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  • All types of birds are included in this class.
  • Study of birds is known as "Ornithology".
  • Dr. Salim Ali was the great ornithologist of India and regarded as "Birdman of India”
  • Birds are also known as "Feathered bipeds or glorified reptiles".

Detailed explanation:

General characteristics of class Aves

Habit and habitat

  • Aerial habitat
  • Well adapted for flying.
  • Though some birds are aquatic too.


  • Their bodies are boat shaped.
  • It is divided into head, neck, trunk and tail.
  • Neck is long and flexible.
  • Body is covered with feathers or plumage.
  • Forelimbs are modified into wings, which help in flying.
  • Scales are found only on hind limbs.

Digestive system

  • Digestive tract is complete and has additional chambers
  • called Crop and Gizzard.
  • Oesophagus is modified into Crop for quick food ingestion and storage.
  • The function of Gizzard is crushing the food which is swallowed unmasticated.
  • A three chambered cloaca is present in the birds.
  • Pigeon or crop milk is secreted by both sexes (Crop product).
  • Their jaws are modified into horny beaks or rhamphotheca, which are toothless.

Respiratory system

  • Spongy lungs are present for respiration.
  • Inelastic air sacs are found.
  • Air sac is connected to lungs for supplement respiration.
  • Sound producing organ at the junction of trachea and bronchi of birds is called syrinx.

Circulatory system

  • Heart is four chambered.
  • Double circulation is found.
  • Hepatic portal system is well developed in birds, but the renal portal system is ill developed.
  • Sinus venosus is absent.
  • Only Right aortic arch is present.
  • R.B.Cs are nucleated,biconvex and Haemoglobin filled.

Excretory system

  • Kidneys are metanephric.
  • Ureters open into cloaca.
  • These are mostly Uricotelic.
  • Most of the birds do not have a urinary bladder and copulatory organ.


  • Bony endoskeleton.
  • Monocondylic skull
  • Body is covered by keratinised plumage,while hindlimbs have a covering on scales.
  • Last four caudal vertebrae fuse to form pygostyle which helps in wagging of the tail.
  • Long bones are hollow, with air filled cavities and these bones are called pneumatic bones.These make the body light in weight and help in flying.
  • Sternum is large.
  • Their swollen basal part of the sternum is called "Keel" which provides sites for attachment of flight muscles


  • Forelimbs are modified into wings.
  • Pectoralis muscles or flight muscles are well-developed for flying
  • Hind limbs are best adapted for clasping the branches of trees or for perching or for walking and swimming.
  • Two bones, clavicle and interclavicle fuse to form V - shaped bone called furcula or Wish bone or Merry-thought bone. It acts as a spring between two pectoral girdles.
  • Pygostyle, Keel and Furcula are absent in flight less birds.

Nervous system

  • Brain is large, smooth, highly developed.
  • Cerebellum is well developed for aerial mode of life.
  • Cranial nerves are 12 - Pairs.

Sense organs

  • Dirhynous nose
  • Olfactory organs are less - developed.
  • Eyes are large and a nictitating membrane is present in the eye.
  • Monocular vision
  • A specific comb like structure, Pecten is found in the eyes of all birds except kiwi.
  • Pecten helps in accommodation of the eye and provides nutrition to eyeballs. Acute vision and telescopic vision of birds is due to pecten.
  • External ears are present but ear pinnae are absent. Only one ossicle columella (Stapes) is found in the middle ear.

Reproductive System

  • Birds are unisexual.
  • Sexual dimorphism is a characteristics feature.
  • Birds are monodelphic i.e. only the left ovary and left oviduct is functional in females
  • Fertilisation is internal.
  • They are oviparous.
  • Amniotes.
  • Cleidoic eggs present.


  • Development is direct.
  • All the birds from nests and Parental care is a characteristic feature.

Special adaptive features

  • The characteristic features of birds are the presence of feathers all over the body and most of them can fly except flightless birds.
  • Feathers keep them warm and also make body weight light.
  • Feathers are modification of epidermal scales.
  • They are warm-blooded (homoiothermous) animals, i.e., They are able to maintain a constant body temperature.
  • Skin is dry and without glands but oil glands or Preen glands are found on the tail or Uropygium. These glands secrete oil, which lubricates feathers.
  • Their vision is monocular.

Classification of aves

Class Aves is divided into two subclasses:

1. Archaeornithes.

  • They are Extinct.
  • Bones were non-pneumatic.
  • Toothed beak
  • Their tail is long lizard-like.
  • Example - Archaeopteryx

2. Neornithes

  • Modern as well as extinct birds
  • Teeth are absent
  • Tail is short.
  • Example - Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture).

Common examples

1. Archaeopteryx (Lizard bird)

  • They were extinct in the Cretaceous period.
  • Its fossil was discovered by Andreas Wagner in 1861 from Bavaria (Germany).
  • They are considered as the connecting links between reptiles and birds.

2. Aptenodytes

  • They are commonly known as Penguin or "sea bird of Antarctica".
  • Their forelimbs are modified into flippers for swimming.

3. Struthio

  • They are commonly known as African ostrich or Camel-bird.
  • It is the largest living bird of the modern period.


4. Rhea (American ostrich)

5. Apteryx

  • They are commonly known as Kiwi.
  • It is the smallest flightless bird.
  • It is the National bird of New zealand.
  • It has hair like feathers all over its body.

6. Pavo - cristatus

7. Psittacula


8. Columba livia

9. Passer domesticus

10. Peacock

  • It is the national bird of India.

11. Humming bird or sunbird

  • It is the smallest flying bird which is found in cuba.

Frequently Asked Questions: FAQs

Q1. Which scientist is regarded as the “Bird-man of India”?
Ans :

  • Dr. Salim Ali was the great ornithologist of India and regarded as "Birdman of India”

Q2. What is the function of crop ?
Ans :

  • Crop is found in birds for quick food ingestion and storage.
  • In pigeon it is modified into crop glands for milk secretion.

Q3. What is the function of Gizzard ?
Ans :

  • The function of Gizzard is crushing the food which is swallowed unmasticated.

Q4. What is the excretory product of birds ?
Ans :
Uric acid

Q5. What is the voice box of birds called ?
Ans :

  • Syrinx is called the voice box of birds which is present at the junction of trachea and bronchi of birds.

Q6. What are pneumatic bones?
Ans :

  • Pneumatic bones are the long bones that are hollow, with air filled cavities.

Q7. Why are birds monodelphic ?
Ans :

  • These condition is found to make their bodies light.

Q8. What is the hematopoietic organ of the birds?
Ans :

  • Bursa of fabricius

Q9. What do you mean by Rhampothaeca?
Ans :

  • Their jaws are modified into horny beaks or rhamphotheca, which are toothless.

Q10. What is plumage ?
Ans :

  • They are used for the feathery coat for birds.
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