Why a woodpecker's beak doesn't get stuck in a tree like a nail

When you hammer a nail into wood, it's usually difficult to remove it. But woodpeckers hammer away at trees all day long. How can they so easily pull their beaks out of the holes they create? University of Antwerp biologists and their colleagues used high-speed video of black woodpeckers to solve the mystery. Turns out the trick is moving their upper and lower beaks in opposite directions. From Science:

Once the tip of the woodpecker's bill hits the wood, the bird's head rotates to the side ever so slightly, lifting the top part of the beak and twisting it a bit in the other direction, the videos reveal. This pull opens the bill a tiny amount and creates free space between the beak tip and the wood at the bottom of the punctured hole, so the bird can then easily retract its beak.

"How woodpeckers manage to retract their beak quickly after it got stuck in wood" (Society for Integrative and Comparative Biology)