bird

Birds are believed to be extant members of a group of dinosaurs called maniraptors (other maniraptors include Velociraptor and Oviraptor). They share with dinosaurs such characteristics as a foot with three primary toes and one accessory toe held high in back. Early avians include such primitive birds as Arachaeopteryx, the rooster-sized Patagopteryx, and the ichthyornithiforms, skillful flyers with toothed beaks. The fossil remains of the Archaeopteryx, which date to the Jurassic period, show reptilian tails, jaws with teeth, and clawed wings, but feathers were well developed. Pterosaurs, another group of flying reptiles, did not share the common characteristics of birds and dinosaurs and are not considered birds. Whether the capacity for flight arose in tree-living dinosaurs that glided from branch to branch (the “trees-down” hypothesis) or in fast-running terrestrial dinosaurs (the “ground-up” hypothesis) continues to be debated. Indeed, the inclusion of birds in the dinosaur family tree, although accepted by most paleontologists, is debated by some, and the identification (2000) of the oldest known feathers on 220-million-year-old, four-legged reptile fossil, Longisquama insignis, raised questions concerning the theory.

Birds are of enormous value to humanity because of their destruction of insect pests and weed seeds. Many are useful as scavengers. The game birds hunted for food and sport include grouse, pheasant, quail, duck, and plover. The chief domestic birds are the chicken (see poultry), duck, goose, turkey, and guinea fowl. Parrots and many members of the finch family are kept as pets.

Characteristic Features and Behaviors

Like mammals, they have a four-chambered heart, and there is a complete separation of oxygenated and deoxygenated blood. The body temperature is from 2° to 14° higher than that of mammals. Birds have a relatively large brain, keen sight, and acute hearing, but little sense of smell. Birds are highly adapted for flight. Their structure combines lightness and strength. Body weight is reduced by the presence of a horny bill instead of heavy jaws and teeth and by the air sacs in the hollow bones as well as in other parts of the body. Compactness and firmness are achieved by the fusion of bones in the pelvic region and in other parts of the skeleton. The heavier parts of the body—the gizzard, intestines, flight muscles, and thigh muscles—are all strategically located for maintaining balance in flight. Feathers, despite their lightness, are highly protective against cold and wet. The flight feathers, especially, have great strength. Feathers are renewed in the process of molting. Some birds, such as the ostrich, the penguin, and the kiwi, lack the power of flight and have a flat sternum, or breastbone, without the prominent keel to which the well-developed flight muscles of other birds are attached. The bills of birds are well adapted to their food habits. Specialized bills are found in the crossbill, hummingbird, spoonbill, pelican, and woodpecker.

In the majority of species there are differences between male and female in plumage coloring. In these birds the male (except in the phalarope) is usually the more brilliant or the more distinctly marked and is the aggressor in courtship. Unusual courtship displays are performed by several species, particularly by the ruffed grouse, the bird of paradise, the crane, the pheasant, and the peacock. Birdsong reaches its highest development during the breeding season, and singing ability is usually either restricted to or superior in the male. Most birds build a nest in which to lay their eggs. Some birds, such as the oriole, weave an intricate structure, while others lay their eggs directly on the ground or among a few seemingly carelessly assembled twigs. Eggs vary in size, number, color, and shape. In spring and fall many birds migrate. Not all of the factors motivating this behavior are fully understood. These trips often involve flights of hundreds and even thousands of miles over mountains and oceans (see also migration of animals).

Bibliography

Among the periodicals devoted to the study of bird life are the Auk, the Condor, and the Wilson Bulletin. Books on birds include the many guides by R. T. Peterson; the life histories of North American birds in F. Gill and A. Poole, ed., The Birds of North America (1992–2003); R. M. De Schauensee, A Guide to the Birds of South America (1970); A. Rutgers and K. A. Norris, ed., Encyclopaedia of Aviculture (Vol. I, 1971, and Vol. II, 1972); U.S. Bureau of Sports Fisheries and Wildlife, Birds in Our Lives (1970); J. Van Tyne and A. J. Berger, Fundamentals of Ornithology (1971); S. Cramp, ed., Handbook of the Birds of Europe, the Middle East, and North Africa (5 vol., 1977–88); M. Walters, Birds of the World (1980); B. King et al., The Collins Field Guide to the Birds of South-East Asia (1988); J. Farrand, Jr., Eastern Birds (1988) and Western Birds (1988); S. Chatterjee, The Rise of Birds (1997); P. Shipman, Taking Wing (1998); D. Attenborough, The Life of Birds (1998); D. A. Sibley, The Sibley Guide to Birds (2000).

Birds Fossil range: Late Jurassic - Recent
Superb Fairy-wren, Malurus cyaneus, juvenile
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Class: Aves Linnaeus, 1758
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 period, c 200 to 150 Ma (million years ago), and the earliest known bird is the Late Jurassic Archaeopteryx, c 155–150 Ma. Around 10,000 living and recently (after 1500) extinct species of birds compose the class Aves, making them the most diverse terrestrial vertebrates. They inhabit ecosystems across the globe, from Arctic terns to Antarctic penguins. Birds range in size from the tiny hummingbirds to the huge Ostrich.

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, though the ratites and several others, particularly endemic island species, have lost the ability to fly. Birds also have unique digestive and respiratory systems that are highly adapted for flight.

Many species of bird undertake long distance annual migrations, and many more perform shorter irregular movements. Birds are social and communicate using visual signals and through calls and song, and participate in social behaviours including cooperative hunting, cooperative breeding, flocking and mobbing of predators. The vast majority of bird species are socially monogamous, usually one breeding season at a time, sometimes for years, and rarely for life. Other species have breeding systems that are polygynous ("many females") or, rarely, polyandrous ("many males"). Among some monogamous species, extra-pair copulations are common. Eggs are usually laid in a nest and incubated and most birds have an extended period of parental care after hatching.

Birds are economically important to humans: many are important sources of food, acquired either through hunting or farming, and they provide other products. Some species, particularly songbirds and parrots, are popular as pets. Birds figure prominently in all aspects of human culture from religion to poetry and popular music. About 120–130 species have become extinct as a result of human activity since 1600, and hundreds more before this. Currently around 1,200 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 birds was developed by Francis Willughby and John Ray in their 1676 volume, Ornithologiae.^[1] Carolus Linnaeus modified that work in 1758 to devise the taxonomic classification system still in use.^[2] Birds are categorised as the biological class Aves in Linnean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda.^[3] Aves and a sister group, the order Crocodilia, together are the sole living members of the reptile clade Archosauria. Phylogenetically, Aves is commonly defined as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica.^[4] 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,^[5] in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.

Modern birds all sit within the subclass Neornithes, which is divided into two superorders, the Paleognathae (mostly flightless birds like ostriches), and the wildly diverse Neognathae, containing all other birds.^[3] Depending on the taxonomic viewpoint, the number of species cited varies anywhere from 9,800^[6] to 10,050^[7] known living bird species in the world.

Dinosaurs and the origin of birds

Main article: Origin of birds

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.^[8] As more non-avian theropods that are closely related to birds are discovered, the formerly clear distinction between non-birds and birds becomes blurred. Recent discoveries in Liaoning Province of northeast China, demonstrating that many small theropod dinosaurs had feathers, contribute to this ambiguity.^[9]

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 it open to considerable doubt whether this was a bird ancestor.^[10]

The dromaeosaurids Cryptovolans and Microraptor may have been capable of powered flight to an extent similar to or greater than that of Archaeopteryx. Cryptovolans had a sternal keel and had ribs with uncinate processes. In fact, Cryptovolans makes a better "bird" than Archaeopteryx which is missing some of these modern bird features. Because of this, some palaeontologists have suggested that dromaeosaurs are actually basal birds, and that the larger members of the family are secondarily flightless, i.e. that dromaeosaurs evolved from birds and not the other way around.^[11] Evidence for this theory is currently inconclusive, as the exact relationship among the most advanced maniraptoran dinosaurs and the most primitive true birds are not well understood.

Although ornithischian (bird-hipped) dinosaurs share the hip structure of birds, birds actually originated from the saurischian (lizard-hipped) dinosaurs, and thus evolved their hip structure independently.^[12] 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,^[13] a theory which is contested by most palaeontologists and evidence based on feather development and evolution.^[14]

Early evolution of birds

See also: Fossil birds

During the Cretaceous Period, birds diversified into a wide variety of forms.^[15] Many of these groups retained primitive characteristics, such as clawed wings and teeth, though the latter was lost independently in a number of bird groups, including modern birds (Neornithes). While the earliest birds retained the long bony tails of their ancestors (birds such as Archaeopteryx and Jeholornis),^[15] more advanced birds shortened the tail with the advent of the pygostyle bone in the clade Pygostylia.

The first large, diverse lineage of short-tailed birds to evolve were the Enantiornithes, or "opposite birds", so named because the construction of their shoulder bones was the reverse of the condition seen in modern birds. Enantirornithes occupied a wide array of ecological niches, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters.^[15] More advanced lineages also specialized in eating fish, like the superficially gull-like subclass of Ichthyornithes ("fish birds").^[16] 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.^[15]

Radiation of modern birds

See also: Sibley-Ahlquist taxonomy and dinosaur classification

Modern birds are classified in the subclass Neornithes, which are now known to have evolved into some basic lineages by the end of the Cretaceous (see Vegavis).^[17] The Neornithes are split into two superorders, 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, rheas, 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).^[18]

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.^[19] This disagreement is in part caused by a divergence in the evidence, with molecular dating suggesting a Cretaceous radiation and fossil evidence supporting a Tertiary radiation. Attempts to reconcile the molecular and fossil evidence have proved controversial.^[19][20]

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,^[21] although frequently debated and constantly revised. A preponderance of evidence seems to suggest that the modern bird orders constitute accurate taxa.^[22]

But scientists disagree about the relationships between orders; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem but no strong consensus has emerged. More recently, new fossil and molecular evidence is providing an increasingly clear picture of the evolution of modern bird orders.

Modern bird orders

This is a list of the taxonomic orders in the subclass Neornithes, or modern birds. This is 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.^[19] By 2006, increasing evidence made it possible to verify the major proposals of the taxonomy. For example, see Charadriiformes, Gruiformes or Caprimulgiformes.

Distribution

The range of the House Sparrow has expanded dramatically due to human activities.^[23]

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.^[24] 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  kilometres ( mi) inland in Antarctica.^[25] 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^[26] and some penguins recorded diving as deeply as  metres ( ft).^[27] 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.^[28] Others are accidental, such as the Monk Parakeets that have escaped from captivity and established breeding colonies in a number of North American cities.^[29] Some species, including the Cattle Egret,^[30] Yellow-headed Caracara^[31] and Galah,^[32] 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 bones which are very light. They have large pneumatic (air-filled) cavities which connect with the respiratory system.^[33] The skull bones are fused and do not show cranial sutures.^[34] 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.^[35] The last few are fused with the pelvis to form the synsacrum.^[34] 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.^[36]

Like the reptiles, birds are primarily uricotelic, that is their kidneys extract nitrogenous wastes from their bloodstream and excrete it as uric acid instead of urea or ammonia. The uric acid is excreted along with feces as a semisolid waste and they do not have a separate urinary bladder or opening.^[37][38] Some birds such as hummingbirds however can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia.^[39] They also excrete creatine rather than creatinine as in mammals.^[34] This material, as well as the output of the intestines, emerges from the bird's cloaca.^[40][41] 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.^[42] 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.^[43] Most are highly adapted for rapid digestion, an adaptation to flight.^[44] Some migratory birds have the additional ability to reduce parts of the intestines prior to migration.^[45]

Birds have one of the most complex respiratory systems of all animal groups.^[34] When a bird inhales, 75% of the fresh air bypasses the lungs and flows directly into a posterior air sac which extends from the lungs and connects with air spaces in the bones and fills them with air. The other 25% of the air goes directly into the lungs. When the bird exhales, the used air flows out of the lung and the stored fresh air from the posterior air sac is simultaneously forced into the lungs. Thus, a bird's lungs receive a constant supply of fresh air during both inhalation and exhalation.^[46] 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.^[47] 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).^[34] The postcava receives blood from the limbs via the renal portal system. Birds, unlike mammals, have nucleated erythrocytes, that is, red blood cells which retain a nucleus.^[48]

The nervous system is large relative to the bird's size.^[34] 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,^[49] vultures^[50] and the tubenoses.^[51] The visual system is usually highly developed. Water birds have special flexible lenses, allowing accommodation for vision in air and water.^[34] 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.^[52] This allows them to perceive ultraviolet light; which is used in courtship. Many birds show plumage patterns in ultraviolet that are invisible to the human eye; so that some birds, whose sexes appear similar are distinguished by the presence of ultraviolet reflective patches of feathers. Male Blue Tits have an ultraviolet reflective crown patch which is displayed in courtship by posturing and raising of their nape feathers.^[53] 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.^[54] The eyelids of a bird are not used in blinking, instead the eye is lubricated by the nictitating membrane, the third eyelid that moves horizontally.^[55] The nictitating membrane also covers the eye and acts as a contact lens in many aquatic birds.