Dr. Craig Riddell
Department of Pathology
Western College of Veterinary Medicine
University of Saskatchewan
Saskatoon, Saskatchewan

 Musculoskeletal System

        Many peculiarities of the avian skeleton are related to flight. Others are related to egg production. As a two-legged animal some chicken bone defects may be similar to defects in man. Many avian bones are hollow and contain air sacs. Air sacs in bones are of practical significance in repairing fractures in small birds and in making a decision in meat inspection when a bird is affected with airsacculitis. Inside other bones marrow occurs and an accessory medullary bone is formed during egg production in the female. This medullary bone grows under the influence of estrogen as trabeculae of primitive bone almost filling the medullary cavity and in the pigeon undergoes breakdown as the egg shell is being formed. In the chicken no clear cut breakdown of medullary bone associated with egg shell production has been demonstrated but it is implied that some breakdown does occur. Hens when laying at a high rate utilize from their skeletons considerable calcium for egg shell formation.

        The spinal column of the chicken is fused in the thoracic to sacral areas except for T6. The pectoral girdle is well developed with a scapula, coracoid bone and clavicle (wishbone). The latter two bones are missing or rudimentary in many mammals. This pectoral girdle and the large keel bone are important in flight. The pelvic girdle has no pubis symphysis facilitating egg laying. The tarsal bones are fused with other bones giving the tibiotarsus and tarsometatarsus. Four digits are present and an accessory structure, the metatarsal spur develops in males.

        The young chick at hatching has a very immature skeleton and cones of cartilage persist in the end of the long bones. These cones are not broken down until 5-7 days of age when a growth plate can be recognized. The growth plate in the chicken is more irregular than in the mammal. Most epiphyses in the growing chicken are cartilaginous and secondary centres of ossification only form in the proximal and distal tibia and proximal metatarsus. In the distal tibia and proximal metatarsus the second centres of ossification probably represent fused tarsal bones.

        In the chicken the parathyroid glands are found close to the posterior poles of the thyroid. In addition the chicken has discrete small ultimobranchial glands posterior to the thyroids and parathyroids. These latter glands produce calcitonin. They have received considerable attention recently because of the importance of calcium metabolism in the chicken and also because the glands are discrete rather than incorporated in the thyroid as in the mammal.

        The chicken has dark and red muscles related to the different types of fibres. The chicken breast muscle has mainly white fibres which contain relatively little myoglobin while some leg muscles contain more red fibres with myoglobin. Certain birds such as the hummingbird have only red fibres and studies indicate that these fibres have high mitochondrial density and stored fat. It is hypothesized that they are designed for sustained flight in migration while white fibres are used for short rapid flight. There are intermediate fibres between red and white which are extremes of a continuous gradation. Many muscles contain a mixture of different types of fibres. Red fibres receive a more copious blood supply than white fibres and are capable of greater aerobic metabolism.

Integumentary System

        Skin pigment in birds depends on breed, feed and rate of production in laying birds. Accessory structures such as wattles, comb and snood are highly vascular and prone to injury and are often removed surgically at an early age.

        Feather follicles undergo cyclical periods of rest and growth. Following a rest period germinal cells at the base of the follicle start a new feather which pushes the old feather out. These germinal cells can be stimulated by plucking feathers. There is an ordered symmetrical sequence of shed of feathers. Most wild birds molt gradually and are never unable to fly while some others, particularly waterfowl, molt rapidly and are unable to fly for a period of time. Shedding and renewal of feathers in mature birds occurs normally once a year, but variation from twice a year to once every two years does occur in some types of birds. All feathers are not replaced at the same time and overlap between molts occurs in young birds.

        A young chick is covered with down feathers but a second generation of feathers starts to appear within a few days of hatching and cover the bird by 4 to 6 weeks of age. A third plumage starts to appear at this age and takes 2-3 months to replace the second plumage. A fourth plumage develops at sexual maturity. Laying hens normally molt when they cease egg production but modern strains of birds can molt without ceasing egg production. Modern husbandry, particularly lighting procedures, tends to prevent molting in the laying hen. Feather formation is retarded by high levels of estrogen. Molting in the laying hen generally follows a certain order, head, neck, body, wings and tail. Wing feathers are dropped in a definite order.

Digestive System

        In selecting its food, the chicken utilizes taste, vision and tactile sense. The sense of smell is considered to be poorly developed in the chicken. The sense of taste is different from that in man and is more acute when additives are in water rather than in feed. The chicken can discern shape, size and colour and can feel material with its tongue and mouth. Taste, colour, size, shape and consistency can, therefore, be important to feed consumption. Chickens do have food preferences which in part can be learned. This latter is important when feed is changed. Appetite is influenced by contractions of an empty crop, an empty rectum and cold and visual stimuli such as the sight of food. Factors influencing the amount of food consumed are position in the social order, social stimulation, availability of food and form of the food.

        Chickens have no teeth or soft palate and there is no mastication as occurs in mammals. The beak is used in grasping the food which is then propelled backward to the esophagus where it is carried by peristalsis, aided by gravity, down to the crop. The crop is a large dilatation in the esophagus at the foot of the neck which stores food allowing the chicken to eat its daily ration in a short period and digest it later. The crop is very distensible and chickens can use it to eat large amounts of food in a short period which has important implications in restricted feeding programs. As the crop can be readily seen and palpated in a live chicken, it is useful in providing information on the feeding habits of chickens. Little digestion occurs in the crop which allows small amounts of food to pass onto the proventriculus. Considerable microbial growth occurs in the crop and there is a possibility that this may be beneficial to the bird. The time food is in the crop depends on the amount, consistency, hardness and amount of water. A full crop of grain will take up to 24 hours to empty while a full crop of mash will take much less time.

        The proventriculus, a glandular organ, corresponds to the stomach of other animals as it produces a gastric juice containing acid and enzymes. It differs in that little mixing or holding of food occurs in it. From the proventriculus, the food moves to the gizzard, also called the ventriculus, the familiar muscular organ with a hard lining. In the gizzard the food is ground and mixed with the gastric juice. The muscular development of the gizzard is influenced primarily by diet and less by the presence of grit which helps the grinding process and increases the digestibility of certain feeds. Round grit which will not fragment is most desirable.

        In the small intestine most of the breakdown of food occurs and the nutrients are absorbed. In the ceca some previously indigestible plant fibre is broken down by microbial action but how important that is in providing absorbable nutrients in the chicken has not been established. Surgical removal of the ceca is not harmful to the chicken.

        The rectum and cloaca are used in disposing of the residue. Overall passage of food ranges from 2 to 24 hours. It is fastest in growing and laying chickens. The ureters empty into the cloaca and the white, almost solid urine of the chicken is voided at the same time as the intestinal contents giving the white cap to the chicken faeces. The chicken empties its ceca approximately every eight hours producing the brown frothy droppings which are distinct from the other faeces. White droppings in the chicken may imply that only urine is being voided.

        The avian liver changes in colour and consistency during the life of the bird. The liver of the newly hatched chick is very pale and contains a large amount of fat. It changes to a more normal brownish red colour at 5-7 days of age at which time the yolk sac has been completely reabsorbed. Fat again accumulates in the liver of the female chicken when it is starting egg production. This change is physiological and under the control of estrogen. With fat infiltration the liver of a laying hen tends to be larger, paler and more friable than that of a male bird of the same age. In some instances this fat infiltration may become pathological.

Circulatory Systems

        The chicken heart is four chambered and the circulation is the same as the mammalian heart. Between the right atrium and ventricle there is a muscular flap which acts as the right A-V valve. Plaques of cartilage are normally found around the base of the aorta. The heart rate is about 200-300+ beats per minute in the chicken. In some small birds such as the canary, it exceeds 1,000. The chicken has a renal portal systems in which venous blood from the legs flows to the kidneys and from the kidneys to the posterior vena cava. It is suggested that injection of antibiotics into leg muscles of chickens should be avoided as they could be rapidly excreted via the renal portal system.

        Blood volume of an adult chicken is about 6% of body weight and 4% can be obtained by bleeding out. Blood cells of a chicken are very different from a mammal and little is known of the function of these various cells. Figures tabulated below are very approximate and do not consider variations which occur with sex, age and breed.

Cell No. in Thousands
per cubic m.m.
Hematocrit Characteristics
Erythrocyte 3,000 30-40 Elongate & nucleated
Thrombocytes 30   Nucleated - no multinucleated megakaryocytes
Total Leukocytes 20-30    


Cell No. in Thousands
per cubic m.m.
% Count
Heterophils 3-4 10-25 Polymorphonuclear with rod shaped eosinophilic granules
Eosinophils 0.25-1 1-3 Polymorphonuclear with spherical eosinophilic granules
Basophils 0.5 2 Polymorphonuclear with Basophilic granules
Lymphocytes 10-24 65-80 Round nucleus variable agranular cytoplasm
Monocytes 1.5 5 Large cells, round indented nuclei and abundant cytoplasm

        As the erythrocytes and the thrombocytes are nucleated and the same size as small lymphocytes techniques used in preparing cells for counting in chicken hematology are different from those used in mammalian hematology. The heterophil may be the equivalent of the mammalian neutrophil but appears to lack proteolytic enzymes. This may explain why the chicken does not form liquid pus but rather a caseous exudate. In the acute inflammatory response in the chicken heterophils predominate and are present in large numbers along with mononuclear cells and basophils within 12 hours. This is followed by lymphoid hyperplasia and lymphocytes are dominant within 36 hours.

Blood chemistry differs in several ways from that of mammals. The main values are tabulated below:

milliequivalents per litre

Na 140
K 4
Cl 100


Ca Total 10-30 (high level in laying hens)
Ultrafiltrable 6
Phosphate 4-8 (variable in laying hens)
Mg 3


Plasma protein 4.00
Lipophosphoprotein complex
(20% protein, 80% liquid)

2.00 (only in laying hens)

Most of the unusual figures are related to egg production when estrogen results in the production of phosphoprotein which binds calcium and raises the level of calcium.

Urogenital System

        The kidney in the chicken is multilobulated with no separation into cortex and medulla. The collecting tubules empty into the ureters which empty directly into the cloaca, a common vestibule into which the digestive and reproductive tracts also empty. No bladder is present. The kidney has a renal portal circulation as described previously. Uric acid is the main nitrogenous waste product excreted in birds. Urine of birds is usually cream coloured and viscous but under certain conditions it may be thin and watery. It is voided at the same time as the faeces and diuresis may give so-called wet droppings.

        Birds have a special gland located above the eye with ducts draining to the nose. This salt gland excretes excess salt in marine birds and may be important in electrolyte homeostasis.

Reproductive System

        The left ovary and oviduct develop in the chicken before maturity the ovary contains many thousands of oocytes. Only a few of these mature into follicles and are ovulated. The yolk in the chicken's egg is an ovum and it is picked up and passed down the oviduct.

Part Time egg remains here Function
Infundibulum ---- Catches ovum
Magnum 3 hours Produces egg white
Isthmus 1 hour Produces shell membranes
Uterus (shell gland) 20 hours Adds water and shell
Vagina ---- Expels egg

        Blood will often be found on the shell of the first egg. The eggs are laid in clutches (one each day) and generally in the morning. With very prolific layers clutches may be very large and this places a tremendous strain on the bird's metabolism. In particular, the bird may not be able to obtain sufficient calcium from its diet and in spite of the presence of medullary bone, the negative calcium balance may lead to osteoporosis.

        The male reproductive system is relatively simple. There are no seminal vessicles or prostate. The phallus is small and has associated lymph glands which produce a fluid which is added to the semen. The sperm after mating is stored in folds in the oviduct by the hen. Good fertility may be obtained for as long as 6 days after insemination. Fertile eggs have been obtained from turkeys 10 weeks after insemination, but this may have been affected by the fact that parthenogenesis can occur in turkeys. Freezing avian semen has not yet proved practical. Males in mating may tear females and inside claws are removed from most breeder males.

        Caponizing of males for meat production has been done surgically and by use of estrogens. Estrogen implantation in chickens has been prohibited in many countries including Canada because of undesirable effects when offal from these birds are fed to other livestock, and because of public health considerations.

        The baby chick has limited control of its body temperature for the first two weeks of life and brooding temperatures are therefore critical. If kept at suitable temperatures, it can survive 3 days or longer without food or water. The yolk sac is not completely absorbed until 5-7 days of age.

Respiratory System

        At the bifurcation of the trachea there is a wall modification which gives the syrinx. It produces sound for the bird. The trachea gives two primary bronchi (mesobronchi) which pass through the lungs to the abdominal air sacs. Secondary bronchi branch from the primary bronchi and go to the other air sacs. From the secondary bronchi, tertiary bronchi (parabronchi) branch and anastomose. From the tertiary bronchi a network of thin walled tubes (air capillaries) are in intimate contact with blood capillaries allowing gaseous exchange. There are no dead end alveoli.

        There are nine air sacs in the chicken and some of these have recurrent bronchi which anastomose with bronchi in the lungs. The air sacs are thin but contain vessels and nerves. They have diverticuli extending into the bones. They contribute extra respiratory reserve for the bird and it is believed that they allow gaseous exchange to occur both on inspiration and expiration. Because of this more efficient gaseous exchange birds may be more susceptible to volatile anesthetics and toxic gases.

        The air sacs allow spread of respiratory infections beyond the lungs to the abdominal cavity and into bones. Some of the air sacs are ventral to the lungs and may collect infectious particles in a similar manner to the anterior and ventral lobes of the lung of the 4-legged mammal.

Nervous System and Special Senses

        The cerebral hemispheres are considerably larger in birds than in reptiles but are almost homologous to the mammalian basal nuclei. Optic lobes are prominent. The spinal cord in the area of the sciatic plexus has a unique structure, the glycogen body, lying in a sinus in the dorsal half of the cord.

        The eye has cartilage and/or bone in the sclera and vascular pigmented structure, the pecten, arising from the retina. The function of the pecten is not known. Visual acuity is comparable to man's, but assimilation is more rapid. The chicken's eye is more sensitive at the red end of the spectrum and blue lights are used to catch birds.

        The ear has no external pinnae but has an external auditory meatus and can be divided into external, middle and inner ear. Two bones, the columella and stapes, are found in the middle ear, and the inner ear has semi-circular canals, utriculus, sacculus and a straight cochlea. The chickens frequency range and discrimination of frequencies is similar to man's.

        Taste and smell have been discussed under digestions.

Immune System

        This system has been extensively studied as an experimental model and split into two components:

1. The thymus, a paired lobulated gland along the neck of the chicken which is considered to be the source of T cells or small lymphocytes which mediate the rejection of hemografts, graft-virus-host reaction and delayed hypersensitivity.

2. The bursa of fabricius, an organ situated dorsally to the cloaca which is considered to be the source of B cells which forms plasma cells and produce antibody.

        Both the thymus and bursa regress as the bird approaches maturity and may also regress with disease. Early damage to the bursa has been shown to increase susceptibility to certain diseases. The chicken has no lymph glands but mural lymphoid nodules have been found along lymphatics and ectopic lymphoid foci are found in most tissues. Grossly recognizable lymphoid tissue equivalent to Peyer's patches are found along the intestinal tract of the chicken, particularly in the ileum, at Meckel's diverticulum and at the base of the ceca (cecal tonsils). Other important accumulations of lymphoid tissue are found under respiratory epithelium, in Harderian glands, and around lacrimal and nasal gland ducts. Bursal-dependent lymphoid tissue appears in histological section as well circumscribed immune bodies (follicles). The avian spleen has white and red pulp. Its functions are similar to those of the mammalian spleen except that there is some argument as to whether or not it functions as a blood reservoir.

Body Temperature

        Normal body temperature is 105-107oF. This may explain why the chicken is refractory to some diseases such as anthrax. Chickens will stand temperatures up to 100oF at a relative humidity of 65%. Most of the heat loss is by respiration and birds will pant when the temperature is too high. Birds will also spread their wings, but if environmental temperature is high, little heat is lost from the body surface. Leghorns are more tolerant to heat than other breeds.


        Chickens have a well developed social order (peck order) and birds at the bottom may become culls if there is insufficient feed or water space. Close confinement of birds in cages may aggravate this problem. If birds are mixed, considerable fighting to re-establish the peck order may occur. Temperament is a breed characteristic. Leghorns are more flighty than White Rocks.


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