There are many species of flightless birds ranging from rails to penguins to the ostrich. Flightless birds evolved from birds that could fly but in areas where flight was not needed for survival. Flightless bird’s bones are more dense and heavier than flighted birds. With the exception of penguins, which have adapted to “flying” through the water, most flightless birds lack a keeled sternum for flight muscle attachment. These skeletal adaptations, along with the absence of flight feathers, permanently ground these birds.
Ratite is term used to name a diverse group of large, flightless birds. This group includes the ostrich, emu, rhea, cassowary, and kiwi. In the prehistoric past there were many more members of this group including the elephant bird and the giant moa. The word ratite is Latin for raft, which refers to the shape of their keel-less sternum. Birds that fly possess a large keel on their sternum for muscle attachment.
The order Sphenisciformes consists of a single group of birds, the penguins. Found only in the Southern Hemisphere, penguins are flightless marine birds with wings that have adapted to be more flipper-like to aide in aquatic locomotion. There are approximately 20 species of penguins alive today.