How Do Birds Fly?


Flight in birds depends on four physical forces – thrust, drag, lift and weight – which must be balanced for stable flight to occur.

It is one of the most complex forms of movement in the animal kingdom and as birds have adapted to specific environments they have evolved varied forms of wings and varied forms of flight.

Birds fly in a similar manner to aircraft, using lift and drag to sustain their flight. A bird uses its strong breast muscles to flap its wings, which are aerofoils, up and downwards as it flies through the air. As it moves it holds its wings at a slight angle to deflect air downwards which means the air flows faster over the wing than underneath. This causes air pressure to build up underneath its wings, while the pressure above the wings is reduced and it is this difference in pressure that causes the wing to lift.

To see this in effect, blow across a sheet of paper. The faster moving air across the top of the paper lowers the air pressure and the higher air pressure below the paper causes the paper to lift upwards.

To get the air to move over the wing requires thrust, an upward and forward force which counteracts the weight and drag of air resistance. The downstroke of the flap provides the majority of the thrust and during this ‘power stroke’ the wing is angled downwards more steeply.

The downstroke of the flap is also called the “power stroke”, as it provides the majority of the thrust. During this, the wing is angled downwards even more steeply so air is deflected downwards and to the rear.

This stroke is like a very brief downward dive through the air using the bird’s own weight to move forward. But as the wings generate lift, the bird stays airborne. During each upstroke, the angle of the wing reduces, and it folds inwards slightly to reduce resistance.

To help them achieve thrust, birds may use gravity, such as taking off from a tree, or they may make a running take-off from the ground, vigorously flapping their wings as they go.

The size and shape of birds’ wings effect their flying techniques. The increased wind speed over larger wings creates a longer path of air so the air is moving more quickly reducing air pressure and creating more lift. This means that birds with bigger wings produce greater lift and can fly slower to maintain the same lift as birds with smaller wings.

Wing loading is the weight of the bird divided the total surface area of the wings and a smaller wing loading number means the bird can fly more slowly while still maintaining lift. A smaller wing loading number also means the bird can manoeuvre more easily.

Other contributing physical factors that give birds the ability to fly include lightweight, smooth feathers, that reduces the forces of weight and drag, light bones and a beak, instead of a heavy jaw, which reduces the force of weight, a rigid skeleton to provide firm attachments for the breast muscles and a streamlined body to reduce the force of drag.

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