Bird

Birds (class Aves) are bipedal, warm-blooded, egg-laying vertebrate animals. Birds evolved from theropod dinosaurs during the Jurassic period, and the earliest known bird is the Late Jurassic Archaeopteryx. Ranging in size from tiny hummingbirds to the huge Ostrich and Emu, there are around 10,000 known living bird species in the world, making them the most diverse class of terrestrial vertebrates.

Modern birds are characterized 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 birds have forelimbs modified as wings and most can fly, though the ratites and several others, particularly endemic island species, have also lost the ability to fly. Birds also have unique digestive and respiratory systems that are highly adapted for flight.

Many species of bird undertake long distance annual migrations, and many more perform shorter irregular movements. Birds are social and communicate using visual signals and through calls and song, and participate in social behaviors including cooperative hunting, cooperative breeding, flocking and mobbing of predators. Birds are primarily socially monogamous, with engagement in extra-pair copulations being common in some species; other species have polygamous or polyandrous breeding systems. Eggs are usually laid in a nest and incubated and most birds have an extended period of parental care after hatching.

Birds are economically important to humans: many are important sources of food, acquired either through hunting or farming, they also provide other products. Some species, particularly songbirds and parrots, are popular as pets. Birds figure prominently in all aspects of human culture from religion to poetry and popular music. About 120–130 species have become extinct as a result of human activity since 1600, and hundreds more prior to this. Currently around 1,00 species of birds are threatened with extinction by human activities and efforts are underway to protect them.

The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume, Ornithologiae. Carolus Linnaeus modified that work in 1758 to devise the taxonomic classification system still in use.

Birds are categorised as the biological class Aves in Linnean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda.  Aves and a sister group, the order Crocodilia, together are the sole living members 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. Archaeopteryx, from the Kimmeridgian stage of the Late Jurassic (some 155-150 million years ago), is the earliest known bird under this definition. Others have defined Aves to include only the modern bird groups, excluding most groups known only from fossils, in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.

Modern birds all sit within the subclass Neornithes, which is divided into two superorders, the Paleognathae (mostly flightless birds like ostriches), and the wildly diverse Neognathae, containing all other birds.  Depending on the taxonomic viewpoint, the number of species cited varies anywhere from 9,800  to 10,050  known living bird species in the world.

This article uses "bird" to denote members of the Aves, but primarily deals with the living birds which are members of the subclass Neornithes and thus unquestionably Aves. In popular science, the term "bird" is often used in an informal sense, denoting any theropod with feathers and wings.  Thus, animals such as Microraptor and Rahonavis are sometimes called "birds" in news articles,  although most scientists would not consider them to belong to Aves based on current evidence.

There is significant evidence that birds evolved from theropod dinosaurs, specifically, that birds are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others. As more non-avian theropods that are closely related to birds are discovered, the formerly clear distinction between non-birds and birds becomes blurred. Recent discoveries in Liaoning Province of northeast China, demonstrating that many small theropod dinosaurs had feathers, contribute to this ambiguity.

The basal bird Archaeopteryx from the Jurassic era is well-known as one of the first "missing links" to be found in support of evolution in the late 19th century, though it is not considered a direct ancestor of modern birds. Confuciusornis is another early bird; it lived in the Early Cretaceous. Protoavis texensis may be even older although the fragmentary nature of this fossil leaves it open to considerable doubt whether this was a bird ancestor. Other Mesozoic birds include the Enantiornithes, Yanornis, Ichthyornis, Gansus and the Hesperornithiformes, a group of flightless divers resembling grebes and loons.

The dromaeosaurids Cryptovolans and Microraptor may have been capable of powered flight to an extent similar to or greater than that of Archaeopteryx. Cryptovolans had a sternal keel and had ribs with uncinate processes. In fact, Cryptovolans makes a better "bird" than Archaeopteryx which is missing some of these modern bird features. Because of this, some palaeontologists have suggested that dromaeosaurs are actually basal birds, and that the larger members of the family are secondarily flightless, i.e. that dromaeosaurs evolved from birds and not the other way around. Evidence for this theory is currently inconclusive, as the exact relationship among the most advanced maniraptoran dinosaurs and the most primitive true birds are not well understood.

Although ornithischian (bird-hipped) dinosaurs share the hip structure of birds, birds actually originated from the saurischian (lizard-hipped) dinosaurs, and thus evolved their hip structure independently. In fact, the bird-like hip structure also developed a third time among a peculiar group of theropods, the Therizinosauridae.

An alternate theory to the dinosaurian origin of birds, espoused by a few scientists (most notably Larry Martin and Alan Feduccia), states that birds (including maniraptoran "dinosaurs") evolved from early archosaurs like Longisquama, a theory which is contested by most palaeontologists and evidence based on feather development and evolution.

During the Cretaceous Period, birds diversified into a wide variety of forms.  Many of these groups retained primitive characteristics, such as clawed wings and teeth,  though the latter was lost independently in a number of bird groups, including modern birds (Neornithes).  While the earliest birds retained the long bony tails of their ancestors (birds such as Archaeopteryx and Jeholornis),  more advanced birds shortened the tail with the advent of the pygostyle bone in the clade Pygostylia.

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 the reverse of the condition seen in modern birds.  Enantirornithes occupied a wide array of ecological niches, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters.  More advanced lineages also specialized in eating fish, like the superficially gull-like subclass of Ichthyornithes ("fish birds").  One order of Mesozoic seabirds, the Hesperornithiformes, became so well adapted to hunting fish in marine environments that 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. For a comprehensive listing of prehistoric bird groups, see Fossil birds.

Modern birds are classified in the subclass Neornithes, which are now known to have evolved into some basic lineages by the end of the Cretaceous  (see Vegavis). The Neornithes are split into the Paleognathae and Neognathae. The paleognaths include the tinamous of Central and South America and the ratites. The ratites are large flightless birds, and include ostriches, cassowaries, kiwis and emus (though some scientists suspect that the ratites represent an artificial grouping of birds which have independently lost the ability to fly in a number of unrelated lineages).

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.  It is agreed that the Neornithes evolved in the Cretaceous and that 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. This disagreement is in part caused by a divergence in the evidence, with molecular dating suggesting a Cretaceous radiation and fossil evidence supporting a Tertiary radiation. Attempts to reconcile the molecular and fossil evidence have proved controversial.

The classification of birds is a 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. A preponderance of evidence seems to suggest that the modern bird orders constitute accurate taxa.  But scientists disagree about the relationships between orders; 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. See also: Sibley-Ahlquist taxonomy and dinosaur classification.

Birds breed on all seven continents, with the highest diversity occurring in tropical regions; this may be due either to higher speciation rates in the tropics or to higher extinction rates at higher latitudes. They are able to live and feed in most of the world's terrestrial habitats, reaching their southern extreme in the Snow Petrel's breeding colonies, found as far as 440 km inland in Antarctica. Several families of birds have adapted to life both on the world's oceans and in them, with some seabird species coming ashore only to breed and some penguins recorded diving as deeply as 300 m. Many species have established naturalised breeding populations in areas to which they have been introduced by humans. Some of these introductions have been deliberate; the Ring-necked Pheasant, for example, has been introduced around the world as a game bird. Others are accidental, such as the Monk Parakeets that have escaped from captivity and established breeding colonies in a number of North American cities. Some species, including the Cattle Egret, Yellow-headed Caracara  and Galah,  have spread naturally far beyond their original ranges as agricultural practices created suitable new habitat.

External anatomy of a bird: 1 Beak, 2 Head, 3 Iris, 4 Pupil, 5 Mantle, 6 Lesser coverts, 7 Scapulars, 8 Median coverts, 9 Tertials, 10 Rump, 11 Primaries, 12 Vent, 13 Thigh, 14 Tibio-tarsal articulation, 15 Tarsus, 16 Feet, 17 Tibia, 18 Belly, 19 Flanks, 20 Breast, 21 Throat, 22 WattleCompared with other vertebrates, birds have a body plan that shows many unusual adaptations, mostly to facilitate flight.

The skeleton consists of bones which are very light. They have large pneumatic (air-filled) cavities which connect with the respiratory system. The skull bones are fused and do not show cranial sutures.  The orbits are large and separated by a bony septum. The spine has cervical, thoracic, lumbar and caudal regions with the number of cervical (neck) vertebrae highly variable and especially flexible, but movement is reduced in the anterior thoracic vertebrae and absent in the later vertebrae. The last few are fused with the pelvis to form the synsacrum.  The ribs are flattened and the sternum is keeled for the attachment of flight muscles, except in the flightless bird orders. The forelimbs are modified into the wings.

Like the reptiles, birds are primarily uricotelic, that is their kidneys extract nitrogenous wastes from their bloodstream and excrete it as uric acid instead of urea or ammonia. The uric acid is excreted along with feces as a semisolid waste and they do not have a separate urinary bladder or opening. Some birds such as hummingbirds however can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia. They also excrete creatine rather than creatinine as in mammals.  This material, as well as the output of the intestines, emerges from the bird's cloaca. The cloaca is a multi-purpose opening: their wastes are expelled through it, they mate by joining cloaca, and females lay eggs out of it. In addition, many species of birds regurgitate pellets.

Birds have one of the most complex respiratory systems of all animal groups. When a bird inhales, 75% of the fresh air bypasses the lungs and flows directly into a posterior air sac which extends from the lungs and connects with air spaces in the bones and fills them with air. The other 25% of the air goes directly into the lungs. When the bird exhales, the used air flows out of the lung and the stored fresh air from the posterior air sac is simultaneously forced into the lungs. Thus, a bird's lungs receive a constant supply of fresh air during both inhalation and exhalation. Sound production is achieved using the syrinx, a muscular chamber with several tympanic membranes, situated at the lower end of the trachea where it bifurcates. The bird's heart has four chambers and the right aortic arch gives rise to systemic aorta (unlike in the mammals where the left arch is involved). The postcava receives blood from the limbs via the renal portal system. Birds, unlike mammals, have nucleated erythrocytes, i.e. red blood cells which retain a nucleus.

The digestive system of the bird is unique, with a crop for storage and a gizzard that contains swallowed stones for grinding food, given the lack of teeth. Most are highly adapted for rapid digestion, an adaptation to flight. Some migratory birds have the additional ability to reduce parts of the intestines prior to migration.

The nervous system is large relative to the bird's size.  The most developed part of the brain is the one that controls the flight related function while the cerebellum coordinates movement and the cerebrum controls behavior patterns, navigation, mating and nest building.  Birds with eyes on the sides of their heads have a wide visual field while birds with eyes on the front of their heads like owls have binocular vision and can estimate field depth. Most birds have a poor sense of smell with notable exceptions including kiwis, vultures and the tubenoses. The visual system is usually highly developed.  Water birds have special flexible lenses, allowing accommodation for vision in air and water.  Some species also have dual fovea.  Birds are tetrachromatic, possessing ultraviolet cone cells in the eye as well as green, red and blue ones. This allows them to perceive ultraviolet light; which is used in courtship. Many birds show plumage patterns in ultraviolet that are invisible to the human eye; so that some birds, whose sexes appear similar are distinguished by the presence of ultraviolet reflective patches of feathers. Male Blue Tits have an ultraviolet reflective crown patch which is displayed in courtship by posturing and raising of their nape feathers. Ultraviolet light is also used in foraging; kestrels have been shown to search for prey by detecting the UV reflective urine trail marks left on the ground by rodents. The eyelids of a bird are not used in blinking, instead the eye is lubricated by the nictitating membrane, the third eyelid that moves horizontally. The nictitating membrane also covers the eye and acts as a contact lens in many aquatic birds.  When sleeping the lower eyelids are raised.  The bird retina has a fan shaped blood supply system called the pecten.  The avian ear lacks external pinnae but is covered by feathers, although in some birds (the Asio, Bubo and Otus owls, for example) these feathers form tufts which resemble ears. The inner ear has a cochlea but it is not spiral as in mammals.

Some birds use chemical defenses against predators. Some Procellariiformes can eject an unpleasant oil against an aggressor, and some species of pitohui, found in New Guinea, secrete a powerful neurotoxin in their skin and feathers.

The one characteristic that distinguishes birds from all other living groups is the covering of feathers. Feathers are epidermal growths attached to the skin that serve a variety of functions to birds: they aid in thermoregulation by insulating birds from cold weather and water, they are essential to bird flight, and they are also used in display, camouflage and signaling. There are several different types of feather that serve different purposes. Feathers need maintenance, and birds preen or groom their feathers daily,  using their bills to brush away foreign particles, and applying waxy secretions from the uropygial gland, which protects feather flexibility and also acts as an anti-microbial agent, inhibiting the growth of feather-degrading bacteria. This may be supplemented with the secretions of formic acid from ants, which birds apply in a behavior known as anting in order to remove feather parasites.

The arrangement and appearance of feathers on the body is known as plumage. Within species plumage can vary between age groups,  by social status,  with higher ranked individuals displaying their status, or most commonly  by sex.  Plumage is regularly molted, the standard plumage of a bird that has molted after breeding is known as the non-breeding plumage, or in the Humphrey-Parkes terminology, 'basic plumage'; breeding plumages or variations of the basic plumage are known under the Humphrey-Parkes system as 'alternate plumages'. Molt is annual in most species  but some species may have two molts a year,  while large birds of prey may molt once in two or three years.  Ducks and geese molt their primaries and secondaries simultaneously and become flightless for about a month. Different groups of birds have different molting patterns and strategies. Some drop the feathers starting sequentially from outward-in  while others replace feathers inwards-out  and the rare others lose all their feathers at once.  The first or centripetal molt as termed for the molt of tail feathers is seen for instance in the Phasianidae.  The second or centrifugal molt is seen for instance in the tail feathers of the woodpeckers and treecreepers,  although it begins with the second innermost pair of tail-feathers and the central pair of feathers is molted last, so as to permits the continuous presence of a functional climbing tail. The general pattern seen in the passerines is that the primaries are replaced outward, secondaries inward, and the tail from center outward.

Feathers do not arise from all parts of the bird skin but grow in specific tracts or pterylae. The distribution pattern of these feather tracts or pterylosis is used in taxonomy and systematics. Prior to nesting, the females of most bird species gain a bare brood patch by loss of feathers close to the belly. The skin here is well supplied with blood vessels and helps in incubation.

Flight characterizes most birds, and distinguishes them from almost all other vertebrates with the exception of mammalian bats and the extinct pterosaurs. As the main means of locomotion for most bird species,  flight is used for breeding, feeding, and predator avoidance and escape. Birds have a variety of adaptations to flight, including a lightweight skeleton, two large flight muscles, the pectoralis (which accounts for 15% of the total mass of the bird) and the supercoracoideus and a modified forelimb (the wing) serving as an aerofoil.  Wing shape and size primarily determines the type of flight each species is capable of. Many birds combine powered or flapping flight with less energy intensive soaring flight. About 60 species of extant birds are flightless, and many extinct birds were also flightless. Flightlessness often arises in birds on isolated islands, probably due to the lack of land predators and limited resources, which rewards the loss of costly unnecessary adaptations. Penguins, while flightless, use similar musculature and movements to "fly" through the water, as do auks, shearwaters and dippers.

Most birds are diurnal, but some birds, such as many species of owls and nightjars, are nocturnal or crepuscular (active during twilight hours), and many coastal waders feed when the tides are appropriate, by day or night.

Feeding adaptations in beaks. A:Nectarivore, B:Insectivore, C:Granivore, D:Specialist Seed-eater, E:Fishing, F:Netting, G:Filter feeding, H:Surface probing, I:Probing, J:Surface skimming, K:RaptorialBirds feed on a variety of materials, including nectar, fruit, plants, seeds, carrion, and various types of small animals including other birds. Because birds have no teeth, the digestive system of birds is specially adapted to process deal with unmasticated food items that are usually swallowed whole.

Various feeding strategies are used by birds. Gleaning for insects, invertebrates, fruit and seeds is used by many species.  Sallying from a branch and flycatching for insects is used by many songbirds.  Nectar feeders such as hummingbirds, lorikeets,  sunbirds, honeyeaters  and some other songbirds  is facilitated by specially adapted brushy tongues and in many cases bills designed to fit co-adapted flowers. Probing for invertebrates is used by kiwis  and shorebirds  with long bills; in the case of shorebirds length of bill and feeding method are associated with niche separation.  Pursuit diving is used by falcons and accipiters in the air,  and by loons, diving ducks and penguins in the water.  Plunge diving is used by sulids, kingfishers and terns.  Three species of prion, the flamingos and some ducks are filter feeders.  Geese and dabbling ducks are primarily grazers. Some species will engage in kleptoparasitism, stealing food items from other birds; frigatebirds, gulls, and skuas  employ this type of feeding behavior. Kleptoparasitism is not thought to play a significant part of the diet of any species, and is instead a supplement to food obtained by hunting; a study of Great Frigatebirds stealing from Masked Boobies estimated that the frigatebirds could at most obtain 40% of the food they needed, and on average obtained only 5%. Finally, some birds, such as gulls and vultures, are scavengers.  Some birds may employ many strategies to obtain food, or feed on a variety of food items and are called generalists, while others are considered specialists, concentrating time and effort on specific food items or having a single strategy to obtain food.

The routes of satellite tagged Bar-tailed Godwits migrating north from New Zealand. This species has the longest known non-stop migration of any species, up to 10,00 kmMany bird species migrate to take advantage of global differences of seasonal temperatures to optimise availability of food sources and breeding habitat. These migrations vary among the different groups. Many landbirds, shorebirds and waterbirds undertake annual long distance migrations, usually triggered by length of daylight as well as weather conditions. These are characterised by a breeding season spent in the temperate or arctic/antarctic regions, and a non-breeding season in the tropical regions or opposite hemisphere. Prior to migration, birds substantially increase body fats and reserves and reduce the size of some of their organs. Migration is highly energetically demanding, particularly as birds need to cross deserts and oceans without refuelling; landbirds have a flight range of around 2500 km and shorebirds can fly up to 4000 km, although the Bar-tailed Godwit is capable of non-stop flights of up to 10,00 km. Seabirds also undertake long migrations, the longest annual migration being those of Sooty Shearwaters, which nest in New Zealand and Chile and spend the northern summer feeding in the North Pacific off Japan, Alaska and California, an annual round trip of 64,000 km. Other seabirds disperse after breeding, traveling widely but having no set migration route. Albatrosses nesting in the Southern Ocean often undertake circumpolar trips between breeding seasons.

Birds also display other types of migration. Some species undertake shorter migrations, traveling only as far as is required to avoid bad weather or obtain food. These include irruptive species, which may be quite common some years and almost absent in others.  This type of migration is normally associated with food availability. Boreal finches, arctic owls, and waxwings are most commonly identified as irruptive species.  Species may also travel shorter distances over part of their range, with individuals from higher latitudes traveling into the existing range of conspecifics; others undertake partial migrations, where only a fraction of the population, usually females and subdominant males, migrates. Partial migration can form a large percentage of the migration behavior of birds in some regions; in Australia surveys found that 44% of non-passerine birds studied were partially migratory and 32% of passerines were. Altitudinal migration is a form of short distance migration, in which birds spend the breeding season at higher altitudes elevations, and move to lower ones during suboptimal conditions. It is most often triggered by temperature changes and usually occurs when the normal territories become inhospitable also due to lack of food. Some species may also be nomadic, holding no fixed territory and moving according to weather and food availability. Parrots as a family are overwhelmingly neither migratory nor sedentary but considered to either be dispersive, irruptive, nomadic or undertake small and irregular migration.

The ability of birds to return to precise locations across vast distances has been known for some time; in an experiment conducted in the 1950s a Manx Shearwater released in Boston returned to its colony in Skomer, Wales within 13 days, a distance of 5,150 kilometres (3,00 mi). Birds navigate during migration using a variety of methods. For diurnal migrants the sun is used to navigate by, at night a stellar compass is used instead. Birds that use the sun compensate for the changing position of the sun during the day, by the use of an internal clock. Orientation with the stellar compass depends on the position of the constellations surrounding Polaris. These are backed up in some species with the ability to sense the Earth's geomagnetism through specialised sensitive photoreceptors.

Birds communicate principally using visual and auditory signals. Signals can be interspecific (between species) and intraspecific (within species).

The startling display of the Sunbittern mimics a large predatorVisual communication in birds serves a number of functions and is manifested in both plumage and behavior. Plumage can be used to assess and assert social dominance, display breeding condition in sexually selected species, even make a threatening display, such as the threat display of the Sunbittern, which mimics a large possible predator. This display is used to ward off potential predators such as hawks, and to protect young chicks. Variation in plumage also allows for identification, particularly between species.

Visual communication includes ritualized displays, such as those which signal aggression or submission, or those which are used in the formation of pair-bonds. These ritualised behaviors develop from non-signaling actions such as preening, adjustments of feather position, pecking or other behaviors. The most elaborate displays are shown during courtship, such as the breeding dances of the albatrosses, where the successful formation of a life-long pair-bond requires both partners to practice a unique dance, and the birds-of-paradise, where the breeding success of males depends on plumage and display quality. Male birds can demonstrate their fitness through construction; females of weaver species, such as the Baya Weaver, may choose mates with good nest-building skills, while bowerbirds attract mates through constructing bowers and decorating them with bright objects.

In addition to visual communication, birds are renowned for their auditory skills. Calls, and in some species song, are the major means by which birds communicate with sound; though some birds use mechanical sounds, for example driving air through their feathers, as do the Coenocorypha snipes of New Zealand, the territorial drumming of woodpeckers, or the use of tools to drum in Palm Cockatoos.0 Bird calls and songs can be very complex; sounds are created in the syrinx, both sides of which, in some species, can be operated separately, resulting in two different songs being produced at the same time.

Calls are used for a variety of purposes, several of which may be tied into an individual song.0 They are used to advertise when seeking a mate, either to attract a mate, aid identification of potential mates or aid in bond formation (often with combined with visual communication). They can convey information about the quality of a male and aid in female choice.0 They are used to claim and maintain territories. Calls can also be used to identify individuals, aiding parents in finding chicks in crowded colonies or adults reuniting with mates at the start of the breeding season.0 Calls may be used to warn other birds of potential predators; calls of this nature may be detailed and convey specific information about the nature of the threat.0

While some birds are essentially territorial or live in small family groups, other birds often form large flocks. The benefits of aggregating in flocks are varied and flocks will form explicitly for specific purposes. Flocking also has costs, particularly to socially subordinate birds, which are bullied by more dominant birds; birds may also sacrifice feeding efficiency in a flock in order to gain other benefits.06 The principal benefits are safety in numbers and increased foraging efficiency. Defense against predators is particularly important in closed habitats such as forests where predation is often by ambush and early warning provided by multiple eyes is important, this has led to the development of many mixed-species feeding flocks.0 These multi-species flocks are usually composed of small numbers of many species, increasing the benefits of numbers but reducing potential competition for resources. Birds also form associations with non-avian species; plunge diving seabirds associate with dolphins and tuna which push shoaling fish up towards the surface,0 and a mutual relationship has evolved between Dwarf Mongooses and hornbills, where hornbills seek out mongooses in order to forage together, and warn each other of birds of prey and other predators.0

The high metabolic rates of birds during the active part of the day is supplemented by rest at other times. Sleeping birds often utilize a type of sleep known as vigilant sleep, where periods of rest are interspersed with quick eye-opening 'peeks' allowing birds to be sensitive to disturbance and enable rapid escape from threats. It has been widely believed that swifts may sleep while flying, however this is not supported by experimental evidence. It is however suggested that there may be certain kinds of sleep which are possible even when in flight.

In many birds, half of the brain falls asleep or awakens when the opposite eyelid is closed or opened. Opposite eyelids are involved due to decussation of the optic nerves at the optic chiasm. Avian sleep is so closely associated with eyelid closure that it is assumed that the eyelids "close only in sleep," and that "blinking" is a good behavioral indication of sleep.

Many sleeping birds bends their heads over their backs and tuck their bills in their back feathers, others cover their beaks among their breast feathers. Many birds rest on one leg, some may pull up their legs into their feathers, especially in cold weather. Communal roosting is common, it lowers the loss of body heat and decreases the risks associated with predators. Roosting sites are often chosen with regard to thermoregulation and safety.

Perching birds roost on twigs and their tarsal muscles have a ratchet mechanism that locks their toes. Many ground birds such as quails and pheasants roost in trees. A few parrots of the genus Loriculus roost hanging upside down. Some Hummingbirds go into a nightly state of torpor with a reduction in their metabolic rates, as around a hundred other species, including owlet-nightjars, nightjars, and woodswallows;  one species, the Common Poorwill, even enters a state of hibernation.

Red-necked Phalaropes have an unusual polyandrous mating system where males care for the eggs and chicks and brightly colored females compete for males. 1The vast majority (95%) of bird species are socially monogamous; although polygyny (2%) and polyandry (< 1%), polygamy, polygynandry (where a female pairs with several males and the male pairs with several females) and promiscuity systems also occur. Some species may use more than one system depending on the circumstances. Monogamous species of males and females pair for the breeding season; in some cases, the pair bonds may persist for a number of years or even the lifetime of the pair.

The advantage of monogamy for birds is bi-parental care. In most groups of animals, male parental care is rare, but in birds it is quite common; in fact, it is more extensive in birds than in any other vertebrate class. In birds, male care can be seen as important or essential to female fitness; in some species the females are unable to successfully raise a brood without the help of the male. Polygamous breeding systems arise when females are able to raise broods without the help of males. There is sometimes a division of labor in monogamous species, with the roles of incubation, nest site defence, chick feeding and territory defence being either shared or undertaken by one sex.

While social monogamy is common in birds, infidelity, in the form of extra-pair copulations, is common in many socially monogamous species. These can take the form of forced copulation (or rape) in ducks and other anatids, or more usually between dominant males and females partnered with subordinate males. It is thought that the benefit to females comes from getting better genes for her offspring, as well as an insurance against the possibility of infertility in the mate. Males in species that engage in extra-pair copulations will engage in mate-guarding in order to ensure parentage of the offspring they raise.

Breeding usually involves some form of courtship display, most often performed by the male. Most are rather simple, and usually involve some type of song. Some displays can be quite elaborate, using such varied methods as tail and wing drumming, dancing, aerial flights, and communal leks depending on the species. Females are most often involved with partner selection, although in the polyandrous phalaropes the males choose brightly colored females. Courtship feeding, billing and preening are commonly performed between partners, most often after birds have been paired and mated.

Many birds actively defend a territory from others of the same species during the breeding season. Large territories are protected in order to protect the food source for their chicks. Species that are unable to defend feeding territories, such as seabirds and swifts, often breed in colonies instead; this is thought to offer protection from predators. Colonial breeders will defend small nesting sites, and competition between and within species for nesting sites can be intense.

All birds lay amniotic eggs with hard shells made mostly of calcium carbonate. The color of eggs is controlled by a number of factors, those of hole and burrow nesting species tend to be white or pale, while those of open nesters such as Charadriiformes are camouflaged.  There are many exceptions to this pattern, however; the ground nesting nightjars have pale eggs, camouflage being provided instead by the bird's plumage.  Species that are victims of brood parasites like the Dideric Cuckoo will vary their egg colors in order to improve the chances of spotting a cuckoo's egg, and female cuckoos need to match their eggs to their hosts.

The eggs are usually laid in a nest, which can be highly elaborate, like those created by weavers and oropendolas, or extremely primitive, like some albatrosses, which are no more than a scrape on the ground. Some species have no nest, the cliff nesting Common Guillemot lays its egg on bare rock and the egg of the Emperor Penguin is kept between the body and feet; this is especially prevalent in ground nesting species where the newly hatched young are precocial. Most species build more elaborate nests, which can be cups, domes, plates, beds scrapes, mounds or burrows. Most nests are built in shelter and hidden to reduce the risk of predation, more open nests are usually colonial or built by larger birds capable of defending the nest. Nests are mostly built out of plant matter, some species specifically select plants such as yarrow which have chemicals that reduce nest parasites such as mites, leading to increased chick survival. Nests are often lined with feathers in order to improve the retention of heat.

Incubation, which regulates temperature to keep it optimum for chick development, usually begins after the last egg has been laid. Incubation duties are often shared in monogamous species; in polygamous species a singe parent undertakes all duties. Warmth from parents passes to the eggs through brood patches, areas of bare skin on the abdomen or breast of the incubating birds. Incubation can be an energetically demanding process, for example adult albatrosses lose as much as 83 g of body weight a day. The warmth for the incubation of the eggs of megapodes comes from the sun, decaying vegetation or from volcanic sources. Incubation periods last between 10 days (in species of woodpeckers, cuckoos and passerine birds) to over 80 days (in albatrosses and kiwis).

Chicks can be helpless or independent at birth, or be at any stage in between. The helpless chicks are known as altricial, and tend to be born, small, naked and blind; chicks that are mobile and feathered at birth are precocial, chicks can also be semi-precocial and semi-altricial. Altricial chicks require help in thermoregulation and need to be brooded for longer than precocial chicks.

The length and nature of parental care varies widely amongst different orders and species. At one extreme, parental care in megapodes ends at nest building; the newly-hatched chick digs itself out of the nest mound without parental assistance and can fend for itself immediately. At the other extreme many seabirds have extended periods of parental care, the longest being Great Frigatebird, the chicks of which take up to six months to fledge and are fed by the parents for up to another 14 months.

In some species the care of young is shared between both parents, in others it is the responsibility of just one sex. In some species other members of the same species will help the breeding pair in raising the young. These helpers are usually close relatives such as the chicks of the breeding pair from previous breeding seasons.  Alloparenting is particularly common in the corvids, but has been observed in as different species as the Rifleman, Red Kite and Australian Magpie.

The point at which chicks fledge varies dramatically. The chicks of the Synthliboramphus murrelets, like the Ancient Murrelet, leave the nest the night after they hatch, following their parents calls out to sea, where they are raised away from terrestrial predators. Some other species, especially ducks, move their chicks away from the nest at an early age.  In most  species chicks leave the nest soon after, or just before, they are able to fly. Parental care after fledging varies; in albatrosses chicks leave the nest alone and receive no further help, other species continue some supplementary feeding after fledging. Chicks may also follow their parents during their first migration.

Although some insects and fish engage in brood parasitism, most brood parasites are birds. Brood parasites are birds which lay their eggs in the nests of other birds. These eggs are often accepted and raised by the host species, often at the cost of their own brood. There are two kinds of brood parasite, obligate brood parasites, which are incapable of raising their own young and must lay their eggs in the nests of other species; and non-obligate brood parasites, which are capable of raising their own young but lay eggs in the nests of conspecifics in order to increase their reproductive output. The most famous obligate brood parasites are the cuckoos, although in total 100 species of cuckoos, honeyguides, icterids, estrildid finches and ducks are obligate parasites. Some brood parasites are adapted to hatching before their hosts and pushing their hosts eggs out of the nest, destroying the egg or killing their chicks, ensuring that all the food brought to the nest is fed to them.

The diverse food habits and life-histories of birds are associated with a range of ecological positions.0 While some birds are generalists, others are highly specialized in their habitat or food requirements. Even within a habitat such as a forest, the niches occupied by different groups of birds are varied with some species using the forest canopy, others using the space under the canopy, while still others may use the branches and so on.  In addition forest birds may be classified into different feeding guilds such as insectivores, frugivores and nectarivores.  Aquatic birds show other food habits such as fishing, plant eating and piracy or kleptoparasitism.  The birds of prey specialize in hunting mammals or other birds while the vultures have specialized as scavengers.

Some nectar-feeding birds are also important pollinators of plants and many frugivores play a key role in seed dispersal. Numerous plants have adapted to using birds as their primary pollinators, and both flower and plant have coevolved together, in some cases to the point where the flower's primary pollinator is the only species capable of reaching the nectar.

Birds have important impacts on the ecology of islands. In many cases they reach islands that mammals do not, and in which they may fulfill ecological roles played by larger animals; for example in New Zealand the Moas were important browsers, as are the Kereru and Kokako today. Today the plants of New Zealand retain the defensive adaptations evolved to protect them from the extinct moa. Large concentrations of nesting seabirds also have an impact on the ecology of islands and the surrounding seas, principally through the concentration of large quanities of guano, which can have appreciable impacts on the richness of the local soil, and of the surrounding seas.

Birds are highly visible and common animals, and humans have had a long relationship with them. In some cases the relationship has been mutualistic, such as the cooperative relationship between honeyguides and tribesmen in obtaining honey, or commensal, as found in the numerous species that benefit indirectly from human activities.  For example, the common pigeon or Rock Pigeon thrives in urban areas around the world. Human effects can also be detrimental, where species are threatened by human activities.

Birds also have many effects on humans. They can act as vectors for spreading diseases such as psittacosis, salmonellosis, campylobacteriosis, mycobacteriosis (avian tuberculosis), avian influenza (bird flu), giardiasis, and cryptosporidiosis over long distances. Some of these are zoonotic diseases which can also be transmitted to humans. They can also be commercially important pests on agricultural crops5 as well as hazardous to aviation due to the risk of bird strikes.  They are also important food and income sources.

In some ecosystems, birds are at the apex of food chains making them very sensitive indicators of pollution. The decline in bird populations in the US as a result of pesticide use is a famous example. Birds and their diversity have therefore been considered as good indicators of ecosystem health and in the UK, bird diversity is used as one of 15 quality of life indicators.

Birds are an important food source for humans. The most commonly eaten species is the domestic chicken and its eggs, and geese, pheasants, turkeys, ducks and quail are also widely domesticated and eaten. Hunting remains an important method of obtaining birds, as it has been throughout history, and has led to the extinction or endangerment of dozens of species. However, muttonbirding in Australia and New Zealand is an example of an ongoing sustainable harvest of two seabird species.

Besides meat and eggs, birds provide feathers for clothing, bedding and decoration, guano-derived phosphorus and nitrogen used in fertiliser and gunpowder, and the central ingredient of bird's nest soup.  In former times, the long wing feathers of geese and other birds were used for writing,  and the word pen is derived from the Latin for feather penna.  Colorful birds (e.g. parrots, and mynas) are often bred in captivity or kept as pets, and this practice has led to the illegal trafficking of some endangered species. CITES, an international agreement adopted in 1963, has worked to reduce the trafficking in the bird species.

Other birds have long been used by humans to perform tasks; falcons for hunting, and cormorants to catch fish. Pigeons were used as a messenger as early as 1 AD, according to Pliny and played an important role as recently as World War II. Today, such activities are more commonly undertaken as a hobby, or for entertainment and tourism, or for sport including pigeon racing, which evolved from  the tradition of messenger pigeons.

The scientific study of birds is called ornithology. Birds are among the most extensively studied of all animal groups; chickens and pigeons are popular as experimental subjects, and are often used in biology and comparative psychology research. Hundreds of academic journals and thousands of scientists  are devoted to bird research, while amateur enthusiasts (called birdwatchers, twitchers or, more commonly, birders) number in the millions. Many homeowners erect bird feeders near their homes to attract various species. Bird feeding has grown into a multimillion dollar industry; for example an estimated 75% of households in Britain provide food for birds at some point during the winter.

Birds feature prominently in folklore, religion and popular culture, in which they fulfil a number of roles. In religion they may serve as messengers or priests and leaders for a deity, such as in the cult of Make-make where the Tangata manu (bird men) of Easter Island served as chiefs, or as attendants, as in the case of Hugin and Munin, two Common Ravens which whisper news into the ears of the Norse god Odin. They may also serve as religious symbols, for example the symbolism of Jonah as a dove (יוֹנָה), with its various associated meanings, fright, passivity, mourning and beauty. Birds can themselves be deified, as occurred to the Common Peacock by the Dravidians of India, who perceived the peacock as Mother Earth. Birds have also been perceived as monsters, including the legendary Roc  and the Māori legends about the Pouākai, a giant bird capable of snatching humans, based on the extinct Haast's Eagle. In some parts of the world many birds are regarded with suspicion; in parts of Africa owls are associated with bad luck, witchcraft and death.6

Birds feature in culture and art and have done so since prehistoric times. Birds are represented in early cave paintings along with other animals. Later birds came to be used in religious or symbolic art and design; among the most magnificent of these was the (now lost) Peacock Throne of the Mughal and Persian emperors of India.  With the advent of scientific interest in birds many paintings of birds were commissioned for books, amongst the most famous bird artists was John James Audubon, who's paintings of North American birds were a great commercial success in Europe and who later lent his name to the National Audubon Society. Birds are also important in poetry; Homer incorporated Nightingales into the Odyssey, and poets have continued to use that species ever since. The relationship between an albatross and a sailor is the central theme of Samuel Taylor Coleridge's The Rime of the Ancient Mariner, the significance of which has increased with the adoption of the term as a metaphor for a 'burden'. Birds serve as other metaphors in the English language, for example vulture funds and vulture investors, where vultures are perceived as unpleasant and possibly unethical.  Perceptions of individual bird species vary from culture to culture; while owls are considered bad luck in some parts of Africa they are regarded as wise across much of Europe,  and Hoopoes were considered sacred in Ancient Egypt, symbols of virtue in Persia, thieves across much of Europe and harbingers of war in Scandinavia.

Humans have had a large impact on many bird species. Human activities have in some cases allowed some species to dramatically expand their natural ranges, in other species ranges have decreased and have even resulted in many extinction. Over a hundred species have gone extinct in historical times, although the most dramatic human caused extinctions occurred in the Pacific Ocean as people colonized the islands of Melanesia, Polynesia and Micronesia, during which an estimated 750-1800 species of bird went extinct. Many bird populations are currently declining worldwide, with 1,21 species listed as threatened by Birdlife International and the IUCN. The biggest cited reason surrounds habitat loss.6 Other threats include overhunting, accidental mortality due to structural collisions and as long-line fishing bycatch, pollution (including oil spills and pesticide use), competition and predation by nonnative invasive species, and climate change. Governments, along with numerous conservation charities, work to protect birds, either through laws to protect birds, preserving and restoring bird habitat or establishing captive populations for reintroductions. The efforts of conservation biology have met with some success, a study estimated that between 1994 and 2004 16 species of bird that would otherwise have gone extinct were saved.

See Late Quaternary prehistoric birds for taxa which disappeared in prehistoric and early historic times, usually due to human activity (i.e., starting with the Upper Paleolithic Revolution). For birds having gone extinct in modern times (since 1500), see Extinct birds.