There are about 40 flightless species of birds living today. All of these evolved from ancestors that flew and most evolved in the absence of predators. There are two key differences between birds that fly and those that can't:

• The wing bones of flightless birds are relatively small.
• Flying birds have a keel on their breastbone. The keel anchors the strong muscles that move a bird's wings during flight. Flightless birds have a flat breastbone or a greatly reduced keel.

Flightlessness is an adaptation that usually evolves on islands where predators are absent. However, not all flightless birds are found on islands. Ostriches live on the African savannas. Since ostriches can't fly away to escape predators, ostriches have evolved another defense. Ostriches can use the claws on their feet as dangerous weapons when threatened.