This is a female with black on the crown. It is the subspecies M. f. bairdi which ranges down the West Coast from Oregon to Baja. It is the largest subspecies. M. f. formicivorus of Arizona and New Mexico is decidedly smaller. My Arizona Acorn Woodpecker photo shows a small red spot on the chest which we rarely see in California. This character is a color morph which shows up occasionally in all populations, but which is most often seen in birds from the tropics of Central America. Acorn Woodpeckers have one of the most bizarre mating systems of any bird in the world. It is called Polygynandry.
Olympus D-550z / Nikon FieldScope 3 / 30XWA (hand-held - no adapter).