The ability to sleep while flying is a remarkable adaptation found in some species of birds. Known as “unihemispheric slow-wave sleep,” this phenomenon allows birds to rest half their brain while keeping the other half awake for flight. There are a few different types of birds capable of sleeping on the wing, but the common swift is particularly well-known for this ability. In this article, we’ll take a closer look at how the common swift can sleep while airborne and the advantages this grants the species. We’ll also examine some of the other avian species capable of in-flight slumber.
The Common Swift’s In-Flight Slumber
The common swift (Apus apus) is a small bird found throughout much of Europe and Asia. It is a member of the swift family, known for its superb flying skills and aerial lifestyle. The common swift spends almost its entire life airborne, feeding, mating, and even sleeping during flight. But how does this bird manage to snooze while cruising hundreds of feet above the ground?
The secret lies in the common swift’s ability to put one brain hemisphere “to sleep” while keeping the other hemisphere awake. Scientists have found that swift’s brains show what is known as “asymmetrical slow-wave sleep,” meaning one half exhibits low brain activity associated with deep sleep, while the other maintains higher activity levels similar to wakefulness. This allows the bird to maintain aerodynamic control and navigation with its “awake” side while permitted the other side to get much-needed rest.
Researchers have also discovered that common swifts keep one eye open when napping in flight. By alternating which eye is open and which is closed, the bird can continue scanning for obstacles and make aerodynamic adjustments even while half-asleep. The swift’s ability to fly continuously for years at a time relies heavily on being able to rest and recover while airborne. Unihemispheric slow-wave sleep provides an ingenious means of doing just that.
Advantages of In-Flight Sleep
For the common swift, the ability to sleep while flying confers several key advantages:
– **Maximizes feeding and breeding time** – By sleeping on the wing, swifts minimize time spent grounded and maximize time spent foraging and raising young. Even a few hours on the ground each day represents lost feeding opportunities.
– **Avoids predation** – Swifts are extremely vulnerable on the ground, lacking the strong legs and feet needed for walking or perching. Sleeping in flight keeps them safely out of reach of most predators.
– **Saves energy** – Landing, taking off, and getting airborne again are energetically costly. Sleeping in the air eliminates these expenditures.
– **Enables migration** – Swifts can migrate extraordinary distances without needing substantial stopovers to rest. In-flight sleep allows them to refuel and rest simultaneously by sleeping on days with favorable winds.
In-Flight Sleep in Other Birds
While common swifts are masters of aerial slumber, they aren’t the only birds capable of this feat. Some other birds known to exhibit unihemispheric slow-wave sleep and sleep while flying include:
– **Frigatebirds** – These large seabirds famed for their aerial piracy are able to soar for weeks at a time, sleeping on the wing.
– **Alpine swifts** – Close relatives of common swifts, alpine swifts display a similar ability to sleep hundreds of meters above the ground.
– **Great frigatebirds** – Males of this species sleep during their months-long courtship flights high above breeding colonies.
– **Mallard ducks** – Surprisingly, some evidence suggests mallards can sleep unihemispherically during long flights, keeping half the brain vigilant for obstacles.
Mechanisms of Unihemispheric Sleep
To understand how birds like the common swift can sleep with half their brain at a time, we need to examine the mechanisms that allow for unihemispheric sleep:
– **Asynchronous brain waves** – In unihemispheric sleep, brain wave activity is asymmetric. One hemisphere shows slow waves characteristic of deep sleep, while the other exhibits faster waves associated with wakefulness.
– **Lateralized eye closure** – Birds close only one eye during unihemispheric sleep, enhancing the awake state of the opposing hemisphere. Information from the open eye keeps its counterpart alert.
– **Preferential eye use** – Many birds favor one eye for certain tasks like feeding or scanning for predators. This lateralization may make asymmetrical sleep more feasible.
– **Unilateral muscle control** – Some birds can independently control muscle groups on either side of their body, permitting one half to rest even as the other continues beating wings.
The Common Swift’s Amazing Flying Abilities
The common swift’s adaption for in-flight slumber is intimately tied to the bird’s incredible aerial skills. A few of the common swift’s most impressive flying abilities include:
Speed and Agility
– Common swifts are among the fastest birds on Earth. In a steep dive, they can reach speeds of over 110 miles per hour. Even in routine flight, they cruise at speeds around 70 mph.
– Their slender, curved wings allow great agility and the ability to instantly change direction, ideal for catching flying insects.
– Swifts’ rapid fluttering wingbeats, up to 10 per second, grant them the maneuverability to fly through narrow spaces and dodge obstacles with ease.
Time Spent Aloft
– Common swifts spend an incredible 10 months of the year entirely on the wing, landing only to nest. Even during breeding months, they rarely touch down.
– Over its lifetime, a common swift may spend a full 2-3 years airborne in total. No other bird spends more of its life in flight.
– The longest recorded nonstop flight by a common swift is 6 months without landing, an amazing feat made possible by sleeping while aloft.
Migration
– Common swifts migrate from Europe all the way to southern Africa each year, up to 10,000 miles round trip. Their ability to sleep in flight enables these long journeys.
– Individual swifts may log over 3 million miles migrating during their lifetime, distance equivalent to 6 roundtrips to the moon!
– Swifts routinely fly 200-400 miles in a single day during migration. Nonstop flights of up to 1,000 miles have been recorded.
Threats Facing the Common Swift
While an incredibly accomplished flier, the common swift faces a number of threats in the modern world:
Habitat Loss
– Development eliminates the clifftops, crevices, and chimneys swifts rely on for nest and roost sites. Collisions with buildings and towers also pose risks.
– Declines in flying insect populations due to pesticide use may reduce critical food resources for swifts.
Climate Change
– Unusually wet, cold springs can decrease the aerial insects swifts depend on to provision young. Nestlings may starve.
– Extended droughts in Africa may curtail swifts’ overwintering food supplies, reducing survival.
Light Pollution
– Bright city lights can disorient and ground migrating swifts, leaving them vulnerable to collisions with buildings or vehicles.
– Streetlights attract masses of insects, distracting swifts from higher-quality natural feeding areas. This may decrease foraging efficiency.
Conservation Measures
To aid the common swift’s continued survival, some recommended conservation actions include:
Protect and Create Nest Sites
– Preserve old buildings with suitable cavities, crevices, and unused chimneys for nesting.
– Install specialized swift nest or roost boxes on buildings, towers, or bridges.
– Leave stand-alone dead or dying trees which provide ideal natural hollows for nesting.
Reduce Pesticide Usage
– Encourage organic agriculture and sustainable gardening practices on private and public lands.
– Advocate for policies limiting the most harmful insecticides, especially neonicotinoids.
Alter Lighting to Reduce Disorientation
– Turn off unnecessary lights during peak migration periods to minimize attraction and confusion.
– Install blinking or red aviation lights on towers to make structures more visible and avoid collisions.
| Scientific Name | Apus apus |
|---|---|
| Wingspan | 16.5 – 17.5 in |
| Length | 6 – 6.5 in |
| Weight | 1.75 – 2 oz |
| Maximum Speed | 69 mph in normal flight over 110 mph in dives |
| No. of Eggs | 2 – 3 |
| Incubation Period | 19 – 20 days |
| Nestlings Fledge In | 42 – 49 days |
| Typical Lifespan | 5 years in wild 21 years in captivity |
Fascinating Details about the Common Swift
Beyond its impressive flying skills, there are many fascinating details about the unique common swift:
Breeding
– Common swifts are completely devoted to aerial living, even mating on the wing! Pairs will mate midair, clutching each other with their feet.
– Swifts cement their nests to vertical surfaces using their sticky saliva, sometimes reusing the same nest for many years.
– Amazingly, a female swift can produce eggs within 36 hours of arrival at breeding grounds after migrating thousands of miles.
Feeding
– Common swifts feed exclusively on insects caught mid-flight, including flies, aphids, mosquitoes, midges, and others. A swift can eat up to 20,000 insects a day!
– They drink by skimming across ponds and lakes and gulping water on the fly, without landing.
– Swifts can even gather feathers, cotton, and other materials for their nests while in flight.
Vocalizations
– The swift’s shrill, twittering screams are thought to maintain contact between paired birds as theypursue prey. Their wide gape enhances these loud calls.
– Because they spend so little time on land, swifts lack the beautiful songs of many other passerines. Their primary vocalizations are simple, loud screams.
– Nestlings beg for food from parents with an eerie, pulsating whistle thought to grab the parent’s attention.
Comparison to Other Aerial Sleepers
While the common swift is the bird best known for sleeping on the wing, how do its abilities compare to other avian in-flight sleepers like frigatebirds and alpine swifts?
| Common Swift | Frigatebird | Alpine Swift | |
|---|---|---|---|
| Maximum Speed | 110 mph | 95 mph | 105 mph |
| Wingspan | 16.5 – 17.5 in | 6.5 – 8.2 ft | 15.7 – 17.7 in |
| Time Aloft Annually | 10 months | 8 – 10 months | 10 months |
| Longest Single Flight | 6 months | 2 months | 3 months |
| Migration Distance | 10,000 mi | 3,100 mi | 3,100 mi |
While the common swift is the smallest of these species, it migrates the farthest and spends a larger percentage of its life airborne. Frigatebirds have the advantage in raw size and wingspan to aid soaring flight. Overall, the common swift arguably best epitomizes aerial living of these avian aerial sleepers.
Conclusion
The common swift provides a fascinating example of evolution producing birds exquisitely adapted to life aloft. Their ability to sleep while flying grants them advantages in migration, foraging, and evading predators that make aerial living feasible year-round. Swifts push the boundaries of avian biomechanics in terms of flight speeds, maneuverability, and time spent airborne. While modern hazards from buildings to pesticides pose increasing threats, these aerial masters continue surviving thanks to their supreme in-flight sleeping skills and nonstop flight. If you’re lucky enough to glimpse a swift overhead, take a moment to appreciate the wonder of an animal that lives its entire life on the wing.
