|The Cassin's Vireo winters in Mexico and Central America and breed in the western United States and Canada. Cassin's Vireo have a drab olive green plumage with white wingbars and prominent "spectacles" around the eyes. If you look for them in an old field guide, they were formerly known as the Solitary Vireo. Solitary Vireo was split by the American Ornithological Union into the Cassin's Vireo inhabiting the Pacific States, the Plumbeous Vireo of the Great Basin and Southwest and the Blue-headed Vireo living east of the Rocky Mountains.
Around the state, Cassin's Vireo inhabits mainly dry coniferous forests. It is interesting to note that it used to like the oak prairies that were once much more common in the southern Puget Sound region. On Vashon it's generally found along riparian areas in mixed woods where conifers form the dominant part of the mix. When the island was originally cleared of coniferous forests, the Cassin's population would have taken a significant hit, as much as 25-30%. Their ability to survive in stands other than pure conifers and the regrowth of some stands even as others were cut are possible reasons for their survival in earlier times on the islands.
Likely places to check for Cassin's Vireos include the Island Center Wetlands with their stand of cottonwoods and lodgepole pine as well as the forests of southern Vashon and Maury Island. Rayna and Jay Holtz host them most years in their forested property off Cove Road. Dan Willsie found a nest in Paradise Valley for the sole breeding record but they likely breed each year. The first one I noticed this year appeared along the ridge north of the horse park at Paradise Ridge.