Bird anatomy

Anatomy of birds

Bird anatomy, or the physiological structure of birds' bodies, shows many unique adaptations, mostly aiding flight. Birds have a light skeletal system and light but powerful musculature which, along with circulatory and respiratory systems capable of very high metabolic rates and oxygen supply, permit the bird to fly. The development of a beak has led to evolution of a specially adapted digestive system.

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 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 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 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 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 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 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.

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.

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.

The free vertebrae immediately following the fused sacro caudal vertebrae of the synsacrum are known as the caudal vertebrae. Birds have between 5 and 8 free caudal vertebrae.

In birds, the last 5 to 6 caudal vertebrae are fused to form the pygostyle. The keeled sternum serves as an attachment site for the muscles used in flying or swimming. Swimming birds have a wide sternum, walking birds have a long sternum, and flying birds have a sternum that 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). As 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. 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 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 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.

The shoulder consists of the scapula (shoulder blade), coracoid, and humerus (upper arm). The humerus joins 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 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 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 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, 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 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 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 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 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 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.

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 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 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 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. 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 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 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 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;
Cancella – minute scales which are really just a thickening and hardening of the skin, crisscrossed with shallow grooves. Scutella – scales that are not quite as large as scutes, such as those found on the caudal, or hind part, of the chicken metatarsus. Scutes – the largest scales, usually on the anterior surface of the metatarsus and dorsal surface of the toes. 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 the suggestion that these are actually feather buds that were arrested early in development. All extant birds can move the parts of the upper jaw relative to 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 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, 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 airsacs 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 bird lungs are smaller than those of mammals of comparable size, 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. The walls of 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. 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 the trachea to the exterior. Blood or air with a high oxygen content is shown in red; oxygen poor air or blood is shown in various shades of purple blue. ]]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 blood flow around the parabronchi (and their atria), forms a cross current gas exchanger (see diagram on the left).

Birds have a four chambered heart, in common with mammals, and some reptiles (mainly the crocodilia). This adaptation allows for an efficient nutrient and oxygen transport throughout the body, providing birds with energy to fly and maintain high levels of activity. A ruby throated hummingbird's heart beats up to 1200 times per minute (about 20 beats per second).

Many birds possess a muscular pouch along the esophagus called a crop. The crop functions to both soften food and regulate its flow through the system by storing it temporarily. The size and shape of the crop is quite variable among the birds. Members of the family Columbidae, such as pigeons, produce a nutritious crop milk which is fed to their young by regurgitation. The acid converts the inactive pepsinogen into the active proteolytic enzyme, pepsin, which breaks down specific peptide bonds found in proteins, to produce a set of peptides, which are amino acid chains that are shorter than the original dietary protein. The gastric juices (hydrochloric acid and pepsinogen) are mixed with the stomach contents through the muscular contractions of the gizzard.

The gizzard is composed of four muscular bands that rotate and crush food by shifting the food from one area to the next within the gizzard. The gizzard of some species of herbivorous birds, like turkey and quails, However, unlike mammals, many birds do not excrete the bulky portions (roughage) of their undigested food (e. g. feathers, fur, bone fragments, and seed husks) via the cloaca, but regurgitate them as food pellets.

Male birds have two testes which become hundreds of times larger during the breeding season to produce sperm. The testes in birds are generally asymmetric with most birds having a larger left testis. Female birds in most families have only one functional ovary (the left one), connected to an oviduct — although two ovaries are present in the embryonic stage of each female bird. Some species of birds have two functional ovaries, and the kiwis always retain both. Birds do not have male accessory glands. Most male birds have no phallus. In the males of species without a phallus, sperm is stored in the seminal glomera within the cloacal protuberance prior to copulation. During copulation, the female moves her tail to the side and the male either mounts the female from behind or in front (as in the stitchbird), or moves very close to her. The cloacae then touch, so that the sperm can enter the female's reproductive tract. This can happen very fast, sometimes in less than half a second. The sperm is stored in the female's sperm storage tubules for a period varying from a week to more than 100 days, depending on the species. Then, eggs will be fertilized individually as they leave the ovaries, before the shell is calcified in the oviduct. After the egg is laid by the female, the embryo continues to develop in the egg outside the female body. Many waterfowl and some other birds, such as the ostrich and turkey, possess a phallus. This appears to be the ancestral condition among birds; most birds have lost the phallus. The length is thought to be related to sperm competition in species that usually mate many times in a breeding season; sperm deposited closer to the ovaries is more likely to achieve fertilization. The longer and more complicated phalli tend to occur in waterfowl whose females have unusual anatomical features of the vagina (such as dead end sacs and clockwise coils). These vaginal structures may be used to prevent penetration by the male phallus (which coils counter clockwise). In these species, copulation is often violent and female co operation is not required; the female ability to prevent fertilization may allow the female to choose the father for her offspring. When not copulating, the phallus is hidden within the proctodeum compartment within the cloaca, just inside the vent. After the eggs hatch, parents provide varying degrees of care in terms of food and protection. Precocial birds can care for themselves independently within minutes of hatching; altricial hatchlings are helpless, blind, and naked, and require extended parental care. The chicks of many ground nesting birds such as partridges and waders are often able to run virtually immediately after hatching; such birds are referred to as nidifugous. The young of hole nesters, though, are often totally incapable of unassisted survival. The process whereby a chick acquires feathers until it can fly is called "fledging". Some birds, such as pigeons, geese, and red crowned cranes, remain with their mates for life and may produce offspring on a regular basis.

Avian kidneys function in almost the same way as the more extensively studied mammalian kidney, but with a few important adaptations; while much of the anatomy remains unchanged in design, some important modifications have occurred during their evolution. The three sectioned kidneys are placed on the bilateral side of the vertebral column, and there are connected to the lower gastrointestinal tract. Depending on the bird species, the cortex makes up around 71–80% of the kidney's mass, while the medulla is much smaller at about 5–15% of the mass. Blood vessels and other tubes make up the remaining mass. Unique to birds is the presence of two different types of nephrons (the functional unit of the kidney): both reptilian like nephrons located in the cortex; and mammalian like nephrons located in the medulla. Reptilian nephrons are more abundant but lack the distinctive loops of Henle seen in mammals. Because of the absence of the loop of Henle in birds, their ability to concentrate water does not depend heavily on it. Water reabsorption depends entirely on the coprodeum and the rectum.

Birds evolved from an ancestor that had lost the taste bud type called T1R2, which allows other animals, like alligators, to taste sweet. After many birds adapted to a diet with high sugar content, they modified their umami taste receptors (T1R1 T1R3) to also sense sweet tastes. TR2, used to detect bitter tastes, is reduced in birds.

The immune system of birds resembles that of other jawed vertebrates. Birds have both innate and adaptive immune systems. Birds are susceptible to tumours, immune deficiency and autoimmune diseases.

The bursa of fabricius, also known as the cloacal bursa, is a lymphoid organ which aids in the production of B lymphocytes during humoral immunity. The bursa of fabricius is present during juvenile stages but curls up, and in the sparrow is not visible after sexual maturity. The medulla is separated from the lumen by the epithelium and this aids in the transport of epithelial cells into the lumen of the bursa. There are 150,000 B lymphocytes located around each follicle.