Your liberal arts education continues. How is it that a bird can sleep while perched on a branch, yet not fall off the limb? It comes down to the mechanics of their talons, or claws:
The avian talon works through a “pulley system of tendons,” according to the animal morphology blog Ars Anatomica, and it can lock into place. “The bird’s foot closes and grasps automatically as the ankle and knee joints are bent,” we read. “This grasp cannot be released until the limb is straightened again.” So, instead of expending precious energy holding the muscles tight – as you would if you were hanging onto a branch with your fists/arms – the system simply physically locks in place.