Avian Anatomy

Avian Anatomy

 Domestic Birds

The indigenous domestic birds include the domestic fowl, duck, goose, turkey, pigeon and guinea fowl.


The domestic fowl (Gallus domesticus) will be used to describe the anatomy of domestic birds.

Skeletal System


In adult birds, the medullary cavity of long bones and the medullary spaces of the spongy bone are only partly filled by blood forming bone marrow.


Diverticula of the pulmonary air sac system enter medullary cavities through pneumatic foramina during early growth so that a large proportion of the birds’ skeletal system contains air-resulting in presence of pneumatic bones.


Pneumatic skull bones are ventilated through the mouth cavity.






Body trunk skeleton


Consists of the vertebral column, ribs and sternum. Functionally, the pelvis may be included since it shows bony fusion with many vertebrae.


Vertebral column


Cervical vertebrae are S-shaped. Has 14 vertebrae in the fowl and duck. The atlas articulates with the hemispherical occipital condyle and dens and cranial articular processes of the axis.


Vertebral column cont….


Thoracic vertebrae are 7 in number in the fowl and pigeon. Have a plate-like spine. The vertebral body has a prominent ventral crest in some cases.


In the fowl and pigeon, the second to fifth thoracic vertebrae are fused into a bony column known as notarium. The sixth thoracic vertebra is free while the seventh is fused to the synsacrum.


Vertebral column cont….


The caudal region of the vertebral column consists of 14-15 vertebrae, depending on species, that are fused into a bony rod known as synsacrum (L., with sacrum).


Synsacrum includes the bones of the sacrum, last thoracic, lumbar and first few caudal vertebrae.


There is a bony connection between the synsacrum and ilium that is made by spines and transverse processes cranially and transverse processes of the synsacrum caudally.


Vertebral column cont….


The synsacral transverse processes are fused into a continuous bony plate.


There are five coccygeal vertebrae in all domestic birds. The vertebrae are connected to each other by movable joints.


The last coccygeal vertebra is the pygostyle. The pygostyle is a ploughshare- or sickle-shaped bony plate that results from the fusion of several vertebrae.








The sternum (Gr., chest) is a large bone that is dorsally concave and ventrally convex. Forms a ventral cover to more than half of the body cavity.


The area for muscle attachment is further increased by the ventrally projecting sternal crest.


The sternum articulates with the coracoid and ribs (sternocostal bones). The sternum also has lateral cranial and lateral caudal processes, and the thoracic process.




Fowls have seven pairs of ribs. Ribs articulate with thoracic vertebrae.


All the ribs, except the first pair, articulate with the sternocostal bones (correspond to the costal cartilages of mammals).


Distal to the middle of a rib’s length is the flat uncinate process (L., hook) that points in a caudodorsal direction. The process lies against the lateral surface of the next rib and is lacking from the 1st rib and the last two ribs in the fowl.



Skeleton of the forelimb


Pectoral girdle-connects the trunk to forelimbs. Consists of scapula, coracoid and clavicle.


The scapula is a long sword-like bone with sharp borders. It runs close to and roughly parallel to the vertebral column.


The strongest bone of the shoulder girdle is the coracoid.


The two clavicles unite distally forming the wishbone or furcula (L., dimunitive of fork).



Forelimb skeleton cont….


The three bones of the pectoral girdle meet mecially and are joined to the shoulder joint, leaving an opening between them, the foramen triosseum or trioseal canal.


The tendons of the deep pectoral and supracoracoid muscles pass through the foramen triosseum to join the lateral tuberosity of the humerus.



Forelimb skeleton cont….


The skeleton of the wings of birds consists of the humerus, radius and ulna, carpus, metacarpus and digits.


The humerus is a strong and largely pneumatic bone. It has a well developed lateral tuberosity.


The ulna is much larger than the radius. The two bones are of about equal length and are separated by a wide interosseus space.


Forelimb skeleton cont….


The carpus of birds has two carpal bones: the ulnar and radial carpal bones.


Carpometacarpal bones has three bones that have fused with distal carpal bones.


There  are three digits. The second digit is the most developed.



Pelvic limb skeleton


The pelvic girdle is large. The pelvis is open ventrally.


The pelvis has two hip bones that each have the ilium, pubis and ischium.


Bones of the pelvic limb include the femur, tibia and fibula, tarsometatarsus and digits.



Pelvic limb skeleton cont….


The femur is a strong tubular bone. Has a large trochanter major. The distal extremity of the femur is very large. Is associated with a small patella.


Tibiotarsus is as a result of fusion of the tibia and the proximal row of tarsal bones.


The fibula has a prominent head. The bone runs on the lateral surface of the femur and narrows distally into a needle-like pointed bone.


Pelvic limb skeleton cont….


Tarsometatarsus is a single bone that results from the fusion of metatarsal bones II, III and IV together with the distal row of tarsal bones.


Metatarsal bone I is a rudimentary and independent bone that forms a ligamentous union with the tarsometatarsus medially.


In the cockerel, the tarsometatarsus has a medially directed, pointed, bony process that forms the core of the spur.



Pelvic limb skeleton cont….


The digits of domesticated birds have four toes.


The first toe points in a medial and planter direction while the other three point forwards.


Number of phalanges per digit increase from two in the 1st phalanx to three in the 2nd  phalanx, four in the 3rd phalanx and five in the 4th phalanx.







Head skeleton


The skull has the neurocranium and splanchnocranium. Many of the bones present in the skull of mammals are present in the avian skull.


The splanchnocranium forms a movable articulation with the neurocranium.


The bird can move its upper jaw vertically, thus producing a vertical movement of the upper beak (kinesis).


Head skeleton cont….


Cranial kinesis refers to significant movement of skull bones relative to each other in addition to movement at the joint between the upper and lower jaw.


Kinesis is initiated by appropriate muscles that move the quadrate. Movement of the quadrate is transmitted in a horizontal direction by the zygomatic, pterygoid and palatine bones to the upper beak.


The upper beak is movable at its base in the craniofacial hinge.







Head skeleton cont….


The quadrate bone (L., square) lies between the neurocranium and the maxillopalatine apparatus. The quadrate exerts considerable influence on the kinesis of the maxillopalatine apparatus.


The three processes of the quadrate bone articulate with squamous temporal bone and mandible while the third process points towards the orbit.




Muscular system


The muscles of the bird show various degrees of development depending on their function.


The pectoral muscles are the most developed followed by those of the pelvic girdle and proximal part of the hind limb.


Prominent pectoral muscles are:


  • Superficial pectoral muscle-arises from the ventral border of the lateral crest and the process of the sternum, clavicle and sternal ribs.


Muscular system cont….


The superficial pectoral muscle inserts on the crest of the lateral tuberosity of humerus.


The muscle peforms the downstroke of the wing.


  • Deep pectoral muscle-originates from the lateral surface of the sternum. Inserts on lateral tuberosity of the humerus and its crest. Its terminal tendon passes through the foramen triosseum.


The deep pectoral muscle lifts the wing.


Muscular system cont….


  • Supracoracoid muscle-originates from the cranial border and outer surface of the coracoid bone.


The tendon of insertion of the supracoracoid muscle passes through  the foramen triosseum before inserting on the lateral tuberosity of the humerus.


Contraction of the supracoracoid muscle lifts the wing.




Body cavities of birds


There is no partition between the thoracic and abdominal cavities similar to the diaphragm.


Birds have aponeurotic membranes, the horizontal and oblique septa, that divide the body cavity into several enclosed compartments that are named according to the they accommodate.


Horizontal septum is a strong tendinous sheet. It arises from the ventral crests of the thoracic vertebrae and is fused to the ventral surface of the lungs.


Body cavities cont….


The horizontal septum runs in a lateral direction towards ribs. It encloses lungs in the pulmonary cavity. There are several openings in the horizontal septum that allow communication with other organs.


The oblique septum is found at the caudal end of the lung and arises from the horizontal septum that attaches to the vertebral column and the abdominal wall, creating the subpulmonary cavity.


Body cavities cont….


The subpulmonary cavity surrounds thoracic air sacs and is bordered dorsally by the horizontal septum and laterally by the thoracic and abdominal walls.


The cavity is surrounded ventromedially by the oblique septum.


Cardioabdominal cavity is the largest body cavity and contains the heart, liver, proventriculus, spleen, urogenital organs, gizzard and intestines.


Digestive system


Is divided into the following parts:


  • Mouth cavity
  • Pharynx
  • Esophagus
  • Stomach
  • Intestines
  • Liver
  • Pancreas





Digestive system cont….


Since birds lack teeth, the function of the mouth parts has been reduced to prehension and incomplete breaking up of the food.


Food in the mouth is lubricated by oral glands.


The roof of the mouth has a palatine cleft which is narrow at its apical end. This slit represents a permanent communication between the oral and nasal cavities.


Digestive system cont….


Since birds lack a soft palate, the mouth and pharyngeal cavities are referred to as oropharyngeal cavity.


Esophagus-has a longer cervical part and shorter thoracic part. The cervical part is very distensible.


Crop-is an extremely distensible part of the esophagus and is capable of storing ingested food for a short time. Is located to the right in the fowl and in front of the furcula.


Digestive system cont….


Stomach-consists of the spindle-shaped glandular stomach (proventriculus) and muscular stomach (gizzard or ventriculus).


There is a distinct constriction between the proventriculus and ventriculus. The proventriculus has longitudinal folds.


The gizzard is shaped like a biconvex lens, is firm and red in colour. It is found to the left of the left lower part of the body cavity.




Digestive system cont….


The wall of the gizzard consists of a large mass of smooth muscle which is dark red or bluish-red in colour.


The muscles arise from tendinous aponeuroses that are located on medial and lateral walls of the gizzard.


The inner surface of the gizzard is lines by a keratinoid layer that forms a hard covering of the mucosa. This layer is produced by glands of the gizzard before solidification on the inner surface of the gizzard.


Digestive system cont….


The ketatinoid layer is smooth in the tendinous part of the gizzard whereas it forms ridges in other parts of this stomach.


The ridges, with the aid od the very powerful musculature of the gizzard are very effective grinding plates. Presence of small stones effectively assists in the grinding process.


Digestive system cont….




Divided into small and large intestines.


Small intestines start from the pylorus of the gizzard. Divided into the duodenum, jejunum and ileum.


The duodenum forms a U-shaped loop. The two limbs of the loop are connected by a strip of mesentery and contain the elongated pancreas.


Digestive system cont….


The bile and pancreatic ducts open the ascending limb of the duodenum.


The jejunum is the longest segment of the intestines. It occupies most of the right caudal quarter of the body cavity.


The ileum runs from the jejunum to the large intestine.


Digestive system cont….


Large intestines comprise of the paired caeca and the colon. The colon is short and terminates at the distinct cloacal sphincter that separates it from the ampulla-like dilated first part of the cloaca-known as the coprodeum.


The two caeca originate at the area of transition between the ileum and colon. The caeca are bound to the iliocaecal ligament.


Colon-a direct continuation of the ileum and is of similar diameter. Runs into the cloaca.



Digestive system cont….


Cloaca (L., cleanse)-is the terminal part of the intestinal tract. It has three compartments that are separated by contractile ring of folds. The compartments are:


  • Coprodeum-is the first ampulla-like compartment. Stores faeces from the colon temporarily.


  • Urodeum-small second compartment. The two ureters open into this compartment in both sexes. Other openings include the seminal duct in the male and the oviduct in the female.


Digestive system cont….


  • Proctodeum-is the third compartment of the cloaca. In the cockrel it has an elevation of the mucous membrane that serves as a copulatory organ.


The bursa of Fabricius opens on the dorsal wall of the proctodeum. It contains numerous lymphoid cells that are important in immunological mechanisms.


Respiratory system


Differs from that of mammals as far as demands of flight and voice production are concerned. Parts of the avian respiratory system include:


  • Nasal cavities
  • Cranial larynx
  • Trachea
  • Caudal larynx or syrinx
  • Lumgs
  • Air sacs




Nasal cavities


The floor of the nasal cavity is formed by the palatine processes of the palatine bone and the vomer.


Surrounding the nasal cavity are the nasal process of the incisive bone dorsally and nasal and lacrymal bones both laterally and dorsally.


The floor of the nasal cavity has a palatine cleft. There are two nasal turbinates: the larger ventral and the smaller dorsal conchae.


Respiratory system cont….


Cranial larynx-is situated on the floor of the pharyngeal cavity. It is supported by the cricoid and paired arytenoid cartilages.


The trachea leads from the larynx, ventrally along the neck and dorsal to the oesophagus. Is covered only by skin and has tracheal cartilages.


The trachea passes between the two rami of the clavicle to enter the thoracic cavity. Above the base of the heart, it divides into two main bronchi.


Respiratory system cont….


The syrinx or caudal larynx is the modified terminal part of the trachea and the first part of the main bronchi. It is enclosed by the clavicular air sac.


Between the trachea and the main bronchi is the tympanum. It consists of the last fused tracheal cartilages. This part leads into the elastic tympaniform membrane.


The tympaniform membrane is analogous to the vocal cords of mammals. The membrane is activated by musculature that is highly differentiated in singing birds.




Respiratory system cont….


The pessulus is a sagittally placed comb-like protruding cartilage that is found in the tympanum between the two openings that lead into the main bronchi.


Only two muscles are associated with the activation of the tympaniform membranes in domestic birds.



Respiratory system cont….


Lungs of birds are two and are relatively small in size. The lungs are bright red in colour. The lungs are not lobed and are similar in shape and size.


The dorsal border of the lungs is blunt and is closely associated with the vertebral column while the ventral border is sharp.


The lateral surface of the lungs is closely applied to ribs so that the sharp-edged parts of ribs form deep grooves in the lung, especially on the blunt border.


Respiratory system cont….


The lungs are fused by fibrous tissue to the adjacent lateral wall of the body cavity.


Lungs don’t change much in volume during breathing in birds.


The two main bronchi enter the lungs though the hilus together with blood vessels.


Respiratory system cont….


Within the lung, the main bronchus or mesobronchus runs to the caudal part of the lung into the abdominal air sac.


The main bronchus gives off the following branches:


  • Four ventrobronchii that include the following:


1st ventrobronchus-gives off branches to the cervical air sac and the cranioventral part of the lung.



Respiratory system cont….


2nd ventrobronchus supplies the cranioventral part of the lung.


3rd ventrobronchus supplies the caudal half of the ventral part of the lung and clavicular and cranial thoracic air sacs.


4th ventrobronchus is small and has no connection with the air sacs.



Respiratory system cont….


  • Dorsobronchi (are 7-10 in number) that ramify in a fan-like fashion on the dorsal and lateral surfaces of the lung.


  • Laterobronchi are variable in numbers and course the lungs ventrolaterally. The 1st lateral bronchus supplies the caudal thoracic air sac.


Respiratory system cont….


Parabronchi or tertiary bronchi arise close to one another from secondary bronchi and the main bronchus.


Parabronchi occupy more than half of the lung volume. They lead to the freely branching thin walled air capillaries that act as gas exchange sites.


Air capillaies are surrounded by a dense network of capillaries.


Respiratory system cont….


The air capillaries of birds are lined by a single layer of epithelial cells and are the gas exchange sites of birds.


The narrow intercommunicating air capillaries provide birds with an enormously large respiratory surface area despite the relatively small total volume of the lungs.



Air sacs


Air sacs are joined to the lungs and arise from them during development.


Air sacs have thin walls that are covered on the exterior by a serosa and in places they are fused to their surroundings by connective tissue.


Air sacs are passively distensible.


Air sacs cont….


The walls of air sacs consist of fibrous and elastic connective tissue together with muscle fibres that are variable in amount and distribution.


Internally, air sacs are lined by a simple squamous epithelium.


Air sacs have a poor blood supply and as a whole contribute to the reduction of the specific gravity.


Air sacs cont….


The primary function of air sacs is to compensate for the total capacity of the bird’s lung.


With the aid of respiratory muscles, air sacs act as bellows to ventilate the lung in such a manner that there is a continuous passage of air through the air capillaries both during inspiration and expiration.


The caudal thoracic and abdominal air sacs mainly carry out the role of ventilating lung.


Air sacs cont….


Other roles played by air sacs include:


  • Regulation of body temperature.
  • Even distribution of body weight during flight.
  • Voice production.


Air sacs cont….


Birds have four paired and one unpaired air sac located deep in the body cavity and between the internal organs.


The air sacs have variously shaped diverticula that are located between certain groups of muscles and project into the pneumatic bones of the skeleton of the trunk, the pectoral and pelvic girdles and the limbs.

Air sacs cont….


The air sacs include:


  • The unpaired clavicular air sac that is bilaterally symmetrical. Lies cranial to the heart and lung and is enclosed by the bony shoulder girdle and sternum.


  • Cervical air sacs-are bilaterally symmetrical and are ventilated by the 1st


Air sacs cont….


  • The cranial thoracic air sacs are connected to the 3rd


  • Caudal thoracic air sacs are connected to the laterobronchus of their own side.


  • Abdominal air sacs are connected to the mesobronchus. They are larger than the other air sacs. The right air sac is larger than the left.


Abdominal air sacs extend from the caudal end of lungs to the pelvic cavity and are in contact with the kidneys.


Urinary system


Consists of a pair of kidneys and ureters that empty directly into the urodeum of the cloaca.


Birds have attained a further reduction in body weight by the absence of a renal pelvis, urinary bladder and urethra.


Kidneys are long and consist of three successive lobes: cranial, middle and caudal lobes. The lobes are connected to each other by parenchymatous bridges.


Urinary system cont….


The ureter begins at the cranial lobe of the kidney and runs caudally along its vomentromedial border.


The ureter later runs along the dorsal wall of the abdomen before terminating in the middle compartment of the cloaca, the urodeum.


Reproductive system


The male bird has two testes that remain at their site of development in the body cavity, just below the vertebral column.


The testis is yellowish to greyish-white during the resting stage and almost pure white during the stage of activity.


The ductus deferens arises from the epididymis and runs together with the ureter on the same side. It terminates at a papilla in the urodeum.





Reproductive system cont….


The male copulatory organ or penis is rudimentary in the cockerel.


The everted 3rd segment of the cloaca, the proctodeum, accomplishes the transfer of semen into the cloaca of the female, which is also everted during mating.


The gander and drake have a copulatory organ. This organ arises at the transition between the urodeum and proctodeum on the ventral surface of the cloaca.


Reproductive system cont…..


The copulatory organ of the drake and gander consists of a short connective tissue body and a free tube, reaching to the tip of the organ.


The copulatory organ is covered by a mucous membrane and forms a seminal groove in spiral coils.


During copulation, the groove is closed, forming a tube that transfers semen into the cloaca or terminal part of the oviduct of the female.


Reproductive system cont….


The copulatory organ is protruded from and retracted into the male cloaca by muscular action.


Erection of the organ is achieved by the influx of lymph into the interior of the free tube.


Reproductive system cont….


In the female, during the developmental stages, there are two gonads. However, only the left ovary reaches full development.


The ovary has follicles in various stages of development. These stick out and make the ovary resemble a bunch of grapes.


Only the left oviduct is present while the right one regresses entirely.



Reproductive system cont….


A fully developed oviduct can be divided into the following sections:


  • Infundibulum-is the funnel of the oviduct.
  • Magnum-is the oviduct in a strict sense.
  • Isthmus-the narrow part of the oviduct.
  • Uterus or shell gland.
  • Vagina






Reproductive system cont….


The mucosa of the magnum has tall and coarse folds that are arranged in a spiral.


The uterus is wide and has circular ridges that are formed by musculature of the wall. Has numerous glands that produce a calcium containing secretion.


The mucosa of the vagina has circular ridges and lacks glands. It opens into the urodeum via a slit-like opening.






Avian egg


The avian egg consists of the following:


  • The fertilized or unfertilized egg cell or sphere of yolk. Has alternating layers of yellow and white yolk. The egg has a thin covering of yolk or vitelline membrane and is produced by the ovary.


  • Three layers of egg white or albumen that surround the sphere of yolk.


The layers of albumen are of varying viscosity; the outer and inner layers being less viscous than the middle layer.




Avian egg cont….


  • The chalazae are spirally twisted , cord-like structures that are suspended in the albumen and tightly attached to the yolk membrane.


  • Double-layered shell membrane that lies to the inner side of the calcium shell. At the blunt pole of the egg, between the two layers, is the air chamber or space.


  • Calcium shell.


Avian egg cont….


  • Cuticle-is found to the outside of the calcium shell. The cuticle can be compared to a covering of varnish.


The various segments of the oviduct contribute to the various components of the egg.




Egg formation


The follicle, together with its sphere of yolk and covering of yolk membrane, ruptures from the ovary at the time of ovulation and is guided by the muscular infundibulum into its lumen.


The yolk membrane is then surrounded by a concentrated mucin-containing secretion in the form of a delicate fibre network.


This results in the formation of a loose double-layered membrane around the yolk membrane.


Egg formation cont….


The double-layered membrane surrounding the yolk membrane is semi-permeable. With the existing osmotic gradient, water can enter the yolk from the albumen till the yolk attains its ultimate weight.


In the magnum, about half of the albumen weight is added. The remaining amount will be added by the isthmus and the uterus.


Shell membrane formation starts at the isthmus and ends at the magnum.




Egg formation cont….


The keratin-containing component of the secretions forms the shell membrane. The membrane has a double-layered fibrous structure.


The outer layered membrane will later combine with the calcium shell while the inner layer will be in close contact with the albumen.


The air chamber develops at the blunt end of the egg between the outer and inner layers of the shell membrane after the egg has been laid.


Egg formation cont….


Chalazae develop in the isthmus and uterus. Since the yolk membrane is loosely applied to the yolk, it projects into the albumen towards both the pointed and blunt poles.


The egg normally revolves as it moves through the oviduct and fibrous components of the albumen are added to the projections of the yolk membrane.


The revolving action of the egg results in spiral twisting of the yolk sac projections since they remain firmly attached to the yolk.


Egg formation cont….


Either ends of chalazae lie free in the albumen.


The yolk is suspended by the chelazae such that it can move within the egg. This ensures that the germinal disc that is located at the animal pole is always uppermost.


The heavier vegetative pole lies at the lower position.


Egg formation cont….


The calcium shell is laid down in the uterus. By this time the egg is surrounded by its shell membrane loosely.


The first process involves the passage of a watery protein-containing component of the uterine glands through the semipermeable shell gland into the albumen to complete its formation.


The uterine secretion also provides the calcium shell.


Egg formation cont….


The shell constituents are laid down on the shell membrane.


The shell contains minerals and also some amount of organic matrix composed of a protein-polysaccharide complex.


Coloured shells also contain protoporphyrin (ooporphyrins), a haemoglobin breakdown product.


Egg formation cont….


The egg takes about 24 hours in the oviduct before it is ready for laying.


The outer cuticle covering of the egg is sticky immediately after laying but it dries out rapidly.




Most of the skin of birds, apart from non-feathered areas,  is very thin and feathers protect it from mechanical injury.


The skin is easily movable on the well developed subcutis which is a necessary  prerequisite for the movement of feathers that are implanted on it.


The epidermis is generally thin and consists of only a few layers except that of the sheath of the beak, scales of feet, claws, balls of toes and spurs.


Integument cont….


Black or grey skin is caused by pigment forming melanophores that have migrated into the epidermis.


Yellow or red colouration, for example, of the feet or the beak is due to specific lipochromes which are carotenoids.


In the duck and goose as well as well nourished fowls and turkeys, the subcutis has a store of yellowish fatty tissue.


Integument cont….


The adipose cushions of the digital pads are always well developed and not dependent on the nutritional status of the bird.


The turkey and fowl have prominent brood patches that are prominent at the time of brooding. Brood patches appear as less densely feathered but highly vascularised areas of the skin on the ventral part of the breast.


Brooding patches are regions for transference of a bird’s body heat to the incubating eggs.


Integument cont….


Avian skin lacks sweat glands and has a sparse distribution of sebaceous glands. There are a few ceruminous glands in the skin of the outer ear.


The preen gland produces a sebaceous product comparable to that of mammals.


The preen gland consists of two bilaterally symmetrical halves and is located on the dorsal surface of the last sacral or 1st coccygeal vertebra, between the tail feathers.


Integument cont….


The preen gland can be as large as a bean in the fowl.


The two halves of the gland are surrounded by a connective tissue capsule.


The oily secretion of the preen gland passes into a single pointed papilla.


The bird uses its beak to spread the secretion from the preen gland over its plumage, especially the contour feathers. The secretion acts as a water repellent film.


Integument cont….


The skin of a bird’s foot is characteristic to a species.


On the tarsometatarsus of the fowl are two dorsal rows of larger and two planter rows of smaller scales.


The 2nd, 3rd and 4th toes are connected at their base by a small but strong web. The claws are strong and pointed and their growth rate is same to their regular rate of wear.


Integument cont….


The tarsometatarsus of cockerels and sometimes old hens forms a pointed bony peg that is directed caudomedially and forms the basis of the spur.


The spur is covered by a strong horny sheath which, in old birds, is pointed and curved upwards.


The periodic growth of the horny sheath results in ring formation. The number of rings may be used to estimate the age of the bird.


Integument cont….


The planter surfaces of the metatarsophalangeal and interphalangeal joints are supported by pads or balls that are well developed in the fowl.


The basis of the pad is a cushion of adipose tissue in the subcutis.



Integument cont….


Both sexes in the fowl have a comb. The comb differs in shape and size in various breeds.


The comb, paired wattles and ear lobes are double layers of skin that can distend. The subcutis  contains numerous blood vessels in these areas.


The turkey has a frontal process on its head. It lies on a cushion of adipose tissue in the subcutis and contains tracts of smooth muscle.




Integument cont….




Plays a role in flight and temperature regulation. Birds have various types of feathers that are distributed over most of the body in a particular manner to ensure that the roles of plumage are fulfilled.



Integument cont….


A feather (using the contour) has the following parts:


  • A shaft or quill-is divided into a calamus and a rachis. The calamus is hollow, cylindrical and transluscent. It fits into the feather follicle. The rachis bears the vane (web).


  • Inferior umbilicus-is a depression at the proximal end of the calamus. Accomodates the very vascular dermal papilla.




Integument cont….


The dermal papilla plays a role in the growth of the feather but when such growth is complete, the papilla gradually contracts.


The calamus is eventually left with group of keratinized epidermal cells containing air (pulp caps).


The long rachis has a slightly curved surface and ends distally in a point.



Integument cont….


  • Superior umbilicus-is a small depression that forms a transition between the calamus and vane. An aftershaft or hyporachis with an aftervane may arise from here-forming the afterfeather.


On both sides of the rachis are the rami or barbs that bear proximal and distal barbules at an acute angle. The barbules are arranged in one plane.


The distally directed barbules bear hooklets or hamuli (barbicels). The hooklets fix onto the proximal hookless barbules.




Integument cont….


There is a firm but flexible union between the proximal and distal barbules of the vane.


There are various types of feathers:


  • Down feathers
  • Semiplumes
  • Contour feathers
  • Bristles feathers
  • filoplumes





Integument cont….


Contour feathers-have the feather parts described (true feathers) and cover most of the body.


Contour feathers grow only in circumscribed skin areas known as tracts or pterylae. The neighbouring bald patches or apteria have down feathers.


Arrangement of contour feathers is known as pterylosis and it shows a certain species specificity.


Integument cont….


Down feathers-have a thin and short rachis. Their vanes are loose and soft since they lack hooklets. The feathers are overlaid by contour feathers.


Semiplumes are feathers that are intermediate in structure between true and down feathers.


The distal part of the vanes of semiplumes carry hooklets while the structure of the proximal part resembles down feathers.



Integument cont….


Filoplumes have a delicate and almost vaneless rachis.


At the end of its growth, a feather dies. A dead feather is prone to natural but irreparable wear. A dead feather will drop out of a follicle and will be replaced by formation of a new feather by the papilla.


A bird’s plumage can only be maintained by partial or complete replacement. This process is gradual over the body and is known as the moult.


Nervous system


The fundamental structure of the nervous system of birds corresponds to that of other vertebrates.




The most prominent segment of the brain is the cerebrum. The cerebrum is divided into two halves, the hemispheres, by the longitudinal cerebral fissure, a median longitudinal cleft.


Nervous system cont….


The surface of the cerebrum is smooth-lacks gyri and sulci.


The cerebellum is analagous to the middle part of the mammalian cerebellum.


The cerebellum is the central organ for locomotion and maintainance of balance and consequently has attained a considerable size and degree of differentiation in birds.


Nervous system cont….


The midbrain of birds is the integration site of optic acoustic, vestibular and protopathic stimuli.


Like the mammalian cerebrum, the avian cerebrum coordinates stimuli that originate from the environment.


Through reticular formation the cerebrum controls the whole of the body’s movements and the somatomotor responses that may be expressed in flight and defense reactions.


Nervous system cont….


In the avian midbrain, all structures that are concerned with vision and balance are well developed.


The rounded optic bulb, representing the tectum, sticks out between the cerebrum and cerebellum.


The optic lobes correspond to the rostral colliculi of the corpora quadrigemina of mammals.




Circulatory system


Birds have a body temperature of 40-42°C which gives them a high basal and energy metabolism. Birds have rapid heart rates (200-300 heart beats per minute in the fowl).


The hearts of birds are relatively large. In the fowl it accounts for 0.5-1.42 % of the body weight.



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