How Do Birds Fly? Wings, Feathers and Lift Explained

How a bird’s wing creates lift

A bird’s wing in cross-section is an airfoil: rounded at the front, thin at the back and slightly curved, with the front edge tilted a little upward. This tilt is called the angle of attack. As the wing moves through the air, it turns the airflow downward.

Two ways of describing the same physics explain why this lifts the bird. Air flowing over the curved upper surface speeds up, and its pressure drops, while the air underneath is at higher pressure, so the wing is pushed upward. At the same time, by deflecting a large mass of air downward, the wing receives an equal and opposite push upward, as Newton’s third law predicts. Both descriptions are correct and go together.

Lift grows with speed and with wing area. That is why heavy birds need either big wings or high speed, and why a bird landing slowly spreads its wings and tail as wide as possible.

How flapping pushes a bird forward

Lift alone would not keep a bird moving, because air resistance, called drag, slows it down. Flapping solves this. On the downstroke the wing sweeps down and forward, and the long primary feathers at the wingtip twist so that they work like the blades of a propeller, producing both lift and forward thrust.

The downstroke is driven by the pectoralis, the big breast muscle that makes up a large share of a flying bird’s weight. The wing is raised again by a smaller muscle, the supracoracoideus, whose tendon passes through a gap in the shoulder bones and pulls the wing up like a rope over a pulley. In most birds the upstroke is a recovery stroke: the wing is partly folded or the feathers part to let air through, so it creates little drag.

Watch a pigeon take off from a city square and you can see this in action: the wings clap together over the back and sweep down hard, producing the loud clatter typical of a startled rock pigeon.

Feathers, bones and a body built for flight

Feathers. Flight feathers are light but stiff, with interlocking barbs that form a smooth surface. The primaries at the wingtip give thrust, the secondaries along the inner wing provide most of the lift, and the tail feathers act as a rudder and brake. A small tuft on the “thumb,” the alula, can be raised to keep air flowing smoothly over the wing at low speeds and prevent a stall.

Bones. Many bird bones are pneumatic: hollow, filled with air from the breathing system and braced inside by thin struts. They are strong and stiff for their weight. Research has shown that bird skeletons are not simply lighter than mammal skeletons of the same body mass; instead the bones are denser and stronger, and the savings come from fused and reduced parts. Birds have no teeth and no heavy jaw, a fused lower back (the synsacrum), a short fused tail bone (the pygostyle) and a wishbone made of fused collarbones.

Breathing and the heart. Flight takes a great deal of energy. Birds have a system of air sacs that keeps air flowing in one direction through rigid lungs, collecting oxygen more efficiently than mammal lungs. A large, powerful heart supplies the flight muscles.

The keel. The breastbone carries a deep ridge called the keel, which anchors the flight muscles. Flightless birds such as ostriches have lost it.

Flapping, gliding and soaring

Birds use several kinds of flight, often in the same journey.

Gliding birds trade height for distance, while soaring birds also gain height from moving air. Over land, broad-winged eagles and storks circle up inside thermals, columns of air warmed by the sun, then glide to the next one. Over the Southern Ocean, the wandering albatross, whose wingspan can exceed 3 m (10 ft), uses dynamic soaring: it swoops into the faster wind higher above the waves, then turns down into the calmer air near the surface, gaining energy on each cycle. Tracking studies have found that its heart rate while soaring is close to its resting rate. A tendon “shoulder lock” lets it hold its wings out without muscular effort.

How hummingbirds hover

Hovering is the most demanding flight of all, and hummingbirds are the experts. A ruby-throated hummingbird beats its wings around 50 times a second. Its wing can rotate almost freely at the shoulder, so the wingtip traces a flat figure eight and the wing flips over on the backstroke.

A study of rufous hummingbirds published in 2005 found that they make about 75% of their lift on the downstroke and about 25% on the upstroke. That is why hummingbirds can hover, and even fly backwards, in a way no other bird can sustain. See can hummingbirds fly backwards? for more.

What wing shape tells you about a bird

Wing shape is a good clue to how a bird lives.

• Long, narrow wings glide efficiently: albatrosses, gulls and swifts.
• Broad wings with spread, finger-like tips suit slow soaring on thermals: eagles, vultures and storks.
• Short, rounded wings give quick takeoffs and tight turns among trees: sparrowhawks, pheasants and many songbirds.
• Pointed, swept-back wings are built for speed: falcons, swallows and pigeons.

Myths about bird flight

A common textbook explanation says that air over the top of the wing must “meet up” with air underneath at the trailing edge, so it travels faster. This “equal transit time” idea is wrong: the air over the top actually arrives first. Lift comes from the wing’s shape and angle turning the airflow downward.

Another myth is that birds need hollow bones to fly. Bats fly well with solid bones, and many diving birds have heavier, less hollow bones than their relatives. Hollow bones help, but flight depends on the whole package: feathers, muscles, breathing and wing shape. Some birds have given it up entirely, as shown in our list of flightless birds.