The offspring may remain with the mother until it is more than a year old, returning to her pouch for nourishment or safety. Birds are the only animals with feathers, many others may have wings to fly with, none have feathers. Most birds are flying creatures, even the now flightless ostriches and penguins evolved from ancestors quite capable of flight. The reason birds are able to fly is largely due to their physical make-up. Putting aside the outwardly obvious, wings and feathers, the internal body parts of all birds, including flightless ones, show the evolution of birds as flying animals.
Birds have lightweight skeletons in which many of the major bones are hollow.
Unique to the skeleton of birds is the furculum, or wishbone. When in flight, the furculum not only aids the animal in breathing, but acts as a shock absorber against the great force of wing movement. Eggs are laid instead of the mother carrying the young inside her. Light skeletons, hollow bones, and of course, feathers. Fish are a diverse group of animals that live and breathe in water.