Bird

For other uses, see Bird (disambiguation).

"Aves" redirects here. For other uses, see Aves (disambiguation).

Birds Temporal range: Late Jurassic – Recent
Superb Fairy-wren, Malurus cyaneus, juvenile
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
(unranked):	Archosauria
Class:	Aves Linnaeus, 1758
Orders
About two dozen - see section below

Birds (class Aves) are bipedal, warm-blooded, vertebrate animals that lay eggs. There are around 10,000 living species, making them the most diverse tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the 5 cm (2 in) Bee Hummingbird to the 2.7 m (9 ft) Ostrich. The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic period, around 150–200 Ma (million years ago), and the earliest known bird is the Late Jurassic Archaeopteryx, c 155–150 Ma.

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 birds have forelimbs modified as wings and most 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.

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 breeding systems that are polygynous ("many females") or, rarely, polyandrous ("many males"). 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: Bird evolution

Archaeopteryx, the earliest known bird

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 currently in use. Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda. Aves and a sister group, the clade 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, including Jacques Gauthier and adherents of the Phylocode system, have defined Aves to include only the modern bird groups, excluding most groups known only from fossils, and assigning them, instead, to the Avialae in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.

All modern birds lie within the subclass Neornithes, which is divided into two superorders: the Paleognathae, containing mostly flightless birds like ostriches, and the wildly diverse Neognathae, containing all other birds. Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800 to 10,050.

Dinosaurs and the origin of birds

Main article: Origin of birds

File:Confuciusornis.jpg

Confuciusornis, a Cretaceous bird from China

There is substantial evidence that birds are theropod dinosaurs—more specifically, that they are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others. As scientists discover more non-avian theropods that are closely related to birds, the previously clear distinction between non-birds and birds has become blurred. Recent discoveries in the Liaoning Province of northeast China, which demonstrate that many small theropod dinosaurs had feathers, contribute to this ambiguity.

The oldest known bird, the Late Jurassic Archaeopteryx, is well-known as one of the first transitional fossils to be found in support of evolution in the late 19th century, though it is not considered a direct ancestor of modern birds. Protoavis texensis may be even older, although the fragmentary nature of this fossil leaves considerable doubt regarding whether it was a bird ancestor.

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

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. In fact, a bird-like hip structure evolved a third time among a peculiar group of theropods known as the Therizinosauridae.

An alternate theory to the dinosaurian origin of birds, proposed by a few scientists (most notably Larry Martin and Alan Feduccia), states that birds (including maniraptoran "dinosaurs") evolved from early archosaurs like Longisquama; this theory is contested by most paleontologists.

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

Birds diversified into a wide variety of forms during the Cretaceous Period. 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 such as Archaeopteryx and Jeholornis, retained the long bony tails of their ancestors, the tails of more advanced birds were shortened 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 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. 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.

Radiation 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 and is split into two superorders, the Paleognathae and Neognathae. The paleognaths include the tinamous of Central and South America and the ratites. The basal divergence from the remaining Neognathes was that 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; molecular dating suggests a Cretaceous radiation, while fossil evidence supports a Tertiary radiation. Attempts to reconcile the molecular and fossil evidence have proved controversial.

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, although it is frequently debated and constantly revised. Most evidence seems to suggest that the modern bird orders constitute accurate taxa, although 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

 

Neornithes	Paleognathae  Struthioniformes Tinamiformes  Neognathae    Other birds Galloanserae  Anseriformes Galliformes

Basal divergences of modern birds
based on Sibley-Ahlquist taxonomy

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
Paleognathae:

• Struthioniformes, Ostriches, emus, kiwis, and allies
• Tinamiformes, tinamous

Neognathae:

• Anseriformes, waterfowl
• Galliformes, fowl
• Gaviiformes, loons
• Podicipediformes, grebes
• Procellariiformes, albatrosses, petrels, and allies
• Sphenisciformes, penguins
• Pelecaniformes, pelicans and allies
• Ciconiiformes, storks and allies
• Phoenicopteriformes, flamingos
• Falconiformes, falcons, eagles, hawks and allies
• Gruiformes, cranes and allies
• Charadriiformes, gulls, button-quail, plovers and allies
• Pteroclidiformes, sandgrouse
• Columbiformes, doves and pigeons
• Psittaciformes, parrots and allies
• Cuculiformes, cuckoos, turacos, hoatzin
• Strigiformes, owls
• Caprimulgiformes, nightjars and allies
• Apodiformes, swifts and hummingbirds
• Coraciiformes, kingfishers
• Piciformes, woodpeckers and allies
• Trogoniformes, trogons
• Coliiformes, mousebirds
• Passeriformes, passerines

The radically different Sibley-Monroe classification (Sibley-Ahlquist taxonomy), based on molecular data, became quite influential as recent molecular, fossil, and anatomical evidence supported the Galloanserae. By 2006, increasing evidence made it possible to verify the major proposals of the taxonomy, such as in Charadriiformes, Gruiformes or Caprimulgiformes.

Distribution

The range of the House Sparrow has expanded dramatically due to human activities.

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 greater 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, which are found as far as 440 kilometres (270 mi) 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 metres (980 ft). 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 Cattle Egret, Yellow-headed Caracara and Galah, have spread naturally far beyond their original ranges as agricultural practices created suitable new habitat.

Anatomy

Main article: Bird anatomy

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 Wattle

Compared with other vertebrates, birds have a body plan that shows many unusual adaptations, mostly to facilitate flight.

The skeleton consists of very lightweight bones. 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, which means that 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 since birds do not have a separate bladder or opening. However, birds such as hummingbirds 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 from it. In addition, many species of birds regurgitate pellets. The digestive system of the bird is unique, with a crop for storage and a gizzard that contains swallowed stones for grinding food to compensate for the lack of teeth. Most are highly adapted for rapid digestion to aid in the bird's flight. Some migratory birds have the additional ability to reduce parts of the intestines prior to migration.

Birds have one of the most complex respiratory systems of all animal groups. Upon inhalation, 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 which is 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. Unlike mammals, the red blood cells in birds retain a nucleus.

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 behaviour patterns, navigation, mating and nest building. Most birds have a poor sense of smell with notable exceptions including kiwis, New World vultures and the tubenoses. The avian 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 (UV) sensitive 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; some birds whose sexes appear similar to the naked eye are distinguished by the presence of ultraviolet reflective patches on their 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, a third eyelid that moves horizontally. The nictitating membrane also covers the eye and acts as a contact lens in many aquatic birds. The bird retina has a fan shaped blood supply system called the pecten. Most birds cannot move their eyes, although there are exceptions, such as the Great Cormorant. Birds with eyes on the sides of their heads have a wide visual field, while birds with eyes on the front of their heads, such as owls, have binocular vision and can estimate the depth of field. The avian ear lacks external pinnae but is covered by feathers, although in some birds, such as the Asio, Bubo and Otus owls, these feathers form tufts which resemble ears. The inner ear has a cochlea, but it is not spiral as in mammals.

A few species are able to use chemical defenses against predators; some Procellariiformes can eject an unpleasant oil against an aggressor, and some species of pitohui from New Guinea secrete a powerful neurotoxin in their skin and feathers.

Feathers and plumage

Main articles: Feather and Flight feather

The plumage of the African Scops Owl allows it to blend in with its surroundings.

Unique to birds, feathers are epidermal growths attached to the skin that serve a variety of functions: they aid in thermoregulation by providing insulation in cold weather and water, are essential to flight, and are also used in display, camouflage and signaling. There are several types of feather, each serving a different set of purposes. Feathers require maintenance and birds preen or groom them daily, spending around an average of 9% of their daily time on this action. The bill is used to brush away foreign particles and to apply waxy secretions from the uropygial gland which protect feather flexibility and act as an antimicrobial 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 behaviour known as anting to remove feather parasites.

Plumage is the term given to the arrangement and appearance of feathers on the body; within species this can vary with age, social status, or most commonly by sex. Plumage is regularly moulted; the standard plumage of a bird that has moulted 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. Moulting is annual in most species, although some species may have two moults a year, and large birds of prey may moult only once in two or three years. Ducks and geese moult their primaries and secondaries simultaneously and become flightless for about a month. Moulting patterns vary across species. Some drop and regrow wing flight feathers, starting sequentially from the outermost feathers and progressing inwards, while others replace feathers starting from the innermost feathers. A small number of species lose all of their flight feathers at once. The first moult (or centripetal moult), as termed for the moult of tail feathers, is seen in the Phasianidae. The second or centrifugal moult is seen, for instance, in the tail feathers of woodpeckers and treecreepers, although it begins with the second innermost pair of tail-feathers and finishes with the central pair of feathers so that the bird maintains 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 a bird's skin, but grow in specific tracts, or pterylae. The distribution pattern of these feather tracts or pterylosis is used in taxonomy and systematics. Before nesting, the females of most bird species gain a bare brood patch by losing feathers close to the belly. The skin here is well supplied with blood vessels and helps the bird in incubation.

Flight

Main article: Bird flight

Common Raven in flight

Flight characterises most birds, distinguishing 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 are conditions that reward the loss of costly unnecessary adaptations. Though flightless, penguins use similar musculature and movements to "fly" through the water, as do auks, shearwaters and dippers.

Behaviour

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.

Diet and feeding

Feeding adaptations in beaks

Birds feed on a variety of things, including nectar, fruit, plants, seeds, carrion, and various small animals, including other birds. Because birds have no teeth, the digestive system of birds is specially adapted to process unmasticated food items that are usually swallowed whole.

Different species of birds use different feeding strategies. Many glean for insects, invertebrates, fruit or seeds. Some hunt insects by sallying from a branch. Nectar feeders such as hummingbirds, sunbirds, lories and lorikeets amongst others are facilitated by specially adapted brushy tongues and in many cases bills designed to fit co-adapted flowers. Kiwis and shorebirds with long bills probe for invertebrates; in the case of shorebirds, length of bill and feeding method are associated with niche separation. Loons, diving ducks, penguins and auks pursue their prey underwater, using their wings or feet for propulsion, while aerial predators such as sulids, kingfishers and terns plunge dive after their prey. 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 behaviour. 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%. Some birds are scavengers, either specialised carrion eaters like vultures or opportunists like gulls, corvids or other birds of prey. 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.

Migration

Main article: Bird migration

Many bird species migrate to take advantage of global differences of seasonal temperatures, therefore optimising 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 the length of daylight as well as weather conditions. These birds 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. Before migration, birds substantially increase body fats and reserves and reduce the size of some of their organs. Migration is highly demanding energetically, particularly as birds need to cross deserts and oceans without refueling. Landbirds have a flight range of around 2,500 km (1,600 mi) and shorebirds can fly up to 4,000 km (2,500 mi), although the Bar-tailed Godwit is capable of non-stop flights of up to 10,200 km (6,300 mi). 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 (39,800 mi). 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.

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 10,200 km (6,300 mi).

Some bird species undertake shorter migrations, traveling only as far as is required to avoid bad weather or obtain food. Irruptive species such as the boreal finches are one such group and can commonly found at a location in one year while absent in others. This type of migration is normally associated with food availability. Species may also travel shorter distances over part of their range, with individuals from higher latitudes travelling 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 behaviour of birds in some regions; in Australia, surveys found that 44% of non-passerine birds and 32% of passerines were partially migratory. 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 also become inhospitable 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 migrations.

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 km (3,200 mi). Birds navigate during migration using a variety of methods. For diurnal migrants, the sun is used to navigate by day, and a stellar compass is used at night. 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 by their ability to sense the Earth's geomagnetism through specialised photoreceptors.

Communication

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 predator

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

Ritualised displays comprise those which signal aggression or submission, as well those used in the formation of pair-bonds. These have developed from non-signaling actions such as preening, the adjustments of feather position, pecking, or other behaviour; they are used as forms of visual communication as indication of a particular trait or mood. 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 nest-site selection and construction; females of weaver species, such as the Baya Weaver, may choose mates with good site selection and 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, although some birds use mechanical sounds such as driving air through their feathers like the Coenocorypha snipes of New Zealand, the territorial drumming of woodpeckers, or the use of tools to drum in Palm Cockatoos. 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.

Call of the House Wren, a common songbird from North America

Calls are used for a variety of purposes, several of which may be tied into an individual song. They are used to advertise when seeking a mate by either attracting a mate, aiding in the identification of potential mates, or aiding in bond formation and are often combined with visual communication. They can convey information about the quality of a male and aid in the female's choice. 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. 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.

Red-billed Queleas, the most numerous species of bird, form enormous flocks—sometimes tens of thousands strong.

Flocking

While some birds are essentially territorial or live in small family groups, other birds may 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 to gain other benefits. The principal benefits are safety in numbers and increased foraging efficiency. Defence against predators is particularly important in closed habitats such as forests where predation is often by ambush and the early warning provided by multiple eyes is important. This has led to the development of many mixed-species feeding flocks. These multi-species flocks are usually composed of small numbers of many species, which increases the benefits of numbers but reduces 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, while a mutualistic relationship has evolved between Dwarf Mongooses and hornbills, where hornbills seek out mongooses to forage together and warn each other of nearby birds of prey and other predators.

Resting and roosting

The high metabolic rates of birds during the active part of the day is supplemented by rest at other times. Sleeping birds often utilise a type of sleep known as vigilant sleep, where periods of rest are interspersed with quick eye-opening 'peeks', thus allowing birds to be sensitive to disturbances and enable rapid escape from threats. Swifts have been widely believed to be able to sleep while flying; however, this has not been confirmed by experimental evidence. However, there may be certain kinds of sleep which are possible even when in flight. Some birds have also demonstrated the capacity to fall into slow-wave sleep one hemisphere of the brain at a time. The birds tend to exercise this ability depending upon its position relative to the outside of the flock. This may allow the eye opposite the sleeping hemisphere to remain vigilant for predators by viewing the outer margins of the flock. This adaption is also known in marine mammals.

Many sleeping birds bend their heads over their backs and tuck their bills in their back feathers, although others place their beaks among their breast feathers. Many birds rest on one leg, while some may pull up their legs into their feathers, especially in cold weather. Communal roosting is common because 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 have a tendon locking mechanism that helps them hold on to the perch when they are asleep. 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. This physiological adaptation is shown in around a hundred other species, including owlet-nightjars, nightjars, and woodswallows. One species, the Common Poorwill, even enters a state of hibernation. Birds do not have sweat glands, but they may cool themselves by moving to shade, standing in water, panting, increasing their surface area, fluttering their throat or by using special behaviours like urohidrosis to cool themselves.

Breeding

Social systems

Red-necked Phalaropes have an unusual polyandrous mating system where males care for the eggs and chicks and brightly coloured females compete for males.

Birds, like mammals, have two sexes: male and female. However, the sex chromosomes of birds differ from those of mammals. Instead of X and Y sex chromosomes, birds have Z and W sex chromosomes. Males are the homogametic sex, carrying two Z chromosomes (ZZ), while females are the heterogametic sex, carrying a W chromosome and a Z chromosome (WZ). In nearly all species, an individual's sex is determined at fertilization. However, a recent study showed that, as is the case with some reptiles, the sex determination of embryonic megapodes may be related to the temperature of their nest; females hatched from eggs incubated at higher mean temperatures.

The vast majority of bird species—some 95 percent—are socially monogamous. These species pair for at least the length of the breeding season, and in some cases, the pair bonds may persist for several years, or even until one of the birds dies. One major advantage of monogamy is that it allows for biparental care; in some species, the female is unable to successfully raise a brood without the help of the male. Among most groups of animals, male parental care is rare. In birds, however, it is quite common—more so than in any other vertebrate class. Though the roles of territory defence, incubation, nest site defence, and chick feeding are often shared by both members of a monogamous pair, there is sometimes a division of labour, with all or most of a particular role undertaken either by the male or by the female.

Infidelity, in the form of extra-pair copulations, is common in many socially monogamous birds. These typically occur between dominant males and females paired with subordinate males, but can also be the result of forced copulation or rape in ducks and other anatids. Unfaithful behaviour offers several potential benefits for the female: the possibilty of getting better genes for her offspring, and some insurance against the possibility of infertility in her mate. Males of species that engage in extra-pair copulations will closely guard their mates to ensure the parentage of the offspring that they raise.

Other mating systems, including polygyny, polyandry, polygamy, polygynandry, and promiscuity, also occur. Polygamous breeding systems arise when females are able to raise broods without the help of males. Some species may use more than one system depending on circumstances.

Breeding usually involves some form of courtship display, typically performed by the male. Most displays are rather simple and involve some type of song. Some displays, however, are quite elaborate. Depending on the species, these may include wing or tail drumming, dancing, aerial flights, or communal lekking. Females are generally the ones that drive partner selection, although in the polyandrous phalaropes, this is reversed: plainer males choose brightly coloured females. Courtship feeding, billing and allopreening are commonly performed between partners, generally after the birds have paired and mated.

Territories, nesting and incubation

Main article: Bird nest

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.

The nesting colonies of the Sociable Weaver are amongst the largest bird-created structures.

All birds lay amniotic eggs with hard shells made mostly of calcium carbonate. The colour 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 those in the order Charadriiformes, are camouflaged. There are many exceptions to this pattern, however; the ground nesting nightjars have pale eggs with camouflage being provided instead by the bird's plumage. Species that are victims of brood parasites like the Dideric Cuckoo will vary their egg colours in order to improve the chances of spotting a cuckoo's egg, thus forcing female cuckoos to match their eggs to that of 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 albatross nests, 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 of the male. 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 are hidden to reduce the risk of predation, while 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 toxins that reduce nest parasites such as mites and therefore lead to increased chick survival. Nests are often lined with feathers in order to improve the retention of heat.

Incubation usually begins after the last egg has been laid and serves to optimise temperature for chick development. Duties are often shared in monogamous species; in polygamous species a single parent undertakes all incubating. 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 (3 oz) of body weight a day. The warmth for the incubation of the eggs of megapodes comes from the sun, decaying vegetation or volcanic sources. Incubation periods last between 10 days (in species of woodpeckers, cuckoos and passerine birds) to over 80 days (in albatrosses and kiwis).

A female Seychelles Sunbird with arachnid prey attending its nest.

Parental care and fledging

Chicks can be anywhere from helpless to independent at hatching. The helpless chicks are known as altricial and tend to be born, small, naked and blind; chicks that are mobile and feathered at hatching are precocial. Chicks at neither of the extremes can be semi-precocial or 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 hatching; 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 that of the Great Frigatebird, whose chicks take up to six months to fledge and are fed by the parents for up to an additional 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 among the Corvida, which includes such birds as the true crows, Australian Magpie and Fairy-wrens, but has been observed in as different species as the Rifleman and Red Kite.

This Reed Warbler is raising the young of a Common Cuckoo, a brood parasite

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, such as 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, while other species continue some supplementary feeding after fledging. Chicks may also follow their parents during their first migration.

Brood parasites

Main article: Brood parasite

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 so that they hatch before their hosts's young and either push their hosts' eggs out of the nest, destroying the egg, or kill their chicks, therefore ensuring that all the food brought to the nest is fed to them.

Ecology

File:Skua and penguin.jpeg

The South Polar Skua (left) is a generalist predator, taking the eggs of other birds, fish, carrion and other animals. This skua is attempting to push an Adelie Penguin (right) off its nest

The diverse feeding habits and life-histories of birds are associated with a range of ecological positions. While some birds are generalists, others are highly specialized in their habitat or food requirements. Even within a single habitat, such as a forest, the niches occupied by different species of birds vary, with some species feeding in the forest canopy, others utilizing the space under the canopy, while still others using the forest floor. In addition, forest birds may be classified into different feeding guilds such as insectivores, frugivores, and nectarivores. Aquatic birds show other feeding habits such as fishing, plant eating, and piracy or kleptoparasitism. The birds of prey specialize in hunting mammals or other birds while the vultures specialize 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 with both flower and bird having 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 have reached islands that mammals have not, and therefore they may fulfill ecological roles typically 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 quantities of guano, which can have appreciable impacts on the richness of the local soil and of the surrounding seas.

Relationship with humans

Industrial farming of chickens.

Since birds are highly visible and common animals, humans have had a relationship with them since the dawn of man. 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 domestic pigeon thrives in urban areas around the world and has increased in population as man has expanded. Many bird species have become commercially significant pests on agricultural crops and pose a hazard to aviation through bird strikes. Human activities can also be detrimental and have threatened numerous bird species with extinction.

Birds 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 that can also be transmitted to humans. Recent research suggests that the saliva of birds is a better indicator of avian influenza than are their faecal samples.

Some birds are apex predators of their respective food chains, which makes them very sensitive indicators of pollution. The decline in bird populations in the United States due to pesticide use is a famous example. Birds and their diversity have therefore been considered good indicators of an ecosystem's health and, in the United Kingdom, it is used as one of the 15 quality of life indicators.

Economic importance

Birds are an important food source for humans. The most commonly eaten species is the domestic chicken and its eggs; other popular species include geese, pheasants, turkeys, ducks and quail. Hunting remains an important method of obtaining birds, as it has been throughout human history; however, hunting has led to the extinction or endangerment of dozens of species. 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 and guano-derived phosphorus and nitrogen that is used in fertiliser and gunpowder. Colourful birds, such as parrots and mynas, are bred in captivity or kept as pets. The popularity of this practice has led to the illegal trafficking of some endangered species. Other birds have long been used by humans to perform tasks, such as falcons for hunting and cormorants for fishing. Pigeons have been recorded as being used as messengers as early as 1 AD and played an important role as recently as World War II. Today, such activities are more common either as hobbies, for entertainment and tourism, or for sports such as pigeon racing.

Cormorants used by fishermen in Southeast Asia. The practice is in steep decline but survives in some areas as a tourism attraction.

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.

"The 3 of Birds" by the Master of the Playing Cards, 16th Century Germany

Religion, folklore and culture

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

Birds have been featured in culture and art since prehistoric times, where they were represented in early cave paintings. As time progressed, birds came to be used in religious or symbolic art and design; among the most magnificent examples of this was the Peacock Throne of the Mughal and Persian emperors. With the advent of scientific interest in birds, many paintings of birds were commissioned for books. Among the most famous of these bird artists was John James Audubon, whose 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 figures in poetry; for example, Homer incorporated Nightingales into the Odyssey and therefore inspired future poets to refer to this bird and Catullus used a sparrow in his Catullus 2 as an erotic symbol. 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 albatross as a metaphor for a 'burden'. Birds serve as the roots of other metaphors in the English language; for example, vulture funds and vulture investors carry a negative connotation, whereas vultures are perceived as unpleasant and possibly unethical. Perceptions of individual bird species may vary from culture to culture. For example, while owls are considered bad luck in some parts of Africa, they are regarded as wise across much of Europe. Also, Hoopoes were considered sacred in Ancient Egypt and symbols of virtue in Persia, but as thieves across much of Europe and harbingers of war in Scandinavia.

Conservation

Main article: Bird conservation

See also: Late Quaternary prehistoric birds and Extinct birds

This Black-browed Albatross has been hooked on a long-line. This type of fishing threatens 19 of the 21 species of albatross, three critically so.

Humans have had a large impact on many bird species. Human activities have in some cases allowed a few species, like the Barn Swallow or European Starling, to dramatically expand their natural ranges, while in other species ranges have decreased because of human activities and some have even become extinct. Over a hundred species have gone extinct in historical times, although the most dramatic of human-caused avian extinctions occurred before historic times in the Pacific Ocean as people colonised the islands of Melanesia, Polynesia and Micronesia, during which an estimated 750–1800 species of bird went extinct. Many bird populations are declining worldwide, with 1,221 species listed as threatened in 2007 by Birdlife International and the IUCN. The most commonly cited reason for the endangerment of birds is habitat loss. Other threats include overhunting, accidental mortality due to structural collisions or as long-line fishing bycatch, pollution (including oil spills and pesticide use), competition and predation from nonnative invasive species, and climate change. Governments, along with numerous conservation charities, work to protect birds, either through laws that preserve and restore bird habitat or by establishing captive populations for reintroductions. The efforts of conservation biology have met with some success; a study estimated that 16 species of bird that would otherwise have gone extinct between 1994 and 2004 were saved due to conservation efforts, including the California Condor and Norfolk Island Green Parrot.

External links

• Avibase – The World Bird Database
• International Ornithological Committee
• Birdlife International – Dedicated to bird conservation worldwide; has a database with about 250,000 records on endangered bird species
• The Internet Bird Collection – A free library of videos of the world's birds
• Birds and Science from the National Audubon Society
• SORA Searchable online research archive; Archives of the following ornithological journals The Auk, Condor, Journal of Field Ornithology, North American Bird Bander, Studies in Avian Biology, Pacific Coast Avifauna, and the Wilson Bulletin.
• Essays on bird biology
• Cornell Lab of Ornithology
• The Institute for Bird Populations, California
• Ornithology
• North American Birds for Kids
• Bird biogeography