Aves (Birds)
CLASS AVES
General Characters
• Birds are highly specialized class, as every part of their bodies is
modified to the aerial life.
• They evolved from some diapsidan reptiles.
• Birds, like mammals, are homoeothermic
• They have feathers, that is not only support the body during flight, but also conserve the
body temperature.
• The fore limbs are modified into wings
• The hind limbs possess four digits (the 5th digit is absent) ends with horny
claws. The upper portions are covered by feathers, whereas the lower
portions are covered by horny scales, similar to those of reptiles.
• The skin gland is uropygial gland (oil gland), which is found at the base of
the tail and secretes a greasy secretion which may be :
a. Waterproof the feathers especially in aquatic birds.
b. Produce a specific recognition scent.
c. Elaborate ergosterol which when exposed on the feathers is
transformed by sun light to vitamin D.
• The teeth are absent from the jaws, that modified to beaks.
• The bones are light, as they contain pneumatic cavities, that enable
them for powerful flight, and mostly fuse together, e.g skull. Fore, hind limb ..etc
• The digestive system is characterized by presence of Crop (part of
esophagus), Stomach (Proventriculus, Gizzard).
• The respiratory system is characterized by the presence of the organ
of voice (syrinx) and air sacs
• The heart is 4-chambered (2 auricles & 2 ventricles). It has a single
right aortic arch in adult.
• The olfactory organs and the olfactory bulbs are very reduced thus
the birds have a weak smell. The cerebral hemispheres (cerebrum)
are large which responsible for the intelligent behavior. The optic
lobes are large which responsible for the sharp sight.
• There is three lobed (tripartite) kidney; no urinary bladder. The R.
ovary & oviduct are atrophied.
• They are oviparous and the eggs are of a large size as they contain a
large amount of yolk (heavy teleolecithal) for nourishing the embryos.
SKIN:
• The skin is loose, dry, thin and composed of an outer
epidermis and inner dermis. The stratified epidermis is
formed of basal columnar Malpighian layer that
produces continuous generations of rounded cells that
flat as they reach to the surface, where they forming a
distinct cornfied layer.
• The dermis contains, C.T., blood vessels, nerve endings
and abundant smooth muscle fibers to raise and lower
the feathers.
• The pigments are confined in feathers and scales. No
glands in the skin of birds except one type (oil gland),
well developed at the base of the tail especially in the
aquatic birds
Cervical A. Sac
Ventriculus
( gizzard)
• DIGESTIVE SYSTEM:
• The digestive system, is unique in many features & peculiar to birds:
• The horny beak bound the mouth. The teeth are absent and the
tongue is triangular in shape and pointed at the tip serving for
swallowing the seeds.
• Taste buds on the tongue are scanty or absent.
• The buccal glands pouring their secretion in the buccal cavity, for
lubricating and moistening the food. These glands are not
salivary in functions.
• The esophagus is long, tube and extends along the neck region,
It dilates to a large crop, in the middle, and immediately front
of the furcula.
• The crop is used as a storage part, moistening of food and has
unique ability to produce "pigeon's milk" in both sexes.
• The pigeon’s milk is white and cheesy secretion formed by the
proliferation and degeneration the cells lining the crop under the
control of prolactin hormone (an adenohypophysial hormone).
It is rich in proteins, fats and vitamins and is produced by both
sexes during the breeding season. This milk is used to nourish
the squabs until they are old enough to eat grains.
• The stomach is divided into two parts, an anterior small thick walled glandular proventriculus and a posterior large
muscular and mechanical ventriculus or gizzard.
• The proventriculus secretes gastric juice and initiates the breakdown
of food before passing to the gizzard.
• The gizzard is bluish-red in color and has a shape of a biconvex lens.
Horny layer lines it and its small cavity always contains
small stones, which aid in grinding up the food and
mixing it with the gastric juice.
• The small intestine is long, coiled and differentiated into the duodenum
and ileum.
• The duodenum leaves the gizzard close to the entrance of the
proventriculus and forms a U-shaped loop, enclosing the pancreas.
• The long coiled ileum passes into a slightly wider large intestine or rectum
and a pair of small rectal caeca marks the junction between them.
These caeca are the site of digestion of certain
vegetable fibers by enzymatic and bacterial action, as
well as absorption of some water.
•
• The rectum is followed by a large cloaca which is divided into three
chambers, coprodaeum, into which the rectum opens; the urodaeum
which receives the urinary and genital ducts, and the proctodaeum
(or preferably the vestibule) which opens externally by the cloacal
opening.
• The bursa Fabricii is a lymphoid thick-walled pouch, lies against the
dorsal wall of the cloaca in young bird only and degenerates in the
adult. Its function is suggested to serve as a cloacal thymus or the
site of maturation the B-lymphocytes when the thymus is inactive.
These cells involved in certain immune reactions and local
protection.
• The liver is large, bilobed gland, the right is the larger than the left.
Each lobe opens into the duodenum by its own duct.
• The gall bladder is absent in the pigeon, but occurs in the hens.
• The pancreas lies in the U-shaped loop of the duodenum and it
consists of three lobes; each opens into the distal limb of the
duodenum by its own pancreatic duct.
Glottis
Cricoid cartilage Larynx
Arytenoid cartilage
Trachea
Syrinx
Cervical A. Sac Interclavicular A. Sac
Axillary A. Sac Anterior thoracic A. S
Humerus Posterior thoracic A. S
Bronchus
R. Lung
Abdominal A. Sac
Bronchiole
•The air sacs
•Are thin-walled, transparent chambers and lying
outside the lungs, in the body cavity.
•They are poorly vascularized, therefore they have no
any role in gas exchange and respiration. However,
These sacs may be act as :
•Bellows at expiration to renew the air in the lung.
•Balloons giving buoyancy in the flight by reducing
the body weight.
•Reduce the specific gravity, due to their warm air.
•Regulate the body temperature: act as cooling
device by losing heat through internal evaporation,
Voice production.
Types of air sacs :
1. Cervical air sacs : are paired , situated at the base of
neck
2. Interclavicular air sac: are paired sacs, united together
forming one median sac, in between the two clavicles. It
gives two lateral axillary air sacs, which communicate
with pneumatic foramens of the two humeri.
3. Anterior thoracic air sacs: are paired sacs, situated on
the ventral surfaces of the anterior parts of the lungs.
4. Posterior thoracic air sacs : are paired sacs, situated
behind the anterior thoracic ones.
5. Abdominal air sacs: the largest air sacs, above the
intestine.
All air sacs are connected to the bronchioles except
the abdominal air sacs, which connect to the primary
bronchi.
