These are hooked extensions of bone which help to strengthen the rib cage by overlapping with the rib behind them. At various times and under various conditions, ceca are the site for 1 fermentation and further digestion of food especially for the breakdown of cellulose and absorption of nutrients, 2 production of antibodies, and 3 the use and absorption of water and nitrogenous components Clench Because their beaks point downward when feeding, gravity must be overcome to get those droplets from the tip of the bird's long beak to its mouth. These scales can be organized into;. A tough nut to crack. Note how particle size of material in the gizzard ventriculus is smaller than in the proventriculus due to the grinding action of the muscular walls plus small pebbles gastroliths. These microflora aid in digestion.
The intrapulmonary bronchus is also known as the mesobronchus. B - Expiration Source: So, air always moves unidirectionally through the lungs and, as a result, is higher in oxygen content than, for example, air in the alveoli of humans and other mammals.
Variation in length of uncinate processes -- Birds with different forms of locomotion exhibit morphological differences in their rib cages: Uncinate processes are shorter in walking species, of intermediate length in typical birds, and relatively long in diving species scale bar, 5 cm. Muscles attached to uncinate processes appendicocostales muscles help rotate the ribs forwards, pushing the sternum down and inflating the air sacs during inspiration.
Another muscle external oblique attached to uncinate processes pulls the ribs backward, moving the sternum upward during expiration. The longer uncinate processes of diving birds are probably related to the greater length of the sternum and the lower angle of the ribs to the backbone and sternum.
The insertion of the appendicocostales muscles near the end of the uncinate processes may provide a mechanical advantage for moving the elongated ribs during breathing Tickle et al. This exchange is very efficient in birds for a number of reasons. First, the complex arrangement of blood and air capillaries in the avian lung creates a substantial surface area through which gases can diffuse. The surface area available for exchange SAE varies with bird size.
For example, the ASE is about 0. However, smaller birds have a greater SAE per unit mass than do larger birds. Among mammals, there is also a negative relationship between SAE and body size, with smaller mammals like shrews having a greater SAE per unit mass than larger mammals. A second reason why gas exchange in avian lungs is so efficient is that the blood-gas barrier through which gases diffuse is extremely thin. This is important because the amount of gas diffusing across this barrier is inversely proportional to its thickness.
Among terrestrial vertebrates, the blood-gas barrier is thinnest in birds. Natural selection has favored thinner blood-gas barriers in birds and mammals because endotherms use oxygen at higher rates than ectotherms like amphibians and reptiles. Among birds, the thickness of the blood-gas barrier varies, with smaller birds generally having thinner blood-gas barriers than larger birds. For example, the blood-gas barrier is 0. Comparison of the mean thickness of the blood-gas barrier of 34 species of birds, 37 species of mammals, 16 species of reptiles, and 10 species of amphibians revealed that birds had significantly thinner blood-gas barriers than the other taxa West Also contributing to the efficiency of gas exchange in avian lungs is a process called cross-current exchange.
Air passing through air capillaries and blood moving through blood capillaries generally travel at right angles to each other in what is called cross-current flow Figure below; Makanya and Djonov As a result, oxygen diffuses from the air capillaries into the blood at many points along the length of the parabronchi, resulting in a greater concentration of oxygen i. A Micrograph of lung tissue from a Brown Honeyeater Lichmera indistincta showing a parabronchi, b blood vessel, and c exchange tissue bar, micrometers.
B Electron micrograph from the lung of a Welcome Swallow Hirundo neoxena showing a blood-air barrier, b air capillary, c blood capillary, and d red blood cell in the blood capillary bar, 2 micrometers.
Vitali and Richardson A Medial view of the lung of a domestic chicken Gallus gallus domesticus. Scale bar, 1 cm. B An intraparabronchial artery i giving rise to blood capillaries c in the lung of an Emu Dromiceus novaehollandiae.
C Air capillaries closely associated with blood capillaries arrows in a chicken lung. D Blood capillaries c closely associated with air capillaries spaces in a chicken lung. An individual air capillary AC surrounded by a dense network of blood capillaries asterisk in a chicken lung. The blood capillaries drain into a larger vein V6 adjacent to an infundibulum IF. Note that the general direction of blood flow through the blood capillaries is perpendicular to the flow of air through the air capillaries, i.
Makanya and Djonov In birds, the thickness of the blood-gas barrier in the 7. Relationship between the harmonic mean thickness of the blood-gas barrier the thickness of the barrier that affects the diffusion of oxygen from air capillaries into blood capillaries against body mass in the lungs of bats, birds, and non-flying mammals.
Birds have particularly thinner barriers than bats and non-flying mammals Maina Light micrographs of a portion of the lung of a chicken A and rabbit B. Note the small diameter of the air capillaries in the chicken lung vs. A In the chicken lung, pulmonary capillaries are supported by 'struts' of epithelium arrows. B In the rabbit lung, pulmonary capillaries are suspended in the large spaces between alveoli Watson et al. Air flow large arrows and blood flow small arrows illustrating the cross-current gas-exchange mechanism operating in the avian lung between the blood capillaries and air capillaries.
Note the serial arrangement of blood capillaries running from the periphery to the lumen of the parabronchus and the air capillaries radially extending from the parabronchial lumen. The exchange of gases simple diffusion of O 2 and CO 2 occurs only between blood capillaries and air capillaries.
As air moves through a parabronchus and each successive air capillary, the partial pressure of oxygen P O2 declines as indicated by the decreased density of the stippling because oxygen is diffusing into the blood capillaries associated with each air capillary.
As a result of this diffusion, the partial pressure of oxygen in the blood leaving the lungs pulmonary vein is higher than that in blood entering the lungs pulmonary artery as indicated by the increased density of the stippling. Relative partial pressures of O 2 and CO 2 1 for air entering a parabronchus initial-parabronchial, P I and air leaving a parabronchus end-parabronchial, P E , and 2 for blood before entering blood capillaries in the lungs pulmonary artery, P A and for blood after leaving the blood capillaries in the lungs pulmonary vein, P V.
The partial pressure of oxygen P O2 of venous blood P V is derived from a mixture of all serial air capillary-blood capillary units. Because of this cross-current exchange the partial pressure of oxygen in avian pulmonary veins P V is greater than that of the air leaving the parabronchus P E ; air that will be exhaled.
In mammals, the partial pressure of oxygen in veins leaving the lungs cannot exceed that of exhaled air end-expiratory gas, or P E Figure adapted from Scheid and Piiper Importantly, the partial pressure of oxygen in blood leaving the avian lung is the result of 'mixing'; blood from a series of capillaries associated with successive air capillaries along the length of a parabronchus is mixed as the blood leaves the capillaries and enters small veins.
As a result, the direction of air flow through a parabronchus does not effect the efficiency of the cross-current exchange because gases are only exchanged between blood capillaries and air capillaries, not between the parabronchus and the blood.
So, in above diagram, reversing the direction of air flow would obviously mean that the air capillary on the far left would have the highest partial pressure of oxygen rather than the air capillary on the far right so the stippling pattern that indicates the amount of oxygen in each air capillary would be reversed.
However, because of the 'mixing' of blood just mentioned, this reversal would have little effect on the P V , the partial pressure of oxygen in blood leaving via pulmonary veins the P O2 would likely be a bit lower because some oxygen would have been lost the first time air passed through the neopulmonic parabronchi. This is important because most birds have neopulmonic parabronchi as well as paleopulmonic parabronchi and, although air flow through paleopulmonic parabronchi is unidirectional, air flow through neopulmonic parabronchi is bidirectional.
Diagram showing the flow of air from the parabronchial lumen PL into the air capillaries not shown and arterial blood from the periphery of the parabronchus into the area of gas exchange exchange tissue, ET.
The orientation between the flow of air along the parabronchus and that of blood into the exchange tissue ET from the periphery is perpendicular or cross-current dashed arrows. The exchange tissue is supplied with arterial blood by interparabronchial arteries IPA that give rise to arterioles stars that terminate in blood capillaries.
After passing through the capillaries, blood flows into the intraparabronchial venules asterisks that drain into interparabronchial veins IPV. These in turn empty into the pulmonary vein which returns the blood to the heart. Maina and Woodward Ventilation and respiratory rate are regulated to meet the demands imposed by changes in metabolic activity e. There is likely a central respiratory control center in the avian brain, but this has not been unequivocally demonstrated.
As in mammals, the central control area appears to be located in the pons and medulla oblongata with facilitation and inhibition coming from higher regions of the brain. It also appears that the chemical drive on respiratory frequency and inspiratory and expiratory duration depend on feedback from receptors in the lung as well as on extrapulmonary chemoreceptors, mechanoreceptors, and thermoreceptors Ludders Central chemoreceptors affect ventilation in response to changes in arterial P CO 2 and hydrogen ion concentration.
Peripheral extrapulmonary chemoreceptors, specifically the carotid bodies located in the carotid arteries , are influenced by P O 2 and increase their discharge rate as P O 2 decreases, thus increasing ventilation; they decrease their rate of discharge as P O 2 increases or P CO 2 decreases.
These responses are the same as those observed in mammals. Unlike mammals, birds have a unique group of peripheral receptors located in the lung called intrapulmonary chemoreceptors IPC that are acutely sensitive to carbon dioxide and insensitive to hypoxia.
The IPC affect rate and volume of breathing on a breath-to-breath basis by acting as the afferent limb of an inspiratory-inhibitory reflex that is sensitive to the timing, rate, and extent of CO 2 washout from the lung during inspiration Ludders Pulmonary surfactant in birds: From birds to humans: The avian respiratory system: Environ Health Perspectives Activity of three muscles associated with the uncinate processes of the giant Canada Goose Branta canadensis maximus. Journal of Experimental Biology Evolution of surface activity related functions of vertebrate pulmonary surfactant.
Clin Exp Pharmacol Physiol. Quantitative analysis of the respiratory system of the House Sparrow, Budgerigar, and Violet-eared Hummingbird. The lung air sac system of birds. Advances in Anatomy, Embryology, and Cell Biology A comparative perspective of the progressive integration of respiratory system, locomotor apparatus and ontogenetic development. Function of intracoelomic septa in lung ventilation of amniotes: Physiological and Biochemical Zoology Inhaled anesthesia for birds.
Recent advances in veterinary anesthesia and analgesia: The morphometry of the avian lung. Themes and principles in the design and construction of the gas exchangers. Structure, function and evolution of the gas exchangers: Journal of Anatomy Functional morphology of the avian respiratory system, the lung-air system: The lung of the Emu, Dromaius novaehollandiae: An allometric study of the pulmonary morphometric parameters in birds, with mammalian comparison.
The bioengineering dilemma in the structural and functional design of the blood-gas barrier. A qualitative and quantitative study of the lung of an Ostrich, Struthio camelus. Three-dimensional serial section computer reconstruction of the arrangement of the structural components of the parabronchus of the Ostrich, Struthio camelus lung. Parabronchial angioarchitecture in developing and adult chickens. Journal of Applied Physiology Anatomy of the lungs and air sacs.
Form and function in birds, vol. Composite cellular defence stratagem in the avian respiratory system: Basic avian pulmonary design and flow-through ventilation in non-avian theropod dinosaurs.
Singing with reduced air sac volume causes uniform decrease in airflow and sound amplitude in the Zebra Finch. Comparative physiology of lung complexity: News in Physiological Science The avian lung-associated immune system: Gas exchange and transport. Bird respiration, volume 1 T. Evidence for avian intrathoracic air sacs in a new predatory dinosaur from Argentina.
Functional significance of the uncinate processes in birds. Evaluation of pulmonary volumetric morphometry at the light and electron microscopy level in several species of passerine birds. Morphometry of the extremely thin pulmonary blood-gas barrier in the chicken lung.
American Journal of Physiology. Lung Cellular and Molecular Physiology Vertebral pneumaticity, air sacs, and the physiology of sauropod dinosaurs. The life of birds, fourth edition. Comparative physiology of the pulmonary blood-gas barrier: European Respiratory Journal Avian Respiratory Dynamics Animation. Zina Deretsky, National Science Foundation Bird-like respiratory systems in dinosaurs -- A recent analysis showing the presence of a very bird-like pulmonary, or lung, system in predatory dinosaurs provides more evidence of an evolutionary link between dinosaurs and birds.
First proposed in the late 19 th century, theories about the animals' relatedness enjoyed brief support but soon fell out of favor.
Evidence gathered over the past 30 years has breathed new life into the hypothesis. O'Connor and Claessens make clear the unique pulmonary system of birds, which has fixed lungs and air sacs that penetrate the skeleton, has an older history than previously realized. It also dispels the theory that predatory dinosaurs had lungs similar to living reptiles, like crocodiles. Use the toolbar to step through the five pages of the diagram. Depending on your browser - you may need to click the toolbar one time or two times to fully activate it.
Some pages have notes that contain anatomical terms that may not be familiar to you. Put your cursor over the labels button furthest right on the toolbar or click on it to see what they refer to. Role of uncinate processes and associated muscles in avian respiration -- Codd et al.
The external intercostal muscles demonstrated no respiratory activity, being active only during running, suggesting they play some role in trunk stabilization. The appendicocostalis and external oblique muscles are respiratory muscles, being active during inspiration and expiration, respectively.
The activity of the appendicocostalis muscle increased when sternal movements were restricted, suggesting that activity of these muscles may be particularly important during prolonged sitting such as during egg incubation. Dorsal view of the trachea circled and the lung of the Ostrich Struthio camelus. The lungs are deeply entrenched into the ribs on the dorsolateral aspects arrowhead.
Filled circle is on the right primary bronchus. Note that the right primary bronchus is relatively longer, rather horizontal and relatively narrower than the left primary bronchus. 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. 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. 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 third and fourth toes the outer 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. 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 — 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.
Most birds have approximately different muscles, mainly controlling the wings, skin, and legs. They provide the powerful wing stroke essential for flight. The muscle medial to underneath the pectorals is the supracoracoideus.
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 skin muscles help a bird in its flight by adjusting the feathers, which are attached to the skin muscle and help the bird in its flight maneuvers.
There are only a few muscles in the trunk and the tail, but they are very strong and are essential for the bird. The pygostyle controls all the movement in the tail and controls the feathers in the tail. This gives the tail a larger surface area which helps keep the bird in the air.
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. 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;. 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.
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. 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 grooming , 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, these are fairly rigid structures, which do not expand and contract as they do in mammals, reptiles and many amphibians.
The structures that act as the bellows which ventilate the lungs, are the air sacs distributed throughout much of the birds' bodies. The walls of these air sacs do not have a good blood supply and so do not play a direct role in gas exchange. They act like a set of bellows  which move air unidirectionally through the parabronchi of the rigid lungs. 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. Three distinct sets of organs perform respiration — 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 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. 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. Each pair of dorso-ventrobronchi is connected by a large number of parallel microscopic air capillaries or parabronchi where gas exchange occurs.
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.
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. 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 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 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: In comparison to the mammalian respiratory tract , the dead space volume in a bird is, on average, 4. In some birds e. 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.
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 syrinx is the sound-producing vocal organ of birds, located at the base of a bird's trachea. As with the mammalian larynx , sound is produced by the vibration of air flowing across the organ. The syrinx enables some species of birds to produce extremely complex vocalizations, even mimicking human speech.
In some songbirds, the syrinx can produce more than one sound at a time. 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 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. 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 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 gastroliths 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 dinosaurs , which left gastroliths as trace fossils. 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. There are three general ways in which birds drink: Fluid is also obtained from food. 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 only other group, however, which shows the same behavior, the Pteroclidae , is placed near the doves just by this doubtlessly very old characteristic. Although this general rule still stands, since that time, observations have been made of a few exceptions in both directions. 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. The elimination of nitrogenous wastes as uric acid 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 testes which become hundreds of times larger during the breeding season to produce sperm. Some species of birds have two functional ovaries, and the order Apterygiformes always retain both ovaries. 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 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 primitive condition among birds, most birds have lost the phallus. 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.
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.
A bird has paired kidneys which are connected to the lower gastrointestinal tract through the ureters. 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. The urine collected by the kidney is emptied into the cloaca through the ureters and then to the colon by reverse peristalsis.
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,, per square mm in Buteos , compared to , for humans , a high number of neurons in the optic nerves , a second set of eye muscles not found in other animals, and, in some cases, an indented fovea which magnifies the central part of the visual field.
Many species, including hummingbirds and albatrosses , have two foveas in each eye. Many birds can detect polarised light. The avian ear is adapted to pick up on slight and rapid changes of pitch found in bird song.