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85-497: Eurostopodus Lyncornis The eared nightjars are a small group of nocturnal birds in the nightjar family, although the taxonomy is uncertain. There are seven species , mainly found in forest and scrub from China to Australia. Five species are placed in the genus Eurostopodus , the other two species in Lyncornis . They are long winged birds with plumage patterned with grey and brown to camouflage them when resting on

170-524: A beak , which is far more lightweight. The beaks of many baby birds have a projection called an egg tooth , which facilitates their exit from the amniotic egg . It falls off once the egg has been penetrated. The vertebral column is divided into five sections of vertebrae : The cervical vertebrae provide structural support to the neck and number between 8 and as many as 25 vertebrae in certain swan species ( Cygninae ) and other long-necked birds. All cervical vertebrae have transverse processes attached except

255-417: A sclerotic eye-ring , a ring of tiny bones. This characteristic is also seen in their reptile cousins. Broadly speaking, avian skulls consist of many small, non-overlapping bones. Pedomorphosis , maintenance of the ancestral state in adults, is thought to have facilitated the evolution of the avian skull. In essence, adult bird skulls will resemble the juvenile form of their theropod dinosaur ancestors. As

340-461: A yoke ) feet have two toes facing forward (digits two and three) and two back (digits one and four). This arrangement is most common in arboreal species, particularly those that climb tree trunks or clamber through foliage. Zygodactyly occurs in the parrots , woodpeckers (including flickers ), cuckoos (including roadrunners ), and some owls . Zygodactyl tracks have been found dating to 120–110 Ma (early Cretaceous ), 50 million years before

425-911: A better sense of smell. A third stage of bird evolution starting with Ornithothoraces (the "bird-chested" avialans) can be associated with the refining of aerodynamics and flight capabilities, and the loss or co-ossification of several skeletal features. Particularly significant are the development of an enlarged, keeled sternum and the alula , and the loss of grasping hands. † Anchiornis † Archaeopteryx † Xiaotingia † Rahonavis † Jeholornis † Jixiangornis † Balaur † Zhongjianornis † Sapeornis † Confuciusornithiformes † Protopteryx † Pengornis Ornithothoraces † Enantiornithes Bird skeleton Birds have many bones that are hollow ( pneumatized ) with criss-crossing struts or trusses for structural strength . The number of hollow bones varies among species, though large gliding and soaring birds tend to have

510-411: A bird in its flight by adjusting the feathers, which are attached to the skin muscle and help the bird in its flight maneuvers as well as aiding in mating rituals. There are only a few muscles in the trunk and the tail, but they are very strong and are essential for the bird. These include the lateralis caudae and the levator caudae which control movement of the tail and the spreading of rectrices, giving

595-637: A comb-like pecten on its inner edge, which may be used for plumage care. The plumage is cryptically patterned with browns and greys, to make these ground-nesting birds difficult to see when resting during the day. Some species have white patches in the wings, and the two in the genus Lyncornis (Great and Malaysian) have "ear tufts" at the rear of the crown. The songs of these birds are three or more repeated notes, sometimes with whistles or bubbling sounds, and are typically given at dawn or dusk. The eared nightjars are found from China through Southeast Asia to Australia. Tropical populations are mostly sedentary, but

680-473: A definition similar to "all theropods closer to birds than to Deinonychus ", with Troodon being sometimes added as a second external specifier in case it is closer to birds than to Deinonychus . Avialae is also occasionally defined as an apomorphy-based clade (that is, one based on physical characteristics). Jacques Gauthier , who named Avialae in 1986, re-defined it in 2001 as all dinosaurs that possessed feathered wings used in flapping flight , and

765-446: A greatly elongate tetradiate pelvis , similar to some reptiles. The hind limb has an intra-tarsal joint found also in some reptiles. There is extensive fusion of the trunk vertebrae as well as fusion with the pectoral girdle . Birds' feet are classified as anisodactyl , zygodactyl , heterodactyl , syndactyl or pamprodactyl . Anisodactyl is the most common arrangement of digits in birds, with three toes forward and one back. This

850-442: A group called Paraves . Some basal members of Deinonychosauria, such as Microraptor , have features which may have enabled them to glide or fly. The most basal deinonychosaurs were very small. This evidence raises the possibility that the ancestor of all paravians may have been arboreal , have been able to glide, or both. Unlike Archaeopteryx and the non-avialan feathered dinosaurs, who primarily ate meat, studies suggest that

935-422: A role in head-bobbing which is present in at least 8 out of 27 orders of birds, including Columbiformes , Galliformes , and Gruiformes . Head-bobbing is an optokinetic response which stabilizes a bird's surroundings as it alternates between a thrust phase and a hold phase. Head-bobbing is synchronous with the feet as the head moves in accordance with the rest of the body. Data from various studies suggest that

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1020-556: A similar underlying structure. Two bony projections—the upper and lower mandibles—covered with a thin keratinized layer of epidermis known as the rhamphotheca . In most species, two holes known as nares lead to the respiratory system. Due to the high metabolic rate required for flight, birds have a high oxygen demand. Their highly effective respiratory system helps them meet that demand. Although birds have lungs, theirs are fairly rigid structures that do not expand and contract as they do in mammals, reptiles and many amphibians. Instead,

1105-406: A sister group, the order Crocodilia , contain the only living representatives of the reptile clade Archosauria . During the late 1990s, Aves was most commonly defined phylogenetically as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica . However, an earlier definition proposed by Jacques Gauthier gained wide currency in the 21st century, and

1190-717: A time, sometimes for years, and rarely for life. Other species have breeding systems that are polygynous (one male with many females) or, rarely, polyandrous (one female with many males). Birds produce offspring by laying eggs which are fertilised through sexual reproduction . They are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching. Many species of birds are economically important as food for human consumption and raw material in manufacturing, with domesticated and undomesticated birds being important sources of eggs, meat, and feathers. Songbirds , parrots, and other species are popular as pets. Guano (bird excrement)

1275-423: Is also known to play a large role in feeding behaviours in fish. The structure of the avian skull has important implications for their feeding behaviours. Birds show independent movement of the skull bones known as cranial kinesis . Cranial kinesis in birds occurs in several forms, but all of the different varieties are all made possible by the anatomy of the skull. Animals with large, overlapping bones (including

1360-540: Is called ornithology . Birds are feathered theropod dinosaurs and constitute the only known living dinosaurs . Likewise, birds are considered reptiles in the modern cladistic sense of the term, and their closest living relatives are the crocodilians . Birds are descendants of the primitive avialans (whose members include Archaeopteryx ) which first appeared during the Late Jurassic . According to recent estimates, modern birds ( Neornithes ) evolved in

1445-562: Is coiled back and forth within the body, drastically increasing the dead space ventilation. The purpose of this extraordinary feature is unknown. Air passes unidirectionally through the lungs during both exhalation and inspiration, causing, except for the oxygen-poor dead space air left in the trachea after exhalation and breathed in at the beginning of inhalation, little to no mixing of new oxygen-rich air with spent oxygen-poor air (as occurs in mammalian lungs ), changing only (from oxygen-rich to oxygen-poor) as it moves (unidirectionally) through

1530-466: Is common in songbirds and other perching birds , as well as hunting birds like eagles , hawks , and falcons . Syndactyly, as it occurs in birds, is like anisodactyly, except that the second and third toes (the inner and middle forward-pointing toes), or three toes, are fused together, as in the belted kingfisher Ceryle alcyon . This is characteristic of Coraciiformes ( kingfishers , bee-eaters , rollers , etc.). Zygodactyl (from Greek ζυγον ,

1615-429: Is harvested for use as a fertiliser. Birds figure throughout human culture. About 120 to 130 species have become extinct due to human activity since the 17th century, and hundreds more before then. Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them. Recreational birdwatching is an important part of the ecotourism industry. The first classification of birds

1700-399: Is highly adapted for flight . It is extremely lightweight but strong enough to withstand the stresses of taking off, flying, and landing. One key adaptation is the fusing of bones into single ossifications , such as the pygostyle . Because of this, birds usually have a smaller number of bones than other terrestrial vertebrates. Birds also lack teeth or even a true jaw and instead have

1785-445: Is in mammals of the same size. Birds with long necks will inevitably have long tracheae, and must therefore take deeper breaths than mammals do to make allowances for their greater dead space volumes. In some birds (e.g. the whooper swan , Cygnus cygnus , the white spoonbill , Platalea leucorodia , the whooping crane , Grus americana , and the helmeted curassow , Pauxi pauxi ) the trachea, which some cranes can be 1.5 m long,

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1870-415: Is nearly equal in width and height. The chest consists of the furcula (wishbone) and coracoid (collar bone) which, together with the scapula , form the pectoral girdle ; the side of the chest is formed by the ribs, which meet at the sternum (mid-line of the chest). Birds have uncinate processes on the ribs. These are hooked extensions of bone which help to strengthen the rib cage by overlapping with

1955-503: Is not considered a direct ancestor of birds, though it is possibly closely related to the true ancestor. Over 40% of key traits found in modern birds evolved during the 60 million year transition from the earliest bird-line archosaurs to the first maniraptoromorphs , i.e. the first dinosaurs closer to living birds than to Tyrannosaurus rex . The loss of osteoderms otherwise common in archosaurs and acquisition of primitive feathers might have occurred early during this phase. After

2040-516: Is synonymous to Avifilopluma. † Scansoriopterygidae † Eosinopteryx † Jinfengopteryx † Aurornis † Dromaeosauridae † Troodontidae Avialae Based on fossil and biological evidence, most scientists accept that birds are a specialised subgroup of theropod dinosaurs and, more specifically, members of Maniraptora , a group of theropods which includes dromaeosaurids and oviraptorosaurs , among others. As scientists have discovered more theropods closely related to birds,

2125-535: Is used by many scientists including adherents to the PhyloCode . Gauthier defined Aves to include only the crown group of the set of modern birds. This was done by excluding most groups known only from fossils , and assigning them, instead, to the broader group Avialae, on the principle that a clade based on extant species should be limited to those extant species and their closest extinct relatives. Gauthier and de Queiroz identified four different definitions for

2210-722: The Late Cretaceous and diversified dramatically around the time of the Cretaceous–Paleogene extinction event 66 million years ago, which killed off the pterosaurs and all non- ornithuran dinosaurs. Many social species preserve knowledge across generations ( culture ). Birds are social, communicating with visual signals, calls, and songs , and participating in such behaviour as cooperative breeding and hunting, flocking , and mobbing of predators. The vast majority of bird species are socially (but not necessarily sexually) monogamous , usually for one breeding season at

2295-669: The Tiaojishan Formation of China, which has been dated to the late Jurassic period ( Oxfordian stage), about 160 million years ago. The avialan species from this time period include Anchiornis huxleyi , Xiaotingia zhengi , and Aurornis xui . The well-known probable early avialan, Archaeopteryx , dates from slightly later Jurassic rocks (about 155 million years old) from Germany . Many of these early avialans shared unusual anatomical features that may be ancestral to modern birds but were later lost during bird evolution. These features include enlarged claws on

2380-509: The acetabulum (hip socket) and articulate with the femur, which is the first bone of the hind limb. The upper leg consists of the femur. At the knee joint, the femur connects to the tibiotarsus (shin) and fibula (side of lower leg). The tarsometatarsus forms the upper part of the foot, digits make up the toes. The leg bones of birds are the heaviest, contributing to a low center of gravity, which aids in flight. A bird's skeleton accounts for only about 5% of its total body weight. They have

2465-400: The anterior air sacs (interclavicular, cervicals, and anterior thoracics), the lungs , and the posterior air sacs (posterior thoracics and abdominals). Typically there are nine air sacs within the system; however, that number can range between seven and twelve, depending on the species of bird. Passerines possess seven air sacs, as the clavicular air sacs may interconnect or be fused with

2550-476: The laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart , and a strong yet lightweight skeleton . Birds live worldwide and range in size from the 5.5 cm (2.2 in) bee hummingbird to the 2.8 m (9 ft 2 in) common ostrich . There are over 11,000 living species and they are split into 44 orders . More than half are passerine or "perching" birds. Birds have wings whose development varies according to species;

2635-453: The notarium of the pectoral girdle. The synsacrum consists of one thoracic, six lumbar, two sacral, and five sacro-caudal vertebrae fused into one ossified structure that then fuse with the ilium. When not in flight, this structure provides the main support for the rest of the body. Similar to the sacrum of mammals, the synsacrum lacks the distinct disc shape of cervical and thoracic vertebrae. The free vertebrae immediately following

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2720-541: The radius and ulna (forearm) to form the elbow. The carpus and metacarpus form the "wrist" and "hand" of the bird, and the digits are fused together. The bones in the wing are extremely light so that the bird can fly more easily. The hips consist of the pelvis, which includes three major bones: the ilium (top of the hip), ischium (sides of hip), and pubis (front of the hip). These are fused into one (the innominate bone ). Innominate bones are evolutionary significant in that they allow birds to lay eggs. They meet at

2805-426: The air sacs account for 15% of the total body volume, whereas in mammals, the alveoli , which act as the bellows, constitute only 7% of the total body volume. Overall, avian lungs have a respiratory surface area that is approximately 15% greater, a pulmonary capillary blood volume that is 2.5-3 larger and a blood-gas barrier that is 56-67% thinner than those in the lungs of mammals of a similar body mass. The walls of

2890-564: The air sacs do not have a good blood supply and so do not play a direct role in gas exchange . Birds lack a diaphragm , and therefore use their intercostal and abdominal muscles to expand and contract their entire thoraco-abdominal cavities, thus rhythmically changing the volumes of all their air sacs in unison (illustration on the right). The active phase of respiration in birds is exhalation, requiring contraction of their muscles of respiration. Relaxation of these muscles causes inhalation. Three distinct sets of organs perform respiration —

2975-445: The ancestors of modern birds) have akinetic (non-kinetic) skulls. For this reason it has been argued that the pedomorphic bird beak can be seen as an evolutionary innovation. Birds have a diapsid skull, as in reptiles, with a pre-lachrymal fossa (present in some reptiles). The skull has a single occipital condyle . The shoulder consists of the scapula (shoulder blade), coracoid , and humerus (upper arm). The humerus joins

3060-424: The anterior thoracic sacs. During inhalation, environmental air initially enters the bird through the nostrils from where it is heated, humidified, and filtered in the nasal passages and upper parts of the trachea. From there, the air enters the lower trachea and continues to just beyond the syrinx , at which point the trachea branches into two primary bronchi , going to the two lungs. The primary bronchi enter

3145-428: The appearance of Maniraptoromorpha, the next 40 million years marked a continuous reduction of body size and the accumulation of neotenic (juvenile-like) characteristics. Hypercarnivory became increasingly less common while braincases enlarged and forelimbs became longer. The integument evolved into complex, pennaceous feathers . The oldest known paravian (and probably the earliest avialan) fossils come from

3230-412: The avian lineage has progressed and as pedomorphosis has occurred, they have lost the postorbital bone behind the eye, the ectopterygoid at the back of the palate, and teeth. The palate structures have also become greatly altered with changes, mostly reductions, seen in the ptyergoid, palatine, and jugal bones. A reduction in the adductor chambers has also occurred. These are all conditions seen in

3315-454: The backward deviation of the first toe of the hind limb; in dinosaurs with a long rigid tail, the development of the foot proceeded differently. This process, apparently, took place in parallel in birds and some other dinosaurs. In general, the anisodactyl foot, which also has a better grasping ability and allows confident movement both on the ground and along branches, is ancestral for birds. Against this background, pterosaurs stand out, which, in

3400-406: The bird inhales, tracheal air flows through the intrapulmonary bronchi into the posterior air sacs, as well as into the dorso bronchi (but not into the ventrobronchi whose openings into the intrapulmonary bronchi were previously believed to be tightly closed during inhalation. However, more recent studies have shown that the aerodynamics of the bronchial architecture directs the inhaled air away from

3485-509: The birds that descended from them. Despite being currently one of the most widely used, the crown-group definition of Aves has been criticised by some researchers. Lee and Spencer (1997) argued that, contrary to what Gauthier defended, this definition would not increase the stability of the clade and the exact content of Aves will always be uncertain because any defined clade (either crown or not) will have few synapomorphies distinguishing it from its closest relatives. Their alternative definition

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3570-414: The blood. The blood capillaries leaving the exchanger near the entrance of airflow take up more O 2 than do the capillaries leaving near the exit end of the parabronchi. When the contents of all capillaries mix, the final partial pressure of oxygen of the mixed pulmonary venous blood is higher than that of the exhaled air, but is nevertheless less than half that of the inhaled air, thus achieving roughly

3655-590: The brain case. However, this is more prominent in some birds and can be readily detected in parrots. The region between the eye and bill on the side of a bird's head is called the lore . This region is sometimes featherless, and the skin may be tinted, as in many species of the cormorant family. The beak, bill, or rostrum is an external anatomical structure of birds which is used for eating and for preening , manipulating objects, killing prey, fighting, probing for food, courtship and feeding young. Although beaks vary significantly in size, shape and color, they share

3740-665: The earliest members of Aves, is removed from this group, becoming a non-avian dinosaur instead. These proposals have been adopted by many researchers in the field of palaeontology and bird evolution , though the exact definitions applied have been inconsistent. Avialae, initially proposed to replace the traditional fossil content of Aves, is often used synonymously with the vernacular term "bird" by these researchers. † Coelurus † Ornitholestes † Ornithomimosauria † Alvarezsauridae † Oviraptorosauria   Paraves Most researchers define Avialae as branch-based clade, though definitions vary. Many authors have used

3825-422: The elbow, moving the wing as a whole or in extending or flexing particular digits. These muscles work to adjust the wings for flight and all other actions. Muscle composition does vary between species and even within families. Birds have unique necks which are elongated with complex musculature as it must allow for the head to perform functions other animals may utilize pectoral limbs for. The skin muscles help

3910-710: The evolution of feathers. Bird embryos begin development with smooth skin. On the feet, the corneum , or outermost layer, of this skin may keratinize, thicken and form scales. These scales can be organized into; The rows of scutes on the anterior of the metatarsus can be called an "acrometatarsium" or "acrotarsium". Reticula are located on the lateral and medial surfaces (sides) of the foot and were originally thought to be separate scales. However, histological and evolutionary developmental work in this area revealed that these structures lack beta-keratin (a hallmark of reptilian scales) and are entirely composed of alpha-keratin. This, along with their unique structure, has led to

3995-451: The first avialans were omnivores . The Late Jurassic Archaeopteryx is well known as one of the first transitional fossils to be found, and it provided support for the theory of evolution in the late 19th century. Archaeopteryx was the first fossil to display both clearly traditional reptilian characteristics—teeth, clawed fingers, and a long, lizard-like tail—as well as wings with flight feathers similar to those of modern birds. It

4080-401: The first identified zygodactyl fossils. Heterodactyly is like zygodactyly, except that digits three and four point forward and digits one and two point back. This is found only in trogons , while pamprodactyl is an arrangement in which all four toes may point forward, or birds may rotate the outer two toes backward. It is a characteristic of swifts ( Apodidae ). A significant similarity in

4165-612: The first one. This vertebra (C1) is called the atlas which articulates with the occipital condyles of the skull and lacks the foramen typical of most vertebrae. The neck of a bird is composed of many cervical vertebrae enabling birds to have increased flexibility. A flexible neck allows many birds with immobile eyes to move their head more productively and center their sight on objects that are close or far in distance. Most birds have about three times as many neck vertebrae as humans, which allows for increased stability during fast movements such as flying, landing, and taking-off. The neck plays

4250-540: The fused sacro-caudal vertebrae of the synsacrum are known as the caudal vertebrae. Birds have between 5 and 8 free caudal vertebrae. The caudal vertebrae provide structure to the tails of vertebrates and are homologous to the coccyx found in mammals lacking tails. In birds, the last 5 to 6 caudal vertebrae are fused to form the pygostyle . Some sources note that up to 10 caudal vertebrae may make up this fused structure. This structure provides an attachment point for tail feathers that aid in control of flight. Birds are

4335-490: The ground. They feed on insects caught in flight. A single white egg is laid directly on the ground and incubated by both adults. The chicks can walk soon after hatching. The order Caprimulgiformes contains several families of nocturnal insectivores, these are the frogmouths , the potoos , the oilbird and the nightjars . The latter family is normally split into two subfamilies, the American nighthawks , Chordelinae, and

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4420-430: The help of their forelimbs, and from there they planned, after which they proceeded to flight. Most birds have approximately 175 different muscles, mainly controlling the wings, skin, and legs. Overall, the muscle mass of birds is concentrated ventrally. The largest muscles in the bird are the pectorals, or the pectoralis major, which control the wings and make up about 15–25% of a flighted bird's body weight. They provide

4505-400: The juvenile form of their ancestors. The premaxillary bone has also hypertrophied to form the beak while the maxilla has become diminished, as suggested by both developmental and paleontological studies. This expansion into the beak has occurred in tandem with the loss of a functional hand and the developmental of a point at the front of the beak that resembles a "finger". The premaxilla

4590-440: The lungs to become the intrapulmonary bronchi, which give off a set of parallel branches called ventrobronchi and, a little further on, an equivalent set of dorsobronchi. The ends of the intrapulmonary bronchi discharge air into the posterior air sacs at the caudal end of the bird. Each pair of dorso-ventrobronchi is connected by a large number of parallel microscopic air capillaries (or parabronchi) where gas exchange occurs. As

4675-443: The main reason for head-bobbing in some birds is for the stabilization of their surroundings, although it is uncertain why some but not all bird orders show head-bob. The thoracic vertebrae number between 5 and 10, and the first thoracic vertebra is distinguishable due to the fusion of its attached rib to the sternum while the ribs of cervical vertebrae are free. Anterior thoracic vertebrae are fused in many birds and articulate with

4760-434: The most. Respiratory air sacs often form air pockets within the semi-hollow bones of the bird's skeleton. The bones of diving birds are often less hollow than those of non-diving species. Penguins , loons , and puffins are without pneumatized bones entirely. Flightless birds , such as ostriches and emus , have pneumatized femurs and, in the case of the emu, pneumatized cervical vertebrae . The bird skeleton

4845-429: The night, and eat their prey on the wing. The flight is buoyant and twisting, and may be interspersed with periods of resting on the ground, a road, or in a tree. These birds drink in flight, gliding low over the water and dipping the beak. Birds Birds are a group of warm-blooded vertebrates constituting the class Aves ( Latin: [ˈaveːs] ), characterised by feathers , toothless beaked jaws,

4930-483: The night, but roosts some distance away when the female is brooding. If necessary, the female will attempt to distract the intruder away from the eggs, or perform a defence display with spread wings, puffed throat and hissing sounds. The eggs hatch in three to four weeks, and the young can walk soon after hatching. The chicks are fed by both parents. All eared nightjars feed almost entirely on insects caught in flight, typically moths and beetles. They hunt at twilight and in

5015-523: The only known groups without wings are the extinct moa and elephant birds . Wings, which are modified forelimbs , gave birds the ability to fly, although further evolution has led to the loss of flight in some birds , including ratites , penguins , and diverse endemic island species. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly seabirds and some waterbirds , have further evolved for swimming. The study of birds

5100-410: The only living vertebrates to have fused collarbones and a keeled breastbone . The keeled sternum serves as an attachment site for the muscles used in flying or swimming. Flightless birds, such as ostriches , lack a keeled sternum and have denser and heavier bones compared to birds that fly. Swimming birds have a wide sternum, walking birds have a long sternum, and flying birds have a sternum that

5185-399: The openings of the ventrobronchi, into the continuation of the intrapulmonary bronchus towards the dorsobronchi and posterior air sacs ). From the dorsobronchi the air flows through the parabronchi (and therefore the gas exchanger) to the ventrobronchi from where the air can only escape into the expanding anterior air sacs. So, during inhalation, both the posterior and anterior air sacs expand,

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5270-402: The outermost half) can be seen in the evolution of maniraptoromorphs, and this process culminated in the appearance of the pygostyle , an ossification of fused tail vertebrae. In the late Cretaceous, about 100 million years ago, the ancestors of all modern birds evolved a more open pelvis, allowing them to lay larger eggs compared to body size. Around 95 million years ago, they evolved

5355-442: The parabronchi. Avian lungs do not have alveoli as mammalian lungs do. Instead they contain millions of narrow passages known as parabronchi, connecting the dorsobronchi to the ventrobronchi at either ends of the lungs. Air flows anteriorly (caudal to cranial) through the parallel parabronchi. These parabronchi have honeycombed walls. The cells of the honeycomb are dead-end air vesicles, called atria , which project radially from

5440-399: The pectorals together make up about 25–40% of the bird's full body weight. Caudal to the pectorals and supracoracoideus are the internal and external obliques which compress the abdomen. Additionally, there are other abdominal muscles present that expand and contract the chest, and hold the ribcage. The muscles of the wing, as seen in the labelled images, function mainly in extending or flexing

5525-419: The posterior air sacs filling with fresh inhaled air, while the anterior air sacs fill with "spent" (oxygen-poor) air that has just passed through the lungs. During exhalation the intrapulmonary bronchi were believed to be tightly constricted between the region where the ventrobronchi branch off and the region where the dorsobronchi branch off. But it is now believed that more intricate aerodynamic features have

5610-401: The powerful wing stroke essential for flight. The muscle deep to (underneath) the pectorals is the supracoracoideus, or the pectoralis minor. It raises the wing between wingbeats. Both muscle groups attach to the keel of the sternum. This is remarkable, because other vertebrates have the muscles to raise the upper limbs generally attached to areas on the back of the spine. The supracoracoideus and

5695-536: The previously clear distinction between non-birds and birds has become blurred. By the 2000s, discoveries in the Liaoning Province of northeast China, which demonstrated many small theropod feathered dinosaurs , contributed to this ambiguity. The consensus view in contemporary palaeontology is that the flying theropods, or avialans , are the closest relatives of the deinonychosaurs , which include dromaeosaurids and troodontids . Together, these form

5780-413: The process of unsuccessful evolutionary changes, could not fully move on two legs, but instead developed a physical means of flight that was fundamentally different from birds. Changes in the hindlimbs did not affect the location of the forelimbs, which in birds remained laterally spaced, and in non-avian dinosaurs they switched to a parasagittal orientation. At the same time, the forelimbs, freed from

5865-399: The rib behind them. This feature is also found in the tuatara ( Sphenodon ). The skull consists of five major bones: the frontal (top of head), parietal (back of head), premaxillary and nasal (top beak ), and the mandible (bottom beak). The skull of a normal bird usually weighs about 1% of the bird's total body weight. The eye occupies a considerable amount of the skull and is surrounded by

5950-452: The same biological name "Aves", which is a problem. The authors proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants, which corresponds to meaning number 4 below. They assigned other names to the other groups.   Lizards & snakes   Turtles   Crocodiles   Birds Under the fourth definition Archaeopteryx , traditionally considered one of

6035-449: The same effect. The contracting posterior air sacs can therefore only empty into the dorsobronchi. From there the fresh air from the posterior air sacs flows through the parabronchi (in the same direction as occurred during inhalation) into ventrobronchi. The air passages connecting the ventrobronchi and anterior air sacs to the intrapulmonary bronchi open up during exhalation, thus allowing oxygen-poor air from these two organs to escape via

6120-400: The same systemic arterial blood partial pressure of oxygen as mammals do with their bellows-type lungs . The trachea is an area of dead space : the oxygen-poor air it contains at the end of exhalation is the first air to re-enter the posterior air sacs and lungs. In comparison to the mammalian respiratory tract , the dead space volume in a bird is, on average, 4.5 times greater than it

6205-551: The second toe which may have been held clear of the ground in life, and long feathers or "hind wings" covering the hind limbs and feet, which may have been used in aerial maneuvering. Avialans diversified into a wide variety of forms during the Cretaceous period. Many groups retained primitive characteristics , such as clawed wings and teeth, though the latter were lost independently in a number of avialan groups, including modern birds (Aves). Increasingly stiff tails (especially

6290-409: The structure of the hind limbs of birds and other dinosaurs is associated with their ability to walk on two legs, or bipedalism . In the 20th century, the prevailing opinion was that the transition to bipedalism occurred due to the transformation of the forelimbs into wings. Modern scientists believe that, on the contrary, it was a necessary condition for the occurrence of flight. The transition to

6375-494: The structures that act as the bellows that ventilate the lungs are the air sacs , which are distributed throughout much of the birds' bodies. The air sacs move air unidirectionally through the parabronchi of the rigid lungs. The primary mechanism of unidirectional flows in bird lungs is flow irreversibility at high Reynolds number manifested in asymmetric junctions and their loop-forming connectivity. Although avian lungs are smaller than those of mammals of comparable size,

6460-404: The suggestion that these are actually feather buds that were arrested early in development. Collectively, the scaly covering present on the foot of the birds is called podotheca. The bills of many waders have Herbst corpuscles which help them find prey hidden under wet sand, by detecting minute pressure differences in the water. All extant birds can move the parts of the upper jaw relative to

6545-416: The support function, had ample opportunities for evolutionary changes. Proponents of the running hypothesis believe that flight was formed through fast running, bouncing, and then gliding. The forelimbs could be used for grasping after a jump or as "insect trapping nets", animals could wave them, helping themselves during the jump. According to the arboreal hypothesis, the ancestors of birds climbed trees with

6630-402: The tail a larger surface area which helps keep the bird in the air as well as aiding in turning. Muscle composition and adaptation differ by theories of muscle adaptation in whether evolution of flight came from flapping or gliding first. The scales of birds are composed of keratin, like beaks, claws, and spurs. They are found mainly on the toes and tarsi (lower leg of birds), usually up to

6715-454: The tibio-tarsal joint, but may be found further up the legs in some birds. In many of the eagles and owls the legs are feathered down to (but not including) their toes. Most bird scales do not overlap significantly, except in the cases of kingfishers and woodpeckers . The scales and scutes of birds were originally thought to be homologous to those of reptiles; however, more recent research suggests that scales in birds re-evolved after

6800-441: The trachea to the exterior. Oxygenated air therefore flows constantly (during the entire breathing cycle) in a single direction through the parabronchi. The blood flow through the bird lung is at right angles to the flow of air through the parabronchi, forming a cross-current flow exchange system (see illustration on the left). The partial pressure of oxygen in the parabronchi declines along their lengths as O 2 diffuses into

6885-409: The two Australian species (spotted and white-throated nightjars) are partial migrants . These are birds of open woodland or forest clearings and edges. No nest is built, the single white egg is laid directly on to the ground or leaf litter . The female incubates the egg during the day, relying mainly on the excellent camouflage of the plumage to avoid predators. The male takes over incubation during

6970-630: The typical nightjars Caprimulginae. The eared nightjars are sometimes considered a subfamily Eurostopodinae of the Caprimulgidae but some studies have them as a sister group, while others treat them as a clade within the caprimulgids; others consider that the genus Eurostopodus may not be monophyletic. The eared nightjars consist of seven extant species in two genera, Eurostopodus and Lyncornis : The eared nightjars are large compared to many nightjars, but otherwise are similar in structure. They are long-winged and long-tailed, and are light for

7055-650: The use of only the hind limbs for movement was accompanied by an increase in the rigidity of the lumbar and sacral regions. The pubic bones of birds and some other bipedal dinosaurs are turned backward. Scientists associate this with a shift in the center of gravity of the body backward. The reason for this shift is called the transition to bipedality or the development of powerful forelimbs, as in Archaeopteryx . The large and heavy tail of two-legged dinosaurs may have been an additional support. Partial tail reduction and subsequent formation of pygostyle occurred due to

7140-415: The wing area, making them powerful and agile in flight. An important difference from typical nightjars is the lack of bristles around the beak. They are nocturnal and have a reflective tapetum lucidum at the back of the eye. The beaks are small, but these birds have a very large gape for catching insects in flight. The feet and legs are small and weak, and the toes are partly webbed. The middle toe's claw has

7225-466: Was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae . Carl Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use. Birds are categorised as the biological class Aves in Linnaean taxonomy . Phylogenetic taxonomy places Aves in the clade Theropoda as an infraclass or a subclass, more recently a subclass. Aves and

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