What is it that makes an animal a bird?

An animal is classified as a bird primarily by its feathers, which are unique to this group, and its ability to lay eggs with hard shells. Birds also possess a lightweight skeletal structure, a beak without teeth, and a high metabolic rate to support flight. Most birds have adaptations for flight, such as elongated wings and a keeled breastbone for muscle attachment. Additionally, birds are warm-blooded, allowing them to maintain a stable internal body temperature.