Birds can travel for days by snoozing while they are mid-flight

Birds really can sleep while in flight, according to new research.

The strange phenomenon has long been suspected, but scientists at last have the first definitive proof.

They monitored the brainwaves and movements of great frigates from the Galapagos Islands by strapping devices to the heads of females over the course of ten days.

Sometimes they were just half asleep with one hemisphere of their brain awake with the respective eye open to watch for potential threats or obstacles.

But at other times the birds were also able to keep both brain hemispheres asleep without crashing.

The research, which is published in Nature, comes from an international team of scientists, led by the Max Planck Institute for Ornithology in Seewiesen, Germany.

Despite being able to engage in all types of sleep in flight, the birds slept less than an hour a day, a mere fraction of the time spent sleeping on land.

Given the adverse effect sleep loss has on performance, it is commonly assumed that these birds must fulfill their daily need for sleep on the wing.

The researchers suggest that the birds might only switch off half of their brain at a time, to prevent theme from colliding with obstacles or falling from the sky.

When ducks sleep on the edge of a group, they keep one cererbal hemisphere awake, and the corresponding eye open to look out for potential threats.

Based on these findings, it is commonly assumed that birds also rely on this sort of autopilot to navigate and maintain aerodynamic control during flight.

To understand how birds continuously fly, the researchers recorded the changes in brain activity during the two types of sleep found in birds: slow wave sleep (SWS) and rapid eye movement (REM) sleep.

The team developed a small device, known as a flight data recorder, to measure electric changes in brain activity and head movements in flying birds.

The flight data recorder was attached to frigatebirds - large seabirds that fly in search of fish - for ten days, as they continuously flew a distance of 3000 kilometres.

The results showed that during the day, the birds stayed awake actively searching for foraging opportunities.

However, at night, the electric signals showed that the birds went into a SWS pattern for periods lasting up to several minutes while they were soaring, in either one or both brain hemispheres.

This suggests that birds are able to maintain aerodynamic control, even when both hemispheres are asleep.

Looking at the GPS data, the researchers found that when the birds circled, they kept one eye open, suggesting that they were watching where they were going.

Niels Rattenborg, who led the study, said: "The frigatebirds may be keeping an eye out for other birds to prevent collisions much like ducks keep an eye out for predators."

As well as SWS sleep, which is light, the birds also went through bouts of REM (deep) sleep, in which their heads dropped momentarily, but their flight pattern remained the same.

Overall, the results showed that the frigatebirds only slept for 42 minutes a day.

This is surprising, as when they are on land, the birds can sleep for over 12 hours a day.

While the reasons for sleep deprivation during flight are still unclear, the researchers think that the birds may stay awake to make sure they do not miss good foraging conditions.

The researchers hope to use the findings to determine how birds are able to sustain adaptive performance on such little sleep.

Mr Rattenborg said: "Why we, and many other animals, suffer dramatically from sleep loss whereas some birds are able to perform adaptively on far less sleep, remains a mystery."