Birds are feathered, winged, bipedal, endothermic, egg-laying, vertebrate animals. Around 10,000 living species makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from the 5 cm Bee Hummingbird to the 2.75 m Ostrich. The fossil record indicates birds evolved from theropod dinosaurs during the Jurassic period, around 150-200 million years ago, and the earliest known bird is the Late Jurassic Archaeopteryx, around 150-145 million years ago. Most paleontologists regard birds as the only clade of dinosaurs to have survived the Cretaceous-Tertiary extinction event approximately 65.5 million years ago.
Modern birds are characterised by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. All living species of birds have wings - the now extinct flightless Moa of New Zealand being the only exception. Wings are evolved forelimbs, and most bird species can fly, with some exceptions, including ratites, penguins, and a number of diverse endemic island species. Birds also have unique digestive and respiratory systems that are highly adapted for flight. Some birds, especially corvids and parrots, are among the most intelligent animal species. A number of bird species have been observed manufacturing and using tools, and many social species exhibit cultural transmission of knowledge across generations.
Many species undertake long distance annual migrations, and many more perform shorter irregular movements. Birds are social; they communicate using visual signals and through calls and songs, and participate in social behaviors, including cooperative breeding and hunting, flocking, and mobbing of predators.