I looked through the Stokes Field Guide to Birds, Western Region. I got the western region book to take to Colorado when we go there. The closest match I could come up with was the Dickcissel (Spiza americana
). The dickcissel had the band above the eye and same beak shape as your bird, but its range is not listed as being as far west as California. The dickcissel common name comes from the sound of its song.
I couldn't find a picture exactly like yours. Some reasons why include different colors and appearances in breeding season or in juveniles. Some birds have different "races" based on location. Here in western PA, for example, the junco is a gray and white bird, called the "slate-colored" race. In Colorado, it is a brown and gray bird, called the "gray-headed" race.
One time I saw a bird native to Texas in my backyard! Sometimes they do fly outside of their home range.
Spiza americana: http://www.mbr-pwrc.usgs.gov/id/framlst/i6040id.html