Dr Karl › Dr Karl's Great Moments In Science

How do birds sleep?

Nests are for babies - getting a bit of shut eye without falling off the perch or being eaten takes some very special skills. Dr Karl puts a cosy little myth to rest.

Birds, those flying dinosaurs, are all around us. We hear and see them while they're awake, but how do birds sleep?

Most of us have the image that after a hard day of chasing worms and warbling beautiful songs, our bird will retire to their nest. They will pull up a cosy little blanket — maybe a flexible leaf — fluff up their feathers to generate some air insulation for the night and then (very cutely) burrow their head into the feathers around their neck and sleep peacefully until the next morning.

Nope. That's a myth. Birds hardly ever sleep in nests unless they're babies or if it's a cold night and the adult parents cuddle up to the babies to keep them warm.

Nests are for chicks to hatch and grow up in. By the end of the season the nests full of bird poo, spilled food, and sometimes tragically, a dead chick, are simply abandoned.

For birds, sleep is all about safety and warmth.

If they are a smallish songbird they won't sleep on the ground because a cat will get them, and they won't sleep on an exposed branch because an owl will get them. So they hide, beautifully camouflaged inside dense brush or behind foliage.

Waterfowl such as ducks and geese can't sleep in trees because of their webbed feet. They're too slow and clumsy at taking off to safely sleep on the ground, so they might sleep on a small island, or just on the water. They don't have to worry about hawks and eagles because these predators also sleep at night. The waterfowl can also sense vibrations in the water, emitted by predators swimming towards them.

But many birds have another trick — they can switch off half their brain. We humans have two separate brains or hemispheres in our skull joined only by a tiny bundle of nerves called the corpus callosum. Each of our eyes sends information to each brain. But birds do it differently. The information from one eye goes only the brain on the opposite side. So the left eye sends information only to the right brain and vice versa.

So to get a safe night's sleep, birds can close one eye and switch off one brain, and leave the other eye with the corresponding brain fully awake and alert. In fact, they can turn this on and off depending upon the circumstances. So in a large flock of geese roosting on a lake, the geese in the centre of the flock might have both brains asleep while the more vulnerable birds on the perimeter might have one eye open with its corresponding brain alert to look for predators.

Bigger waterfowl might roost in the shallows, again relying on vibrations in the water to warn them of an attack.

Birds such as quail and grouse are unfortunately not very good at flying and are also fat and yummy, so virtually every creature in the animal kingdom wants to eat them. They go for the densest camouflage they can find. But when there's no vegetation, only snow, then birds such as the white-tailed ptarmigan rely on their whiteness to merge them invisibly into the snow.

Other birds such as crows, swallows, swifts and starlings all share amazing communal roosting behaviours. Some of them gather in enormous flocks, usually at dusk, to seek safety in numbers.

But of course, no birds bother the top predators such as owls, hawks and eagles — they're pretty safe as long as they stay off the ground.

But the majority of birds are perching birds from the order passeriformes — sparrows, jays, warblers, cardinals and so. Around dusk, they find a branch, grab it with their clawed feet and squat down. These 'passerines' have special flexor tendons in their legs that automatically tighten onto the branch. So long as their legs are bent they are physically locked onto the branch — and this action takes no muscular work at all.

And so, they can sleep peacefully throughout the night — resting their bird brains in readiness so they can again spread their wings.

^ to top