Well this was somewhat of a hard one, and only a few people got it. The photo is dark, and this dull bird is definitely a confusing one. Let's take a look again.
What do we see? First of all we see a mostly olive-backed bird with a gray hood/head and grayish-olive tertials. This bird is definitely a relatively slender, small-headed passerine with a medium-sized tail. We know the bird is in California, but that doesn't rule out a vagrant. Blue-headed Vireo may come to mind, but they have wing bars (not to mention quite rare in CA in January). They would also show white edging on the tertials, and their heads are usually darker and thus contrast more with the body. Nashville Warbler may also come to mind and is a frequent vagrant to CA, but depending on age/sex they show some to a lot of bright yellow on the underparts. They should be much greener above rather than drab-olive like this individual. They also would be much shorter-tailed. Female Mourning and MacGillivray's Warblers are also gray-headed but would show extensive bright yellow to the underparts.
This really only leaves a few options. To me, from the beginning the bird was either a Tennessee or an Orange-crowned Warbler (but then again, I took the photo). It is relatively slender and small-headed. The tail isn't all that long, but isn't short either. Oreothlypus (formerly Vermivora) warblers like Tennessee and Orange-crowned come right to mind because of the lack of wing-bars, gray head, and the overall drabness. Look closely and see that this bird has some yellow poking out from under the tail (i.e. yellow undertail coverts). Tennessee Warbler never shows this. Even if this yellow tinge is an artifact of the lighting, exposure, etc., Tennessee Warbler's tail is fairly short. This bird is an Orange-crowned Warbler of either the Taiga or Interior West race. If you still don't believe me, look below.
Timothy Schreckengost - PA
Francesca Massarotto - CA
Drew Weber - PA
Mark Dettling - CA
Andrew Longtin - MN
Thanks for playing!