The Domestic Fowl: Gallus domestica

Birds are vertebrates of the class, Aves. They are a unique group of vertebrates as their most distinguished characteristic, the presence of feathers on the body, is not present in any other group of vertebrates. There are two types of birds: Flightless Birds and Flying Birds.

Flightless birds are birds that cannot fly -e.g. emu, kiwi, penguin and ostrich; while flying birds, as the name implies, are those that can fly e.g. eagle, crow, parrot, pigeon, vulture, sparrow, hawk, etc.

Feathers are modified scales, which is evidence of the belief that birds evolutionally descended from reptiles. Birds also share the following features with reptiles: presence of a bony structured head, presence of scales (on legs only in birds) and laying of eggs with shells.

Birds are homoithermic or warm-blooded, as they are capable of keeping their body temperature constant, independent of the temperature of the surroundings or the external environment, unlike fishes, amphibians and reptiles, which are coldblooded or poikilothermic animals. Coldblooded animals are those whose body temperature depends on the temperature of the external environment.

The body 0f the domestic fowl, Gallus domesticus, consists of a head, compact trunk and tail. The body of the domestic fowl is streamline in shape which enables the bird to mOve through the air with reduced resistance during flight. The body is covered with feathers, which are made of the protein keratin.

Heads: The head is relatively small, compared to the rest of the body. It is round and tapers anteriorly to form a horny beak. The beak is thick and pointed and used for picking seeds, insects and worms and also used in punching fruits. It consists of upper and lower mandibles or jaws.

On the upper jaw or mandible is a pair of slitlike openings called nostrils, which are used for smelling and gaseous exchange. There is a pair of round eyes, one on each side of the head. The eyes are protected by upper and lower eyelids and nictitating membrane. The nictitating membrane is used for moistening and cleaning the eye. Behind each eye and Sllghtly 0n a lower level is an ear opening W protected by a patch of small feathers called ear coverts. The ear openings lead to the eardrums or tympana, used for hearing. Birds have an acute sense of hearing, which enables them to pick alarm signals to warn
them of the approach of an enemy.

On top of the 'head along the median line is a thick, fleshy, serrated crest of skin called comb. Below the head and part of the lower mandible of the beak is a pair of fleshy folds of skin called wattles. The comb and wattles are both red in colour and used for sexual recognition. They .are more conspicuous or large in the male fowl called the cockerel. The head is joined to the trunk by a long flexible neck. This enables the domestic fowl to twist or turn the head in all directions independent of the trunk. This gives the bird 3 wide field of Vision.. The head and neck are both covered with feathers but the beak, comb and wattles are not.

Trunk: The trunk is compact and oval in shape. The trunk is also covered with feathers, which are used for flight and insulation against heat losses. The cockerel also uses the feathers for sexual recognition and courtship display.

Attached to the trunk are two pairs of limbs: 21 pair of fore limbs modified into wings and a pair of hind limbs modified into walking or running legs. The fore limbs or wings are Z-shaped and consists of an upper arm, fore arm, wrist and hand, which bears three digits. The upper arm is attached to the trunk by a flap of skin known as patagium (plural: patagia). The prepatagium (plural: prepatagia) is another flap of skin that attaches the upper arm to the forearm. The patagium and prepatagium prevent the bones of the wings from being completely straightened. The bones are light, strong, porous and spongy or hollow and without a bone marrow. This reduces the weight of the bird for easy flight.

The wings are covered with small covert feathers and large contour feathers. Among the contour feathers are quill feathers attached to the wing called remiges (singular: remix). The remiges attached to the hand are called primaries and those attached to the fore arm are called secondaries. At the bases of the remiges on both sides of the wings are small feathers called wing coverts. The thumb is covered by a tuft of feathers called ala spuria, bastard wing or alula. The wings of the domestic fowl have a relatively small surface a ea or wmg span compared to the size of the body The domestic fowl is therefore unable to su tam flight over long distances due to its relatively small wing span.

The pair of hind limbs or legs supports the bird on the ground. They are not entirely straightened but are slightly bent. They are distinguished into a thigh, knee, shank or leg and four opposable digits or toes. Three of the toes point forward and one points backward. Each toe ends in a slightly curled, sharp claw, used for scratching the ground in search of food, such as grains, insects and worms. The toes are spread out to provide stability to the bird. The thighs are covered Wlth contour feathers but the legs and toes are covered with non-removable horny scales.

Tail: The tail of the domestic fowl is short and covered with quill feathers called rectrices (singular: rectrix). On the dorsal surface of the tail is a small oil gland called preen gland. The preen gland produces an oil which is used by the bird to preen or groom the feathers to keep the feathers glossy and water-proof, prevent the bill from becoming brittle and to make the feathers a neat and smooth insulating matrial.

The domestic fowl is omnivorous and feeds on seeds, grains, insects, worms, green leafy vegetables and fruits. The claws are used to scratch the ground in search of food, which is picked with the sharp pointed beak. The tongue may be used to manipulate the food, which is moistened with saliva. There are no teeth in the buccal cavity of the domestic fowl.

The food is swallowed and passes through a long oesophagus into the crop, where it is stored for sometime and softened.

The food enters the stomach from the crop. The stomach is distinguished into a glandular proventriculus and a mascular ventriculus or ‘ gizzard. The proventriculus secretes gastric juice containing peptic enzymes which convert proteins to peptones. The gizzard grinds the food into small pieces with the help of small stones contained in it.

The ground food passes into the duodenum where the digestive process is completed. Proteins or peptones are converted into amino acids by trypsin and erepsin. Maltase converts maltose into glucose while lipase converts fats and oils into fatty acids and glycerol.

The end products of food digestion are absorbed in the ileum while re-absorption water occurs in the caeca. The undigested food materials or faeces'are egested at the cloaca.

Gaseous Exchange:
The organs for gaseous exchange are the lungs, which are connected to large membranous air sacs, which are filled with fresh air during inspiration and sweeps out expired air during expiration. Birds have therefore the most efficient respiratory system.

During inspiration, the intercostals muscles contract to increase the volume of the thoracic cavity to draw air into the lungs and air sacs. Oxygen diffuses into the blood and carbon (IV) oxide evolved during respiration, diffuses into the lungs and air sacs. Oxygen is used to oxidize food to release energy. '

During expiration, the intercostals muscles. relax and abdominal muscles also contract to decrease the volume of the thorac1c cavity. The increased pressure in the thoracic caVIty forces expired air containing carbon (IV) ox1de and water vapour out of the lungs and membranous air sacs into the atmosphere through the pair of nostrils. ln flight, ventilation of the lungs is assisted by pectoral muscles. The membranous air sacs also help to regulate the body temperature of the bird, Which is about 43°C.

The main organs of excretion in the domestic fowl are two elongated tabular kidneys. The nephrons in the kidneys have smaller. and fewer glomeruli and long loops of Henle. The nitrogenous waste substance, like in reptiles, is uric acid. The kidney tubules or nephrons concentrate the urine, which is emptied into the cloaca through the ureters. The cloaca

further concentrates the urine' into white granules or white semi-solid guano. The excretion of uric acid in this form is an adaptation for water conservation.

Carbon (IV) oxide is removed from the body by diffusion through the lungs.

The domestic fowl moves by walking, running or flying over very short distances.

The fowl can walk slowly or run very fast on the ground. In walking, the legs are lifted, the knee points backwards and the ankle points forwards. The lifted leg is then thrust forward and placed on the ground. The toes are spread on the ground to provide stability while walking or running.

The domestic fowl cannot fly over long distances due to its short wing span and heavy weight. They also cannot fly over high altitudes. Other birds however, can fly over high altitudes and long distances due to their relatively lighter weights and wider wing spans or wing spreads. Flying in birds can be distinguished into flapping, gliding and soaring. ‘

In flapping, the inner half of the wings or upper arms move slightly but the outer half or lower arms move repeated upwards and downwards to enable the wings perform upstroke and down stroke movements. During upstroke, the pectoralis major muscles contract and pectoralis minor muscles relax which raises both wings above the back of the trunk or both wings are held stretched over the back. To do this the wings move upwards and backwards.

During downstroke, pectoralis major muscles relax and pectoralis minor muscles contract, which moves the wings downwards and forwards. Hence during one upstroke and downstroke, the tip of each wing moves in a semi-circle to trace out a figure of eight.

The vanes of the feathers tilt at an angle to allow air to pass easily through the wings during upstroke. The air resistance closes the vanes of the feathers to give maximum lift and move the bird upwards and forwards during down stroke.

The tail and bastard wing or alula are both used for steering and prevent stalling during flight. The tail feathers are to stabilize the bird during flight and used as brake when landing. The tail feathers spread. out upwards and fanned to act as a brake.

The breastbone has a flattened ventral extension Called the keel, for attachment of flight muscles pectoralis major and pectpralis minor muscles.

In soaring, the Wings of the bird spread out during both upStrokes and down strokes in order to gain momentum. Once momentum is gained, the wings remain outstretched and do not flap, The body of the bird soars higher and higher, carried by air currents which are Caused by temperature changes in the atmosphere. Sea birds, e.g. sea gull and albatross however, soar by making use of variations in wind velocity at different levels above the sea surface. Soaring therefore enables birds to fly to great heights WIthout flapping their wings. This allows birds to minimize their energy requirement and thus conserve a lot of energy when soaring to great heights.

In gliding, the wings and tail of the bird are spread out; the body slowly moves forwards and downwards and finally swoops down to land on the ground. Birds usually glide just before they land on a branch, tree or on the ground. They may also glide to rest their wings when on a long flight.

The hind limbs are folded, for instance in doves, swallows and pigeons; or stretched out to lie parallel to the tail, example in ducks and herons, when in flight to offer the least resistance possible to air.

The domestic fowl is polygamous as it has several mating partners in the course Of adult reproductive life. Other birds, such as pigeons, are however, monogamous as they find one mate at a time and usually keep that mate for life. Mating is preceded by courtship display by the cockerel, which attracts the matured female or hen and also serves as a threat to ward away other cockerels to ensure successful mating or copulation.

The courtship diSplay involves the cockerel dancing and displaying its beautifully coloured plumage. It then mounts the hen and presses its cloaca tightly against that of the hen.

Sperms are introduced into the female's cloaca, from where it passes into the oviduct Fertilization is internal and occurs in the oviduct. As the fertilized egg passes down the oviduct, it is coated with a proteinaceous substance known as albumen, two fibrous shell membranes and an outer hard, brittle calcareous shell. The shell membranes and calcareous shell are permeable to gases and allow gaseous exchange between the developing embryo and the outside. The eggs are laid in a group called clutch, of about six or more and usually one a day.

The embryo develops from the germinal disc, which is a small round patch on the surface of the yolk. The yolk is suspended in the middle of the albumen by two tough twisted strands of albumen called chalazae (singular: chalaza). The two shell membranes are separated at the blunt end of the egg to form an air space. The albumen and yolk contain sufficient nutrients for food needed by the developing embryo. with the former providing mainly water. The
hen sits on theclutch of eggs to help the . embryos to develop into chicks. This'is known ' as Brooding or Incubation. The incubation period is about twenty one (21) days in the domestic fowls and about seventeen (17) days in the pigeon.

Incubation brings the eggs to almost the same temperature as that of the hen’s body, prevents temperature fluctuation and reduces evaporation.
The feathers on the ventral surface of the trunk of the hen moult or are shed. This ensures that the eggs are brought into direct contact with the skin of the hen for efficient heat transfer.

About a day after incubation, the embryo can be seen to be surrounded with a highly vascularized yolk sac, which carries nutrients from the yolk to the developing embryo. The embryo is also bounded by the chorion and amnion, which encloses the amniotic fluid. The chorion and amnion protect the developing embryo from desiccation or water loss and prevent adhesion of the embryo to the egg shell. A third embryonic membrane, the allantois, connects the embryo to the air space. The allantois is used for gaseous exchange and storage of uric acid by the embryo. It is therefore respiratory and excretory in function.

The fully developed embryo or chick uses its egg-tooth on the upper mandible of the beak or bill to break the shell from the inside. The chick then emerges from the ruptured shell covered with down feathers.

After hatching, the hen continues to provide parental care to the chicks. She searches for food for the chicks, protects them from enemies or attackers and keeps them warm by brooding over them. The latter is achieved by the hen covering her chicks with her body and wings. The chicks eat, grow and learn from the hen. They are able to live by themselves after about four months. Parental care is therefore exclusively provided by the hen. In the pigeon however, both parents are involved in the provision of parental care.

Adaptation to Movement:
The fore limbs are modified into wings for flight. They are supplied with large pectoralis muscles, a rigid skeleton for muscle attachment. The breastbone or sternum is also ventrally extended or bent into a deep keel, which provides a large surface area for attachment of the strong flight muscles. The bones are light, porous and spongy and lack bone marrow. This results in a reduction of the body weight for easy flight. The bird possesses lungs with large membranous air sacs which keep the bird air-borne or buoyant during flight. There is the presence of opposable digits which are used for gripping a branch or twig when perching.

The body is streamlined, which enables the bird to move through the air with reduced air resistance to facilitate flight. Flying is also aided by the overlapping and backward pointing feathers, which offer little resistance to movement in the air.

Adaptation for Nutrition: The domestic fowl uses its hard, sharp Claws to

scratch the ground in search of food. The hard, pointed beak is used to pick food, which includes grains, seeds, insects, worms and green leafy vegetables. The bird lacks teeth in the buccal cavity but possesses a thick muscular ventricums or gizzard which is used to grind its food.

Adaptation for Gaseous Enchange:
Possession 0f two spongy lungs enables the bird. to utilize atmospheric oxygen for r espiration to release energy. The alveoli of the lungs are thin-walled, highly vascularised and have large surface area for efficient absorption and transportation of respiratory gases. The large membranous air sacs ensure efficient ventilation of the lungs by filling them with fresh air during inspiration and sweeping expired air out of the lungs during eXpiration.

The membranous air sacs are also used to regulate the body temperature of the bird, which is about 43°C. Possession of the air sacs therefore, makes the respiratory system of the bird the most efficient.

Adaptation For Excretion:
Possession of fewer and smaller glomeruli reduces the rate of glomerular filtration, which helps the bird to conserve water. Possession of long loops of ilenle and ureter-s, which open into the cloaca, ensure efficient re-absorption of water. Excretion of uric acid as its nitrogenous waste product, in the form of white granules or semisolid guano, ensures the conservation of water.

Adaptation for Reproduction:
Fertilization is internal in the domestic fowl. it also lays large yolky eggs covered with hard calcareous shells. These characteristics enable reproduction and development of the embryo to occur on land. The developing embryo is surrounded with a highly vascularised yolk sac and albumen which nourish it. Possession of chorion and amnion prevent desiccation and adhesion of the embryo to the egg shell. The allantois is used for gaseous exchange and excretion of the nitrogenous waste substance, uric acid.

Feathers are epidermal structures. They are modified scales and cover the entire body of the bird except the legs. They form a light. flexible and firm body covering which insulates the body against excessive heat lost and also prevent desiccation. They trap pockets of air between them. The air is also used to insulate the body against heat lost. The feathers are also used for flight.

Structure of a feather:
Each flight or quill feather consists of a central stalk or rod, which tapers towards the tip with a solid upper portion called shaft or rachis and hollow lower portion called quill. The solid upper portion bears _a smooth, expanded, firm and flattened structure called vane or
vexillum. The vane consists of many narrow, closely spaced, parallel filaments called barbs. The barbs have two rows of interlocking processes called barbules. Barbules on the side of the barb closer to the tip of the feather bears short hooks’while those on the other side of the barb closer to the quill bear grooves or ridges. The books interlock or grasp the grooves of neighbouring barbules to form a stiff vane or vexillum. The barbs at the base of the vane, i.e. closer to the quill, have barbules which lack hooks and ridges or grooves. These barbules therefore do not interlock, resulting in the formation of a soft, fluffy and loose after shaft,at the base of the vane.

There is a hole just below the after shaft on the quill called superior umbilicus, which is used 'for gaseous exchange. Another hole at posterior end of the quill called inferior umbilicus contains a piece of tissue called dermal papilla. The dermal papilla is used for nourishing the feather.

The base of the quill is embedded in a pit in the skin called follicle. At'the base feathers have muscles which control their movements.

Types of Feathers;
There are four types of feathers in the domestic fowl. as well as many other birds. These are the quill feathers, covert feathers, down feathers and flloplumes. The quill and covert feathers establish the contour of the bird's body and are therefore also known as contour feathers or plumae. They are overlapping and backward pointing and thus contribute to the stream lining of the body for easy flight through the air.

The quill feathers are located on the tail and wings. They consist of a quill, shaft, after shaft and an elongated vane. They are large and stiff. They are primarily used for flight but are also used for insulation of the body against heat lost and desiccation.

Covert feathers cover the entire body of the bird except the legs. They consists of a quill, shaft, relatively smaller vane and larger after shaft. They make the body warm and light.

Down feathers are also called plumules. They are soft, fluffy and short and grow beneath the contour feathers. The barbules lack hooks and grooves and therefore do not interlock. They form most of the feathers on the body of chicks but are also present in adult birds. They insulate the body against heat lost and desiccation, but are not used for flight.

The filoplumes or pin feathers are small and hair-like feathers. They consist of short, slender shaft with few loose barbs at their tips.

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