity relates to the overwhelming abundance of the larger Red-eyed Vireo (Vireo olivaceus), which resembles the Philadelphia Vireo in plumage and more strongly in voice. As a result of this similarity, the Philadelphia Vireo's presence in a forest often goes undetected even by experienced observers. Many aspects of this species' behavior are strongly influenced by its larger congener; interactions between the two species have been the subject of most recent studies of the Philadelphia Vireo.
Philadelphia and Red-eyed vireos can coexist in the same places because the Philadelphia Vireo modifies its behavior either to exclude the Red-eyed Vireo from its territory or to avoid it by foraging in areas seldom used by that species. Interactions between these two species provide one of the best available examples of adaptive interspecific territoriality.
The Philadelphia Vireo has the most northerly breeding range of all the vireos and winters in southern Central America. Throughout its range, it is primarily a bird of early- to mid-successional vegetation and, like most vireos, it feeds chiefly on caterpillars gleaned from leaves. It strongly prefers to forage in successional tree species, and its foraging tactics may be better adapted to early-successional than to late-successional forests.
This vireo was described and named by John Cassin ( Cassin 1851 ) from a specimen collected in Philadelphia during migration-hence its name, despite the species' rarity there. A nineteenth-century local name for this species was Brotherly-love Vireo ( Park 1880 ).