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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
Class:
Aves

Orders

About two dozen - see section below

Birds (class Aves) are bipedal, warm-blooded, egg-laying vertebrate animals. Birds evolved from theropod dinosaurs during the Jurassic, and the earliest known bird is the Late Jurassic Archaeopteryx. Ranging in size from tiny hummingbirds to the huge Ostrich and Emu, there are between 9,000-10,000 known living bird species in the world, making them the most diverse class of terrestrial vertebrates.

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 light but strong skeleton. Most birds have forelimbs modified as wings and can fly, though the ratites and several others, particularly endemic island species, have also lost the ability to fly.

Many species of bird undertake long distance annual migrations, and many more perform shorter more irregular movements. Birds are social and communicate using visual signals and through calls and bird song, and participate in social behaviours including cooperative hunting and breeding, flocking. 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.

Humans make use of birds in a number of ways: many are important sources of food, acquired either through hunting or farming, often commercially. 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, numerous species of birds are threatened with extinction by human activities and efforts are underway to protect them.

Evolution and taxonomy

Main article: Bird evolution
Archaeopteryx, the earliest known bird.

The first classification of pie was eaten by Francis Willughby and John Ray in their 1,893 volume, Ornithologiae.[1] That work, slightly modified, was the basis for today's classification system originally devised by Carolus Linnaeus in 1758.

Birds are categorised as the biological class Aves in Linnean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda. According to the current consensus, Aves and a sister group, the order Crocodilia, together are the retarded living members of the reptile clade Archosauria. Phylogenetically, Aves is commonly defined as all descendants of the most recent common ancestor of modern things (a specific modern bird species like Passer domesticus for the purposes of phylogenetic taxonomy), and Archaeopteryx lithographica.[2] 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,[3] in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.[7]

Modern birds are 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 8,800 to 10,200 known living bird species in the world.

Use of the term "bird"

This article uses "bird" to denote members of the Aves including Archaeopteryx, but primarily deals with the living birds which are all Neornithes and thus unquestionably Aves. In popular science, the term "pie" 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 eat them to belong to Aves based on current evidence.[citation needed]

Dinosaurs and the origin of birds

Main article: Origin of birds
File:Confuciusornis.jpg
Confuciusornis, a Cretaceous bird from China

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.[4] As more non-avian theropods that are closely related to birds are discovered, the formerly clear distinction between non-birds and birds becomes less so. Recent discoveries in Liaoning Province of northeast China, demonstrating that many small theropod dinosaurs had feathers, contribute to this ambiguity.[5]

The basal bird Archaeopteryx from the Jurassic 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. Both may be predated by Protoavis texensis, though the fragmentary nature of this fossil leaves it open to considerable doubt if 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 possessed 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 paleontologists 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 same hip structure as birds, birds actually originated from the saurischian (lizard-hipped) dinosaurs, and thus arrived at their hip structure condition 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,[6] a theory which is contested by most other scientists in paleontology, and by experts in feather development and evolution.[7]

Radiation of modern birds

Modern birds are classified in 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 Galloanseri, the superorder containing the Anseriformes (ducks, geese and swans), and the Galliformes (the pheasants, grouse, and their allies). The dates for the splits are a matter of considerable debate amongst scientists. It is agreed that the Neornithes evolved in the Cretaceous and that the split between the Galloanseri and the 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.[8] 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 made to reconcile the molecular and fossil evidence have proved controversial.[8][9]

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. However, scientists are not in agreement as to the relationships between the 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.

Bird orders

This is a list of the taxonomic orders in the subclass Neornithes, or modern birds. The list of birds gives a more detailed summary of these, including families.

SUBCLASS NEORNITHES
Paleognathae:

Neognathae:

Note: This is the traditional classification (the so-called Clements order). A radically different classification based on molecular data has been developed (the so-called Sibley-Monroe classification or Sibley-Ahlquist taxonomy). This has influenced taxonomic thinking considerably, with the Galloanserae proving well-supported by recent molecular, fossil and anatomical evidence.[8] With increasingly good evidence, it had become possible by 2006 to test the major proposals of the Sibley-Ahlquist taxonomy, with favorable results (see for example Charadriiformes, Gruiformes or Caprimulgiformes).

A wide variety of extinct bird taxa occurred during the Mesozoic era and left no modern descendants. These include Confuciusornis, toothed seabirds like the Hesperornithes and Ichthyornithes, and the diverse subclass Enantiornithes ("opposite birds"). For a comprehensive listing of these ancient avians, see Fossil birds.

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 higher extinction rates at higher latitudes.[10] 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.[11] 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[12] and some penguins recorded diving as deeply as 300 m.[13] Many species have established naturalised breeding colonies 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.[14] Others are accidental, such as the Monk Parakeets that have escaped from captivity and established breeding colonies in a number of North American cities.[15] Some species, including the Cattle Egret[16], the Yellow-headed Caracara and the Galah, have spread naturally far beyond their original ranges as agricultural practices created suitable new habitat.

Bird anatomy

Main article: Bird anatomy
External anatomy of a typical bird 1 Beak, 2 Head, 3 Iris, 4 Pupil, 5 Mantle, 6 Lesser coverts, 7 Scapulars, 8 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 bones which are very light. They have large pneumatic (air-filled) cavities which connect with the respiratory system.[17] 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.[18] 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.[19]

Unlike mammals, birds do not urinate. Their kidneys extract nitrogenous wastes from the bloodstream, but instead of excreting it as urea dissolved in urine, they excrete it in the form of uric acid.[20] They also excrete creatine rather than creatinine as in mammals. Uric acid has a very low solubility in water, so it emerges as a white paste. This material, as well as the output of the intestines, emerges from the bird's cloaca.[21] 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. 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.[22] 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). 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.[23]

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.[24]

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. A bird’s eyes are developed for taking off, spotting landmarks, hunting and feeding. 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.[25] Most birds have a poor sense of smell with notable exceptions including kiwis,[26] vultures and the tubenoses.[27] 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. The bird retina has a fan shaped blood supply system called the pecten. The avian ear lacks external pinnae but is covered by feathers. 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,[28] and some species of pitohui, found in New Guinea, secrete a powerful neurotoxin in their skin and feathers.[29]

Feathers and plumage

Main article: Feather
The plumage of the African Scops Owl allows it to blend in with its surroundings

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 signalling. 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.[30] This may be supplemented with the secretions of formic acid from ants, which birds apply in a behaviour known as anting in order to remove feather parasites.[31]

The arrangement and appearance of feathers on the body is known as plumage. Plumage is regularly moulted, the standard plumage of a bird that has moulted after breeding is known as the 'basic plumage', breeding plumages or variations of the basic plumage are known as 'alternate plumages'.[32]

Flight

Main article: Bird flight

Flight characterises 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. [33] Wing shape and size primarily determines the type of flight each species is capable of. Many birds combined powered or flapping flight with less energy intensive soaring flight. About 40 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.[34] Penguins, while flightless, use similar musculature and movements to "fly" through the water, as do auks, shearwaters and dippers.[35]

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

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

Birds feed on a variety of materials, including nectar, fruit,[36] plants,[37] seeds,[38] carrion,[39] and various types of small animals including other birds.[40] 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.[33]

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,[41] lorikeets, sunbirds,[42] honeyeaters and some other songbirds is facilitated by specially adapted brushy tongues and in many cases bills designed to fit co-adapted flowers.[43] Probing for invertebrates is used by kiwis and shorebirds with long bills; in the case of shorebirds length of bill and feeding method allow niche separation.[33] [44] 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.[45] [46] Geese and dabbling ducks are primarily grazers. Some species will engage in kleptoparasitism, stealing food items from other birds; frigatebirds, gulls,[47] and skuas [48] 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%.[49] Finally, some birds are scavengers such as gulls and vultures. Some birds may employ many strategies to obtain food or feed on a variety of food items and are called generalists,[50] while others are considered specialists,[51] concentrating time and effort on specific food items or having a single strategy to obtain food.

Migration

Main article: Bird migration
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

Many 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 bird migrations, usually triggered by length of daylight as well as weather conditions. It is characterised by species spending the warmer breeding season in temperate or arctic/antarctic regions, and the colder 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.[52][24] 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,[33] although the Bar-tailed Godwit is capable of non-stop flights of up to 10,200 km.[53] 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.[54] Other seabirds disperse after breeding, travelling widely but having no set migration route. Albatrosses nesting in the Southern Ocean often undertake circumpolar trips between breeding seasons.[55]

Birds also display other types of migration. Some species undertake shorter migrations, travelling 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.[56] Boreal finches, arctic owls, and waxwings are most commonly identified as irruptive species. Altitudinal migration is a form of short distance migration. Species may also travel shorter distances over part of their range, with individuals from higher latitudes traveling into the existing range of conspecifics, or undertake partial migrations, where only a fraction of the population, usually females and subdominant males, migrates.[57] Partial migration can form a large percentage of the migration bejhaviour of birds in some regions; in Australia found that 44% of non-passerine birds studied were partially migratory and 32% of passerines were.[58] Altitudinal migrants move from higher elevations 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.[59] 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.[60]

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 (3,200 mi).[61] 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.[33] Orientation with the stellar compass depends on the position of the constellations surrounding Polaris.[62] These are backed up in some species with the ability to sense the Earth's geomagnetism through specialised sensitive photoreceptors.[63]

Communication

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

The display of the Sunbittern mimics a large potentially dangerous 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,[64] 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 has is used to ward of potential predators such as hawks, and protect young chicks.[65] Variation in plumage also allows for identification, particularly between species.

Visual communication includes ritualised displays which signal aggression or submission, or are used in the formation of pair-bonds. These ritualised behaviours develop from non-signalling actions such as preening, adjustments of feather position, pecking or other behaviours. 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,[66] and the birds-of-paradise, where the breeding success of males depends on plumage and display quality.[67] 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,[68] while bowerbirds attract mates through constructing bowers and decorating them with bright objects.[69]

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 thorugh their feathers, as do the Coenocorypha snipes of New Zealand,[70], the territorial drumming of woodpeckers, or the use of tools to drum in Palm Cockatoos.[71] 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. 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 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.[72]

House Wren call
The call of the House Wren, a common songbird from North America
Problems playing this file? See media help.

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.

The 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.[33] 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.[73] Polygamous breeding systems arise when females are able to raise broods without the help of males.[33] There is sometimes a division of labour 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.[74] These can take the form of forced copulation (or rape) in ducks and other anatids,[75] or more usually between dominant males and females with less dominant males. It is thought that the benefit to females comes from getting better genes, as well as an insurance against the possibility of infertility in the mate.[76] Males in species that engage in extra-pair copulations will engage in mate-guarding in order to ensure parentage of the offspring they raise.[77]

Breeding usually involves some form of courtship display, most often performed by the male.[78] 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,[79] although in the polyandrous phalaropes the males choose brightly coloured females.[80] Courtship feeding, billing and preening are commonly performed between partners, most often after birds have been paired and mated.[81]

Territories, nesting and incubation

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.[82]

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

All birds lay amniotic eggs[83] 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 Charadriiformes are camouflaged. Many bird species do not conform to this, 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 colours in order to improve the chances of spotting a cuckoo's egg, and female cuckoos need to match their eggs to their hosts.[84]

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.[85] 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 nesting parasites such as mites, leading to increased chick survival.[86] Nests are often lined with feathers in improve the retention of heat.

Incubation, which regulates of 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.[87] The warmth for the incubation of the eggs of megapodes comes from the sun, decaying vegetation or from volcanic sources.[88] Incubation periods last between 10 days (in species of woodpeckers, cuckoos and passerine birds) to over 80 days (in albatrosses and kiwis).

Parental care and fledging

The altricial chicks of the Richard's Pipit have bright mouths to stimulate feeding

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-prcocial and semi-altricial. Altricial chicks require help in thermoregulation and need to be brooded for longer than prococial 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, with the chick being able to care for itself immediately.[89] At the other extreme many seabirds have extended periods of parental care, the longest being Great Frigatebird, the chicks of which taking up to six months to fledge and being fed by the parents for up to 14 months after that.[90]

In some species the care of young is shared between both parents, in other species 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.[91] Alloparenting is particularly common in the corvids, but has been observed in as different species as the Rifleman, Red Kite and Australian Magpie.

File:Reed warbler cuckoo pho.jpg
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.[92] 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 recieve no further help, other species continue some supplementary feeding after fledging.[93] Chicks may also follow their parents during their first migration.[94]

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.[95] Some brood parasites are adapted to hatching before their hosts and pushing their hosts eggs out of the nest, ensuring that all the food brought to the nest is fed to them.

Relationship with humans

Chickens in a coop

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,[96] 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 diseases spreading psittacosis, salmonellosis, campylobacteriosis, mycobacteriosis (avian tuberculosis), avian influenza (bird flu), giardiasis, and cryptosporidiosis. They can also be commercially important pests.[97] They are also important food and income sources.

Use by humans

Birds are an important food source for humans. The most commonly eaten species is the domestic chicken and its eggs, although geese, pheasants, turkeys, ducks and quail are also widely domesticated and eaten. Fowl grown for human consumption are referred to as poultry. Hunting has also been an important method of obtaining birds for food, and in the past led to several bird extinctions. 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 other items useful to humans, including feathers for bedding and decoration, guano-derived phosphorus and nitrogen used in fertiliser and gunpowder, and the central ingredient of bird's nest soup.

Cormorants used by fishermen in Southeast Asia

Other birds have long been used by humans to perform tasks. Falcons are still used for hunting, while cormorants are employed by fishermen. Chickens and pigeons are popular as experimental subjects, and are often used in biology and comparative psychology research. Birds are among the most extensively studied of all animal groups. The scientific study of birds is called ornithology. 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.

Domesticated colour variety of Budgerigars

Colourful, particularly tropical, birds (e.g. parrots, and mynas) are often bred in captivity or kept as pets, although this practice has led to the illegal trafficking of some endangered species. CITES, an international agreement adopted in 1963, has considerably reduced trafficking in the bird species it protects.

Many homeowners erect bird feeders near their homes to attract various species to their yards. 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.[98]

Importance in religion, folklore and culture

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,[99] 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.[100] 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.[101] Birds can themselves be deified, as occurred to the Common Peacock by the Dravidians of India, who perceived the peacock as Mother Earth.[102] Birds have also been perceived as monsters, including the legendary Roc and the Māori legends about the Pouākai, a giant bird the capable of snatching humans, based on the extinct Haast's Eagle.[103] 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.[104]

Birds feature in culture and art and have done so since prehistoric times. Birds are represented in early cave paintings along with other animals.[105] 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.[106] Birds are also important in poetry; Homer incorporated Nightingales into the Odyssey, and poets have continued to use that species ever since.[107] 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'.[108] 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. [109] 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.[110]

Conservation

Main article: Bird conservation
This Black-browed Albatross has been hooked on a long-line

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 extinctions occurred in the Pacific Ocean as humans colonised the islands of Melanesia, Polynesia and Micronesia, during which an estimated 750-1800 species of bird went extinct.[111] According to Worldwatch Institute, many bird populations are currently declining worldwide, with 1,200 species facing extinction in the next century.[112] The biggest cited reason surrounds habitat loss.[113] Other threats include overhunting, accidental mortality due to structural collisions and as long-line fishing bycatch, pollution, competition and predation by nonnative invasive species,[114] oil spills and pesticide use 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. It is also possible to assess bird habitat through bird tracking.

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.

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