Birds
Birds are unique in having feathers, the major characteristic that makes them different from all other animals. Birds are endothermic, which means that they produce their own body heat and are warm blooded vertebrates more related to reptiles than to mammals and that they have a four-chambered heart (as do mammals), forelimbs modified into wings (a trait shared with bats), a hard-shelled egg, and keen vision, the major sense they rely on for information about the environment.
Modern birds are characterized by feathers, a beak with no teeth, a high metabolic rate, and a lightweight but strong skeleton. All living species of birds have wings. Wings are evolved forelimbs, and most bird species can fly; exceptions include the ostriches, emus and relatives, and penguins. Their hollow, light weight bones enable them to fly.
Modern birds are characterized by feathers, a beak with no teeth, a high metabolic rate, and a lightweight but strong skeleton. All living species of birds have wings. Wings are evolved forelimbs, and most bird species can fly; exceptions include the ostriches, emus and relatives, and penguins. Their hollow, light weight bones enable them to fly.