Andean condors are the heaviest soaring bird in the world, with a single individual weighing up to about 16 kilos (or 35 pounds). When it comes to keeping these hefty bodies aloft, the sky is very much the limit, according to new research.

Getting off the ground is the hardest part for these South American condors (Vultur gryphus), but once the giant birds are airborne, researchers have found they barely ever flap their wings. Instead they glide, soaring for up to 99 percent of their flight time, mostly on winds and thermal updrafts.

Attaching bio-logging devices, or 'daily diaries', to eight juvenile condors, researchers obtained more than 230 hours of catalogued flight time. In all that time, only 1 percent of it was spent flapping, and most of that was simply for take-off.

"The extraordinary low investment in flapping flight was seen in all individuals, which is notable, as none were adult birds," the authors write.

"Therefore, even relatively inexperienced birds operate for hours with a minimal need to flap."

One young condor actually flew for more than five hours without beating its wings once, covering over 170 kilometres (100 miles) using air currents alone.

"The finding that [Andean condors] basically almost never beat their wings and just soar is mind-blowing," David Lentink, an expert in bird flight from Stanford University, who was not involved in the study, told The Associated Press.

Soaring birds are usually the largest ones, because the energy required for powered flight is so much greater for heavier creatures. While lighter species, like hummingbirds, flap their wings at a crazy rate, the condor's 'marine counterpart', the albatross, spends 1.2 to 14.5 percent of its flight slowly flapping.

The Andean condor does even less. For example, on a 50-minute journey, juvenile condors spend nearly the same amount of energy gliding, soaring, and occasionally flapping as they do during their 3.3-minute take-off.

In fact, the cost of flapping for these large birds was deemed by the authors to be some 30 times greater than their resting metabolic costs, which means it's probably as energy-efficient as sprinting for mammals.

Using continuous data from the bio-loggers, researchers identified each and every wingbeat from all eight juvenile condors in various wind and thermal conditions.

Even over mountains, where there are complex airflow interactions, these young condors were able to navigate invisible currents of air with very little movement.

"Human glider pilots can soar all day if the conditions are right, so in some senses the condor's performance may not seem surprising," biologist Emily Shepard from Swansea University told the BBC.

"But glider pilots look at the weather and decide whether or not it is good for flying."

Condors don't have that luxury. They usually go soaring to find food, which isn't always located in easy-to-access spots, especially when you're predominantly riding air currents to get there.

While it takes a lot of energy for condors to take off, it requires finesse for them to land, so these giant birds are selective about where they touch down.

If a condor wanted to head towards a juicy carcass on the ground, for instance, it would have to jump from updraft to updraft, moving towards warm rising air. Sometimes, bridging those gaps requires an occasional flap.

What's more, these atmospheric 'hot spots' aren't always hot. Their power and frequency change with weather, topography, and season, so predicting them isn't always easy when heading for ground.

"This is a critical time as birds need to find rising air to avoid an unplanned landing," explains Sergio Lambertucci, a biologist at the National University of Comahue in Argentina.

"These risks are higher when moving between thermal updrafts. Thermals can behave like lava lamps, with bubbles of air rising intermittently from the ground when the air is warm enough. Birds may therefore arrive in the right place for a thermal, but at the wrong time."

Even in winter, when conditions for strong winds and thermal updrafts aren't as good, the authors found Andean condors are still similarly unwilling to take a path that requires them to flap.

"This suggests that decisions about when and where to land are crucial, as not only do condors need to be able to take off again, but unnecessary landings will add significantly to their overall flight costs," says movement ecologist Hannah Williams, now at the Max Planck Institute for Animal Behaviour.

Understanding how giant birds navigate invisible obstacles in the sky can doesn't just tell us about atmospheric conditions, but can also shed light on how absolutely massive extinct birds, like Argentavis magnificens, once kept their 72-kilogram bodies aloft.

"It has always been assumed that Argentavis would have been incapable of sustained flapping flight and thus entirely dependent on soaring," the authors write.

It's likely, therefore, that they too soared the skies much like the Andean condor, flapping their wings as a safety net, and only when absolutely necessary.

The study was published in PNAS.