Birds come bipedal, warm-blooded, egg-laying vertebrates characterized primarily by feathers, forelimbs modified as wings, and hollow bones.

Birds zero in size from either a bantam hummingbirds to the huge Ostrich and Emu. Based in taxonomical viewpoint, there are all astir 8,800–10,200 residing bird metal money (+ about 120–130 that use get extinct in a span of human history) in the globe, making the two the virtually all diverse class of terrestrial vertebrates.

Birds come the super differentiated class, by having a bit of feeding in nectar, plants, seeds, insects, rodents, fish, carrion, or more birds. Virtually all birds come diurnal, or active when you took a day. Occasionally birds, like a owls and nightjars, are nocturnal or crepuscular (active during twilight hours). Several birds migrate long distances to utilise optimal home ground (e.g., Arctic Tern) while others spend everthing but all their instance confounded (e.g. a Wandering Albatross). A select few, like frigatebirds, stay aloft for days at once, possibly dormant in flight.

Most common characteristics of birds include the pinched beak with no dentition, a egg laying of arduous-shelled eggs, high metabolic rate, and the weak however heavy skeleton. Virtually all birds come characterized by flight, though the ratites are flightless, & many more mintage, particularly in islands, keep around too wasted this ability. Flightless birds include a penguins, Ostrich, kiwi, and a out Dodo. Flightless mintage come vulnerable to extinction once homo or even a mammals they introduce arrive in their home ground, e.g. a Great Auk, flightless rails, and a moa of New Zealand.

Bird orders
This occurs as listings of the taxonomical orders in the class Aves. List of birds gives a extra elaborate sum-up, including families. Struthioniformes, Ostrich, emus, kiwis, and allies Tinamiformes, tinamous Anseriformes, waterfowl Galliformes, fowl Sphenisciformes, penguins Gaviiformes, loons Podicipediformes, grebes Procellariiformes, albatrosses, petrels, and allies Pelecaniformes, pelicans and allies Ciconiiformes, storks and allies Phoenicopteriformes, flamingos Accipitriformes, eagles, hawks and allies Falconiformes, falcons Turniciformes, button-quail Gruiformes, cranes and allies Charadriiformes, plovers and allies Pteroclidiformes, sandgrouse Columbiformes, doves and pigeons Psittaciformes, parrots and allies Cuculiformes, cuckoos Strigiformes, owls Caprimulgiformes, nightjars and allies Apodiformes, swifts Trochiliformes, hummingbirds Coraciiformes, kingfishers Piciformes, woodpeckers and allies Trogoniformes, trogons Coliiformes, mousebirds Passeriformes, passerines

Note: This is a traditional classification (the thus-supposed Clement's the correct sequence). a extra recent, radically different classification according to molecular information has been developed (the and then-alleged Sibley's the correct sequence) & is gaining acceptance.

Evolution
Birds come usually considered to stand evolved from either theropod dinosaurs. Specifically, birds come members of Maniraptora, a class action of bird-footed dinosaur which includes dromaeosaurs and oviraptorids. When supplementary non-avian theropod dinosaur that come closely related birds come found, a at one time clear distinction between non-birds & birds becomes less and then. Recent discoveries inside N-nor'-east China (Liaoning Province) demonstrating that many little theropod dinosaurs had feathers contribute to this ambiguity.

A radical bird Archaeopteryx, from a Jurassic, is well-referred to as one of a number one "missing links" to exist as obtained within trend lines of evolution in the late 19th century. It remains a virtually all primitive known bird. More Mesozoic birds include the Confuciusornithidae, Enantiornithes, Ichthyornis, and Hesperornithiformes, a class action of flightless divers resembling grebes and loons.

A recently found dromaeosaur, Cryptovolans, was capable of powered flight, contained a keel and had ribs by using uncinate processes. As a matter of fact, Cryptovolans makes a better "bird" than Archaeopteryx which is missing some one modern bird features. Because of this, a select few palaeontologist stand suggested that dromaeosaurs come actually basal birds whose big members are secondarily flightless, i personally.e. dromaeosaurs evolved from either birds & non a more way in. Grounds to believe for this theory is presently inconclusive, however digs prove my point to unearth fossils (especially inside China) of the unknown plumy dromaeosaurs.

It should become noted that although ornithischian (bird-hipped) dinosaurs share a equivalent hip structure as birds, birds actually originated from either a saurischian (lizard-hipped) dinosaurs, and so hit their hep structure problem independently. In point of fact, a bird-birdlike hip to structure developed the third period among a peculiar class action of theropod dinosaur, the Therizinosauridae.

Modern birds come classified inside Neornithes, which are then split into a Paleognathae & Neognathae. A paleognaths include a tinamous (found merely around Exchange & South Us) & a ratites. A flightless bird come big flightless birds, & include ostriches, cassowaries, kiwis & emus; a few man of science suspect that a flightless bird represent an unreal grouping of birds which st& independently wasted a ability to fly, others contend that a flightless bird never experienced a ability to fly and come additional directly related the dinosaurs. A radical rate of change from either a unexpended Neognathes was that of a Galloanseri, a superorder containing the Anseriformes (ducks, geese and swans), and a Galliformes (the pheasants, grouse, and their allies). Watch a chart.

A classification of birds occurs as contentious issue. Sibley & Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds (although frequently debated and constantly revised). Grounds to believe for the various orders seems to become fairly dependable, however a relationships between the orders come within a state of disarray. Grounds to believe from either modern bird anatomy, fossils & DNA stand completely been brought to bear on the condition however there are no hard consensus has emerged. Look at likewise: Sibley-Ahlquist taxonomy.

Reproduction
Although virtually all male birds keep close at hand there is no external reproductive organ, a male does develop deuce testes which become hundreds of days big in a period of the breeding year to develop sperm. A female's ovaries also become big, although lone a left ovary actually functions.

In the males of metal money forswearing the phallus (see beneath), spermatozoon is stored inside a proctodeum compartment within a cloaca prior to copulation. When you took copulation, a female moves her backside to a side & the male either mounts the female from either behind or even moves super or so her. He moves a opening of his sewerage, or even vent, some hers, and so that a sperm cell might enter a female's sewer, inside what is known as the cloacal kiss. This might happen around no time, occasionally in less than a single 2nd.

A sperm cell is stored in the female's sewerage for anywhere from either the week to a year, based on the coinage of bird. So, a single by of these, eggs may descend from either a female's ovaries & get fertilized per male's spermatozoan, prior to existence later on placed per female. A eggs might so prove my point their development in the nest.

Several waterfowl & some other birds, like a ostrich and turkey, do possess the phallus. Except in a period of intercourse, these are hidden inside the proctodeum compartment within a sewer, upright in a vent. A avian phallus differs from either a mammalian penis in several ways, virtually all importantly therein these are strictly the coital organ & is non utilized for dispelling piss.

When a eggs hatch, parent birds provide variable degrees of care within terms of food & protection. Precocial birds can care for themselves independently inside minutes of hatching; altricial hatchlings are helpless, unsighted, & naked, & postulate extended parental care. A chicks of numbers of ground-incubation birds like partridges and waders come typically breathe to rerun virtually immediately when hatching; such birds are known as nidifugous. A young of hole-nesters, but then, come typically completely incapable of single-handed survival. "Fledging" is the run of the chick getting feathers until it might fly.

Occasionally birds, like pigeons, geese, & Red-crowned Cranes, remain with their mates for even life (or for an extended time period) & will develop offspring regularly.

Mating systems and parental care

Witharound virtually all groups of fauna, male parental care is unmost common, however within birds these are quite common; these are extra extensive in birds than in any more vertebrate class. A single cause for this big percentage of monogamousness by owning birds is due to the fact that male birds come even when whizz at parental care as females. when a matter of fact, male care may be seen as crucial or even essential to female fitness. "In one form of monogamy such as with obligate monogamy a female cannot rear a litter without the aid of a male" (Gowaty, 1983). These are assumed that whenever such male care is necessary, monogamousness might develop directly sustaining this. It seems unknown so that the male’s ability to defend mates & resources relies in his ability to separate himself from either parental care.

A parental behavior virtually all associated by owning monogamousness was male incubation. This is very interesting, because male nesting is the virtually all confining male parental behavior. It non single consumes instance, however as well will necessitate physiologic changes that interfere by having common union. By having a extreme loss of union chance, there is a reduction in the generative profits among males. "This information then suggests that sexual selection may be less intense in taxa where males incubate, hypothetically because males allocate more effort to parental care and less to mating" (Ketterson & Nolan, 1994). These are understood so that a females associated by owning these males base their guide of mate in parental behaviors like than physical appearance.

These are surprising that using a total of instance & energy required, that agnatic care is selected for the least bit. Evolution suggests that a female should buy mates that confer maximal fitness. So, whenever males contribute to the care of the offspring each male & female fitness can be maximized. "High paternal care has been seen among many species of monogamous mammals and several authors suggest that male investment is one of the primary advantages of monogamous mating (Zeveloff and Boyce, 1980). This suggests then that this mating system whether costly or not must be advantageous in some way or these females would not be selecting males with this behavior.

It seems likely that fitness is increased in some way by this behavior of male parental care, but not so extreme that it is adopted by most other species. Surprisingly, there appears to be a lot of research to show that monogamy and paternal behavior are actually maladaptive due to its cost to fitness. However, is this due to the use of male care in this mating system? While the answer may be yes, it also seems to prevail in bird populations for some reason. What this reason is though may never be pin pointed to an exact answer. Some observations are thrown out by a couple of researchers but with no conclusive data or results. The only thing concluded seems to be that it is unnecessary. For now, until more research is done, we must instead be content to just know it persists.

Respiration
Birds respire by means of crosscurrent flow: the air flows at a 90 degree angle to the flow of blood in the lungs capillaries. In addition to the lungs themselves, birds have posterior and anterior air sacs (typically nine) which control air flow through the lungs, but do not play a direct role in gas exchange. There are three parts involved in respiration: the anterior air sacs (interclavicular, cervicals, and anterior thoracics), the lungs, and the posterior air sacs (posterior thoracics & abdominals).

It takes a bird two full breaths (inhaling and exhaling), to completely cycle the air from each inhalation through the lungs and out again. The air flows through air sacs and lungs as follows. First inhalation: air flows through the trachea and bronchi into the posterior air sacs. First exhalation: air flows from the posterior air sacs to the lungs. Second inhalation: air flows from the lungs to the anterior air sacs. Second exhalation: air flows from the anterior sacs back through the trachea and out of the body.

In birds, at each inhalation and each exhalation fresh air flows through the lungs in only one direction. Birds are therefore able to diffuse more oxygen into their blood. Unlike humans and other mammals, there is no mixing of oxygen rich air and carbon dioxide rich air. Thus, the partial pressure of oxygen in a bird's lungs is the same as the environment. This is also why you would more likely see a bird on Mount Everest than, say, a mouse.

Avian lungs do not have alveoli, as mammalian lungs do, but instead contain millions of tiny passages known as parabronchi, connected at either ends by the dorsobronchi and ventrobronchi. Air flows through the honeycombed walls of the parabronchi and into air capillaries, where oxygen and carbon-dioxide are traded with cross-flowing blood capillaries by diffusion.

Other anatomy
Birds possess a ventriculus, or gizzard, that is composed of four muscular bands that act to rotate and crush food by shifting the food from one area to the next within the gizzard. Depending on the species, the gizzard may contain small pieces of grit or stone that the bird has swallowed to aid in the grinding process of digestion. For birds in captivity, only certain species of birds require grit in their diet for digestion. The use of gizzard stones is a similarity between birds and dinosaurs, which left gizzard stones called gastroliths as trace fossils.

Birds and humans

Birds are an important food source for humans. The most commonly eaten species is the domestic chicken and its eggs, although geese, pheasants, turkeys, and ducks are also widely eaten. Other birds that have been utilized for food include emus, ostriches, pigeons, grouse, quails, doves, woodcocks, songbirds, and others, including small passerines such as finches.

At one time swans and flamingos were delicacies of the rich and powerful, although these are generally protected now.

Many species have become extinct through over-hunting, such as the Passenger Pigeon, and many others have become endangered or extinct through habitat destruction, deforestation and intensive agriculture being common causes for declines.

Numerous species have come to depend on human activities for food and are widespread to the point of being pests. For example the common pigeon or Rock Dove (Columba livia) thrives in urban areas around the world. In North America, introduced House Sparrows, Common Starlings, and House Finches are similarly widespread.

Other birds have been used by humans: for example Homing pigeons to carry messages (many are still kept for sport), falcons for hunting, cormorants for fishing. Chickens and pigeons are popular subjects in experimental research in biology and comparative psychology. As birds are extra-sensitive to toxins, the Canary was often used in coal mines to indicate the presence of poisonous gases, so that the miners could escape.

Colorful, particularly tropical, birds (e.g., parrots, and mynahs) are often kept as pets although this has led to smuggling of some endangered species; CITES does considerable work to deter this.

Bird diseases that can be contracted by humans include these: psittacosis, salmonellosis, campylobacteriosis, Newcastle's disease, mycobacteriosis (avian tuberculosis), influenza, giardiasis, and cryptosporiadiosis.

Trivia
To preen or groom their feathers, birds use their bills to brush away foreign particles. The birds of a region are called the avifauna. Few birds use chemical defences against predators. Tubenoses can eject an unpleasant slime against an aggressor, and some species of pitohui, found in New Guinea secrete a powerful neurotoxin in their feathers. Birds are among the most extensively studied animal groups, with hundreds of academic journals devoted to their study.