The Aves belong to the phylum Chordata of the animal kingdom. It has about 9,000 species. Aves are adapted to fly. All the birds come in the class Aves. They show courtship, parental care, nest building, and territorial behaviour.

Characteristics of Aves :

Birds are warm-blooded animals.

Their forelimbs are modified into wings.

They have well-developed flight muscles that help during the flight.

Their hind limbs are adapted for walking, hopping, perching, grasping, wading and swimming.

There are epidermal scales on their legs.

The endoskeleton is bony with long hollow bones filled with air cavities. known as pneumatic bones.

Their spindle-shaped body minimizes resistance of the wind.

The feathers help in preventing heat loss and reduce air friction by providing passage to the air.

There is no skin gland except the oil gland.

The lower and upper and jaws are modified into a beak.

They have no teeth.

They have sharp eyesight.

The alimentary canal has a crop and a gizzard.

 The crops help in softening food, and the gizzard helps in crushing the food.

Pigeons and other seed-eating birds lack a gall bladder.

They have spongy and elastic lungs for respiration.

The special vocal organ called syrinx is present at the base of trachea.

Their heart is four-chambered.

RBCs are oval, nucleated and biconvex.

12 pairs of cranial nerves are present.

They have a single ovary and oviduct on the left side

All the birds are oviparous and exhibit sexual dimorphism. The eggs have four embryonic membranes- amnion, chorion, allantois, and yolk sac.

Classification of Aves :

The class Aves is divided into two categories:

📌 Archaeornithes :

This class of birds are extinct. They had a toothed beak with a long lizard-like tail.

Eg., Archaeopteryx

📌 Neornithes :

These include extant as well as extinct birds. They have no teeth and a short tail.

Eg., Penguin, Grey Heron, Kingfisher, Duck, etc.

Flight Adaptations in Class Aves:

📌 Spindle-Shaped Body :

It is designed to offer minimum resistance to the wind.

📌 Feathers:

They provide the passage for air and reduce friction to minimum. They also prevent loss of heat and help to maintain a constant temperature.

📌 Wings:

Fore-limbs are modified into wings, which help during flight.

📌 Beak:

Besides procurement of food, the beak is also used for nest-building.

📌 Neck and Head:

Mobile neck and head are very useful for feeding, nest building, offence and defense.

📌 Flight Muscles:

The flight muscles on the breast are greatly developed which help in flight.

📌 Hind Limbs (Legs):

They are well suited for perching.

📌 Endoskeleton:

Most of bones are pneumatic and filled with air instead of bone marrow. It makes the body light. Most of bones are firmly fused together, which help in flight.

📌 Air Sacs:

These are attached to lungs which serve as reservoirs of air. They may also aid as cooling devices in regulation of the temperature of the body.

📌 Warm-Bloodedness:

Birds are warm-blooded animals which is necessary for flight.

📌 Circulatory System:

A large oxygen supply is required for rapid metabolism and warm-bloodedness. It is done by an efficient circulatory system.

📌 Absence of Urinary Bladder:

Except Rhea, urinary bladder is absent in birds. Excreta are passed out at once. This helps in reducing the weight of the body.

📌 Brain and Eyes:

Brain and eyes are well developed. Equilibrium is maintained by well developed cerebellum of the brain.

📌 Single Ovary:

Presence of a single functional ovary on the left side in the female bird also leads to reduction of weight which is so essential for flight.

 Darwin’s Finches:

A group of finches (birds) which live only on the Galapagos Islands except for one species which is also found on the Cocos Island. They are named after the naturalist Charles Darwin who visited the islands during his famous voyage on HMS Beagle (Ship) in 1835.

Each finch has a different shape of bill to suit its method of

 feeding. Darwin’s observations on these birds were an important influence in helping him to draw up his theory of evolution.