Orange-crowned Warbler, Oreothlyps celata

Orange-crowned Warbler, 10/10/10, Sunnyvale Baylands Park
The picture above and the two below show bright males of the Pacific Coast subspecies lutescens. The (often faint) dark eye-line through a broken eye-ring is a good mark to distinguish this species from dull Yellow and Wilson's Warblers, which can be similar in color to Orange-crowned.

Orange-crowned Warbler, lutescens male  singing, 4/23/10, Picchetti Ranch

Orange-crowned Warbler, lutescens male, 4/23/09, Stevens Creek Park

Orange-crowned Warbler, orestera female, 9/30/09, Palo Alto Baylands
The interior west orestera subspecies, duller in hue than lutescens, is regular but
uncommon in the Bay Area in migration; the still-duller eastern celata is much
less likely. In general, females and immatures are duller than males, basic plumage
is duller than alternate, and birds are duller as they nest further east. So the dullest
Orange-crowned Warbler is a fall immature female celata, the brightest a spring adult
male lutescens. Shades vary in brightness from gray through olive-green to bright yellow.

Orange-crowned Warbler, lutescens, 10/7/06, Guadalupe River south of 85
I suspect this Bay Area bird in fall, with a dull yellow-olive rather than
a solid gray head, is a drab lutescens rather than an adult/male orestera.
I believe the bird in the two pictures below, photographed in South Texas
in early spring, is an orestera, the predominant subspecies found there.

Orange-crowned Warbler, 3/1/11, Ramirez Ranch, Roma, TX

Orange-crowned Warbler, 3/1/11, Ramirez Ranch, Roma, TX