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Some birds are known to be able to make incredibly long flights, which until now remained a mystery to scientists. Some researchers have hypothesized that migratory birds somehow manage to sleep during the flight. New experiment, conducted by an international team of scientists, proved this theory by demonstrating that birds can indeed take a nap while continuing to fly by hovering on ascending air currents.

Tracking devices were put on frigates.

The results of the study by Niels Rattenborg of the Max Planck Institute and his colleagues from a number of other institutions have been published in the journal Nature Communications. Scientists have presented evidence that during flight, birds can sleep, either by keeping one half of the brain active or temporarily turning off both hemispheres of the brain. It is noteworthy that migratory birds retain their navigational ability even during the so-called "REM sleep", in which the body temporarily loses muscle tone.

It is well known that birds such as swifts and waders are able to cover great distances during migrations. Therefore, scientists suggest, such birds should have developed the ability to turn off one half of the brain for some time, giving rest to the other, without risking crashing in flight. Dolphins have a similar sleep regulation mechanism. It allows them to stay afloat without sinking when they sleep.

However, until now, there has been no concrete scientific evidence for this assumption. To confirm their theory, Rattenborg and his colleagues directly recorded the brain activity of birds during flight. Their goal was to establish what type of sleep - slow wave or REM - is present in birds during long flights.

Together with researchers from the University of Zurich and the Swiss Federal Institute of Technology, Rattenborg's team developed a small device that was strapped to a bird's head and recorded brain activity as well as the movement of the bird's head.

Frigatebirds that nest on the Galapagos Islands were chosen as objects of study. These birds often have to spend several weeks in flight over the ocean in search of prey. As a result, as part of the study, with a small device on their heads, the frigates flew about 3,000 kilometers without stopping to rest.

After removing the recorders and analyzing the flight data, the researchers made amazing discoveries. It turned out that the birds were awake only during daylight hours, but after sunset they switched to the hovering mode (in contrast to the active foraging mode), and the device began to record slow-wave sleep, which could last up to several minutes.

It turned out to be curious and completely unexpected for scientists that slow-wave sleep could be recorded both in one hemisphere (as the researchers initially assumed), and in both hemispheres at the same time. The researchers concluded that, in general, birds do not need constant activity of one of the hemispheres of the brain for aerodynamic control. However, such a dream occurred within the framework of this study quite often, when the birds, circling, rose up on the air currents. This suggests that the birds literally slept with one eye while watching with the other so as not to run into obstacles.

As for the phase of REM sleep, it should be noted that it differs in birds from a similar type of sleep in mammals. Unlike humans, whose REM sleep phases are long and cause a complete loss of muscle tone, in birds this phase lasts only a few seconds. However, due to the loss of muscle tone, the head of the birds drops during REM sleep, but this does not affect the flight itself.

Despite this amazing ability sleep during the flight, the total duration of sleep of frigates turned out to be extremely small. On average, these birds slept only 42 minutes per night. In contrast, these same birds typically sleep 12 hours a day when they are on land. Such a striking contrast in the behavior of birds remains a mystery to scientists.

Have you ever wondered how animals sleep? For example, how do birds doze and sleep? How do they manage to do this, because they cover long distances for several days or weeks without "halts"?

Birds and mammals have a lot in common in this matter.

It is somewhat surprising that birds, which have more in common with reptiles, are similar to mammals in matters of sleep. In fact, members of the Bird class are the only animals (other than mammals) that are characterized by slow and fast phases of sleep. Despite these common features, they have several unique features.

When the sleep of birds was studied using EEG to determine changes in brain activity, specific changes were noted that indicate a change in sleep phases. In the phase of slow sleep, the EEG shows a high voltage. The same happens in other animals, including humans.

Birds can sleep with one eye open while flying

Birds are interesting in that during slow sleep they can sleep with one eye open, however, like reptiles. How is this possible? This may be because birds and other migrating animals can sleep while one half of the brain is working. This phenomenon is called hemispherical sleep.

By closing just one eye, the bird can sleep and at the same time control the state of its environment, see the approach of a dangerous predator.

Studies show that when birds feel any danger, they try to sleep with one eye open. Incidentally, for various reasons, people can also sleep with their eyes open.

Such sleep, when one part of the brain is active, has certain advantages. We have already talked about protection from predators. But other activities can be improved by keeping the brain active for long periods of time. For example, this phenomenon allows birds to sleep during flight and fly continuously for many days, weeks. And there is absolutely no need to land to rest.

What do birds dream about?

In many animals, rapid eye movement can be seen during sleep, and birds are no exception. When observed with the help of EEG, it was noted that the phase of REM sleep resembles insomnia, as in humans. In addition, signs such as rapid eye movements, muscle twitching, and a decrease in temperature are observed. However, there are several significant differences.

During a typical REM phase, the muscles are in a relaxed state, thereby preventing the performance of those actions that are dreamed of. If you dream that you are jumping out of a window, in fact you do not want to do this while being unconscious.

Failures of this normal state can occur as a result of a disruption in the flow of REM sleep. Observing the condition of birds using EEG, during REM sleep, you can see that the degree of their muscle activity is high, relaxation or so-called muscle paralysis is rarely observed. Nevertheless, some decrease in muscle tone should still be noted. For example, when birds are in REM sleep, their head is slightly tilted.

In addition, REM sleep is often much shorter in birds than in mammals. Each episode is brief, often lasting less than 10 seconds. When the entire sleep period of the birds was assessed, it was concluded that they spend most of their sleep in the slow phase.

Conclusion

In many birds, but not all, periods of REM sleep become longer in the morning, and this happens in other animals. Probably no one knows what birds actually see in their dreams, but it could be assumed that they re-experience all the events that happened to them throughout the day, as it happens with people.

An international team of scientists led by researchers from the Institute of Ornithology of the Max Planck Society (Germany) has found the answer to a question that has haunted researchers for decades. And the question is simple: can birds sleep during the flight?

They can, scientists proved by conducting the most unusual study to follow the daily routine of birds that are in the air for hours.

It has long been known that some species of swifts, songbirds, snipes, and seabirds can make non-stop flights lasting days, weeks or more. Considering what negative consequences leads animals to lack of sleep, many scientists believed that birds somehow manage to sleep in flight. However, it has recently been shown that some birds are indeed able to stay awake for weeks, minimizing sleep time, and this has further added to the confusion about the possibility of sleeping in the air.

The main problem in answering this question is the lack of studies of the brain of birds during flight, which could shed light on this problem. How can a bird sleep in the air without colliding with its relatives and without falling down? According to one version, birds do not sleep completely, but with separate hemispheres, first the left, then the right. There are works showing that this is how mallards sleep if there is danger on land. In these birds, if they are on the edge of the flock, one hemisphere of the brain is always awake, and the corresponding eye is always directed towards possible threats. Be that as it may, in order to record sleep, scientists need to measure the electrical activity of the brain, and this requires the placement of special devices on the heads of birds.

To solve the problem, scientists needed to identify the moments when the bird is awake, and when it falls into one of the two known types of sleep - slow wave sleep (SWS) and REM sleep (Rapid Eye Movement, REM). To do this, Nils Rattenborg, the author of the work, together with Alexei Vysotsky from the University of Zurich, developed a miniature device,

taking an encephalogram of the brain of birds and fixing head movements during the flight.

They decided to study the large frigatebirds nesting in the Galapagos Islands. These birds are known to spend whole weeks above the ocean in search of fish, which are driven by predators to the surface. Birds with instruments placed on their heads sometimes flew away for 10 days, during which they flew up to 3 thousand km. All this time, the device recorded encephalograms of both hemispheres of birds, head movements, route and flight altitude using a GPS receiver. Upon returning, the instruments were removed from the birds and the process of processing the received data began. “Like many other animals in the Galapagos, the frigatebirds were quite calm and continued to sleep when I approached them a second time,” said co-author Bryson Voirin.

Illustration: nature.com

Analysis of the data showed that birds do sleep in flight, but sleep in both intended and unusual ways. During the day they are awake, looking for food. After sunset, the habitual encephalogram switched to slow sleep mode, and the bird hovered for several minutes. Scientists noted that at this time, both one and both hemispheres of the brain can be turned off.

This means that birds do not need to be awake in one hemisphere in order to maintain control over the aerodynamics of their flight.

However, another discovery was made by comparing the movements of the head with the records of the encephalogram of the hemispheres. When a bird soars in circles in updrafts, the hemisphere of the brain responsible for the eye, which looks in the direction of the turn while the other sleeps, is awake, which implies that the bird keeps track of where it flies. “Frigates can keep an eye on birds to avoid collision, similar to how ducks track predators,” Rattenborg explained.

Illustration: nature.com

In addition, periods of non-REM sleep are sometimes interrupted by short, just a few seconds, phases of REM sleep, scientists have established. Unlike mammals, in which REM sleep lasts for a long time and is accompanied by complete relaxation of the muscles, in birds it turns on only for a few seconds. Unlike animals, birds in this phase of sleep, although they drop their heads for a while, can continue to stand on one leg.

Similarly, frigatebirds at the moment of REM sleep temporarily nod their heads, but their flight remains unchanged.

Perhaps the main discovery for scientists was the total sleep time: it turned out that during the day the birds allow themselves to fall asleep for only 42 minutes. But, returning to the nest, they sleep off and can sleep for half a day. “Why they sleep so little at night, if they don’t hunt much, remains a mystery,” says Rattenborg. In the future, scientists intend to explain how birds go so long without sleep. “Why we and many other animals suffer so much from sleep deprivation, while some birds adapt so well, remains a mystery,” he added. The work of scientists was published in the journal

The ability of birds to fall asleep during flight. This ability of birds has been suspected for a long time, but only now they were able to prove it. Interestingly, the new study also shows that the way to take a short in-flight nap is much more unusual than thought.

The discovery, the researchers say, will finally help explain how birds manage to fly for days (or even weeks) without getting tired.

Ornithologists did not fully understand what exactly happens to birds during a flight: whether they are awake throughout the journey, or whether they use only one hemisphere of the brain, while the other is resting. So, for example, it was found that ducks are able to go to sleep with only one hemisphere, in order to remain vigilant even in a dream and notice the approach of a predator in time. Previously, the same feature was observed in dolphins. By the way, .

In a new study, German scientists measured the brain activity of a frigatebird sea bird, which, as you know, is able to fly over the ocean for weeks in search of fish. The team developed a small device that monitored electroencephalographic changes in bird brain activity. The device, which they called the "flight recorder", was attached to 15 adult female frigatebirds. The device could record slow wave sleep and rapid eye movement sleep.

The team observed brain activity for ten days, during which the birds flew about three thousand kilometers. In addition, the built-in GPS sensor tracked the position of the birds and the flight altitude. After the birds returned, ornithologists collected "flight recorders" to analyze these records and were very surprised by the results.

During the day, the birds were awake and actively looking for fish, but as soon as the sun set, the birds went into a slow sleep stage and continued to fly. True, such a dream lasted only a few minutes.

Most often birds use only one hemisphere during sleep, which the researchers expected to find. But electroencephalographic data also showed that both hemispheres could immediately fall into slow sleep at the same time, which unexpectedly suggests that birds are able to control the flight even when the entire brain is in "sleep mode".

Full-fledged slow-wave sleep usually occurred while the birds were circling in the updrafts and did not need to flap their wings.

But perhaps the biggest surprise was the fact that, despite this unique opportunity, the frigates themselves were content in most cases with a short sleep. It lasted no more than one hour a day - an average of only 42 minutes. This is less than 10% of the time birds spend sleeping on the ground.

Scientists do not yet understand why this is happening: there is still a lot of research to be done. "Why they sleep so little in flight, even at night, when they rarely find food, is a mystery to us," says study author Niels Rattenborg (Niels Rattenborg).

According to experts, the study of this issue will help people in the future. "It is not completely known why we, like many animals, while some birds may not sleep long time", says Rattenborg.

It turned out to be curious and completely unexpected for scientists that slow-wave sleep could be recorded both in one hemisphere (as the researchers initially assumed), and in both hemispheres at the same time. The researchers concluded that, in general, birds do not need constant activity of one of the hemispheres of the brain for aerodynamic control. However, such a dream occurred quite often in the framework of this study, when the birds, circling, rose up on the air currents. This suggests that the birds literally slept with one eye while watching with the other so as not to run into obstacles.

Despite this amazing ability to sleep during the flight, the total duration of sleep of frigatebirds was extremely short. On average, these birds slept only 42 minutes per night. In contrast, these same birds typically sleep 12 hours a day when they are on land. Such a striking contrast in the behavior of birds remains a mystery to scientists.