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 Cassin's Vireo is similar in appearance (including the white spectacles) and behavior to the Plumbeous Vireo but with more green on the back and with yellow flanks. These features are hard to spot given these views, but present I think on some images. Also, the Cassin's Vireo is a migrant in Summer and more common in fall in the valley. These images were captured at Sweetwater Wetlands in November of 2015, most consistent with their fall presence in Tucson.