WHY don't birds fall when sleeping on a branch?

 While humans are able to fall asleep when their muscles are fully relaxed, birds need to tense their leg muscles in order to fall asleep. These muscles help birds maintain their balance on a branch, and are connected to the fingers by long tendons. When a bird settles down to sleep, the muscles contract, the tendons tighten, and the fingers clench, causing the toes to grip the branch for support. As a result, birds cannot straighten their legs while asleep.