Most paleontologists regard birds as the only clade of dinosaurs that survived the Cretaceous–Tertiary extinction event approximately 65. There are around 10,000 living species, making them the most diverse tetrapod vertebrates. 5 Ma. 7 m Ostrich. The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic period, around 150–200 Ma , and the earliest known bird is the Late Jurassic Archaeopteryx, c 155–150 Ma. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds are bipedal, endothermic , vertebrate animals that lay eggs. Birds range in size from the 5 cm Bee Hummingbird to the 2
Fossil evidence and intensive biological analyses have demonstrated beyond any reasonable doubt that birds are theropod dinosaurs. More specifically, they are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others. As scientists discover more non-avian theropods that are closely related to birds, the previously clear distinction between non-birds and birds has become blurred. Recent discoveries in the Liaoning Province of northeast China, which demonstrate that many small theropod dinosaurs had feathers, contribute to this ambiguity
All birds have forelimbs modified as wings and most can fly, with some exceptions including ratites, penguins, and a number of diverse endemic island species. 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. Birds also have unique digestive and respiratory systems that are highly adapted for flight. 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
Many species undertake long distance annual migrations, and many more perform shorter irregular movements. Most birds have an extended period of parental care after hatching. Eggs are usually laid in a nest and incubated by the parents. Other species have breeding systems that are polygynous or, rarely, polyandrous . The vast majority of bird species are socially monogamous, usually for one breeding season at a time, sometimes for years, but rarely for life. Birds are social; they communicate using visual signals and through calls and songs, and participate in social behaviours including cooperative breeding and hunting, flocking, and mobbing of predators
Together, these three form a group called Paraves. The consensus view in contemporary paleontology is that the birds, Aves, are the closest relatives of the deinonychosaurs, which include dromaeosaurids and troodontids. This evidence raises the possibility that the ancestor of all paravians may have been arboreal, and/or may have been able to glide. The basal dromaeosaur Microraptor has features which may have enabled it to glide or fly. The most basal deinonychosaurs are very small
About two dozen - see section below
It has even been suggested that Archaeopteryx was a dinosaur that was no more closely related to birds than were other dinosaur groups, and that Avimimus was more likely to be the ancestor of all birds than Archaeopteryx. It is not considered a direct ancestor of modern birds, but is the oldest and most primitive member of Aves or Avialae, and it is probably closely related to the real ancestor. Archaeopteryx has clearly reptilian characters: teeth, clawed fingers, and a long, lizard-like tail, but it has finely preserved wings with flight feathers identical to those of modern birds. The Late Jurassic Archaeopteryx is well-known as one of the first transitional fossils to be found and it provided support for the theory of evolution in the late 19th century
Archaeopteryx, from the Kimmeridgian stage of the Late Jurassic , is the earliest known bird under this definition. Phylogenetically, Aves is commonly defined as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica. Carolus Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use. Birds are categorised as the biological class Aves in Linnaean taxonomy. Aves and a sister group, the clade Crocodilia, together are the sole living members of the reptile clade Archosauria. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda. Others, including Jacques Gauthier and adherents of the Phylocode system, have defined Aves to include only the modern bird groups, excluding most groups known only from fossils, and assigning them, instead, to the Avialae in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs. The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae
The majority of their publications argued that the similarities between birds and maniraptoran dinosaurs were convergent, and that the two were unrelated. Scientists Larry Martin and Alan Feduccia believe that birds are not dinosaurs, but that birds evolved from early archosaurs like Longisquama. Maniraptorans are, instead, flightless, archosaurian, birds. Paul; where maniraptorans are secondarily flightless birds but, in their version, birds evolved directly from Longisquama. In the late 1990s the evidence that birds were maniraptorans became almost indisputable, so Martin and Feduccia adopted a modified version of a hypothesis by dinosaur artist Gregory S. The features cited as evidence of flightlessness are interpreted by mainstream paleontologists as exaptations, or “pre-adaptations”, that maniraptorans inherited from their common ancestor with birds. This theory is contested by most paleontologists. Thus birds are still not dinosaurs, but neither are most of the known species that are currently classified as theropod dinosaurs
Enantiornithes
Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800 to 10,050. All modern birds lie within the subclass Neornithes, which has two subdivisions: the Paleognathae, containing mostly flightless birds like ostriches, and the wildly diverse Neognathae, containing all other birds. These two subdivisions are often given the rank of superorder, although Livezey & Zusi assigned them “cohort” rank
Archaeopteryx
Within the dinosaur camp there were disagreements as to whether ornithischian or theropod dinosaurs were the more likely ancestors. There have been many controversies in the study of the origin of birds. Early disagreements included whether birds evolved from dinosaurs or more primitive archosaurs. Although ornithischian dinosaurs share the hip structure of modern birds, birds are thought to have originated from the saurischian dinosaurs, and therefore evolved their hip structure independently. In fact, a bird-like hip structure evolved a third time among a peculiar group of theropods known as the Therizinosauridae
Confuciusornithidae
Currently about 1,200 species of birds are threatened with extinction by human activities, though efforts are underway to protect them. Birds figure prominently in all aspects of human culture from religion to poetry to popular music. Some species, particularly songbirds and parrots, are popular as pets. Many species are of economic importance, mostly as sources of food acquired through hunting or farming. About 120–130 species have become extinct as a result of human activity since the 17th century, and hundreds more before then. Other uses include the harvesting of guano for use as a fertiliser
Critics have indicated that the fossil is poorly preserved, extensively reconstructed, and may be a chimera . The braincase is most likely that of a very early coelurosaur. Protoavis texensis, was described in 1991 as a bird older than Archaeopteryx