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Aves

Contributed by: Walter J. Bock

Publication year: 2014

The class of consisting of the . Modern birds ( Fig. 1 ) are characterized by being feathered, warm-blooded (homeothermic), and bipedal (two-legged), with the forelimb modified into a wing which, together with the tail attached to the short tail, forms the flight mechanism, and by having a very high metabolic rate. Such a characterization, however, as with any group of vertebrates, holds for the living forms and most fossil members of this class, but is blurred by the early fossil record, which contains species with characteristics closer to those of the reptilian ancestors of birds. The feathers of birds are lightweight modifications of the outer skin possessing remarkable aerodynamic qualities. They serve not only as surfaces to generate lift and thrust, and as a streamlined outer surface of the body, but also as insulation to maintain high body temperatures. In addition, birds have lightweight, hollow bones; a well-developed air-sac system and

flow-through lungs; a wishbone or furcula (fused clavicles); and a hand reduced to three digits (comparable to digits 2, 3, and 4 of the human hand). Birds have most likely evolved from an ancestor within the large group of ancient diapsid reptiles known as archosaurs (including alligators, snakes and lizards, and , among others). However, debate still centers on whether birds are derived from a basal archosaurian stock or arose later directly from the later and more derived theropod dinosaurs (carnivores such as Allosaurus and Velociraptor ).

The latter theory gained support by the discovery of fossils from the end of the Mesozoic Era (“Age of Reptiles”) that some researchers reported to be feathered dinosaurs; however, other researchers think they were birds that had already became secondarily flightless and represent “Mesozoic kiwis.”

Feathers

Feathers are unique to birds. These lightweight structures made of keratin are the most complex appendages produced by the skin of any vertebrate. The vanes of a are supported by a central shaft, or rachis, which is the structural backbone. They are composed of specialized filaments called barbs, which possess secondary, tiny

filaments called barbules, or barbs. The barbs are bound together in “Velcro” fashion by small hooklets, or hamuli, located on the barbs. Body-contour feathers of many species have an associated smaller secondary shaft—an aftershaft—which results from the developmental process of the feather and which adds to the insulating property of the plumage. Feathers grow from specialized dermal follicles and are generally renewed once a year during the postbreeding molt. Body feathers of some birds are molted twice a year; the large wing feathers of some raptors are not replaced every year. In addition to the typical vaned form, feathers come in many other forms, such as down, power down, bristles, and filoplumes. However, the tuft of epidermal outgrowths, the beard present on the breast of male turkeys, are not feathers. AccessScience from McGraw-Hill Education Page 2 of 11 www.accessscience.com

ImageFig. 1 ( aof ) :1 Calypte anna , Anna’s ( photo by Dr. Lloyd Glenn Ingles , copyright © California Academy of Sciences ), ( b ) : Red-headed woodpecker ( Melanerpes erythrocephalus ) in Desoto National Wildlife Refuge, Iowa ( p hoto by Dave Menke ∕ U.S. Fish and Wildlife Service ), ( c ) Strigiformes: Northern spotted ( photo by John and Karen Hollingsworth ∕ U.S. Fish and Wildlife Service ), ( d ) : Osprey ( Pandion haliaetus ) [ p hoto by Glenn and Martha Vargas , copyright © California Academy of Sciences ], ( e ) Phoenicapteriformes: Phoenicopterus ruber , greater flamingo ( photo by Dr. Lloyd Glenn Ingles , copyright © California Academy of Sciences ), ( f ) : Phalaropus fulicaria , the red phalarope ( p hoto by Gerald and Buff Corsi , copyright © California Academy of Sciences ), ( g ) : Pelecanus erythrorhynchos , American white pelican ( p hoto by Dr. Lloyd Glenn Ingles , copyright © California Academy of Sciences )

Body feathers in most birds are arranged in definite feather tracts, leaving large parts of the skin free of feathers.

Muscles attaching to the base of the feather, the calamus, move the feathers, thereby allowing the , for example, to fluff the body feathers into a loose ball around its entire body for maximum insulation or to move courtship plumes in precise and complicated ways.

The wing feathers, the remiges, produce lift and forward thrust in flight. They are asymmetric, with a smaller outer vane and a larger inner vane, which are the building blocks of slotted wings. The bird wing comprises two main sets of flight feathers, the outer primary feathers which are attached to the hand, and the inner secondary feathers which are attached to the ulna. The several feathers attached to the moveable second finger constitute the ulna which can form a slot to prevent stalling at low speeds, similar to those in airplane wings.

Contour feathers provide smooth aerodynamic surfaces of the body, resulting in laminar airflow during flight.

Typically, the body contour feathers have symmetrical vanes. Most of the vane is stiff and tightly bound, like

flight feathers, and is known as the pennaceous portion. However, the part of the vane near the base, known as the plumaceous portion, lacks hooklets and is loosely bound. This basal portion can by fluffed up to trap body AccessScience from McGraw-Hill Education Page 3 of 11 www.accessscience.com

heat next to the skin. In warm conditions the body feathers can be flattened to allow heat to escape. Thus, feathers form an insulating plumage to cover the surface of the avian body.

Tail feathers (rectrices) resemble the flight feathers of the wing and provide lift in flight. The tail feathers of modern birds are attached to a vertically flattened bone known as the pygostyle, formed by a number of fused caudal vertebrae. The pygostyle (sometimes called the plowshare bone) also supports the uropygial or oil gland that provides a rich oil used to preen the feathers and to maintain their moistness and flexibility.

Not all feathers have stiff vanes. Typically, newly hatched birds are covered by a coat of plush down feathers, which are replaced by the adult contour feathers. Down feathers lack a central shaft but have long, loosely connected barbules that provide an insulating layer next to the skin.

In addition to flight and insulation, feathers can serve other functions, ranging from providing color patterns and structural forms serving for species recognition and courtship displays (including sound production by wing or tail feathers) to cryptic color patterns for protection (as is the case with the woodcock, which has feathers that blend with the dead leaves of the forest floor). Another unusual adaptation of feathers is seen in the sand , a desert bird of Africa and Asia that makes long flights to a water hole to drink. The male sand grouse also obtains water for the young, storing it in his specialized flattened and coiled barbules on the contour feathers of the belly.

This feature allows the male to hold water in flight and transport it to the nest, which may be some 20 mi (30 km) away. The young drink by squeezing the wet feathers in their bills. See also: FEATHER .

Beak

The original toothed jaws of ancestral birds have evolved into the light toothless beak in which the upper and lower jaws are covered by a horny rhamphotheca, and which may vary in texture from the strong beaks of predatory raptors and seed-eating and finches to relatively soft beaks of shorebirds and ducks. Beaks have a great variety of adaptive forms, including the flesh-tearing hooked beaks of hawks and eagles, the filter-feeding straining beaks of flamingos and ducks, the fish-trapping beaks of pelicans, the climbing and nut-cracking beaks of parrots, the hammering beaks of woodpeckers, and the seed-eating beaks of finches. As in most vertebrates

(many fish and reptiles), in addition to the usual moveable lower jaw, the upper jaw of all birds is kinetic, that is, moveable with respect to the brain case, which enables many important functional properties of the avian feeding apparatus in contrast to the akinetic jaw apparatus of mammals. The structure of the tongue also varies greatly depending on the diet of the species.

Birds have developed a muscular gizzard (also found in their relatives, crocodiles and dinosaurs) for grinding and processing food into small pieces. The grinding is often assisted by the addition of ingested gizzard stones. The ancient giant moas of New Zealand are preserved with masses of stones in the stomach region, and are known to have subsisted on a diet of leaves. AccessScience from McGraw-Hill Education Page 4 of 11 www.accessscience.com

Wing

Birds are characterized as flying vertebrates with the forelimb modified as a wing with strong flight feathers. They also possess a large, strong pectoral girdle and bony sternum for the attachment of the large flight muscles and for the transfer of the body weight to the wings when the bird is flying. Considerable variation exists in the morphology of the wing (size and shape of the aerodynamic surface formed by the flight feathers) and of the entire muscle-bone system of the pectoral girdle and limb. But it is a mistake to conclude that all activities of birds are associated with flight; indeed for most birds, flight occupies a minor part of their daily activities. The important biological roles of flight in most birds are associated with quick escape from predators, reaching secure nesting places, larger separation of the nesting and foraging sites, and long-distance migration. Yet a few birds

(such as swifts and ) are able to spend many days or even weeks in the air, apparently being able to sleep during flight. And some birds (such as waterfowl and shorebirds) are able to make very long distance migration

flights of up to 3000–4000 km (1800–2400 mi).

Hind limb

The leg and foot structures of birds differ greatly depending on their nonflying locomotion: from the long legs of wading birds to the strong, shorter legs of predators and aquatic birds to the almost rudimentary legs of the swifts and . The most primitive avian foot, found in the earliest definitively known bird, , is the anisodactyl perching arrangement of toes, also found in most modern tree-dwelling birds. Three toes point forward, and a reversed first toe, or hallux, opposes them in perching on a branch.

Other modifications include the zygodactyl feet (fourth toe reversed) of woodpeckers, , and parrots, and the heterodactyl foot (second toe reversed) of the with two forward- and two rearward-pointing toes.

The webbed feet of ducks and many other swimming birds have three forward-pointing toes united by a web that serves as a paddle. Members of the order Pelecaniformes have a foot in which all four toes are united by webbing, a totipalmate foot. Ostriches are unique among living birds in having a foot with only two toes.

Other distinctive features

The avian neck is long and flexible, consisting of many vertebrae with unique saddle-shaped articulating surfaces, allowing the bird to reach most parts of its plumage with its beak for preening. Modern flying birds have a well-developed pectoral girdle including a large, bony sternum with a keel, or carina, for the attachment of the large flight musculature. The pelvic girdle is large with the bones fused together with the synsacrum (a number of completely fused sacral vertebrae) to provide firm support for the bipedal hind leg. The ankle and foot bones are fused and elongated, so that the avian leg consists of three long segments—the femur, the tibiotarsus plus the reduced fibula, and the tarsometatarsus (the fused ankle and foot bones, which is equivalent to the human foot)—and finally the toes. Thus, birds walk on their toes, which are four in most birds but have been reduced to three, with the loss (usually) of the posterior hallux, or two (hallux and fourth) in the ostrich. AccessScience from McGraw-Hill Education Page 5 of 11 www.accessscience.com

In many respects, birds have more advanced sensory and physiological systems than mammals. They have a keen sense of vision (color vision based on four different cone cells, allowing color vision into the ultraviolet and

fluorescent colors; the plumage of many birds looks very different to a bird than to a human) and hearing. The sense of smell (olfaction) is not particularly well developed, although some birds possess a good olfactory sense

(for example, the vulture, Cathartes aura ). Birds have developed a small, rigid, flow-through lung using tubular air passages for a greater oxygen exchange surface than in mammals, and an extensive air-sac system serving as the pumping mechanism and as an internal system to lose excess heat. Nitrogenous wastes are lost in the form of uric acid, which requires much less water and a simpler kidney than in mammals.

Migration

Birds are found over the entire Earth. One of the most intriguing aspects of avian biology is the ability to migrate exceptional distances. Birds possess highly specialized directional senses for orientation, navigation, homing, and migration, including the ability to detect the Earth’s magnetic field. These uncanny abilities permit birds to occupy widely separated wintering and nesting grounds, thus expanding their usable habitats. Some migrations, such as that of the Arctic , involve a circum-Atlantic route from Alaska to the South Pole. See also: FLIGHT ;

MIGRATORY BEHAVIOR .

Taxonomy

There are about 10,000 species of birds living today, of which more than 5500 belong to the order Passeriformes.

Many avian species are particularly well known; however, the relationships of the higher categories of birds

(orders, etc.) are still debated. Recognition and arrangement of avian orders and of their contained families has traditionally been based on study of morphological, and to a much lesser extent behavioral, features. Over the last few decades, much new information has been added using molecular features and especially with comparison of nucleotide sequences of nuclear and mitochondrial DNA. The number of avian orders, and the contained families and their arrangement are still controversial; even today, the relationships of these orders are almost unknown. Many authors advocate different arrangements. Because the situation is in flux, a fairly conservative system is used below (fossil groups are designated by a dagger).

Class Aves

† Subclass Sauriurae,

† Infraclass Archaeornithes,

† Order Archaeopterygiformes (Late reptile-birds, Archaeopteryx ),

† Confuciusornithiformes (Lower , beaked reptile-birds, Confuciusornis ), AccessScience from McGraw-Hill Education Page 6 of 11 www.accessscience.com

† Infraclass (archaic Mesozoic land birds),

Subclass Ornithuriae

Infraclass Neornithes (or Carinata)

Superorder Incertae Sedis

† Order Hesperornithiformes (Cretaceous toothed divers, Hesperornis ),

† Ichthyornithiformes (gull-like, Mesozoic toothed birds, ),

† Incertae Sedis ( Gansus , Chaoyangia , etc., archaic modern-type birds),

Superorder (modern birds)

Order: Struthioniformes (58 species)

Galliformes (chickens and allies, 282)

Anseriformes (waterfowl, 161)

† Gastornithiformes (giant ground-birds, 2),

Sphenisciformes (, 17)

Procellariiformes (tube-nose , 114)

Pelecaniformes (pelicans and allies, 66)

Ciconiiformes ( and allies, 119)

Falconiformes (hawks, eagles, and vultures, 309)

Gruiformes (rails, cranes and allies, 214)

Podicipediformes (, 22)

Order: Charadriiformes (shorebirds, gulls and allies, 349)

Phoenicopteriformes (flamingos, 5)

Gaviiformes (, 5) AccessScience from McGraw-Hill Education Page 7 of 11 www.accessscience.com

Columbiformes (pigeons and doves, 332)

Psittaciformes (parrots, 360)

Coliiformes (, 6)

Cuculiformes (cuckoos, 165)

Opisthocomiformes (hoatzin, 1)

Strigiformes (, barn owls, 173)

Caprimulgiformes (nightjars and allies, 116)

Apodiformes (hummingbirds and swifts, 425)

Trogoniformes (trogons, quetzals, 39)

Coraciiformes (kingfishers, bee-eaters and allies, 219)

Piciformes (woodpeckers and allies, 407)

Passeriformes (perching birds, songbirds, 5739)

See also: NEOGNATHAE ; NEORNITHES ; VERTEBRATA .

Fossil recor d

The classification system presented above agrees with those presented in most major treatises on birds. The subclass Sauriurae contains the archaic birds of the Mesozoic Era, the Age of Reptiles, which includes the toothed fossil Archaeopteryx , or Urvogel, the oldest known bird (about 145 million years ago). Sauriurae also contains the subsequently discovered Confuciusornis , a beaked bird from Chinese deposits probably of Lower Cretaceous age, but possibly as recent as 120 million years ago. The first recognized specimen of Archaeopteryx was discovered in 1861 from the Solnhofen lithographic limestone of Late Jurassic age in Bavaria, and was named A. lithographica in reference to the rocks in which it was found. The crow-sized specimen from the bottom of a shallow salt-water lagoon was preserved with its wing and tail feathers in place, illustrating that it was indeed a bird and not a reptile. Another specimen was found in 1876 and became known as the Berlin specimen, as it is housed there. Often called an avian “Rosetta Stone,” the Berlin specimen is beautifully preserved, with outstretched wings, and is often cited as the best example of an perfectly intermediate between two classes of vertebrates, in this case reptiles and birds. It fulfilled Darwin’s expectation that such forms should exist, and this specimen has played a large role in the debate on evolution over the years. The jaws of Archaeopteryx possess rounded teeth with large root crowns, unlike the flattened, recurved, serrated teeth of theropod AccessScience from McGraw-Hill Education Page 8 of 11 www.accessscience.com

dinosaurs. The wing feathers are typical of modern birds, showing that feathers remained essentially unchanged structurally in the 150 million years since Archaeopteryx lived. As in modern birds, the wing feathers are divided into primary and secondary flight feathers, which exhibit asymmetric vanes, indicative of aerodynamic function, probably gliding because of the overall weakness of the pectoral girdle suggesting small flight muscles. The wings end in three sharply clawed fingers, the claws being virtually identical to the pedal claws of tree-trunk-climbing birds such as woodpeckers. Because the curvature of the foot claws of Archaeopteryx is very similar to that of perching birds, it has been assumed that Archaeopteryx was a tree-dweller that used its clawed hands and feet to ascend tree trunks; its flying ability was mainly gliding, with some very limited flapping flight. There are now seven skeletal specimens of Archaeopteryx . See also: ARCHAEOPTERYX ; REPTILIA .

A controversial fossil, Protoavis texensis , was described from the Late Jurassic of Texas from very fragmentary, crushed material. It does have clearly cervical vertebrae with the typical heterocelous (saddle-shaped) articulating surfaces that have been considered to be unique to birds. If this fossil is confirmed as an ancestral bird, it would push the known record of birds back nearly 100 million years and the to the earliest members of the archosaurian radiation. However, more and better material is needed before the relationships of this fossil can be determined with any assurance.

Confuciusornis sanctus , discovered in 1994 in China, is not as old as Archaeopteryx and could be as recent as

130–120 million years, but it shares many features with the Urvogel, including a wing with three sharply clawed

fingers. It is clearly a perching bird like Archaeopteryx but has better-developed flight architecture and exhibits a typical avian beak without teeth. Many hundreds of specimens of C. sanctus have been found, some with an elongated pair of narrow central tail feathers and presumed to be males. See also: CONFUCIUSORNITHIDAE .

During the past few decades, a large diversity of Cretaceous fossil birds have been discovered in China, including some controversial fossils from the same deposits that produced Confuciusornis ; these include two feathered but flightless creatures named Protarchaeopteryx and Caudipteryx . They are proclaimed to be feathered dinosaurs by Chinese and some western scientists, and said to prove the dinosaurian ancestry of birds; however, others believe that these are merely flightless Mesozoic birds and have little to do with avian origins. Despite their superficial resemblance to small dinosaurs, they show many of the features associated with flightlessness in modern birds. Other birds had become flightless early in avian history such as Hesperornis (Cretaceous) and

Diatryma ().

The Sauriurae contains a group known as the enantiornithines, or opposite birds. These archaic land birds of the

Mesozoic are now known throughout the world from the Cretaceous Period, and are called “opposite birds” because their foot bones fuse in the opposite direction to that of modern birds. They also have a different type of formation of the triosseal canal in the pectoral girdle which is produced by the bones associated with the flight apparatus and through which passes the tendon of the elevator muscle of the wing. These archaic birds are particularly well known from the Lower Cretaceous of Spain and China. All opposite birds became extinct at the end of the Cretaceous Period. AccessScience from McGraw-Hill Education Page 9 of 11 www.accessscience.com

Other Mesozoic birds included the ancient ornithurine birds more closely allied with the modern radiation of birds, among them such forms as the hesperornithiforms ( Hesperornis , Baptornis , etc.), the Cretaceous toothed divers, which superficially resembled loons. They became extinct at the end of the Cretaceous along with their gull-like contemporaries, Ichthyornis and Apatornis . The Lower Cretaceous Ambiortus from Mongolia was a fully volant (capable of flight) ornithurine bird about the size of a pigeon, possessing a well-developed sternal keel and other features of the pectoral region typical of modern birds, which showed that true flying birds existed some 12 million years after the appearance of Archaeopteryx .

The Struthioniformes contain the living ratites—flightless birds such as the ostrich, rhea, emu, cassowary, and —as well as their South American chickenlike relatives, the , which are fully capable of flight.

Recently, a group of fossil birds, the Lithornithidae, has been described from the Paleocene and Eocene of North

America and Europe. These chickenlike forms were fully volant, and are closely related to the living tinamous and the ratites. They now are thought to be the ancestral stock that gave rise to the Struthioniformes. The more reasonable explanation for the disjunct distribution of living ratites is that their ancestors, the lithornithids, flew to remote parts of the world and gave rise to various lineages of flightless birds. The huge elephant birds lived on

Madagascar contemporaneously with the native peoples, and probably became extinct in historic times. The same was true of New Zealand, where 11 or 12 species of large moas lived. They too survived until the arrival of

Polynesians in the late thirteenth century and subsequently became extinct years ago. See also: RATITES ;

STRUTHIONIFORMES .

By the Eocene, approximately 50 million years ago, all the major orders of modern birds were present. By the

Oligocene, most of the families were present; and by the , some genera of modern birds were well established.

Economic significance

Birds have a huge economic importance in terms of domesticated species, such as chickens and turkeys, and hunting. Today, however, the economics of birds for entertainment, such as birdwatching, ecotourism, and simply backyard feeding and watching, is far more important than for hunting. The interest of humans in observing birds is perhaps the major driving force in conservation efforts for the past century.

Walter J. Bock

Bibliography

W. J. Bock, Aves, in S. P. Parker (ed.), Synopsis and Classification of Living Organisms , vol. 2, 1982

J. Cracraft et al., Phylogenetic relationships among modern birds (Neornithes), pp. 468–489, in J. Cracraft and M.

J. Donoghue (eds.), Assembling the Tree of Life , Oxford University Press, 2004 AccessScience from McGraw-Hill Education Page 10 of 11 www.accessscience.com

J. del Hoyo et al. (eds.), Handbook of Birds of the World , vols. 1–9, 1992–2004

E. C. Dickinson (ed.), The Howard and Moore Complete Checklist of the Birds of the World , 3d ed., 2003

A. Feduccia, The Origin and , 2d ed., 1999

F. B. Gill, , 2d ed., 1995

D. P. Mindell (ed.), Avian Molecular Evolution and Systematics , Academic Press, 1997

N. S. Proctor and P. J. Lynch, Manual of Ornithology: Avian Structure and Function , 1993

C. G. Sibley and J. E. Ahlquist, Phylogeny and Classification of Birds: A Study in Molecular Evolution , 1990

C. G. Sibley and B. L. Monroe, Distribution and Taxonomy of Birds of the World , 1990

Additional Readings

A. C. Carruthers et al., Mechanics and aerodynamics of perching manoeuvres in a large bird of prey, Aeronaut. J. ,

114(1161):673–680, 2010

L. Christidis and W. E. Boles, Systematics and Taxonomy of Australian Birds , CSIRO Publishing, Collingwood,

VIC, , 2008

S. M. Kisia, Vertebrates: Structures and Functions , CRC Press, Enfield, NH, 2010

D. T. Ksepka and J. A. Clarke, The basal (Aves: Sphenisciformes) Perudyptes devriesi and a phylogenetic evaluation of the penguin fossil record, B. Am. Mus. Nat. Hist. , 337(1):1–77, 2010

DOI: http://doi.org/10.1206/653.1

G. B. Olea and M. T. Sandoval, Embryonic development of Columba livia (Aves: Columbiformes) from an altricial-precocial perspective, Rev. Colomb. Cienc. Pec. , 25(1):3–13, 2012

P. Olsen and L. Joseph, Stray Feathers: Reflections on the Structure, Behaviour and Evolution of Birds , CSIRO

Publishing, Collingwood, VIC, Australia, 2011 AccessScience from McGraw-Hill Education Page 11 of 11 www.accessscience.com

R. O. Prum et al., Molecular diversity, metabolic transformation, and evolution of carotenoid feather pigments in cotingas (Aves: Cotingidae), J. Comp. Physiol. B , 182(8):1095–1116, 2012

DOI: http://doi.org/10.1007/s00360-012-0677-4

J. T. Springer and D. Holley, An Introduction to Zoology , Jones & Bartlett Publishers, Burlington, MA, 2013