Another consideration is rotational sheer forces.
A hummingbird can absorb 300g’s of impact when pecking a tree because it keeps its head and brain in a perfectly straight line while in motion. Conversely, an elephant has no neck and must turn it’s entire body to face a threat. The larger the brain case, the slower the rotation speed allowed as the
brain cannot be allowed to actually rotate within the case.
If the rotational speed equals sheer speed, the animal experiences a black out.
Did you mean woodpecker? I read recently where the woodpecker's brain is encased in a "net" that actually suspends the brain and absorbs the shock.