All birds are bipedal warm-blooded animals with wings, four-chambered hearts, tough beaks instead of teeth and feathers growing from their skin. Most species have hollow or otherwise lightened bones, except for heavier species that cannot fly. Most birds can fly, and even those that cannot evolved from species that could.
Many of the features universal to birds exist as adaptations for flight. In addition to their light bones, beaks are also much lighter than the teeth they replace. Feathers are also specially designed for flight, although in non-flying groups, their structure is often very simplified. The major groups of flightless birds are ostriches and rheas, emus and cassowaries, kiwis and penguins. Individual flightless species exist in some other groups, but the large majority are flying.
Penguins, as a group, vary more from the normal features of birds than any other group, although they retain the essential features that define them as birds. They are true amphibious organisms that spend up to 75 percent of their time in the water and feed exclusively on aquatic prey. Their eggs are laid and their young are raised on land, however. Their wings are stiffened and modified into flippers, allowing them to fly through the water as other birds do through the air.