Done My Hair Up Big

The red-bellied woodpecker on the left is defending it's feeding territory, and it's "hair" is standing on end. Head feathers are normally flat like those of the woodpecker on the right. Except for photos or slow motion video we may not see signs of aggressive behavior such as head feathers standing on end.

One can also see the red blush on the belly of the bird on the left, which is why the species is called red-bellied.