Modern bird

Birds Temporal range: Early Cretaceous–Present, 125.0 – 0 Mya PreЄ Є O S D C P T J K Pg N
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Ornithurae
Class:	Aves Linnaeus, 1758
Extant Orders
Infraclass Palaeognathae Struthioniformes Rheiformes Tinamiformes Casuariiformes Apterygiformes Infraclass Neognathae Superorder Galloanserae Galliformes Anseriformes Superorder Neoaves Phoenicopteriformes Podicipediformes Columbiformes Mesitornithiformes Pteroclidiformes Apodiformes Caprimulgiformes Cuculiformes Otidiformes Musophagiformes Opisthocomiformes Gruiformes Charadriiformes Gaviiformes Procellariiformes Sphenisciformes Ciconiiformes Suliformes Pelecaniformes Eurypygiformes Phaethontiformes Cathartiformes Accipitriformes Strigiformes Coliiformes Leptosomiformes Trogoniformes Bucerotiformes Coraciiformes Piciformes Cariamiformes Falconiformes Psittaciformes Passeriformes
Synonyms
Neornithes Gadow, 1883

Birds (Aves) are a group of endothermic vertebrates, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. Birds live worldwide and range in size from the 5 cm (2 in) bee hummingbird to the 2.75 m (9 ft) ostrich. They rank as the class of tetrapods with the most living species, at approximately ten thousand, with more than half of these being passerines, sometimes known as perching birds. Birds are the closest living relatives of crocodilians. Birds are descendants of extinct dinosaurs with feathers, making them the only surviving dinosaurs according to cladistics.

The fossil record indicates that birds evolved from feathered ancestors within the theropod group, which are traditionally placed within the saurischian dinosaurs, though a 2017 paper has put them in a proposed clade Ornithoscelida, along with the Ornithischia. True birds first appeared during the Cretaceous period, around 120 million years ago. DNA-based evidence finds that birds diversified dramatically around the time of the Cretaceous–Palaeogene extinction event 66 million years ago, which reduced the Pterosaurs, and killed off all the non-avian dinosaur lineages. Birds, especially those in the southern continents, survived this event and then migrated to other parts of the world while diversifying during periods of global cooling. Primitive bird-like dinosaurs that lie outside class Aves proper, in the broader group Avialae, have been found dating back to the mid-Jurassic period, around 170 million years ago. Many of these early "stem-birds", such as Archaeopteryx, were not yet capable of fully powered flight, and many retained primitive characteristics like toothy jaws in place of beaks, and long bony tails.