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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
Many - see section below.

Birds are bipedal, warm-blooded, oviparous vertebrate animals characterized primarily by feathers, forelimbs modified as wings, and (in most) hollow bones. All birds reproduce sexually, although parthenogenetic eggs are known to be produced by the domesticated turkey on occasion and suspected to occur in its wild ancestor.

Birds range in size from the tiny hummingbirds to the huge Ostrich and Emu. Depending on the taxonomic viewpoint, there are about 8,800–10,200 living bird species (and about 120–130 that have become extinct in the span of human history) in the world, making them the most diverse class of terrestrial vertebrates.

Birds feed on nectar, plants, seeds, insects, fish, mammals, carrion, or other birds.

Most birds are diurnal, or active during the day, but some birds, such as the 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.

Many birds migrate long distances to utilise optimum habitats (e.g., Arctic Tern) while others spend almost all their time at sea (e.g. the Wandering Albatross). Some, such as Common Swifts, stay aloft for days at a time, even sleeping on the wing.

Common characteristics of birds include beak with no teeth, the laying of hard-shelled eggs, high metabolic rate, a 4-chambered heart, and a light but strong skeleton. Most birds are characterised by flight, though the ratites are flightless, and several other species, particularly on islands, have also lost this ability. Flightless birds include the penguins, ostrich, kiwi, and the extinct Dodo. Flightless species are vulnerable to extinction when humans or the mammals they introduce arrive in their habitat. The Great Auk, flightless rails, and the moa of New Zealand, for example, all became extinct due to human influence.

Birds are among the most extensively studied of all animal groups. Hundreds of academic journals and thousands of scientists are devoted to bird research, while amateur enthusiasts (called birdwatchers, twitchers or, more commonly, birders) probably number in the millions.

High-level taxonomy

Birds are categorised as a biological class, Aves. The earliest known species of this class is Archaeopteryx lithographica, from the Late Jurassic period. Modern phylogenies place birds in the dinosaur clade Theropoda. According to the current consensus, Aves and a sister group, the order Crocodilia, together are the sole living members of an unranked "reptile" clade, the Archosauria.

Phylogenetically, Aves is usually defined as all descendants of the most recent common ancestor of modern birds (or of a specific modern bird species like Passer domesticus), and Archaeopteryx.

Modern birds are divided into two superorders, the Paleognathae (mostly flightless birds like ostriches), and the wildly diverse Neognathae, containing all other birds.

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:

• Struthioniformes, Ostrich, 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

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 taxonomical thinking considerably, with the Galloanserae proving well-supported by recent molecular, fossil and anatomical evidence. With increasingly good evidence, it has become possible by 2006 to test the major proposals of the Sibley-Ahlquist taxonomy. The results are often nothing short of astounding, see e.g. Charadriiformes or Caprimulgiformes.

Extinct bird orders

A wide variety of bird groups became extinct during the Mesozoic era and left no modern descendants. These include the orders Archaeopterygiformes, Confuciusornithiformes, toothed seabirds like the Hesperornithes and Ichthyornithes, and the diverse subclass Enantiornithes ("opposite birds").

For a complete listing of prehistoric bird groups, see Fossil birds.

Evolution

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

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 recently (2002) discovered dromaeosaur Cryptovolans was capable of powered flight, 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 whose larger members are secondarily flightless, i.e. that dromaeosaurs evolved from birds and not the other way around. Evidence for this theory is currently inconclusive, but digs continue to unearth fossils (especially in China) of the strange feathered dromaeosaurs. At any rate, it is fairly certain that avian flight existed in the mid-Jurassic and was "tried out" in several lineages and variants by the mid-Cretaceous.

Although ornithischian (bird-hipped) dinosaurs share the same hip structure as birds, birds actually originated from the saurischian (lizard-hipped) dinosaurs (if the dinosaurian origin theory is correct), 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 Lary Martin and Alan Feduccia), states that birds (including maniraptoran "dinosaurs") evolved from early archosaurs like Longisquama, a theory which is contested by most other scientists in paleontology, and by experts in feather development and evolution such as R.O. Prum. See the Longisquama article for more on this alternative.

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 (found only in 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). See the chart for more information.

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.

Bird anatomy

Birds have a body plan that shows so many unusual adaptations (mostly aiding flight) that birds have earned their own unique class in the vertebrate phylum.

The skeleton consists of bones which are very light. They have large pneumatic 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.

Unlike mammals, birds do not urinate. Their kidneys extract nitrogenous wastes from the bloodstream, but instead of excreting it as urea dissolved in urine as we do, they excrete it in the form of uric acid. 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. The cloaca is a multi-purpose hole for birds: their wastes come out of it, they have sex by putting their cloacas together, and females lay eggs out of it.

Birds have one of the most complex lung systems of all organisms. Air enters the bird and immediately 75% of the 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 air from the posterior air sac is forced into the lungs; thus birds receive a supply of air during both inhalation and exhalation. The gas exchange then takes place in the capillaries. The 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.

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 digestive system of the bird is unique and has a crop for storage and a gizzard for grinding food. Most are highly adapted for rapid digestion, an adaptation to flight. Migratory birds have the additional ability to reduce parts of the intestines prior to migration.

The nervous system, relative to the bird's size, is actually quite large. 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 side of their head have a wide visual field while birds with eyes on the front of their heads like owls have binocular vision and can measure depth.

Most birds have a poor sense of smell except for the kiwis and vultures. The visual system is 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.

Nesting

Eggs

All birds lay amniotic eggs with hard shells made mostly of calcium carbonate. Non-passerines typically have white eggs, except in some ground-nesting groups such as the Charadriiformes, sandgrouse and nightjars, where camouflage is necessary, and some parasitic cuckoos which have to match the passerine host's egg. Most passerines, in contrast, lay coloured eggs, even if, like the tits they are hole-nesters.

The brown or red protoporphyrin markings on passerine eggs reduce brittleness and are a substitute for calcium when that element is in short supply. The colour of individual eggs is genetically influenced, and appears to be inherited through the mother only, suggesting that the gene responsible for pigmentation is on the sex determining W chromosome (female birds are WZ, males ZZ).

The eggs are laid in a nest, which may be anything from a bare cliff ledge or ground scrape to elaboratey decorated structures such as those of the oropendolas.

Social systems and parental care

The three mating systems that predominate among birds are polyandry, polygyny, and monogamy. Monogamy is seen in approximately 91% of all bird species. Polygyny constitutes 2% of all birds and polyandry is seen in less than 1%. Monogamous species of males and females pair for the breeding season. In some cases, the individuals may pair for life.

One reason for the high rate of monogamy among birds is the fact that male birds are just as adept at parental care as females. 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 one form of monogamy such as with obligate monogamy a female cannot rear a litter without the aid of a male".

The parental behavior most closely associated with monogamy is male incubation. Interestingly, male incubation is the most confining male parental behavior. It takes time and also may require physiological changes that interfere with continued mating. This extreme loss of mating opportunities leads to a reduction in reproductive success among incubating males. "This information then suggests that sexual selection may be less intense in taxa where males incubate, hypothetically because males allocate more effort to parental care and less to mating". In other words, in bird species in which male incubation is common, females tend to select mates on the basis of parental behaviors rather than physical appearance.

Birds and humans

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

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

Besides meat and eggs, birds provide other items useful to humans, including feathers for bedding and decoration, guano-derived phosphorus and nitrogen used in fertilizer and gunpowder, and the central ingredient of bird's nest soup.

Humans have caused the disappearance of some bird species. The Passenger Pigeons were hunted to extinction, even being the world's most common bird at one time, and Dodos, which lay their eggs on the ground, became extinct when humans brought pigs to their habitat which escaped and ate the dodo eggs, and when humans easily hunted them because of the dodos' lack of fear for humans. Many others have become endangered or extinct through habitat destruction (e.g. by deforestation or intensive agriculture).

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

Other birds have long been used by humans to perform tasks. For example, homing pigeons were used to carry messages before the advent of modern instant communications methods (many are still kept for sport). 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. As birds are very sensitive to toxins, the Canary was used in coal mines to indicate the presence of poisonous gases, allowing miners sufficient time to escape without injury.

Colorful, particularly tropical, birds (e.g. parrots, and mynas) are often 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.

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

Threats to birds

According to Worldwatch Institute, bird populations are declining worldwide, with 1,200 species facing extinction in the next century. Among the biggest cited reasons are habitat loss, predation by nonnative species, oil spills and pesticide use, hunting and fishing, and climate change.

Trivia

• To preen or groom their feathers, birds use their bills to brush away foreign particles.
• Some birds use chemical defences against predators. Tubenoses can eject an unpleasant oil against an aggressor, and some species of pitohui, found in New Guinea, secrete a powerful neurotoxin in their skin and feathers.

See also

• Anting
• Archaeopteryx
• Avian pallium
• Bird feeder
• Bird flight
• Bird hybrid
• Bird intelligence
• Bird observatory
• Bird migration
• Bird ringing (banding)
• Bird skeleton
• Bird song
• Birdfeeding
• Birding
• Birdwatching
• Carinatae
• Conservation status
• Egg biology
• Extinct birds
• Language of the birds
• List of birds
• Regional and country bird lists
• Longisquama
• Oology
• Ornithology
• Pinion (feather)
• Prehistoric birds
• Flying and gliding animals

Bird families and taxonomic discussion are given in list of birds and Sibley-Ahlquist taxonomy.

References

1. "Early Adaptive Radiation of Birds: Evidence from Fossils from Northeastern China -- Hou et al. 274 (5290): 1164 -- Science". Retrieved 2006-07-21.
2. "Education - Senior 1". Manitoba Fisheries Sustainable Development. Retrieved 2006-10-09.
3. Gowaty, Patricia Adair (1983). "Male Parental Care and Apparent Monogamy among Eastern Bluebirds (Sialia sialis)". The American Naturalist. 121 (2): 149–160. {{cite journal}}: Unknown parameter |quotes= ignored (help)
4. Ketterson, Ellen D. (1994). "Male Parental Behavior in Birds". Annual Review of Ecology and Systematics. 25: 601–28. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |quotes= ignored (help)
5. "Worldwatch Paper #165: Winged Messengers: The Decline of Birds". Retrieved 2006-07-21. {{cite web}}: Text "Worldwatch Institute" ignored (help)
6. "Help Migratory Birds Reach Their Destinations". Retrieved 2006-07-21.
7. "Protect Backyard Birds and Wildlife: Keep Pet Cats Indoors". Retrieved 2006-07-21.

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
• Birdingonthe.net
• Worldtwitch - Rare bird news around the world
• The Internet Bird Collection - A free library of videos of the world's birds
• Bird Hybrids Database - Search by bird name, use Sibley classification

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• Birds