Imagine rising into the air and flying under your own power, the way a bird does. This is only an imagined possibility, because people cannot fly that way, and anyone intrigued by the idea should never jump from a high place to try it. A useful way to understand the difficulty is to look at how size changes flight in real animals, beginning with a giant bird. Argentavis lived in Argentina about 6 million years ago and had a wingspan of about 6 to 7 meters (20 to 23 feet); it weighed six times as much as an Andean condor and had a wingspan over twice as long. Flapping needs more and more energy as a bird gets bigger, so small birds such as warblers can flap fast, but heavier birds, from about the size of a small hawk, rely on soaring and gliding instead.

Weight is the other half of the problem. Many birds have hollow bones, which keep them light so they can fly. A bird needs a low body weight to fly, because as body weight goes up, the muscle output needed for flying goes up too. In birds the main flight muscles make up a bigger share of body mass than in most mammals, and a bird's pectoral muscles make up about 15 to 25 percent of its body weight. Big birds such as eagles and storks have broad wings that let them soar on rising warm air, so they save the energy of flapping, and the wandering albatross has the widest wings of any living bird, about 11 to 12 feet (3.4 to 3.6 meters) from tip to tip, as of November 2026.

Wandering albatrosses fly while hardly flapping, using dynamic soaring, which exploits wind shear near the ocean surface to gain energy, plus updrafts and turbulence. In dynamic soaring, albatrosses take advantage of the different speeds and directions the wind has at different heights above the ocean, and they spend up to 90 percent of their lives soaring above it. As of December 2022, GPS tracks show wandering albatross airspeed rises with wind speed to a maximum of about 20 meters per second (45 mph). Researchers think albatrosses limit their speed to keep the force on their wings below what the wings can take, and in very light wind they use updrafts over waves. An albatross can stay in the air for hours while hardly flapping its wings. Birds with long, thin wings, such as albatrosses, can glide over the ocean for long periods.

Some flying animals were far larger than any bird alive today. Pterosaurs were the first vertebrates to fly under their own power, and only birds and bats are the other flying vertebrates. Quetzalcoatlus was a giant pterosaur with wings as wide as a small airplane's, and the Quetzalcoatlus northropi had an 11-meter wingspan; researchers found it would have had to jump up to 2.5 meters into the air and flap hard to take off. A bird's flight muscles are big chest muscles that pull the wings down, and they are anchored to a deep breastbone called the keel. A human's arm and chest muscles are not strong enough to flap wings big enough to lift a person. Flapping with human legs could work in theory only if the wings spanned at least 80 feet and weighed much less than the person.

The Smithsonian says the basic barrier to a human ornithopter is the limited muscle power and endurance of humans compared to birds. In August 2010 the Snowbird, a flapping-wing airplane, reportedly flew for 19.3 seconds and covered about 145 meters on its pilot's muscle power alone, after being towed up. Although its wingspan is comparable to that of a Boeing 737, it weighs less than all the pillows on board a 737. Fixed wings have done better: in 1988 the human-powered airplane Daedalus flew about 115 kilometers (72 miles) across the sea from Crete to Santorini in Greece. Because a fixed wing can be built both long enough and light enough, about 100 human-powered airplanes with fixed wings had flown by 2010. Engines change the picture again. As of November 2026, the record for the fastest speed in a body-controlled jet-engine-powered suit is 85.06 mph (136.891 km/h), set by Richard Browning of Gravity Industries on 14 November 2019.

Words to know

• wingspan: tip-to-tip width
• pterosaur: flying reptile
• soaring: gliding on air
• updrafts: rising air
• vertebrates: animals with backbones