WHY don't birds fall when sleeping on a branch?
Humans can fall asleep when their muscles are as relaxed as possible. In birds, it's the other way around.
For them, sleep is only possible when they tense the leg muscles that help them sit steadily on the branch.
These muscles are connected to the fingers by long tendons. When a bird sits down, the muscles contract, the tendons tighten, and the fingers clench. During sleep, the bird cannot straighten its legs, so the toes do not release their support.
