Updated: December 25, 2024

All modern birds lie within the crown group Aves ( alternately Neornithes ), which has two subdivisions: the Palaeognathae, which includes the flightless ratites ( such as the ostriches ) and the weak-flying tinamous, and the extremely diverse Neognathae, containing all other birds. These two subdivisions have variously been given the rank of superorder, cohort, or infraclass. The number of known living bird species is around 11,000 although sources may differ in their precise numbers.