Birds (class The composition of each class is determined by a taxonomist. Often there is no exact agreement, with different taxonomists taking different positions. There are no hard rules that a taxonomist needs to follow in describing a class, but for well-known animals there is likely to be consensus. For example, dogs are usually assigned to the phylum Aves) are winged Flight is the main mode of locomotion used by most of the world's bird species. Flight assists birds while feeding, breeding and avoiding predators, bipedal Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs, or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning "two feet" (from the Latin bi for "two" and ped for "foot"). Types of bipedal movement include walking, running, or, endothermic (warm-blooded In biology, a warm-blooded animal species is one whose members maintain thermal homeostasis; that is, they keep their body temperature at a roughly constant level, regardless of the ambient temperature. This involves the ability to cool down or produce more body heat. Warm-blooded animals mainly control their body temperature by regulating their), egg-laying In most birds, reptiles, insects, fish, and two types of mammals an egg (Latin, ovum) is the zygote, resulting from fertilization of the ovum. To enable incubation the egg is usually kept within a favourable temperature range as it nourishes and protects the growing embryo. When the embryo is adequately developed it breaks out of the egg in the, vertebrate Vertebrates are members of the subphylum Vertebrata, chordates with backbones and spinal columns. About 58,000 species of vertebrates have been currently described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates are comprised of animals from the groups jawless fishs, bony fish, animals. There are around 10,000 living species, making them the most varied of tetrapod Tetrapods are vertebrate animals having four feet, legs or leglike appendages. Amphibians, reptiles, birds, and mammals are all tetrapods, and even the limbless snakes are tetrapods by descent. The earliest tetrapods radiated from the Sarcopterygii, or lobe-finned fish vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant Extant is a term commonly used in biology to refer to taxa that are still in existence (living). The term extant contrasts with extinct. For example, Brandt's Cormorant is an extant species, while the Spectacled Cormorant is an extinct species. Likewise, of the group of molluscs known as the cephalopods, there are approximately 600 extant species birds range in size from the 5 cm (2 in) Bee Hummingbird The Bee Hummingbird or Zunzuncito is a species of hummingbird that is endemic to Cuba and Isla de la Juventud. With a mass of approximately 1.8 grams (0.063 oz) and a length of 5 centimetres (2.0 in), it is the smallest living bird and dinosaur as well to the 2.75 m (9 ft) Ostrich The Ostrich, Struthio camelus, is a large flightless bird native to Africa. It is the only living species of its family, Struthionidae and its genus, Struthio. Ostriches share the order Struthioniformes with the kiwis, emus, and other ratites. It is distinctive in its appearance, with a long neck and legs and the ability to run at maximum speeds. The fossil record indicates that birds evolved Evolution is the change in the inherited traits of a population of organisms through successive generations. After a population splits into smaller groups, these groups evolve independently and may eventually diversify into new species. Ultimately, life is descended from a common ancestry through a long series of these speciation events, from theropod Theropods are a group[clarification needed] of bipedal saurischian dinosaurs. Although they were primarily carnivorous, a number of theropod groups evolved herbivory, omnivory, and insectivory. Theropods first appeared during the Carnian age of the late Triassic period about 230 million years ago (Ma) and included the sole large terrestrial dinosaurs Dinosaurs are a diverse group of reptiles. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous period (about 65 million years ago), when the Cretaceous–Tertiary extinction event caused the extinction of most dinosaur species, except for some birds. The fossil during the Jurassic The Jurassic is a geologic period and system that extends from about 199.6± 0.6 Ma to 145.5± 4 Ma, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic Era, also known as the "Age of Reptiles". The start of the period is marked by the major Triassic– period, around 150–200 Ma (million years ago), and the earliest known bird is the Late Jurassic Archaeopteryx Archaeopteryx , sometimes referred to by its German name Urvogel ("original bird" or "first bird"), is the earliest and most primitive bird known. The name derives from the Ancient Greek ἀρχαῖος (archaīos) meaning "ancient", and πτέρυξ (ptéryx), meaning "wing", c 150–145 Ma. Most paleontologists Paleontology [note 1] is the study of prehistoric life, including organisms' evolution and interactions with each other and their environments (their paleoecology). As a "historical science" it tries to explain causes rather than conduct experiments to observe effects. Paleontological observations have been documented as far back as the 5 regard birds as the only clade A clade[note 1] is a group consisting of an organism and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological classification. In of dinosaurs to have survived the Cretaceous–Tertiary extinction event approximately 65.5 Ma.

Modern birds are characterised A body plan is essentially the blueprint for the way the body of an organism is laid out. An organism's symmetry, its number of body segments and number of limbs are all aspects of its body plan. One of the key issues of developmental biology is the evolution of body plans as different as those of a starfish, a fern, or a mammal, from a common by feathers Feathers are one of the epidermal growths that form the distinctive outer covering, or plumage, on birds. They are considered the most complex integumentary structures found in vertebrates, and indeed a premier example of a complex evolutionary novelty. They are among the characteristics that distinguish the extant Aves from other living groups, a beak The beak, bill or rostrum is an external anatomical structure of birds which is used for eating and for grooming, manipulating objects, killing prey, probing for food, courtship and feeding young. The term beak is also used to refer to a similar mouthpart in some Ornithischian dinosaurs, monotremes, cephalopods, cetaceans, pufferfishes, turtles, with no teeth Teeth are small, calcified, whitish structures found in the jaws (or mouths) of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are covered by gums. Teeth are not made of bone, but rather of multiple tissues of varying density and, the laying Fish , amphibians, reptiles, birds and monetremata also lay eggs of hard-shelled The generalized eggshell structure, which varies widely among species, is a protein matrix lined with mineral crystals, usually of a calcium compound such as calcium carbonate. It is calcium build-up and is not made of cells. Harder eggs are more mineralized than softer eggs eggs, a high metabolic Metabolism is the set of chemical reactions that happen in living organisms to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories. Catabolism breaks down organic matter, for example to harvest energy in cellular rate, a four-chambered heart The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions. The term cardiac (as in cardiology) means "related to the heart" and comes from the Greek καρδιά, kardia, for "heart", and a lightweight but strong skeleton Bird anatomy, or the physiological structure of birds' bodies, shows many unique adaptations, mostly aiding flight. Birds have a light skeletal system and light but powerful musculature which, along with circulatory and respiratory systems capable of very high metabolic rates and oxygen supply, permit the bird to fly. The development of a beak has. All living species of birds have wings - the now extinct flightless Moa The moa were eleven species of flightless birds endemic to New Zealand. The two largest species, Dinornis robustus and Dinornis novaezelandiae, reached about 3.7 m (12 ft) in height with neck outstretched, and weighed about 230 kg (510 lb) of New Zealand New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses , and numerous smaller islands, most notably Stewart Island/Rakiura and the Chatham Islands. The indigenous Māori language name for New Zealand is Aotearoa, commonly translated as The Land of the Long White Cloud. The Realm of New Zealand also were the only exceptions. Wings are evolved Evolution is the change in the inherited traits of a population of organisms through successive generations. After a population splits into smaller groups, these groups evolve independently and may eventually diversify into new species. Ultimately, life is descended from a common ancestry through a long series of these speciation events, forelimbs, and most bird species can fly Flight is the main mode of locomotion used by most of the world's bird species. Flight assists birds while feeding, breeding and avoiding predators, with some exceptions including ratites A ratite is any of a diverse group of large, flightless birds of Gondwanan origin, most of them now extinct. Unlike other flightless birds, the ratites have no keel on their sternum—hence the name from the Latin ratis . Without this to anchor their wing muscles, they could not fly even if they were to develop suitable wings, penguins Penguins are a group of aquatic, flightless birds living almost exclusively in the southern hemisphere, especially in Antarctica. Highly adapted for life in the water, penguins have countershaded dark and white plumage, and their wings have become flippers. Most penguins feed on krill, fish, squid, and other forms of sealife caught while swimming, and a number of diverse endemic As applied to birds, the term "endemic" refers to any species found only in a specific geographical area. There is no upper limit for the area; it would not be incorrect to refer to all bird species as endemic to Earth. But in practice, the largest areas for which the term is in common use are countries or zoogeographical regions and island species. Birds also have unique digestive Digestion is the mechanical and chemical breaking down of food into smaller components that can be absorbed into a blood stream, for instance. Digestion is a form of catabolism: a break-down of larger food molecules to smaller ones and respiratory systems The respiratory system's function is to allow oxygen exchange through all parts of the body. The space between the alveoli and the capillaries, the anatomy or structure of the exchange system, and the precise physiological uses of the exchanged gases vary depending on organism. In humans and other mammals, for example, the anatomical features of that are highly adapted for flight. Some birds, especially corvids Corvidae is a cosmopolitan family of oscine passerine birds that contains the crows, ravens, rooks, jackdaws, jays, magpies, treepies, choughs and nutcrackers. The common English names used are corvids or the crow family (more informally), and there are over 120 species. The genus Corvus, including the jackdaws, crows and ravens, makes up over a and parrots Parrots, also known as psittacines , are birds of the roughly 372 species in 86 genera that make up the order Psittaciformes, found in most warm and tropical regions. The order is subdivided in three families: the Psittacidae ('true' parrots), the Cacatuidae (cockatoos) and the Strigopidae (New Zealand parrots). Parrots have a pan-tropical, are among the most intelligent animal species; a number of bird species have been observed manufacturing and using tools Tools are used by some animals, particularly primates, to perform simple tasks such as getting food or grooming. Originally thought to be a skill only possessed by human beings, tool use requires some level of intelligence. Primates have been observed exploiting sticks and stones to accomplish tasks. Numerous bird species have also been noted as, and many social species exhibit cultural transmission of knowledge across generations.

Many species undertake long distance annual migrations Bird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. These however are usually irregular or in only one direction and are termed variously as nomadism, invasions, dispersal or irruptions. Migration is marked by its, and many more perform shorter irregular movements. Birds are social; they communicate using visual signals and through calls and songs Bird vocalization includes both bird calls and bird songs. In non-technical use, bird songs are the bird sounds that are melodious to the human ear. In ornithology and birding, songs are distinguished by function from (relatively simple) calls, and participate in social behaviours including cooperative breeding Helpers at the nest is a term used in behavioural ecology and evolutionary biology to describe a social structure in which juveniles and sexually mature adolescents of either one or both sexes, remain in association with their parents and help them raise subsequent broods or litters, instead of dispersing and beginning to reproduce themselves and hunting, flocking Flocking behavior is the behavior exhibited when a group of birds, called a flock, are foraging or in flight. There are parallels with the shoaling behavior of fish, the swarming behavior of insects, and herd behavior of land animals, and mobbing Mobbing behavior in animals is an antipredator behavior which occurs when individuals of a certain species mob a predator by cooperatively attacking or harassing it, usually in order to protect their offspring. A simple definition of mobbing is an assemblage of individuals around a potentially dangerous predator. This is most frequently seen in of predators. The vast majority of bird species are socially monogamous Monogamy is the state of having only one sexual partner at any one time. The word monogamy comes from the Greek word monos "μονός", which means one or alone, and the Greek word gamos "γάμος", which means marriage or union. In many cases, the word "monogamy" is used to specifically refer to marital monogamy, usually for one breeding season at a time, sometimes for years, but rarely for life. Other species have breeding systems that are polygynous Polygyny is a form of marriage in which a man has two or more wives at the same time. In countries where the practice is illegal, the man is referred to as a bigamist and or a polygamist. It is distinguished from relationships where a man has a sexual partner outside marriage, such as a concubine, casual sexual partner, paramour, cohabitates with ("many females") or, rarely, polyandrous Polyandry refers to a form of marriage in which a woman has two or more husbands at the same time. The form of polyandry in which a woman is married to two or more brothers is known as "fraternal polyandry", and it is believed by many anthropologists to be the most frequently encountered form ("many males"). Eggs are usually laid in a nest and incubated Incubation is the process by which birds hatch their eggs, and to the development of the embryo within the egg. The most vital factor of incubation is the constant temperature required for its development over a specific period. Especially in domestic fowl, the act of sitting on eggs to incubate them is called brooding. The action or behavioral 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 A songbird is a bird belonging to the suborder Passeri of the perching birds . Another name that is sometimes seen as scientific or vernacular name is Oscines. This group contains some 4000 species, in which the vocal organ typically is developed in such a way as to produce a diverse and elaborate bird song and parrots Parrots, also known as psittacines , are birds of the roughly 372 species in 86 genera that make up the order Psittaciformes, found in most warm and tropical regions. The order is subdivided in three families: the Psittacidae ('true' parrots), the Cacatuidae (cockatoos) and the Strigopidae (New Zealand parrots). Parrots have a pan-tropical, are popular as pets. Other uses include the harvesting of guano Guano is the excrement (feces and urine) of seabirds, bats, and seals. Guano manure is an effective fertilizer and gunpowder ingredient due to its high levels of phosphorus and nitrogen and also its lack of odor. Superphosphate made from guano is used for aerial topdressing. Soil that is deficient in organic matter can be made more productive by (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

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. Phylogenetically, Aves is commonly defined as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica.^[5]

Archaeopteryx, from the Tithonian stage of the Late Jurassic (some 150–145 million years ago), is the earliest known bird under this definition. Others, including Jacques Gauthier and adherents of the Phylocode system, have defined Aves to include only the modern bird groups, the crown group. This has been 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 subclass Neornithes, which has two subdivisions: the Palaeognathae, containing birds that are flightless (like ostriches) or weak fliers, and the wildly 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

Based on fossil and biological evidence, most scientists accept that birds are a specialized sub-group 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 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.^[12]

The consensus view in contemporary paleontology is that the birds, Aves, are the closest relatives of the deinonychosaurs, which include dromaeosaurids and troodontids. Together, these three form a group called Paraves. The basal dromaeosaur Microraptor has features which may have enabled it 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.^[13][14]

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 has clearly reptilian characteristics: teeth, clawed fingers, and a long, lizard-like tail, but it has finely preserved wings with flight feathers identical to those of modern birds. It is not considered a direct ancestor of modern birds, but is the oldest and most primitive known member of Aves or Avialae, and it is probably closely related to the real ancestor.^[15]

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