The majority of birds are active during the day, but there are those who don’t actually use the night-time to sleep, such as owls, herons, and some aptly named nighthawks. These are called nocturnal and will find somewhere during the day to rest and catch up on sleep but are active at dusk and well into the night, winding down again as dawn approaches.
It’s a common misconception that birds return to their nests to sleep.
All other birds are diurnal, like us – awake during the day and trying to sleep when it is dark at night.
Apart from the usual ducks snoozing at the water’s edge, or the occasional owl roosting in the tree branches, it is not very common to see a bird sleeping as they usually won’t do so out in the open and will find somewhere very secretive, as safe as they can get away from any bad weather and of course predators.
Ground dwellers like partridges or kiwis will also try to find somewhere up off the floor in some shrub branches or inside a rocky crevice. It’s a common misconception that birds return to their nests to sleep; in fact, once the young have fledged the nests are usually in a sad state of repair, covered in dropped food or faeces, and may even be infested with mites.
They are normally abandoned and a new one will be built for the next breeding season. For those of you who put out nesting boxes the same will apply, although sometimes during the colder winter months birds will go back and check out of curiosity. If you’ve given it a good clean out, they may just choose that as their bedroom if you’re lucky.
Aside from hiding away in thick foliage, canny crevices or holes in trees or sandy cliffs, birds also employ the safety in numbers tactic; starlings and wagtails gather in huge night-time roosts, flocking in their thousands together at night.
This not only means there are more eyes on the lookout for predators, but also ups the chances of survival if an attack happens. Roosting together also generates more heat for those cold nights.
There are a few in the world, notably hummingbirds, who also go into a torpor – winding down their sugar saturated metabolisms to a much slower rate. This lowers their body temperatures and is perfect for snoozing through the cold nights.
They rely on a very sophisticated early warning system to get their zzzs in.
The one-eyed bird is king
But what about the actual mechanisms of sleep? Do they just close their eyes like us and dream?
We humans can exert some kind of ownership over our sleep, but only externally like setting alarm clocks or locking out cats that want feeding. Birds actually do it with their brains,using their eyes as their alarm systems.
They can be fully asleep with both eyes closed if they consider their environment safe enough, undergoing bilateral sleep like you and I, but most of the time they rely on a very sophisticated early warning system to get their zzzs in.
Birds are the only animals who can control waking up.
Consider the mallards at the river’s edge. These ducks often sleep out in the open, again in larger numbers for security, but their choice of openness as opposed to hiding in bushes is their way of ensuring they can see oncoming danger from far away.
But how, if they are asleep? Well, whilst it looks like their heads are buried under their wings, if you look closer, you’ll see that their beaks are hidden with a small portion of their smooth heads showing.
Look closer still and you should be able to make out one beady little eye, which will be anywhere between fully and just a tiny bit open. This is how all birds regulate their sleep, by utilising their eyelids and allowing a certain amount of light in.
They are able to shut down one side of their brain, by completely shutting one eye, whilst the other side ticks over on standby, via a small window open to the outside world. Studies have shown that the more the eye is closed, the more “fast asleep” the bird truly is.
This half-asleep / half-awake status is called unihemispheric slow-wave sleep (USWS) and is a marvellous adaptation. It is still undergoing extensive research by us mere mortal humans, but we can see from studies monitoring the electrical pulses of a brain undergoing USWS, the asleep half of the brain is in deep, deep sleep, with no REM - rapid eye movement.
REM is what happens when you are dreaming, that limbo-land that is neither deep or light sleep. The other half of the brain exhibits degrees of wakefulness; we can probably conclude that birds don’t dream. They can fly already, so that’s OK.
USWS is so effectively employed that birds like swifts and martins who are constantly active during the summer feeding, raising young and then off on their long migratory flights, manage to sleep for a few seconds at a time whilst in the air throughout much of their lives.
We also know that animals such as dolphins and other cetaceans also use this method, but issues like freely drifting into peril underwater and cold temperatures messing with thermoregulation inevitably provokes these mammals to full wakefulness.
Birds are the only animals who can control waking up with their eyes if danger is detected. When they sleep in a formation like the mallards and the large flocks, a method called “group-edge effect” takes place: all of those on the edges keep their outermost eyes open whilst resting the inner-side half of the brain, and all innermost birds sleep the sleep of kings.
This ability to control sleep developed in line with their physiology – special feathers and complex muscles in their legs ruled out the prospect of external stimuli like trying to keep warm or falling over startling them awake.
Getting comfortable is paramount, and birds are able to do this because of the way their muscles and tendons are arranged in their legs. You’d think standing up and falling asleep is impossible, but we see birds doing this all the time.
That’s because when the bird relaxes and its body weight is drawn down by gravity, the leg muscles push the tendons of the feet (called flexor tendons) tight, closing the toes in a kind of vice around whatever the perch is. ž
This is the same method by which bats and some birds sleep upside down, although of course this time gravity pulls the tendons closed rather than pushes.
Birds also stay very warm when the temperatures drop at night using their down feathers, the dense, soft underlayer beneath their flight and primary feathers.
This layer is so thick that it protects all of the unfeathered parts like the legs, feet and beaks sufficiently enough to withstand very cold temperatures, with the added bonus that with the beak nicely tucked inside, the nostrils will propel that warmed air around the inside of their bodies too, in one highly efficient action known as unidirectional flow.
Despite the reduced overall sleep birds will experience using USWS, there has been no evidence of any detrimental effects. It can be said that nothing beats a good night’s sleep, and they will sleep more efficiently in safe conditions in Cloud Cuckoo Land with both sides of their brains shut down, but if they are in a potentially more dangerous environment, which birds usually are, the positives of this half asleep and half alert state of being far outweighs the alternative of no state of being at all.