Characteristic features of class Aves (the birds)

Birds are cosmopolitan and found in all continents, seas and most islands.
Their wide occurrence is due to their power of flight, which enables them to reach places unreachable to other animals.
Most of them can fly and a few have lost the power of flight.
The birds evolved in the Jurassic period from bipedal dinosaur reptiles.
They are often described as ‘glorified reptiles’ because of their resemblances with and origin from the reptiles and magnificent look.

They are homeothermic (warm blooded) and the body temperature is over 100⁰F which provides high metabolic rate for quick energy supply.
Birds are endothermic, and expend a lot of energy to keep warm.

The body is boat shaped and streamlined.
It is divisible into head, neck, trunk and tail.
The forelimbs are modified into wings for flight which are worked by powerful breast muscles attached to the sternum.
Each fore limb has 1 to 3 digits and each hind limb has 1 to 4 digits.
The hind limbs are used for perching, walking, hopping, wading, swimming etc.
Skin is dry and thin except for uropygial or oil gland on the tail.
Body is covered by epidermal horny skeleton of feathers which conserve body heat, help in flight and provide coloration to the birds.

The endoskeleton is bony, but delicate and light.
Skull is monocondylic, i.e with single occipital condyle.
Sternum is usually large and with a median keel for thee attachment of flight muscles.
Bones are pneumatic, i.e. contain air cavities to reduce weight.

The mouth has a wide gap and jaws are covered with horny sheaths to form strong beaks.
Beaks are adapted to various modes of feeding: seed-crushing, fruit-scooping, fish-tearing, nectar-sipping, wood-chiseling, grain-pickling etc.
There are no teeth and food is swallowed unmasticated.
The crop stores and softens food. Alimentary canal often has additional chambers; crop and gizzard.
The gizzard is muscular to crush and churn the softened food.
Some birds keep stone in the gizzard to effectively crush seeds and grains. Alimentary canal leads to the cloaca.

Respiration takes place only by lungs. The lungs are spongy and inelastic.
A system of thin-walled air sacs lying among the viscera is associated with the lungs.
This system maintains a constant draught of fresh air through the lungs, even during expiration.
Voice box lies at the division of the trachea into bronchi called syrinx. The larynx doesn’t act as a sound box.

The heart is large and fast beating for quick supply of adequate amount of blood during flight.
It is 4 chambered, having two auricles and two ventricles.
Renal portal system is greatly reduced.
RBCs are oval, biconvex and nucleated.

The brain is large with well-developed optic lobes and cerebellum and reduced olfactory lobes.
There are 12 pairs of cranial nerves.
The olfactory sacs open by internal nares into the buccal cavity.
Most birds seem to lack the sense of smell and sharp eyesight help to see prey and other foods, land marks and resting places while flying high.
Ear has an external opening, and a large, curved cochlea with organ of corti.

Kidneys are metanephric and 3-lobed. The ureters open into the cloaca as there is no urinary bladder.
They normally excrete uric acid (uricotelic) and urine is semisolid.

There may be sexual dimorphism and the testes are paired.
The ovary and the oviduct of the right side are absent in females.
Gonoducts lead into cloaca and the fertilization is internal.
They are oviparous. Eggs are large with much yolk and hard calcareous shell which need incubation, at a constant body temperature by parents.
Males generally lack copulatory organs. Copulation occurs by cloacal apposition.
Development is direct and embryonic membranes (amnion, chorion, allantoin and yolk sac) are formed.

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