There are many groups of animals. Birds are in one group, called a Class. All birds share some traits.
Birds have feathers instead of hair or fur. They are the only animal group that has feathers as their body covering.
Birds have a beak or bill instead of teeth. Instead of biting and chewing their food, they tear off pieces with the beak or talons (clawed feet) and swallow it whole. The food is stored in the crop to slow digestion, then passed into the stomach to mix with digestive juices. From there it passes into a muscular gizzard that grinds it down for digestion.
Birds have wings instead of front feet or arms. Many have large, well developed chest muscles for flight. They also have hollow bones to make their skeleton lighter.
Birds are warm blooded.
Birds do not have live young, like mammals, but lay eggs.
Most birds build some kind of nest and take care of their young, feeding them until they are old enough to find their own food.
Many birds fly south to warmer climates in winter (migrate) and return north in summer to feed and nest.
Some birds are flightless, such as the ostrich or penguin. They have other ways to protect themselves, than flight. Some run fast or swim or just have a powerful kick.