When people fall asleep on the tube they start lolling around, how do birds manage to stay on their perches while sleeping

A.� Birds leg muscles are arranged so that when sleeping their claws grip onto the perch. The flexor tendon, in the bird's thigh muscles, extends down through the leg, around the ankle, right through to their toes.

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So that when the bird is resting and its body relaxes, its knees bend, drawing the tendon in and with it the claws on its feet so that they grip the perch and stop the bird falling.

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Q.� Sounds impressive

A.� Yes, apparently it's so effective that birds sometimes even manage to cling onto their perch even after they've died.

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Q.� But can they keep it up for 8 hours

A.� Not all animals share the same sleeping pattern as us. In fact most of them sleep in short bursts and probably not all of the animals body sleeps at the same time.

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Q.� Which bits stay awake

A.� The expression 'half asleep' probably more accurately describes the sleeping pattern of animals other than ourselves.

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Studies of some birds have revealed that only half of the brain naps at a time while the other half is alert. The sleeping half eye closes while the alert half has a open beady eye on what's going on.

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Q.� Do they always sleep half a brain at a time

A.� The whole brain can rest at once. This is particularly the case with birds that sleep in flocks. Those on the outside keep a look out while those tucked safely away in the middle rest their whole brain at the same time.

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Q.� Sounds like a useful way of avoiding predators; do any other animals do it

A.� So far it's only been noted in birds, whales, dolphins, seals and manatees but researchers suspect that many other animals may practice half sleeping, they just haven't been spotted yet.

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Q.� Do birds sleep at night or during the day

A.� On the whole, during the day, for practical reasons. Birds are very visual animals and need daylight to be able to get around.

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by Lisa Cardy