Flying is a balance between two sets of forces, lift and weight, and thrust and drag. Weight is the result of gravity and Lift is generated by the flow of air over the wings.
Bird wings are not flat but are concave below and convex above. The air that passes over the top of the wing has more distance to travel and thus it speeds up, causing the pressure to drop because the same amount of air is exerting its pressure over a greater area above the wing that below the wing. This effectively sucks the wing up. Meanwhile the air going below the wing has the opposite effect.
It slows down, generates more pressure and effectively pushes the wing up. Hence a bird with air moving over its wings is pulled up from above and pushed up from below. The low pressure of air on top of the wings represents a sink that the high pressure air under the wing tries to fill.
This happens most along the thin trailing edges of the wing and causes a spiralling vortex of disturbance at the wing tip, which increase drag. Therefore, the most efficient wings are those which provide lift but reduce drag, such as the crescent shaped wings of swallows and swifts.
Aerodynamic properties are measured by aspect ratio, which is the ratio of wing length divided by wing breadth. Long wings are better for gliding but harder to flap quickly and are therefore not much good at quick acceleration. Wing loading is the relationship between total body-mass measured in grams versus total wing area measured in square centimetres.
Non-flapping Flight or Gliding
Many soaring or gliding birds like vultures hang in the air and gain height without moving the wings. Essentially this means that their wings generate a lot of lift without producing much drag. Large birds have evolved to be gliders partly because gliding becomes easier with larger wings and the mechanical flapping flight become harder with larger wings.
With the exception of Hummingbirds, all birds glide to some extent when flying. As a rule, the smaller the bird, the shorter the distance it can glide and the faster it sinks. Gliding can be observed in game birds. A pheasant ascends from the ground like a rocket with fast wing beats and then glides for some distance down to the nearby woods.
Flapping flight is a more complicated process in which bird’s wing changes shape and angle of attack during both the up and down stroke. Flapping flight is basically rowing in the air with the added effort to generate lift as well. If a Blue Tit stops flapping its wings it better be about to land on a branch or it will fall to the ground. Flapping flight consists of two distinct movements: the power stroke and the back stroke. In the power stroke, the wings move forward and down; the back stroke returns the wings to the position from which the next power stroke will commence.
Soaring differs from gliding flight in that the bird does nor lose altitude and sometimes even climbs up in air. When soaring, a bird uses no energy of its own; instead it depends on external forces called thermal currents, which are rising masses of air that form over areas where the ground warms up rapidly. Obstruction currents are produced when wind currents are deflected by mountains, cliffs, or tall buildings. The resulting upward rise of air lifts birds to high altitudes, providing a base for further gliding. Soaring birds always have large and broad wings, and the ratio of their body weight to the size of the airfoils is low.
In hovering flight, a bird generates its own lift by means of rapid wing beats. Holding its body nearly vertical, with its wings firmly flexed at the elbow joint, a hovering bird moves its wing surfaces forward and back in a horizontal plane; each of the two phases of the stroke generates lift. Hummingbirds’ wings are made in such a way that when in motion they act like lifting rotors. Their pointed wings do not flap and glide as other bird wings do, but propel them through the air by moving up and down, at a rate of 70 times a second.
After feeding at a flower they can fly backward, climb vertically, turn at lightening speed, and come to a sudden standstill in midair. Hummingbirds have been known to fly up to speeds of 60 miles an hour and no bird of prey can ever catch a hummingbird in flight.
Birds have short, light and compact body as compared to other animals.
Most organs and large muscles are located near the center of gravity, which is slightly below and behind the wings to provide better balance during flight.
Contour feathers cover the body and make it streamlined and decrease drag. Down feathers and soft and meant for insulation. Primary feathers are on the wings and are also called remiges, which help in flight and also provide wing shape. Tail feathers are called rectrices which stretch sideways so that tail can be used like a rudder for turning and balancing.
The evolution of flight has endowed birds with many physical features in addition to wings and feathers. One way to reduce weight in birds is by the fusion and elimination of some unnecessary bones and the “pneumatization” of the remaining ones. Not only are some bones of birds hollow but many of the larger ones are connected to the air sacs of the respiratory system.
To keep the cylindrical walls of a bird’s major wing bones from buckling, the bones have internal strut-like reinforcements. Fusion of bones in birds makes the skeleton light as well as strong. Coracoid, furcula, and scapula form a sturdy tripod for supporting the wings and broad surfaces for the attachment of large flight muscles.
One key adaptation is the fusing of caudal bones into single pygostyle which supports the tail feathers. Birds also lack teeth or even a true jaw, instead having evolved a beak, which is far more lightweight.
Birds have uncinate processes on the ribs. These are hooked extensions of bone which help to strengthen the rib cage by overlapping with the rib behind them.
Skull is composed of thin, hollow bones,which is extremely light in proportion to the rest of the body due to elimination of a heavy jaw, jaw muscles, and teeth. The job of chewing has largely replaced by the gizzard. The skull usually represents less than 1 percent of a bird’s total body weight.