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Why don't birds fall off their perches when they sleep?

Sent in by Alan Thompson, Wales

While human feet are pretty much the same from one person to the next, birds' feet can be quite specialized. There are webbed feet for swimming and the talons of a bird of prey for example.

Passerines, or perching birds, stand on their toes, not the flats of their feet as humans do. Depending on the species, birds sleep while standing or sitting with their feet locked on perches, or cling by their claws to tree-trunks.

Different feet for different needs

The average bird foot has four toes, and typically the first toe (the hallux) is turned backward, while the other three toes face forward. This toe arrangement is known as anisodactyly.

Swifts, which may rest hanging on vertical walls, have all their toes turned forward (pamprodactyl), or can turn them forward when they want.

Woodpeckers have their toes arranged in pairs (zygodactyl), the second and third toes in front, the fourth and hallux behind - this enables them to walk up and down tree trunks with ease.

Birds' feet and toes are mostly tough tendons and bones, covered with heavily scaled skin. There is a limited supply of nerves, blood vessels and muscles and this is what enables them to land on cold metal perches when temperatures drop. When passerines roost, their belly feathers cover their feet to keep them warm. If the weather is especially cold, many passerines will squat to cover their feet as they eat.

A matter of life and death

Going to sleep is a serious business for birds and often a matter of life or death. The roost selected is usually sheltered from the wind, as dry as possible, and out of reach of any predators.

The feet of passerines e.g. sparrows, wrens, warblers, thrushes, to name a few - can do almost anything, from walking to hopping and holding onto nearly any object. They are beautifully adapted for grasping the twigs and similar objects on which they perch.

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 lands on a perch, these tendons tighten and so the toes lock around the perch. This involuntary reflex keeps a sleeping bird from falling off a perch. The tendons stay tight until the legs straighten.

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.

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