Birds generally have feathers, wings, a toothless beak, and a lightweight skeleton that favors flying. Birds are also warm-blooded, with a four-chambered heart and a high metabolism; and they lay hard-shelled eggs. Although there are other animals that have some of these characteristics, birds are unique in having all of these features.

Birds as a group are designed around the ability to fly. They have many physiologically adaptations for flight. Wings that enable most bird species to fly. All living bird species have wings, although there were some wingless birds in the past they are all extinct now. Birds also have unique digestive, respiratory and skeletal systems that are specialized adaptations for flight. In addition, birds have feathers with serve as a lightweight body covering, but that are also useful for flying.

However, there are some flightless birds such as ratites and the penguins. These birds have lost the ability to fly as they have evolved to fit into some specialized ecological niche. For example, as penguins evolved to live in the ocean it was more useful to have wings that served as flippers rather than wings that allowed the penguin to fly.

Birds are interesting creatures to watch and study. Many birds are very social animals. Birds use visual signals, such as bright colored feathers, and sounds, such as bird calls and songs, to communicate with each other. Birds show other social behaviors, such as cooperative breeding, predator defense, and foraging, flocking and parental care. Birds exhibit a variety of mating systems, although most bird species are monogamous. Some bird species are polygynous (i.e. one male mates with multiple females) and a few are polyandrous (i.e. one female mates with multiple males).