They are also useul for turning your head, when your eyes are fixed deep in sockets, without having to turn your whole body. Most flightless birds, having no need of the large flight muscles, have therefore, no need of the large keel or sternum that flying birds have. The exception to this rule are penguins who effectively fly through the water with their modified wings and still need large muscles to power them. Bird's inherited their basic skeleton from their reptilian ancestors. The constraints of flight however have meant that birds have had to modify it in several major ways.
Flight means lifting the birds weight, so the first major consideration is reduction in weight.
The main ways birds have lost weight is through the loss of teeth and the large jaw bones needed to support teeth, the loss of nearly all the tail and reduction of the skull. The flapping of wings to achieve flight requires huge muscles and these muscles need to be solidly attached to the skeleton. They also to generate tremendous stresses in the skeleton when working. A bird gets around the first problem by having a greatly enlarged sternum sometimes called a keel or carinawhich we call the breast plate. It is also missing in Archeopteryx, however scientists believe that in the ratites the sternal keel has been lost as a result of the birds having adopted a flightless life style, whereas in Archeopteryx it had not evolved yet.