Answer:

The white eye ring and wingbar patterns, plus shape tell us that it is some kind of vireo. The possible vireos in this area are Bell’s, Gray, Cassin’s, and Warbling.

The dark gray color eliminates Warbling and Bell’s. That leaves Gray, Cassin’s, and Plumbeous. On Gray Vireos, the dark area of the head doesn’t get close to the throat, but becomes lighter. However, we can see that the dark extends much farther down, and continues until just before the bill. So it is either Plumbeous or Cassin’s. Note that Plumbeous’s belly is light gray, whereas this bird’s belly is white. Cassin’s Vireos have a yellow wash on the undertail coverts, which is present. This plumage variance is only found in Cassin’s Vireos. (Also, Cassin’s’ wings are partially green, and Plumbeous’s are grey, but they are blocked by a stick in this photo).

This one is tricky. Congratulations to Riley and Ben for figuring out Cassin’s Vireo!