The Meaning Of Life – Part Forty One Of Forty Two
January 19, 2016
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How do birds sleep?
I must admit a certain fascination with our feathered friends. I admire their beauty, their ability to fly and their song. I’m even corned that our constant invasion of their natural habitat is reducing their numbers to critical levels. But there is much I don’t know about them including when, if and how they sleep.
A common misconception is that they all retire to their cosy nests when the sun goes down, have a bit of a chirrup and then settle down for a good night’s sleep. The principal purpose of a nest is to have somewhere to rear their young and mature birds will only sleep there if it is particularly cold and only to keep their brood warm. If you think about it you can understand why. The nest will pretty soon be full of droppings, undigested food and, perhaps, the carcass of one or more of the chicks that didn’t make it. Not the place you would want to go to to catch up on some zeds.
The principal for a bird that wants to take a nap is that they are a sitting duck for predators. For smaller birds this means that they cannot sleep on the ground as cats or other ground-based hunters will get them and they can’t sleep in branches as owls and hawks will easily get them. So their place of choice for a night’s repose is deep inside a bush which is why you can’t see them.
Some birds’ physical features dictate where they can sleep. Wading birds, ducks and the like have webbed feet which means that they cannot perch on trees or bushes. They are also slow and cumbersome taking off which means that they cannot sleep on the ground safely. Their place of choice is the water and they have evolved sensory powers to detect vibrations in the water to protect them from surprise attacks by predators.
Evolution is a wonderful thing and one of the neatest tricks birds have developed is that they can switch off one half of their brain. Our brain is in two parts conjoined by a bundle of nerves called the corpus callosum. What we see with our left eye is processed by the left side of our brain while the images from the right eye are processed by the right side. For birds it is different. The left eye sends signals to the right side of the brain and vice versa. To get a good night’s sleep they can close one eye and switch off one part of the brain. They can do this at will so if they are sleeping in a large flock, the birds in the centre will sleep with both eyes closed and their brain completely shut off whereas those in the outer reaches of the flock will sleep with one eye closed and one part of the brain switched off.
The other trick that nature has bestowed on those birds which are categorised as passerines, sparrows, warblers and the like, is special flexor tendons. When they settle on a branch the tendons lock automatically on to the branch and provided that they have bent their legs, they are locked into position securely all night.
So now we know!