How do woodpeckers avoid brain damage?
Recent research using high-speed films of acorn woodpeckers (Melanerpes formicivorus) shows that their heads can endure up to 1200 gravities of force when the beak hits the wood. Despite this force, their brains do not suffer damage.
The woodpecker's brain is tightly packed into a capsule of dense but spongy bone tissue, which absorbs some of the force of sudden shocks to the outside of the brain case. There is very little fluid surrounding the brain, so it is not free to jerk around. In addition, there are special muscles in the woodpecker's head that contract at just the right moment to absorb still more of the shock. There are also support structures that pass around the back of the skull, starting near the base of the tongue.
The woodpecker's brain is not the only part of the bird's head that must be protected. If it didn't close its eyes just before each peck, they would fly clear out of their sockets.