When a tree becomes ill, insects move in and start eating the tree. These insects are a major food for woodpeckers, indeed, they are the reason for the common name associated with this group of birds. They don't just eat insects, they also eat acorns. So do squirrels eat acorns. Hence the problem: how can woodpeckers store acorns so squirrels cannot get them?
The answer is something called a "Granary Tree". The bird finds a rotting section of tree, branch, or even exposed root, and pecks a hole into it just large enough to accept an acorn. Then, in the Fall, fill the holes with one acorn each, with the big, blunt, hard to grab end of the acorn facing out. Paws are not going to really help getting this slippery rounded thing out of a hole. Hence acorns stored like this tend to not be obtainable by squirrels. Woodpeckers just peck out the blunt end and eat the seed when they need it.
Here is one example from Rocklin, California earlier today:
That doesn't mean the granary trees don't get guarded... My son seems to have flushed a squirrel into a tree and in fact into a granary area, resulting in the squirrel hunkering down and enduring a dive-bombing and pecking by the woodpecker so calmly seen in the photo above. After a minute or two, I had to remove my son from the area to allow the squirrel to escape, as the woodpecker seemed to have attracted reinforcements that were joining the fracas.
Here is the woodpecker beginning to chase the squirrel:
Followed by squirrel hair collection activities by the same bird:
Food is serious business.
My son summarizes this much more succinctly: "The woodpecker is attacking the squirrel and then it is trying to knock the squirrel off the tree. And if the squirrel is knocked off the tree, maybe I can catch it [Editor/Dad's note - nope!]. And that is my blog post."