Anatomy
Birds have a close evolutionary relationship with reptiles especially crocodiles. Their unique
anatomy has helped the birds to adapt well to the environment. Even though there are numerous
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different species of birds, their anatomy shows a greater uniformity of structure than many single
orders of fishes, amphibians, reptiles and mammals (King and McLelland, 1984). A few salient
features on the anatomy of birds is given below.
Birds skin lack glands except for the uropygeal gland or preen gland present at the base of the
tail. This gland is not found in all birds. Various appendages such as comb, wattles, ear lobes,
snood, caruncles, beard, spurs, claws, scales are present in a variety of birds.
Birds have pneumonic bones and air sacs extend in to the proximal humerus, vertebrae and
pelvic girdle. Laying hens form medullary bone in response to estrogen, which serves as a source
of calcium during laying.
Trachea has complete rings and syrinx is present. Syrinx is large in male waterfowl. Lungs are
attached to the body wall. From trachea the primary bronchi passes through the lungs giving rise
to many secondary bronchi before entering the abdominal air sacs. The secondary bronchi give
rise to tertiary or parabronchioles and from there to atria and air capillaries where air exchange
takes place. Lungs are of ‘paleopulmo’ (unidirectional movement of air) and ‘neopulmo’ (bidirectional)
types. Cervical, clavicular (paired), anterior and posterior thoracic (paired) and
abdominal (paired) air sacs are present.
Birds have four-chambered heart and the right atrioventricular valve is a muscular flap.
Birds have beak, lack teeth, have mucus glands in the oral cavity and most birds have a food
storage organ called crop. Proventriculus is the primary glandular stomach and the muscular
stomach; gizzard is well developed in seed eating birds. It is lined by keratinous material or
koilin. Duodenum in the form of a loop encloses pancreas, a long jejunum and a small ileum
comprises the small intestine. Ceca are paired and their presence varies among different species
of birds. It is well developed in gallinaceous birds and ratites but appear as small knobs in
columbiformes, passeriformes and absent in psittacines. Colon is longer than the entire small
intestine in ostrich. Cloaca is compartmentalized and is the common opening for digestive,
urinary and genital systems. Liver is composed of right and left lobes, which join cranially, and
gall bladder is present. Some birds such as most pigeons, ostriches, and many parrots lack gall
bladder and in some birds such as toucans, woodpeckers and barbets it is exceptionally long.
Birds have no lymph nodes; thymus and bursa are the primary lymphoid organs. Thymus, seven
pairs in chickens are located along the jugular vein. Bursa of Fabricius is pale and round in most
birds elongated in ducks and is located dorsal to the cloaca. Bursa in chickens is present at hatch
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reaches maximum size 6-12 weeks and undergoes involution 18-22 weeks. Spleen is the primary
lymphoid organ. It is round to oval in gallinaceous birds, ducks and psittacines but elongated in
charadiiformes and passeriformes. Ceca have well developed lymphoid tissue, ‘cecal tonsils’ at
the junction of ceca and colon. Cecal tonsils occur as annular bands in waterfowl. Duodenum has
well developed Payer’s patches. Paired lymph node-like structures present in duck and goose,
one at the thoracic inlet near the thyroids and the second pair at the lumbar region. Lymphatics
are present but not well developed.
Kidneys are lobulated, lack renal pelvis and have both mammalian and reptilian nephrons. Renal
portal circulation present, venous blood from the legs, pelvic region and hindgut is carried to the
kidneys. Avian excretory system is uricotelic; uric acid is the end product. Urinary bladder is
absent.
Most avian species have only left ovary and oviduct. Oviduct has five parts, infundibulum,
magnum (albumen secreting), isthmus (shell membranes), uterus (shell formation) and vagina.
Testes are paired and located at the cranial poles of the kidneys. Testes can enlarge greatly
during breeding season. Seminal vesicle, prostate, bulbourethral glands are absent. Ratites,
ducks, geese, swan have intromittent (protruding) phallus.
Thyroids are oval fleshy organs located in association with the common carotid artery at the
thoracic inlet. Parathyroids and ultimobronchial bodies (have C cells) are separate and are
located posterior to the thyroids. Adrenals are located at the anterior poles of the kidneys.
Adrenals lack a distinct cortex (pale staining cells with vacuoles in cytoplasm) and medulla
(basophilic cells) but are ‘scrambled’. Pituitary is present in sella turcica posterior to the optic
chiasm. There is no intermediate lobe in the pituitary. Most of the islets are located in the splenic
lobe of the pancreas.
Brain lacks sulci and gyri but cerebellum has folia. Optic lobes are well developed and
prominent. Spinal cord has ‘glycogen body’ in the rhomboid sinus of lumbar segment.
Eyes have pecten, scleral ossicles, single and multiple fovea. Extra orbital nasal or salt glands
are present in the nasal septum, well developed in marine birds.
Ears lack external pinna and have a well-developed sound conducting structure in the middle ear
called columella.
4. Inflammation