Birds are classified as members of the class Aves and share several key derived characteristics including feathers, wings adapted for flight, a beak with no teeth, and a lightened skeleton. The chapter discusses the origin of birds from feathered dinosaurs like Archaeopteryx as well as the anatomy, physiology, behavior and ecology of modern living birds including their feathers, skeletons, digestive and respiratory systems, migration, mating systems, and nesting behaviors.