Question: How do perching birds lock their feet on branches to keep from falling off when asleep?
Answer: It's automatic and requires no conscious action by the bird, awake or asleep.
Tendons pass from the muscle at the back of the bird's leg, down around the back of its ankle, to the inside of its toes. When a bird settles its weight on a branch, the legs bend. That tightens the tendons so much that the toes are automatically pulled tight and clamped around the perch. All conscious or controllable actions are bypassed, so even when it's asleep, the bird cannot possibly move from the perch.
When the bird straightens its legs, the tendons automatically relax, and the grip is released.