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A Boeing 737 requires a lot of jet fuel to stay up in the air: at least 750 gallons every hour. Flying, as humans have learned, takes a lot of energy. For birds, maintaining their own bodies up in the sky for hours, days, and even months can also be incredibly costly, but they've at least evolved for the task. Small birds like warblers are lightweight enough that they can remain airborne by quickly flapping their wings. For heavier birds, though, flapping takes too much energy. When bird species reach the size of a small raptor, they start to rely on other types of flight: soaring and gliding.

Through soaring, birds gain altitude and travel quickly by taking energy from wind currents in the atmosphere. When they glide, they use the position of their wings to deflect air downward, which creates a force called "updraft" that keeps them up in the air. There are different kinds of soaring and gliding, and birds use them in a variety of ways.

“The air is this amazing environment that's on the move all the time,” says Emily Shepard, a researcher who studies animal movement at Swansea University in Wales. “It's just fascinating to see how it can create both opportunities and risks for different species.” Meet some of the masters of these flying techniques.

Wandering Albatrosses

“Wandering Albatrosses are the ultimate soaring birds,” says Anders Hedenström, an animal flight expert at Sweden's Lund University. When you take a look at their bodies, you understand why: With wings reaching 11 to 12 feet long from tip to tip, they have the largest wingspan of any living bird. Those wings can keep their thin, cigarette-like bodies aloft for .

Wandering Albatrosses spend between 1.2 to 14.5 percent of their flight time slowly flapping to stay in the air, researchers . The rest of the time their wings are splayed wide. Like many other birds, Wandering Albatrosses soar by catching a ride on thermals—hot air rising from the ground—to gain altitude. But what makes them unique is their impressive ability to engage in a type of flight called “dynamic soaring,” which can only happen while flying over the ocean, says Todd Katzner, a wildlife biologist at the U.S. Geological Survey.

In dynamic soaring, albatrosses take advantage of the different speeds and directions wind flows depending on how close it is to the Earth’s surface. The birds start close to the ocean, where they catch a ride upward on a thermal. They slowly climb, and when they reach high altitudes, where wind moves faster, they shift to fly in the same direction of the wind. That way, they can glide at a relatively fast speed while descending. By the time they are at lower altitudes, where wind is moving slowly, they have picked up a lot of momentum and are moving fast. That lets them turn their bodies diagonally in the direction they want to travel, even if it's against the wind. Finally, when they are running out of energy and slowing down, they restart the cycle by catching another thermal.

Accompanied by maneuvers that use the movement of the wind rising over waves, dynamic soaring is how Wandering Albatrosses manage to travel up to using barely any energy.

Andean Condors

If Wandering Albatrosses are the soaring masters of the sea, there is no doubt Andean Condors take the crown for inland birds. Research published recently showed that the Andean Condor—the heaviest soaring bird, at 35 pounds—flaps less during flight than any other free-ranging bird. After following eight Andean Condors for five years, a team of European and Latin American scientists found that even young birds soar about 99 percent of their flight time. “They are the extreme version of a soaring bird. It was fascinating to us,” says Emily Shepard, the lead author of .

The researchers found that these giant birds flap only during take-off and landing. They are so massive that they lose altitude even when flapping as fast as they can. This means that their reliance on wind currents is nearly absolute, says Shepard. So, to stay in the air, Andean Condors mostly use thermals to elevate. Once they’re high enough, they can glide between thermals looking for food. Less frequently they use another type of air current called “orographic updraft” that forms when the wind collides with an object—like a mountain or a building—and changes its direction to go upwards.

The researchers are now focusing on understanding the social aspects of soaring, or how a group of birds using the same air resources influence each other’s decisions—such as when to jump from one thermal to another, or picking the best spot to land or take off, says co-author Hannah Williams.

Great Frigatebirds

Great Frigatebirds make the "sky masters" podium because they can successfully soar through doldrums—areas in the open ocean where the wind doesn’t blow. How do they do it? In 2016, a team of researchers uncovered the mystery.

Like many other soaring birds, Great Frigatebirds use thermals to gain altitude. But unlike the others, they ride powerful thermals inside white and puffy cumulus clouds, which can elevate them 13 feet per second. “It’s the only bird that is known to enter into a cloud intentionally,” Henri Weimerskirch, lead author of the paper describing this behavior, in 2016. By doing so the birds can reach altitudes as high as 13,000 feet.

The amount of time they can soar without stopping is also impressive. While migrating through the Indian Ocean, the birds can stay in the air for up to two months, gliding between thermals while scanning for food on the surface of the sea. One tagged frigatebird traveled 34,000 miles in 185 days, stopping briefly on small islands for just four days! Such an achievement could only be accomplished by the bird with the greatest wing-area-to-body-mass ratio in the world. It also helps that they can sleep on the wing. 

American White Pelicans

American White Pelicans have a trick under their wing that sets them apart from other birds on this list: miniature tornados. White Pelicans migrate in flocks, arranging themselves in a characteristic V formation to save energy together. The separated feathers at the tip of each bird's wing creates a force called a "wingtip vortex," Hedenström says. By flying in a V formation, each bird (except for the leader) can from the wingtip vortex created by the bird ahead of it in line. A study on their close relative, the Great White Pelican on the other side of the Atlantic, showed birds could reduce their energy expenditure by 14 percent flying in this formation.

But that's not the only way American White Pelicans save energy while migrating. They  also change their flight pattern depending on the season, revealed. While they rely on updrafts and thermals during their spring migration from their wintering sites in Alabama, Louisiana, and Mississippi to their breeding sites in the Northern Great Plains, they tend to be carried by a tailwind during the autumn, when air currents are weaker. The different strategies made their migration speed significantly different: about 24 miles per hour during the spring, and 20 miles per hour in the fall.

Golden Eagles

For many birds on this list, soaring is a largely passive activity. They travel in a general direction, guided largely by wind currents, and keep their eyes open for food to scavenge or prey that's relatively easy to catch. Golden Eagles, though, engage in more complex activities while soaring. They hunt small and medium-size mammals. They stake out and defend territories of around 6,000 acres. They even play fetch with themselves. That complexity in behavior sets them apart from other large soaring species like vultures and condors, says Todd Katzner. “Golden Eagles, and other big eagles, are kind of unique because not only do they soar, but they soar to do all this crazy stuff,” he says. They don’t do very much flapping at all: It only takes up between 3 and 15 percent of their flight time.

Katzner studies a  that migrates and winters along the Appalachian Mountains. These birds don’t have access to strong thermals like their European peers residing near the Mediterranean, Katzner says. These birds have had to diversify the types of air currents they use to stay in the air. , they soar in thermals for about 41 percent of their flight time, glide between thermals about 45 percent of the time, and soar using orographic updraft, just like Andean Condors, about 13 percent of the time.

Turkey Vultures

Turkey Vultures are different from other vultures throughout the world. Most vulture species rely heavily on soaring and gliding through the air at very high altitudes (up to 37,000 feet), says Katzner, and they rely heavily on sharp vision to see the carcasses of dead animals from such a height. But Turkey Vultures have adapted to fly at lower altitudes to sniff the best pieces of carrion. In fact, they have one of the best smelling systems of all birds. In 2017, after comparing them with other 32 species, a proved that Turkey Vultures has more mitral cells, which transmit information about odors to the brain, than any of the other measured species.

These birds fly using a type of soaring called “contorted soaring.” Through this technique, Turkey Vultures ride the upward wind generated when air currents collide with treetops. This allows them to stay closer to the ground compared to other carrion eaters, like Black Vultures in North America, giving them an advantage. They can also rock side-to-side while flying to counteract the wind forces in turbulent scenarios, which allows them to have a lot of control and stability in their flight. “They can fly with almost no wind and in very turbulent settings," Katzner says. "They are just one of the coolest species in North American to me." 

White Storks

White Storks spend more time flapping than the other birds on this list: It takes up about 17 percent of their flight time, which is still significantly less than most bird species. What makes them fascinating, says Hannah Williams, is how their social interactions shape the amount of soaring, gliding, and flapping of each bird.

After following a group of 27 White Storks for four years, researchers that, when migrating, the lead bird has a more irregular flight pattern than the rest of the flock following behind. It didn't necessarily fly in a straight line, and spent most of its time exploring thermals. The researchers think that’s because the birdin front is in charge of finding new thermals that the other birds use to gain altitude. Meanwhile, the followers in the flock flapped more often to avoid falling behind the group.

In the end, these differences influenced where follower and leader storks spent the winter.  Frequent-flapping follower birds spent more energy and were able to fly only 621 miles (1,000 kilometers) from Lake Constance—where Germany, Switzerland, and Austria meet—to Spain. The leaders, on the other hand, were able to fly more than 2,485 miles (4,000 kilometers), wintering in northern Africa instead.