What with all that pecking, why aren't most
woodpeckers
retards?
Apparently, this was the subject of neurological research in the 1970s. Both The Lancet (1976) and Archives of Neurology (1979) published studies on this very subject.
Here's the rundown:
In addition to keeping it's eyes closed and striking the exact same point everytime to avoid rotational damage, "shock waves are transmitted less readily in the woodpecker's head than in a human's because the former has a narrow space between the skull and the brain, with very little fluid, and the woodpecker's brain is packed tightly by dense yet spongy bone which buffers the force to the brain. Additionally, some of the muscles in the woodpecker's head contract, which helps to absorb and distribute the shock. Structures from the base of the tongue extend round the brain and may also absorb shocks."
Whew!