Sleeping on the Wing

There is an old Monty Python skit where John Cleese and Graham Chapman play airplane pilots. Presumably on a long, tedious flight, they are clearly bored and keen on amusing themselves at the expense of their passengers.

They find entertainment through relaying worrisome, nonsensical messages. Cleese begins their prank with the truism, “Hello, this is your captain speaking. There is absolutely no cause for alarm.” And after some internal discussion about what there should be no cause for alarm about, they add: “The wings are not on fire.” The messages get more ridiculous, and hilarity (at least for the pilots) ensues.

While fictional pilots can pass the time during long flights by pranking their passengers, real pilots have to find other ways to stay awake. Unless, of course, you happen to be a great frigatebird, in which case you can just sleep on the wing, as a paper published in Nature Communications this August reported.

There are numerous bird species that make long journeys over the ocean, during which time they presumably don’t have the luxury of getting a full night’s sleep. Alexei Vyssotski and colleagues decided to see how great frigatebirds (Fregata minor) handle the pressure of sleepiness while making one of those long trips.

Great frigatebirds are typically found roosting around the Galapagos Islands. So, off to the Galapagos islands Vyssotski’s team went, capturing 14 female birds and surgically implanting EEG, accelerometer, and GPS sensors that allowed data to be recorded onto attached memory cards (the device, called Neurologger, was invented by Vyssotski at the ETH in Zurich, and also allows for collection of electrophysiological data from single neurons).

EEG signals are often used to detect slow-wave sleep, which is characterized by high-amplitude, low frequency oscillations. The accelerometer provided signals related to head movements and wing flapping, while the GPS data in this experiment was used to track the birds’ locations as they made foraging trips over the ocean. (After collecting data, the researchers took the head-mounted data loggers off the birds and released them (the birds)).

The first piece of data from this study, while not exactly news to ornithologists, is still astounding. The GPS data showed that the birds took one or two trips over the ocean, each lasting on average 5.76 days, with an average distance travelled of 1988 km (1235 miles!). Oh the lengths some go to just to catch some fish…

One of the great frigatebirds used in the study reported in Nature Communications, outfitted with EEG sensors, an accelerometer, and GPS.

One of the great frigatebirds used in the study reported in Nature Communications, outfitted with EEG sensors, an accelerometer, and GPS.

The GPS also showed that foraging for food, defined as altitudes under 20 meters, occurred primarily during the day and that the average altitude wasn’t different between day and night (around 136 meters).

The EEG data showed bouts of slow-wave activity characteristic of sleep that sometimes occurred in both hemispheres at the same time (as normal sleep should), and sometimes in one hemisphere at a time (termed unihermispheric slow-wave sleep, USWS). That birds (and some marine mammals) can sleep with one hemisphere of their brain at a time was previously known, but is still an amazing feat. (The sleep researcher Giulio Tononi argued in a 2004 paper that human EEG data shows small patches of slow-wave activity that he termed “local sleep,” although whether that actually constitutes sleep is up for debate.)

The accelerometer data showed that when the birds slept with one hemisphere, they tended to lean in the direction of that hemisphere. Presumably the contralateral eye would have been closed during uni-hemispheric sleep consequently biasing the bird’s flight path in the direction of the open eye.

Perhaps the most fascinating result from this study was not that the birds could sleep with both hemispheres while flying, but that they spent a grand total of 2.89% of their flight time sleeping with one or both hemispheres. This is even more striking when compared with the 50% of the time the birds spent sleeping while on land. The data also showed that once the birds returned from their flights, they slept a little more than usual on land, suggesting that they really do need to catch up on the zzz’s after spending so little time sleeping during flight.

An intriguing next step in this line of research would be to see exactly how alert and capable of learning or paying attention these birds are after several days with so little sleep. Do they just barely get by, soaring over the ocean like zombies? Or are they as sharp as after a full night’s rest? Would they spend so much time sleeping on land if they didn’t make such arduous journeys? Perhaps most exciting, what are the molecular mechanisms that allow the brain to keep functioning under so little sleep?

One defining characteristic of sleep is a lack of responsiveness to stimulation. Unfortunately the researchers could not directly test the birds’ responsiveness by poking them as they coasted over the waves. Lack of responses to stimulation, along with closed eyes (not measured here either) would be two pieces of evidence that really support the claim that the high-amplitude, low frequency activity in the EEG actually reflects sleep. Without this, the EEG data may be interpreted to show the local “sleep” that Tononi described more than a decade ago.

Whether they truly do sleep on the wing or not, these magnificent creatures manage to stay in the air for extraordinary amounts of time, so as far as the concern about them dropping into the ocean from sleepiness goes, John Cleese put it best: there is absolutely no cause for alarm.

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