How Birds Move and Fly
Birds move about either by use of their wings or their legs. Some birds use one method of locomotion more than the other. The Ostrich has lost the power of flight and to compensate has very powerful legs. On the other hand, the legs of some seabirds like petrels are particularly well-adapted for swimming but hardly support the bird’s weight on land. The Swift is another example of a bird which has specialized in flight, to the extent that its legs are so weak that it cannot stand upright and seldom lands on the ground.
It is a bird’s power of flight which sets it apart from most other vertebrate animals. With this ability they are able to exploit the air into which they can escape from terrestial predators and through which they can move freely and hunt. A bird’s wing has a curved surface, which is rounded above and hollow below, with a wedge-shaped profile. As the air flows past the wing it creates what is known as drag and lift. Drag, as the name suggests, results from the resistance of the air to the passage of the wing through it. Lift is caused by the air passing over the curved surface more quickly than under it, because the air under the wing has a slightly greater distance to travel. This difference causes a partial vacuum above the wing and pressure below which, together, creates lift.
The angle which the wing makes with the horizontal is called the angle of attack and at its optimum angle the amount of lift created by the air flowing past the wing counteracts the forces of gravity. As the angle of attack is increased, the smooth flow of air over the wing breaks down and becomes turbulent which slows the bird down, until, when the bird is about to stall, the alula or bastard wing and other slotting devices come into play. The air then rushes through these slots keeping the flow over the wing fast and free of turbulence. Incidentally, this type of slot can be seen on the forewing of aircraft.
Flapping flight is highly complicated and, in fact, is not fully understood yet. Generally speaking, forward propulsion is provided mainly by the wing tips or the primaries and the chief function of the secondary feathers is to give lift – they move relatively little as the wing beats. On the down beat the wings move powerfully not only downwards but forwards with the feathers closed. The flight feathers are flexible and twist like a propeller so that the “downward movement pulls the bird through the air. On the upstroke, which also drives the bird through the air although fractionally less powerfully, the primaries open like a Venetian blind to allow the air through. The bird rotates its wings at the shoulder to increase the angle of attack in order to maintain lift and once again the primaries are bent on the upstroke. If a bird wishes to alter course it can do so by tilting its body or adjusting its wings and tail.
Taking-off can be difficult for birds. Swans have to run to attain the necessary air speed, auks need a vertical cliff to jump off, while others like some of the vultures have difficulty raising themselves from the ground unless it is sufficiently heated to provide strong thermals. Passerines on the other hand can take-off simply by jumping into the air. They obtain the forward thrust they need by flicking their wings backwards during the upstroke and obtain the required lift by using a powerful downstroke.
Once in the air a bird can glide without beating its wings for various lengths of time depending upon the shape of the wings and the body length. Some birds, like the gulls and albatrosses, are able to glide rapidly, while slower gliding birds such as the broad-winged hawks and vultures tend to lose height more slowly. Many birds are able to soar making use of up-currents. Fulmars and gulls make use of these along cliffs; petrels and shearwaters glide along ocean waves; buzzards use the rising currents over hills and vultures and many other species use the warm thermals. Some birds can hover by reducing their speed to that of the wind but real hovering birds such as Kestrels and terns have to take a nearly vertical stance and beat their wings backwards and forwards in a horizontal plane.