Yellow-rumped
Warbler, Dendroica
coronata
Yellow-rumped Warbler in North America has two subspecies, which were once (and may soon again be) considered separate species. The first two pictures show breeding plumage males of both subspecies, Audubon's above, the more common one in the west, and Myrtle below, much more common in the east. The best-known distinction between them is the throat color, yellow in Audubon's, white in Myrtle. However this distinction is not diagnostic, as some Audubon's have white throats. Further distinctions are the distinct eyebrow (supercilium) of the Myrtle, and the shape of the throat patch, which wraps around under the cheek patch (auriculars) in the Myrtle, while in the Audubon's it is confined to the throat. Females, non-breeding, and especially first-winter birds are less bright and contrasty than breeding males in both subspecies. This is the most common warbler in winter in the Bay Area.
Two more breeding plumage males, Myrtle above in Alaska, Audobon's below in Arizona.
The yellow of the Audubon's throat patch can be variably pale in winter, as in the two photos above, and can, as below, be completely absent. The absence of any eyebrow and the confined size of the throat patch identify the bird below as Audubon's.
The white throat patch sweeps around under the cheek patch, and an eyebrow is present, though somewhat indistinct, marking this bird as a non-breeding plumage Myrtle.
This bird is leucistic, lacking much of its feather pigment, but the yellow in the throat is enough to identify it as Audubon's; note also the complete absence of any eyebrow line, apart from the double-half-moon eyering that is common to both subspecies.