The perching birds use a locking system to stay on their perches.
Tendons pass from the muscle at the back of the bird’s leg,
down round the back of its ankle and to the inside of its toes.
When a bird settles it weight on a branch, its legs bend.
The tendons are tightened and the toes automatically grip the perch tightly.
Even when the bird falls asleep, there is no danger of its falling off the perch.
As soon as the bird straightens its legs or jumps from the perch,
the tendon relaxes and the grip of the toes is released.
Amazing Animal Kingdom 