The determining factor in the development of the bird's body is the ability to fly. The key is to reduce the body weight, which makes it to fly is less strenuous and therefore requires less energy. For a more detailed description; especially read on.
The less you weigh, the less energy it takes to lift something. When birds are the wings that lift the body. Therefore the body of the bird is ultra lightweight evolved over the years. To save weight birds have no teeth and jaw muscles. The skeleton also weighs less because many bones are hollow.
Fly Muscles need a lot of oxygen. However, the lungs of birds work in a different way than in mammals. They do not expand and do not draw together, but are connected to air pockets that as a kind of bellows, the air efficiently guided by the respiratory system to go.
Due to the transformation of the front limbs wings is in the center of gravity of the modified bird. The actual position of the legs in comparison with mammals pushed forward, so that birds can walk without tumbling over.
The aerodynamic principle which planes can fly, is copied from the construction of bird wings. The upper side is bent and the speed at which the air passes over the wing, is different from the speed in the area underneath it. As the air pressure above the wing is less than below it, a bird can fly. Heavy birds - such as albatrosses, swans and pelicans - rise not as easy as eg small finches. They build their speed by running across the water to rise then. Strong downward wing beats provide trim and strong chest muscles provide powerful strokes. Once in the air to generate the wing beats driving force.
Once remain in the air even larger birds glide easily. Gieren search columns of rising warm air, also known as thermals. They use their wings almost, but glide hot air columns. Some birds such as the albatross, staying almost all their lives in the air. As flying fish search them in order to create the then out of the water.
If the bird will slow down for landing, he wafts through the air and hold the wings still. He adjusts the position of the wings, so that he can reach the surface or branch. To decelerate rapidly, the bird spreads its tail feathers so that the air resistance becomes larger and increases the inhibitory effect. Swifts can not countries. This has to do with their wing shape, because they can be difficult to fly up when they have landed on the ground. Research has even shown that sleep swifts in the air.