Birds are feathered winged two legged that are warm - blooded Egg-laying vertebrates. Birds in the tetrapod class with most Living species. Approximately ten thousand. Extant birds belong to the subclass Neornithes , living worldwide and ranging in size from the 5 cm bee hummingbird to the 2.75 m ostrich. The fossil record indicates that birds emerged the theropod
Dinosaurs during the Jurassic period around 150 million years
Ago.