Ostriches lack the ability to fly because their wings are far too small to generate sufficient lift for their 200-pound bodies. This is the same reason that their relatives, called the ratites, lack the capacity for flight. To cope with such an apparent competitive disadvantage, these birds have evolved large size and great speed. All ratites live in the Southern Hemisphere, and scientists believe that they share a common ancestor.
Sometimes standing more than 6 feet tall, ostriches usually attain 200 pounds or more in weight. Ostriches have wingspans of nearly 6 feet, but to achieve flight they would need much longer wings. As a comparison, bald eagles have wingspans of approximately 6 feet, yet they only weigh about 10 pounds. Even if ostriches weighed much less than they do, and had wings that were much longer, they lack large enough muscles to move them effectively.
Scientists suspect that the common ancestral species of all ratites had the capability of flight. Over time, the individual populations all evolved to survive in their own habitats, leading to the various species that are currently alive. Along with ostriches, emus, cassowaries, kiwis rheas and the extinct moa are all members of the order Struthioniformes.