It may come as a surprise to some, but yes, birds do sleep.
Sleep is essential when it comes to avian cognitive development, including early development, in birds.
But how do they sleep?
The sleeping antics of birds differ somewhat to that of a mammal. For starters, most birds sleep in short bursts of no more than a few minutes at a time.
Some species even sleep whilst flying.
Birds, along with aquatic animals, are some of the only animals that are capable of shutting down just half their brain whilst they sleep. This means the other half is still aware of its surroundings, and any potential predators.
This article aims to explore how different groups of birds, from passerines to waterfowl, sleep.
How Do Off-shore Birds Sleep
If I ask you to think of a bird, what comes to mind?
A bluebird on a bush? A robin in a tree?
Typically, birds are associated with some form of plant – be it a bush, a hedgerow, or a tree.
After all, some of the most common birds found in the USA are those that live in and around gardens and suburban parks, close to trees and bushes.
However, there are some species that spend years, even decades, out at sea. Meet the seabirds.
These birds are adapted to life on the wing. They have mastered the art of feeding and drinking whilst flying, eliminating the need to touch land.
But what about sleeping?
Do Seabirds Need To Land To Sleep?
Well, not exactly.
In a recent paper, it was found that frigatebirds, a tropical seabird, can stay in continuous flight for up to two months. In this time, they will stay aloft without coming into contact with land or sea.
However, even if they wanted to, the frigate birds couldn’t take a rest on the ocean, as they lack waterproof feathers. This means any contact with water will result in waterlogged feathers, and the risk of drowning.
So, how do they sleep?
The paper, authored by Rattenborg, goes on to explain that data, collected via small devices attached to the birds, shows that the frigate birds do in fact sleep whilst flying.
However, unlike land birds, which typically sleep for up to 12 hours per day for approximately one minute at a time, frigate birds sleep very little – as little as 45 minutes per day, in small 10 second bursts.
The Science Behind Seabird Sleeping Patterns
The main theory as to why seabirds can sleep so little is to do with their brains. With the frigatebird being the species in question, scientists have identified that seabirds sleep unihemispherically.
Don’t worry, I’m not expecting you to know what that means. These pesky scientists and their unnecessarily complicated terms.
In short, an animal that displays unihemispheric sleeping essentially shuts down half of their brain.
A range of animals, especially aquatic mammals, display this kind of behavior. It is a way of getting some rest, whilst still being relatively aware of the surroundings. You know, in case a predator comes on by whilst you’re trying to take a nap.
Amazingly, the eye connected to the awake part of the brain remains open, allowing birds to still visually navigate whilst sleepflying.
Sleepwalking just got an upgrade.
However, it has even been proposed that offshore birds can get a quick REM sleep cycle in, with both eyes closed! No wonder why they sleep for just 10 seconds at a time. To achieve this, the bird simply needs to maintain a state of aerodynamic soaring or a gliding position to maintain balance.
The same principles can be allied to other long-distance offshore birds. A prime example of this is the wandering albatross, which can fly over 10,000km in a single trip.
How Do Passerine Birds Sleep?
The passerines are perching birds. They are characterized by having four toes, three of which point forward, whilst one points back.
Approximately 60% of all known bird species are classed as passerines. This includes common garden species that you may see daily, such as sparrows, cardinals and finches.
But have you ever wondered what happens to these birds when the sun goes down? Do they even sleep? Afterall, how many of us have seen a sleeping bird?
Like most animals, sleep is essential for the development of birds. Sleep disturbances in birds have been shown to impair cognitive performance.
Passerine Species Also Show Unihemispheric Sleep
In a nifty evolutionary trick, sleeping passerine birds undergo unihemispheric slow-wave sleep, or USWS for short – the same process that allows offshore birds to sleep whilst flying.
Unlike the mammalian sleep cycle, USWS allows the sleeping bird to react quickly from a perceived threat, but still maintains satisfactory rest should no risk arise.
It’s a complicated field of research, with scientists still trying to wrap their heads around the matter. For now, all we need to know is that passerine birds can sleep with half their brain still awake, in bursts or around 1 – 4 minutes.
That’s the cognitive reasoning as to how passerine birds sleep, but what about the physiological side of things?
Do Passerines Get Comfy When Sleeping?
Well, arguably, yes.
Many passerine birds exhibit behaviors that make their feathers fluff up more. This is to better cover their body when they sleep and keep themselves warmer when exposed to the elements. At night, many species undergo thermogenesis, whereby they burn calories to generate heat.
Sometimes, in extreme weather conditions, some passerine birds turn up the notch and experience a form of regulated hypothermia, referred to as nocturnal torpor. This behavior, aimed at preserving body energy, is most often seen in small birds, such as hummingbirds and swifts.
In most cases, however, passerine birds will simply settle down for the night in a hole within a tree, or even in disused nesting boxes. Some species add animal fur or grass to increase insulation.
How Do Passerine Birds Not Fall Out Of Trees When Sleeping?
Well, these little guys don’t get their perching name for nothing.
When a bird puts any form of weight on its feet, muscles within the leg force tendons within their feet to tighten, forcing the foot to remain closed.
This involuntary behavior creates a vice-like grip on any surface the bird comes into contact with. So, even when sleeping, a bird will remain perched without the risk of falling.
How Do Migratory Birds Sleep?
Migration is a key evolutionary strategy whereby an animal, or species, leaves one area to travel to another.
Oftentimes, birds migrate to escape unfavorable weather conditions, such as the extreme cold.
Twice yearly, many bird species undertake arduous journeys across the globe to reach new environments.
One of the most famous long-distance migrant is the swift.
In fact, the alpine swift holds the record for the longest recorded uninterrupted flight by a bird – over 200 days in the air!
Typically, migrating species will stop off in passing countries to recuperate and build up energy reserves. However, ornithologists have speculated that swifts spend much of their life on the wing.
And, in 2013, this theory was shown to be correct.
Migration Patterns Of The Alpine Swift
A team of researchers studying alpine swifts discovered that they migrate between their breeding grounds in Switzerland, to their wintering grounds in Western Africa. Using tags that log flight information, the researchers found the swifts were airborne for the duration of the trip.
This behavior is similar to those observed by offshore birds, which spend much of their lives out at sea.
The researchers identified periods of increased and decreased activity, where low wing flapping was recorded, as well as elongated gliding flights. These changes of activity levels whilst in flight suggested periods of some kind of sleep in flight.
However, there are other birds, such as the bar-tailed godwit, that travel exceptional distances without resting. One individual was recorded flying non-stop for over 8,000 miles, over a period of 11 days, between Alaska and Tasmania.
Only once it has arrived in their desired destination will the bird rest.
Some migratory birds use thermals, pockets of warm air, to help them glide to save energy. It is thought that whilst soaring on the thermals, some migrant species may squeeze in a few Z’s.
How Do Waterfowl Sleep?
Waterfowl includes species such as ducks and geese.
Like other species of birds, waterfowl display unihemispheric slow-wave sleep (USWS). They can close down half of their brain to sleep, whilst the other half remains awake and alert.
Rottenburg, the guy who conducted the frigatebird study, also carried out research on mallard ducks.
Placed in a row of four, he noticed the two ducks in the middle slept with both eyes shut, whilst the ducks on the ends of the row would sleep with one eye open.
When rotated, the ducks exhibited the same behavior.
Using electroencephalogram recordings, it was found that the side of the brain that controlled the eye opening had the activity of an awake bird, whilst the side with the closed eye showed similar characteristics of a duck in sleep.
Like passerine birds, this is an anti-predator strategy that enables the ducks to get sufficient levels of rest, whilst being prepared to flee in the event of a threat.
Physical Behaviors of Sleeping Waterfowl
The difference with waterfowl from passerine species is the physicality of sleep.
Sure, there are some ducks that display similar behavior to passerine species, like perching in trees.
These aptly named perching ducks, stay rooted on trees via sharp claws on their feet.
But most other species of waterfowl sleep differently.
When seated, be it on dry land or in the water, waterfowl have a tendency to tuck their bill into their feathers, bending their head backwards doing so.
This is typical of heavier waterfowl, such as muscovy ducks, which rest their head and neck muscles on their bodies, to avoid neck injuries whilst sleeping.
The composition of the feathers between waterfowl and passerine birds are slightly different. Living in or near to water, many waterfowl species have evolved waterproof feathers. Due to the higher oil content to make these waterproof feathers, waterfowl can’t quite as readily fluff up their feathers like passerine species do.
So, by nestling their head into the feathers, waterfowl are able to effectively conserve body heat.
However, other theories suggest the placement of the head on their feathers is to better align their ears and eyes to see any approaching ambush predator.
Final Thoughts
Birds are exceptional creatures that have adapted to a range of habitat types.
However, many bird species are susceptible to predation.
Unfortunately, like most animals, birds need to sleep. But there is a trade-off between getting a restful sleep and watching out for predators.
The solution?
Many bird species, from passerine to waterfowl species, undergo unihemispheric slow-wave sleep. This means they can effectively close one half of their brain down to sleep, whilst the other side, the side connected to an open eye, can stay vigilant.
A pretty nifty compromise, if you ask me.
