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Bird anatomy, or the physiology of ' bodies, shows many unique adaptations, mostly aiding bird 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 Digestion.
| + Sections of the vertebral column in anatomical bird diagrams !Color !Vertebral section | |
| Pink | Cervical vertebrae |
| Orange | Thoracic/dorsal vertebrae |
| Yellow | Synsacrum |
| Green | Caudal vertebrae |
| Blue | Pygostyle |
Broadly speaking, avian skulls consist of many small, non-overlapping bones. Heterochrony, 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 Theropoda ancestors. 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 bone bones. A reduction in the adductor chambers has also occurred. These are all conditions seen in the juvenile form of their ancestors. The bone has also Hypertrophy 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 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 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.Wing, Leonard W. (1956) Natural History of Birds. The Ronald Press Company.
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 Hip 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 Shoulder girdle.
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 (, , Coraciidae, 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 , (including flickers), (including Geococcyx), and some . 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 , 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).
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.
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 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.
Bird embryos begin development with smooth skin. On the feet, the stratum corneum, or outermost layer, of this skin may keratinize, thicken and form scales. These scales can be organized into;
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.
Collectively, the scaly covering present on the foot of the birds is called podotheca.
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.
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 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, 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 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 — the anterior air sacs (interclavicular, cervicals, and anterior thoracics), the , 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. possess seven air sacs, as the clavicular air sacs may interconnect or be fused with the anterior thoracic sacs.
During inhalation, environmental air initially enters the bird through the 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 Bronchus, going to the two lungs. The primary bronchi enter 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 the bird inhales, tracheal air flows through the intrapulmonary bronchi into the posterior air sacs, as well as into the dorsobronchi (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 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, 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 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 the trachea to the exterior. Oxygenated air therefore flows constantly (during the entire breathing cycle) in a single direction through the parabronchi.
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 O2 diffuses into the blood. The blood capillaries leaving the exchanger near the entrance of airflow take up more O2 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 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 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, 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 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 the parabronchi. The atria are the site of gas exchange by simple diffusion. The blood flow around the parabronchi (and their atria), forms a cross-current gas exchanger (see diagram on the left). 2. Left Atrium 3. Left Ventricle 4. Right Ventricle 5. Right Atrium.
In chickens and others birds, the superior cava is double.]]
All species of birds with the exception of the penguin, have a small region of their lungs devoted to "neopulmonic parabronchi". This unorganized network of microscopic tubes branches off from the posterior air sacs, and open haphazardly into both the dorso- and ventrobronchi, as well as directly into the intrapulmonary bronchi. Unlike the parabronchi, in which the air moves unidirectionally, the air flow in the neopulmonic parabronchi is bidirectional. The neopulmonic parabronchi never make up more than 25% of the total gas exchange surface of birds.
In order for birds to produce sound, they use an organ located above the lungs called the syrinx, which is composed of tracheal rings, syringeal muscles, Tympaniform membrane, and internal bony structures that contribute to the production of sound. Air then passes through this organ, resulting in the vocalization of birds. Sound can then be produced through the movement of the Tympaniform membrane. Pitch can also be changed by opening and closing of the Tympaniform membrane, allowing for higher and lower production of sound.
Most birds are unable to swallow by the "sucking" or "pumping" action of peristalsis in their esophagus (as humans do), and drink by repeatedly raising their heads after filling their mouths to allow the liquid to flow by gravity, a method usually described as "sipping" or "tipping up".
The notable exception is the family of pigeons and doves, the Columbidae; in fact, according to Konrad Lorenz in 1939:
In addition, specialized nectar feeders like sunbirds (Nectariniidae) and hummingbirds (Trochilidae) drink
by using protrusible grooved or trough-like tongues, and parrots (Psittacidae) lap up water.
Many seabirds have glands near the eyes that allow them to drink seawater. Excess salt is eliminated from the nostrils. Many desert birds get the water that they need entirely from their food. Uricotelic reduces the physiological demand for water, as uric acid is not very toxic and thus does not need to be diluted in as much water.
Male birds have two testicle which become hundreds of times larger during the breeding season to produce spermatozoon. A study of the seasonal changes in avian testes Alexander Watson, J. Physiol. 1919;53;86–91, 'greenfinch ( Carduelis chloris)', "In early summer (May and June) they are as big as a whole pea and in early winter (November) they are no bigger than a pin head" 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 Apterygiformes always retain both. Birds do not have male accessory glands.
Most male birds have no Bird penis. In the males of species without a phallus, sperm is stored in the semen within the 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 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 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 bird penis. 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 and 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.
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 Renal 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.
The urine collected by the kidney is emptied into the cloaca through the ureters and then to the colon by reverse peristalsis.
Circulatory system
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 1,200 times per minute (about 20 beats per second).
Digestive system
Crop
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.
Proventriculus
The avian stomach is composed of two organs, the proventriculus and the gizzard that work together during digestion. The proventriculus is a rod shaped tube, which is found between the esophagus and the gizzard, that secretes hydrochloric acid and pepsinogen into the digestive tract. The acid converts the inactive pepsinogen into the active protease, pepsin, which breaks down specific found in , to produce a set of , which are amino acid chains that are shorter than the original dietary protein. (1995). 9780716720096, W.H. Freeman. ISBN 9780716720096 The gastric acid (hydrochloric acid and pepsinogen) are mixed with the stomach contents through the muscular contractions of the gizzard.
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, contains small pieces of grit or stone called that are swallowed by the bird to aid in the grinding process, serving the function of teeth. The use of gizzard stones is a similarity found between birds and , which left gastroliths as .
Intestines
The partially digested and pulverized gizzard contents, now called a bolus, are passed into the intestine, where pancreatic and intestinal enzymes complete the digestion of the digestible food. The digestion products are then absorbed through the intestinal mucosa into the blood. The intestine ends via the large intestine in the vent or cloaca which serves as the common exit for renal and intestinal excrements as well as for the laying of eggs. 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. (1998). 9781868721047, Struik Publishers. ISBN 9781868721047 (1998). 9781859741009, New Holland Publishers (UK) Ltd.. ISBN 9781859741009
Drinking behaviour
There are three general ways in which birds drink: using gravity itself, sucking, and by using the tongue. Fluid is also obtained from food.
one recognizes the order by the single behavioral characteristic, namely that in drinking the water is pumped up by peristalsis of the esophagus which occurs without exception within the order. The only other group, however, which shows the same behavior, the Pteroclidae, is placed near the doves just by this doubtlessly very old characteristic.K. Lorenz, Verhandl. Deutsch. Zool. Ges., 41 Zool.: 69–102, 1939
Although this general rule still stands, since that time, observations have been made of a few exceptions in both directions.
Reproductive and urogenital systems
reproduction system.
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Kidney
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.
Nervous system
Brain
Birds have a large brain to body mass ratio. This is reflected in the advanced and complex bird intelligence.
Vision
Birds have acute eyesight—raptors (birds of prey) have vision eight times sharper than humans—thanks to higher densities of photoreceptors in the retina (up to 1,000,000 per square mm in , compared to 200,000 for humans), a high number of in the , a second set of eye muscles not found in other animals, and, in some cases, an indented Fovea centralis which magnifies the central part of the visual field. Many species, including and , have two foveas in each eye. Many birds can detect polarised light.
Hearing
The avian ear is adapted to pick up on slight and rapid changes of pitch found in Bird Song. General avian tympanic membrane form is ovular and slightly conical. Morphological differences in the middle ear are observed between species. Ossicles within Greenfinch, blackbirds, Song thrush, and House sparrow are proportionately shorter to those found in Pheasant, Mallard, and Seabird. In song birds, a syrinx allows the respective possessors to create intricate melodies and tones. The middle avian ear is made up of three semicircular canals, each ending in an Osseous ampulla and joining to connect with the macula sacculus and lagena, of which the cochlea, a straight short tube to the external ear, branches from.
Taste
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.
Immune system
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.
Bursa of Fabricius
Function
The bursa of Fabricius, also known as the cloacal bursa, is a lymphoid organ which aids in the production of during humoral immunity. The bursa of Fabricius is present during juvenile stages but curls up. For example, the bursa is not visible after sexual maturity in different species of sparrow. For comparison, B lymphocytes in mammals are developed in the bone marrow.
Anatomy
The bursa of Fabricius is a circular pouch connected to the superior dorsal side of the cloaca. The bursa is composed of many folds, known as plica, which are lined by more than 10,000 follicles encompassed by connective tissue and surrounded by mesenchyme. Each follicle consists of a cortex that surrounds a medulla. The cortex houses the highly compacted B lymphocytes, whereas the medulla houses loosely. 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.
See also
External links
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