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Birds Take Short Snoozes When They Fly

July 11, 2017

Story at-a-glance

  • After studying frigatebirds from the Galapagos Islands, scientists revealed that certain birds take naps while they’re flying
  • Mini Global Positioning System (GPS) devices were equipped with electroencephalographic (EEG) technology so researchers could monitor the journeys of frigatebirds for as long as 10 days and around 1,800 miles
  • During the day, frigatebirds fly and feed over the ocean, but at night switch to a brain mode called short-wave sleep, or SWS, using both hemispheres of their brain, a phenomenon called unihemispheric sleep
  • Other birds such as wild ducks, peregrine falcons and blackbirds have the same ability, while bottlenose dolphins and beluga whales shut down one side of their cranium to sleep while continually surfacing to breathe

By Dr. Becker

For many years, scientists who study birds have pondered the phenomenon of birds and their ability to fly long distances that would take hours to complete, especially over water. Finally, they've revealed the secret: Some birds take in-flight naps.

In a detailed and complex undertaking, scientists tackled the phenomenon. Frigatebirds from the Galapagos Islands were the study subjects; the researchers were from the Max Planck Institute for Ornithology in Germany, who tested the hypothesis that sleeping while flying had to be the only way these birds could get from point A to point B. How did they do it? They were able to "outfit" the birds with specially designed, miniscule and lightweight devices to measure their brain activity. According to Seeker:

"Frigatebirds are the perfect test subjects to wear the gear. The seabirds make long foraging flights of weeks at a time over ocean water, looking for flying fish and squid that have been harassed out of the water by animals such as whales or predatory fish."1

Similar to the black box flight recorders on planes, the mini devices were equipped with Global Positioning System (GPS) technology so researchers could monitor the birds' journeys for as long as 10 days and around 1,800 miles.

GPS Tracking for Birds in Flight

The GPS devices they used were so sophisticated, the scientists could read electroencephalographic (EEG) information from both hemispheres of the birds' brains and track their head movements at the same time. Some of the most fascinating intelligence came when the birds returned to their nests, still wearing the recorders.

Head-mounted data logger devices for recording the EEGs showed scientists their flight patterns, comprised of their typical circling (soaring) and straight (gliding) flight modes."Running the numbers," as it were, it became evident to the scientists that the birds really did take snooze sessions while flying.

In the daytime, they flew while conducting their usual food-foraging activities. At night, however, they "switched" to another brain mode lasting several minutes at a time, a mechanism scientists call short-wave sleep, or SWS. One of the most interesting aspects of the experiments was the discovery that this in-flight sleep mode could occur in both sides of the birds' brain hemispheres at the same time.

The scientists found that surprising. Most of the time, however, the frigatebirds were able to switch one half of their brain, ostensibly to rest, while the other half of their brains remained in fine flying form. This process is referred to as unihemispheric sleep, which is also used by dolphins and other species.

Unihemispheric Sleep, a Survival and Food-Gathering Feature

Wild ducks, aka mallards, peregrine falcons, blackbirds, white-crowned sparrows and other birds share this fascinating characteristic. Bottlenose dolphins also effectively shut down their brains in order to sleep with one side of their cranium, and continue their inborn survival skill of surfacing to breathe when necessary. So do the stub-headed species of beluga whales. The smallest version of the manatee, known as the Amazonian variety, is another mammal that lives in water and exercises this interesting ability.

It's possible that crocodiles also use this modus operandi to "sleep with one eye open," although scientists, who are still hoping to verify this occurrence, believe this may be an evolutionary feature of the reptilian species. One study tackling this instinctive attribute noted that:

" … The behavior and health of aquatic mammals and birds does not seem at all impaired by the reduction of sleep. The neural mechanisms of unihemispheric sleep are unknown, but assuming that the neural structures involved in sleep in cetaceans, seals, and birds are similar to those of terrestrial mammals, it is suggested that they involve the interaction of structures of the hypothalamus, basal forebrain, and brain stem.

The neural mechanisms promoting wakefulness dominate one side of the brain, while those promoting sleep predominates the other side."2

Like Humans in Cars, Birds Need Unihemispheric Sleep for the Same Reason

If you think about it for a moment, there's an even more practical reason why the remarkable knack of double-dimensional brain waves while sleeping are important for birds in flight: Collision avoidance. Seeker said:

"The team observed that when the birds made a circle off a rising air current, the brain hemisphere associated with the eye closest to the direction of the turn stayed awake, meaning the birds kept watch, with one eye, while they performed the maneuver."3

There's one more thing these birds need to keep at least one eye peeled for, and that's the ocean below them. If they fly too low, they might find themselves barefoot skiing, but when they go for the unihemispheric sleep option, they instinctively fly higher and look for rising air currents, Nature Communications explained.4

It should be noted that the sleeping-in-flight stints didn't last long at any given time, nor were they very deep. The time period recorded for most of the frigatebirds averaged around 42 minutes a day. On land, when the feature wasn't needed, they could sleep for up to 12 hours. Here's another amazing thing these birds can do in the in-flight "twilight zone:" they follow ocean currents that form circular patterns known as "eddies," because that's where their food will be at daybreak.

Scientists aren't yet sure if it's something in the atmosphere, their olfactory senses or visual cues that help them ascertain these dining opportunities. However, part of the frigatebirds' skill has to do with their canniness in watching for large predatory fish, which "drive" smaller fish and squid above the surface for an easier mark.5

In the near future, members of the research team hope to continue studying frigatebirds more extensively and figure out how they're able to function so well in the foraging category with so little real shut-eye.

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Sources and References

  • 1, 3 Seeker August 4, 2016
  • 2 Nat Sci Sleep. 2016;8:221-238
  • 4, 5 Nature Communications August 3, 2016
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