AVES

Birds (class Aves ) are feathered, winged, bipedal, endothermic (warm-blooded), egg-laying , vertebrate dinosaurs. With around 10,000 living species, they are 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 (2 in) Bee Hummingbird to the 2.75 m (9 ft) Ostrich. The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic period, around 160 million years (Ma) ago. Paleontologists regard birds as the only clade of dinosaurs to have survived the Cretaceous–Paleogene extinction event 65.5 Ma 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 was 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 behaviours, including cooperative breeding and hunting, flocking , and mobbing of predators. The vast majority of bird species are socially monogamous , usually for one breeding season at a time, sometimes for years, but rarely for life. Other species have polygynous ("many females") or, rarely, polyandrous ("many males") breeding systems. Eggs are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching. Many species are of economic importance, mostly as sources of food acquired through hunting or farming. Some species, particularly songbirds and parrots, are popular as pets. Other uses include the harvesting of guano (droppings) for use as a fertiliser. Birds figure prominently in all aspects of human culture from religion to poetry to popular music. About 120–130 species have become extinct as a result of human activity since the 17th century, and hundreds more before then. Currently about 1,200 species of birds are threatened with extinction by human activities, though efforts are underway to protect them. Evolution and taxonomy Main article: Evolution of birds While Archaeopteryx is often dubbed the "oldest known bird," it is more closely related to theropod dinosaurs than modern birds The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae.[2]Carolus Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use. [3] Birds are categorised as the biological class Aves in Linnaean taxonomy . Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda.[4] Aves and a sister group, the clade Crocodilia, contain the only living representatives of the reptile clade Archosauria. During the late 20th century, Aves was commonly defined phylogenetically as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica . [5] However, an alternate definition proposed by scientists including Jacques Gauthier and adherents of the Phylocode system defined Aves to include only the modern bird groups, the crown group . This was done by excluding most groups known only from fossils, and assigning them, instead, to the Avialae ,[6] in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs. 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.[4] These two subdivisions are often given the rank of superorder,[7] although Livezey and Zusi assigned them "cohort" rank. [4] Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800[8] to 10,050.[9] Dinosaurs and the origin of birds Main article: Origin of birds Confuciusornis, a Cretaceous bird from China Based on fossil and biological evidence, most scientists accept that birds are a specialized subgroup of theropod dinosaurs.[10] More specifically, they are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids , among others. [11] As scientists discover more nonavian theropods closely related to birds, the previously clear distinction between nonbirds and birds has become blurred. Recent discoveries in the Liaoning Province of northeast China, which demonstrate many small theropod dinosaurs had feathers, contribute to this ambiguity. [12] The consensus view in contemporary paleontology is that the birds, or avialans , are the closest relatives of the deinonychosaurs , which include dromaeosaurids, troodontids and possibly archaeopterygids .[13] Together, these three form a group called Paraves . Some basal members of this group, such as Microraptor and Archaeopteryx , have features which may have enabled them to glide or fly. The most basal deinonychosaurs are very small. This evidence raises the possibility that the ancestor of all paravians may have been arboreal, may have been able to glide, or both.[14] [15] Unlike Archaeopteryx and the feathered dinosaurs, who primarily ate meat, recent studies suggest that the first birds were herbivores . [16] 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 was the first fossil to display both clearly reptilian characteristics: teeth, clawed fingers, and a long, lizard-like tail, as well as wings with flight feathers identical to those of modern birds. It is not considered a direct ancestor of modern birds, though it is possibly closely related to the real ancestor. [17] Alternative theories and controversies Early disagreements on the origins of birds included whether birds evolved from dinosaurs or more primitive archosaurs. Within the dinosaur camp, there were disagreements as to whether ornithischian or theropod dinosaurs were the more likely ancestors. [18] Although ornithischian (bird-hipped) dinosaurs share the hip structure of modern birds, birds are thought to have originated from the saurischian (lizard-hipped) dinosaurs, and therefore evolved their hip structure independently.[19] In fact, a bird-like hip structure evolved a third time among a peculiar group of theropods known as the Therizinosauridae. A small minority of researchers, such as paleornithologist Alan Feduccia of the University of North Carolina, challenge the majority view, contending that birds are not dinosaurs, but evolved from early archosaurs like Longisquama. [20][21] Early evolution of birds See also: List of fossil birds Aves Archaeopteryx Pygostylia Confuciusornithidae Ornithothoraces Enantiornithes Ornithurae Hesperornithiformes Neornithes Basal bird phylogeny simplified after Chiappe, 2007[22] Birds diversified into a wide variety of forms during the Cretaceous Period.[22] Many groups retained primitive characteristics, such as clawed wings and teeth, though the latter were lost independently in a number of bird groups, including modern birds (Neornithes). While the earliest forms, such as Archaeopteryx and Jeholornis, retained the long bony tails of their ancestors, [22] the tails of more advanced birds were shortened with the advent of the pygostyle bone in the clade Pygostylia. In the late Cretaceous, around 95 million years ago, the ancestor of all modern birds also evolved better olfactory senses. [23] The first large, diverse lineage of short-tailed birds to evolve were the Enantiornithes, or "opposite birds", so named because the construction of their shoulder bones was in reverse to that of modern birds. Enantiornithes occupied a wide array of ecological niches, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed- eaters.[22] More advanced lineages also specialised in eating fish, like the superficially gull-like subclass of Ichthyornithes (fish birds). [24] One order of Mesozoic seabirds, the Hesperornithiformes, became so well adapted to hunting fish in marine environments, they lost the ability to fly and became primarily aquatic. Despite their extreme specializations, the Hesperornithiformes represent some of the closest relatives of modern birds.[22] Diversification of modern birds See also: Sibley-Ahlquist taxonomy and dinosaur classification Containing all modern birds, the subclass Neornithes is, due to the discovery of Vegavis , now known to have evolved into some basic lineages by the end of the Cretaceous [25] and is split into two superorders, the Palaeognathae and Neognathae. The paleognaths include the tinamous of Central and South America and the ratites. The basal divergence from the remaining Neognathes was that of the Galloanserae, the superorder containing the Anseriformes (ducks, geese, swans and screamers) and the Galliformes (the pheasants, grouse, and their allies, together with the mound builders and the guans and their allies). The dates for the splits are much debated by scientists. The Neornithes are agreed to have evolved in the Cretaceous, and the split between the Galloanseri from other Neognathes occurred before the K–T extinction event , but there are different opinions about whether the radiation of the remaining Neognathes occurred before or after the extinction of the other dinosaurs.[26] This disagreement is in part caused by a divergence in the evidence; molecular dating suggests a Cretaceous radiation, while fossil evidence supports a Tertiary radiation. Attempts to reconcile the molecular and fossil evidence have proved controversial. [26][27] The classification of birds is a contentious issue. Sibley and Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds, [28] although it is frequently debated and constantly revised. Most evidence seems to suggest the assignment of orders is accurate,[29] but scientists disagree about the relationships between the orders themselves; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem, but no strong consensus has emerged. More recently, new fossil and molecular evidence is providing an increasingly clear picture of the evolution of modern bird orders. Modern bird orders: Classification See also: List of birds Neornithes Palaeognathae Struthioniformes Tinamiformes Neognathae Other birds (Neoaves ) Galloanserae Anseriformes Galliformes Basal divergences of modern birds based on Sibley-Ahlquist taxonomy Cladogram showing the most recent classification of Neoaves, based on several phylogenetic studies. [30] This is a list of the taxonomic orders in the subclass Neornithes, or modern birds. This list uses the traditional classification (the so-called Clements order), revised by the Sibley-Monroe classification. The list of birds gives a more detailed summary of the orders, including families. Subclass Neornithes The subclass Neornithes has two extant superorders – Superorder Palaeognathae: The name of the superorder is derived from paleognath, the ancient Greek for "old jaws" in reference to the skeletal anatomy of the palate, which is described as more primitive and reptilian than that in other birds. The Palaeognathae consists of two orders which comprise 49 existing species. Struthioniformes — ostriches, emus, kiwis , and allies Tinamiformes—tinamous Superorder Neognathae: The superorder Neognathae comprises 27 orders which have a total of nearly ten thousand species. The Neognathae have undergone adaptive radiation to produce the staggering diversity of form (especially of the bill and feet), function, and behavior that are seen today. The orders comprising the Neognathae are: Anseriformes —waterfowl Galliformes—fowl Charadriiformes—gulls, button-quails, plovers and allies Gaviiformes —loons Podicipediformes—grebes Procellariiformes— albatrosses, petrels, and allies Sphenisciformes—penguins Pelecaniformes—pelicans and allies Phaethontiformes — tropicbirds Ciconiiformes—storks and allies Cathartiformes —New World vultures Phoenicopteriformes— flamingos Falconiformes—falcons, eagles, hawks and allies Gruiformes—cranes and allies Pteroclidiformes— sandgrouse Columbiformes—doves and pigeons Psittaciformes —parrots and allies Cuculiformes—cuckoos and turacos Opisthocomiformes— hoatzin Strigiformes—owls Caprimulgiformes— nightjars and allies Apodiformes —swifts and hummingbirds Coraciiformes—kingfishers and allies Piciformes—woodpeckers and allies Trogoniformes—trogons Coliiformes—mousebirds Passeriformes—passerines The radically different Sibley- Monroe classification (Sibley- Ahlquist taxonomy ), based on molecular data, found widespread adoption in a few aspects, as recent molecular, fossil, and anatomical evidence supported the Galloanserae.