Costa's and Anna's females seem to look the practically identical--both are
stubby, short-winged and green-backed; both may show white behind the eye,
both may have grayish undersides, but the throat patch is supposedly
diagnostic for Anna's and there seems to be a faint greenish area on her
side below the wing (which is also supposedly diagnostic for Anna's and not
found in Costa's females). Both species can have the white feathers showing
above the on the shoulder above the wing (see the Cornell photo of the male
in B.C.). Since color isn't reliable in a bird whose colors change,
depending on the light and its whims, and since the throat patch is supposed
to be diagnostic and the fact that we're in Anna's range and not in Costa's
range and there are hundreds of Anna's around right now, I would say Chris's
photo is an Anna's.

However, the Cornell page shows a Costa's female in Baja California that
clearly has a dark throat patch and green-tinged sides, so maybe females
can't be identified by coloration alone. It's entirely possible that they
are indistinguishable in appearance.

Allison