Why Doesn’t a Sleeping Bird Fall off Its Perch?
A sleeping bird cannot fall off a twig or branch because its feet are firmly locked in place. The feet of passerines e.g. sparrows, wrens, warblers, thrushes, to name a few – are adapted for grasping the twigs and similar objects on which they perch.
The toes on a bird’s feet are connected to cord like tendons inside its legs. Two thin tendons, called flexor tendons, extend from the leg muscles down the back of the tarsus bone and attach to the toes.
When a bird settles down to sleep on its perch, its body slumps down on its feet. When the bird’s legs are bent in this position the tendons automatically stretch tight and clamp the toes around the perch in a vise-like grip. As a result, the feet are securely locked to the perch and even in sleep the bird cannot lose its grip.
Its toes cannot be unclamped until the bird awakens and stands up to fly away. As the bird stands up, it jumps up, its legs straighten, the tendons relax and the toes unlock to release the feet. Falling asleep doesn’t change the grip, as the weight of the bird keeps the leg in the locked position.