How does a woodpecker go about smacking it's head into a tree without killing itself?
Various features of brain,skull and beak anatomy help to achieve protection.
A paper was published in PLoSOne in 2011 on this very topic:
There is also a very readable summary on the BBC website. I advise that you read the whole article, but here is a quotation which lists the main findings:
Gibson (2006) identified three characteristics that help woodpeckers avoid brain injury:
Combined with Wang et al.'s findings, maybe that makes six features.