Bushtit

I am still convinced that it is a Wrentit: 1. The position of the tail changes as the bird moves around and will only be held up some of the time. 2. The color of a bird's iris is very difficult to see or photograph and requires the right angle and lighting. In this case since the bird is facing away the color of the iris cannot be seen. 3. Another indication that it is a Wrentit is the slightly droopy aspect of the skin above the eye. 4. Boreal Chickadee can be ruled out because the bird does not have the black chin or gray cap of a Boreal Chickadee. Furthermore, Boreal Chickadees do not normally occur in Southern California, their range includes most of Canada and only occur in the lower 48 states in certain northern areas like the Adirondacks of New York, northern Wisconsin, Montana, Idaho etc.

The tail is also too long for a warbler. I agree with wrentit.

The bill is too short to be a warbler. It looks like a Wrentit (Chamaea fasciata) to me. Notice the large head, short heavy bill like a titmouse, and long tail. It has the grayish color of the southern populations.

I would hazard a guess of Warbler subsp., but which one, no idea...