Bell's Vireo (Vireo bellii), at Campbell Cove on 4 October 2015. This is the first record for this species in Sonoma County.
Gene Hunn spotted this bird on 2 October 2015, so I felt lucky that it was still there today.
Bell's Vireos breed in the central and southwestern U.S. and winter in central/southern Mexico and in Baja California. They're rare in northern California. This individual appears to be the eastern subspecies (Vireo bellii bellii), which is even rarer in California. This subspecies is greener above, yellower below, with shorter wings and shorter tail.
The next picture shows the green coloration above. Although there isn't much in the picture for scale, note that this vireo is small — ~4.75 inches (120 mm) long. [The Birds of North America account mentions that Bell's Vireos are sometimes called "greenlets" — probably due to their greenish coloration and small size.]
Note also the two wings bars, with the lower wing bar being more obvious and the upper wing bar being very faint.
There is a narrow white eye ring and a pale stripe above the lores (between the eye and the bill), giving the appearance of "spectacles" from certain angles:
This bird was very yellow below...had thick, gray legs and feet (in contrast to a kinglet)...and was very actively feeding low in shrubs — primarily Coyote Brush (Baccharis pilularis) and California Wax Myrtle (Morella californica).
What a nice way to be welcomed back to California!