A wing seen end-on. Air flowing over the curved top speeds up and presses less; air leaving the wing is bent downward and pushes the wing up. Flapping adds the forward push, and the bird's weight and the drag of the air are what the wings have to beat.
π¦ A bird flaps its wings. The wings push the air down. The air pushes the bird up.
πͺΆ Feathers are light and strong. Bird bones are light too, like straws.
βοΈ Big birds can stop flapping and glide, like a paper plane!
Wings push the air
A bird's wing is curved on top and flatter underneath. When the bird flaps down, the wing pushes air down and back. The air pushes back the other way, so the bird goes up and forward. That upward push is called lift.
Built to be light
Feathers are strong but weigh almost nothing. Many bird bones are hollow inside, like drinking straws. A bird's chest has big muscles that pull the wings down, and its heart beats very fast.
Gliding and hovering
Big birds like eagles spread their wings and ride warm air that rises from the ground, so they float without flapping. Tiny hummingbirds do the opposite. They beat their wings about fifty times every second and can hover in one spot, or even fly backwards!
The Shape of a Wing
Look at a bird's wing from the side and you see a shape scientists call an airfoil: curved on top, flatter underneath, thick at the front and thin at the back. As the bird moves forward, the wing tilts a little and bends the air flowing past it downward. Air pushed down pushes the wing up. At the same time the air racing over the curved top thins out, so there is less pressure above the wing than below it. Both effects push the bird up, and together they are called lift.
Flap Down, Fold Up
Lift only works while air is moving over the wing, so a bird has to keep moving. That is what flapping is for. On the downstroke the wing sweeps down and forward, and the long feathers at the tip twist like little propellers, pushing air backward so the bird goes forward. On the upstroke the bird folds the wing partly in and lets the feathers separate so air slips between them, which is why the upstroke does not push the bird back down.
Built for the Job
Everything about a bird is built to be light and strong. Feathers weigh almost nothing but lock together with tiny hooks, like a zipper. Many bones are hollow with struts inside, like a bridge. The chest muscles that pull the wings down can be a fifth of the bird's weight, and they hang from a big keel on the breastbone. A bird's heart beats hundreds of times a minute, and its lungs pull air through in one direction so every breath brings fresh oxygen.
Try This!
Hold a strip of paper just under your lower lip and blow across the top of it. The paper rises, because fast air over the top presses less than the still air below. That is one of the two tricks a wing uses.
Four Forces
A flying bird is pulled down by its weight and held back by drag, the friction of air on its body, and to stay up and keep going it has to make two forces of its own, lift and thrust, and unlike an aeroplane, which has wings for the first and engines for the second, a bird makes both with the same pair of wings. Lift comes from the wing's shape and angle. An airfoil, curved above and flatter below, is tilted so that its trailing edge sits a little lower than its leading edge, and as air flows past, the wing turns that air downward; by Newton's third law, air pushed down pushes the wing up. The curve also speeds the air over the upper surface, and faster-moving air has lower pressure, so the wing is pushed up from below more than it is pushed down from above. These are two descriptions of one event, not two separate forces. The popular version, that air over the top must arrive at the back at the same moment as air under the bottom, is simply wrong: the air over the top gets there first.
Where Thrust Comes From
Thrust is the job of the downstroke. The wing sweeps down and forward, and the primary feathers at the tip, which are the longest and stiffest, twist under the load so that each one acts like a small propeller blade, throwing air backward and driving the bird ahead. The inner wing, closer to the body, keeps a steadier angle and provides most of the lift. On the upstroke the bird flexes the wrist, folds the wing partly in and lets the primaries rotate apart so that air passes between them, which cuts the downward push that a rigid wing would produce. Small birds flap continuously. Larger birds alternate bursts of flapping with glides, because the muscle cost of flapping rises steeply with size.
The Machinery
The pectoralis muscles that power the downstroke are the largest in the bird's body, 15 to 25 percent of its mass, anchored to a deep keel on the breastbone; a smaller muscle beneath them, the supracoracoideus, runs through a pulley in the shoulder and raises the wing. The skeleton is fused and stiffened where a mammal's would flex, and many bones are hollow tubes braced with internal struts, which makes them stiff for their weight. Feathers are marvels: a central shaft, barbs branching from it, and barbules on the barbs with microscopic hooks that zip neighbouring barbs together, so a feather knocked apart can be preened back into a smooth surface. Breathing is one-way. Air sacs pump fresh air through the lungs on both the in-breath and the out-breath, so the blood is loaded with oxygen continuously, which is how a bar-headed goose can flap over the Himalaya where a climber would need bottled oxygen.
Soaring, Diving, Hovering
Birds have pushed this one design in every direction. Eagles and storks spread slotted wingtips that cut drag and circle in thermals, columns of warm rising air, gaining height for free and gliding to the next one. An albatross skims the waves, climbing into the wind and turning to dive downwind, harvesting the difference in wind speed near the sea surface so that it can travel thousands of kilometres with barely a flap. A peregrine falcon folds its wings into a teardrop and stoops at over 300 kilometres per hour, faster than any other animal moves. A hummingbird's shoulder rotates so far that the wing makes lift on the upstroke as well, about a quarter of the total, which is what lets it hang in the air and back away from a flower.
Think About It
An ostrich has wings and feathers and cannot fly. A penguin cannot fly either, yet it flaps through water at about the speed a bird of its size would fly in air. What has to change about a wing when the fluid it works in is 800 times denser?
The Aerodynamics, Stated Carefully
A wing produces lift by turning the airflow: the air that leaves the trailing edge has a downward component of velocity that it did not have upstream, and the reaction to that momentum change, by Newton's third law, is an upward force on the wing. The same event can be described through pressure: the flow over the cambered upper surface accelerates, and by Bernoulli's principle its static pressure falls, so the wing is pushed up by the higher pressure below. These are not two mechanisms but one, described in the language of momentum and in the language of pressure, and the textbook claim that the upper and lower streams must reunite at the trailing edge, which would require the upper flow to be faster because it has farther to go, is false: the upper stream arrives first, and an airfoil with no camber at all, a flat plate at a positive angle of attack, makes lift perfectly well. Lift scales with the square of airspeed, with wing area and with a lift coefficient set by shape and angle of attack, and increasing the angle raises lift until the flow separates from the upper surface and the wing stalls. Drag has two parts that pull in opposite directions with speed: friction and pressure drag grow as the square of speed, while induced drag, the price of the wingtip vortices that lift inevitably creates, falls as speed rises, so every bird has a speed at which the sum is least and a slightly higher speed at which distance per unit of energy is greatest.
Flapping as Propulsion
An aircraft separates lift from thrust; a bird's wing does both, and the division of labour runs along the wing. The inner wing, the arm section carrying the secondary feathers, moves through a small arc and keeps a modest angle of attack, so it behaves like a fixed wing and supplies most of the lift. The outer wing, the hand section carrying the primaries, sweeps through a large arc, so its resultant airflow comes from below and ahead on the downstroke and the local lift vector tilts forward into thrust, exactly as a propeller blade's does. The primaries are asymmetric and twist under load, each tip acting as a small propeller, and slots between spread primaries in soaring birds break the single tip vortex into several weaker ones and cut induced drag. The upstroke is managed rather than powered: wrist flexion shortens the wing, the primaries rotate to let air through, and in most birds the upstroke produces little net force, while in hummingbirds the shoulder rotates through nearly 180 degrees, the wing inverts and the upstroke supplies about a quarter of the weight support.
Physiology at the Limit
Flapping flight is the most metabolically expensive form of locomotion per unit time in the animal kingdom, and the bird's body is arranged around paying for it. The pectoralis, which powers the downstroke, is 15 to 25 percent of body mass and anchors to the keel; the supracoracoideus lies beneath it and lifts the wing by way of a tendon that passes through a pulley in the shoulder girdle, so both major flight muscles sit low and central and the wing above them stays light. The respiratory system is unlike a mammal's: rigid lungs are ventilated by a set of air sacs that act as bellows, air passes through the gas-exchange surfaces in one direction on both inspiration and expiration, and the blood flows across the airflow rather than into blind pockets, an arrangement that extracts oxygen from thin air well enough for bar-headed geese to cross the Himalaya, where they have been tracked above 7,000 metres. Skeletal weight is often misdescribed: bird skeletons are not lighter than those of equally heavy mammals, but their bones are denser and shaped as hollow, strutted tubes, which buys stiffness and strength for the same mass.
Records and Their Explanations
The extremes all follow from the same physics. A peregrine falcon in a stoop folds its wings to shrink its frontal area and lift coefficient and falls under gravity at over 300 kilometres per hour, with one trained bird measured at 389, the fastest recorded speed of any animal. An albatross flies by dynamic soaring, climbing into the wind through the layer near the sea surface where wind speed rises steeply with height, turning and descending downwind, and gaining energy on each loop from the wind gradient rather than from its muscles, which is how it can cover thousands of kilometres with a heart rate close to resting. Bar-tailed godwits leave Alaska in autumn and fly non-stop to New Zealand: a satellite-tagged female flew 11,680 kilometres in about eight days in 2007, and a later bird was tracked over 13,000 kilometres in eleven days, burning stored fat and even shrinking its digestive organs before departure. Arctic terns tracked with light-level loggers average about 71,000 kilometres a year between the two polar summers. Ibises flying in a V were shown in 2014 to place themselves precisely in the upwash off the wingtips of the bird ahead and to time their flaps to it, and pelicans in formation had heart rates 11 to 15 percent lower than birds flying alone.
What Is Still Unknown
The flow around a flapping wing is unsteady, three-dimensional and only partly captured by the fixed-wing theory above, and measuring it on a living bird is hard; the exact contribution of the upstroke in mid-sized birds, how the nervous system tunes thousands of feather positions in flight, and why the four-winged Microraptor and the first birds chose the wing arrangements they did remain open questions, and the answers are arriving from wind-tunnel particle imaging, high-speed video and a fossil record that keeps growing.
Why Your Kid Asked
Because a pigeon at the bus stop does something a child cannot, without apparent effort, and the child has already tried flapping.
The Short Version
A wing makes lift by bending the air downward as it passes; the reaction pushes the wing up, and the same event seen as pressure is that fast air over the curved top presses less than the slower air beneath. Flapping supplies the forward push: on the downstroke the outer wing and its long primary feathers sweep down and forward and act like propeller blades, while the inner wing keeps a steady angle and supplies most of the lift, and on the upstroke the bird folds the wing partly in and lets the feathers part so it does not undo the work. The rest of the bird is built around the cost: chest muscles that are a fifth of its mass on a keeled breastbone, hollow strutted bones, feathers that zip and can be repaired by preening, and a one-way lung ventilated by air sacs that keeps extracting oxygen on the out-breath as well as the in-breath. Big birds save effort by soaring in rising air or in the wind gradient over the sea; small birds flap continuously; hummingbirds rotate the wing far enough to make lift on both strokes and hover. That is the whole story, and the part that is not settled is the detail of the flapping flow, not the principle.
The Numbers
The pectoralis is 15 to 25 percent of body mass. A ruby-throated hummingbird beats its wings about 53 times a second and gets about a quarter of its weight support from the upstroke. A peregrine's stoop exceeds 300 kilometres per hour, with 389 measured for a trained bird. A bar-tailed godwit flew 11,680 kilometres non-stop in about eight days, and a later one over 13,000 in eleven. Arctic terns average about 71,000 kilometres a year. Bar-headed geese have been tracked above 7,000 metres. Pelicans in a V had heart rates 11 to 15 percent lower than solo fliers. Archaeopteryx is about 150 million years old.
Two Common Misconceptions Worth Correcting
First, "the air over the top has farther to go, so it must speed up to meet the air below at the back": the two streams do not meet, the upper one arrives first, and a flat kite at an angle makes lift with no curve at all. The honest simple version is that the wing throws air downward and the air throws the wing up, with the pressure difference as the same thing seen from the other side. Second, "birds are light because their bones are hollow": a bird's skeleton weighs about the same fraction of its body as a mammal's, and its bones are denser; the hollow tube with struts is about stiffness for the weight, not about saving it. The real weight saving is in what a bird leaves out, such as teeth, a heavy jaw and a bladder, and in fat burned before a long flight.
When to Pay Attention
The question usually comes with a bird on the ground. A fledgling, fully feathered and hopping with a short tail, is normal: it has left the nest before it can fly well and its parents are feeding it nearby, so leave it, keep the cat in, and watch from a distance. A nestling, with bare skin or fluff and eyes half open, belongs back in the nest if you can reach it; the parents will not reject it for your scent. A bird that has struck a window and sits stunned can be left in a ventilated box in a quiet dark place for an hour and released if it recovers; a bird with a drooping wing, blood or a cat's attention needs a wildlife rehabilitator, and handling should be brief, with washed hands afterwards.
Something to Do Together
Cut a strip of paper, hold one end under the lower lip and blow over the top; the strip lifts, and that is the pressure half of the story. Then hold a hand flat out of a slowly moving car window, tilt the leading edge up, and feel it rise; that is the deflection half, with no curve needed. Finish at the park: count a pigeon's wingbeats as it takes off, watch it fold its wings on the upstroke, and look for the first gull that stops flapping and rides the wind up the face of a building.
Sources
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- Ponitz, B., Schmitz, A., Fischer, D., Bleckmann, H. and BrΓΌcker, C. "Diving-flight aerodynamics of a peregrine falcon (Falco peregrinus)." PLOS ONE 9 (2014): e86506.
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