I know that Bushtits in the southern part of their range (southwest U.S., Mexico, Central America) can have dark masks like this, but I haven't seen one with this coloration in California yet.
Here's the photo:
Have you seen or heard of Bushtits with this type of coloration in California? How common is it? Any information would be greatly appreciated.
P.S. To compare with other photos of Bushtits, here are links to a few older posts: