Weight reduction by birds
Although we have a poor fossil record describing the evolution of birds there is little doubt that they evolved directly from a small coelurosauriandinosaur. However, the conquest of the air by birds was not only achieved with the adaptation of the feathers and powerful wing muscles, but also necessitated considerable weight reductions. The bones of birds are extremely thin and hollow inside, with structural strength being created by cross struts. The heavy extension of the spine that supported Archaeopteryx's tail has been replaced with stout quilled feathers. The heavy jaw with teeth has been replaced with a beak composed of lightweight protein called keratin.