Birds are included in the phylum Chordata because they possess the defining characteristics of this group throughout their life cycle. Chordates are characterized by having a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail at some stage of their development.
In birds, the notochord is present during the embryonic stage and later develops into the vertebral column (spine). The dorsal hollow nerve cord becomes the central nervous system, which is crucial for coordination and movement. The pharyngeal slits in birds evolve into structures associated with the respiratory system, such as the gills in closely related aquatic species and later into the lungs in birds. The post-anal tail is also retained during the embryonic development of birds, indicating their place in the chordate family.
Moreover, birds share other characteristics with vertebrates, such as having a skeleton, a complex reproductive system, and advanced brain development. These shared traits solidify their classification within the phylum Chordata, showcasing their evolutionary history and biological characteristics that connect them to other members of this phylum.