This is apparently a variant Slate-colored type Junco showing whitish wing-bars. It resembles the subspecies J. h. aikeni (accidental in California), but this individual is not pale or large enough for that race. Slate-colored Junco (Hyemalis group) rarely acquires white wing-bars (about 3% of individuals). Slate-colored Junco is an uncommon winter visitor to California, so having this variant here is exceptional. The brownish back suggests a female. Nikon P510 Coolpix.
Reference:
Nolan Jr., V., E. D. Ketterson, D. A. Cristol, C. M. Rogers, E. D. Clotfelter, R. C. Titus, S. J. Schoech, and E. Snajdr (2020). Dark-eyed Junco (''Junco hyemalis''), version 1.0. In Birds of the World (A. F. Poole and F. B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.daejun.01