I think the Cornell Website says it all:
Reminiscent of a troupe of wide-eyed clowns, Acorn Woodpeckers live in large groups in western oak woodlands. Their social lives are endlessly fascinating: they store thousands of acorns each year by jamming them into specially made holes in trees. A group member is always on alert to guard the hoard from thieves, while others race through the trees giving parrotlike waka-waka calls. Their breeding behavior is equally complicated, with multiple males and females combining efforts to raise young in a single nest.
These were found on Lone Tree Road in Hollister, CA.