Vireos are passerines (songbirds) comprising 15 species in 1 genus, all in the family Vireonidae. They are small stocky birds with large hooked bills and short strong legs. They are drab in appearance with subtle markings. For a good review of this family, see David Allen Sibley, The Sibley Guide to Birds, 2000, Knopf, p. 340. They are a New World family, but include the shrike-babblers and erpornis of SE Asia (see Lovette and Fitzpatrick, The Cornell Lab of Ornithology Handbook of Bird Biology, Third Edition, p. 53). For a concise on-line reference see This Wikipedia page.
The Plumbeous Vireo is lead-gray (name was a tip-off!) with bold white spectacles. The bird winters in AZ, New Mexico and Mexico, and summers as far north as Utah, Colorado and Nevada, all west of the plains states. In SE Arizona it summers above 3500 feet, wintering in lower elevations.
These two images were captured in Summerhaven toward Marshall Gulch in August of 2015.