Characteristics of Aves 

Question: Give characteristics of class Aves ( birds ) in detail.

Answer :

Birds constitute a well-defined group of vertebrate animals. They possess a series of strongly marked characters hardly seem in any other class. 

Main characteristics of Aves :

The major characteristics of Aves, which define them are:

1. They are homeothermous, endothermous or warm-blooded animals. Their body temperature is generally 42°C and may reach 45°C in some cases.

2. Body : 

Their body is streamlined, spindle and divisible into 4 distinct regions i.e. head, neck (long size), trunk and tail (reduced and stumpy). 

Jawbones prolonged into a toothless beak or bill. Mouth apertures are guarded by toothless upper and lower beaks.

The lower jaw or mandibles consists of 5 or 6 bones and articulate with quadrates.

3. Limbs :

They have two pairs of pentadactyle limbs. Forelimbs are modified to form wings/ feathers.

Hind limbs are rotated under the trunk and oriented vertically. They are modified for perching, walking, running, scratching, food capturing, swimming or wading etc.

Limbs end up into 4 clawed toes of which the first or hallux is directed backwards. The lower part of the hind limbs is guarded by epidermal salts.

4. Exoskeleton :

The exoskeleton is epidermal and horny, represented by :

Feathers: which give warmth by covering the body.

( Related QnA

Feathers: Structure and Types

Feather: Development and Uses )

Scales: scales are present on the lower part of hind Limbs ( similar to those of reptiles ).

Claws: claws are present on the toes.

Sheaths: sheaths on the toothless beaks ( ramphotheca ).

5. Skin : 

Skin is dry, devoid of glands except for the oil or preen gland at the root of the short tail, uropygium ( used for dressing feathers ).

6. Pectoral muscles of flight are well developed.

7. Endoskeleton :

The endoskeleton is bony, fully ossified, light but strong and without epiphyes. 

Bones are light and pneumatic (with air cavities) and with internal supportive trusses and no marrow. All these characteristics help to reduce body weight. 

A bird's entire skeleton weighs less than its plumage and is articulated to the atlas. There is a fusion of bones. 

Flight adaptations in birds ( ossified exoskeleton and pneumatic bones, a defining characteristic of Aves

8. Skull :

The skull is smooth monocondylic ( bearing a single occipital condyle ). The cranium is large and dome-like. Sutures indistinct.

Skull bones and vertebrae of the trunk and tail are fused.

9. Vertebral column :

Centra of vertebrae heterocoelous (saddle-shaped). Numerous cervical vertebrae bearing small cervical ribs. Some thoracic vertebrae are fused.

A synsacrum results from the fusion of posterior thoracic, lumbar, sacral and anterior caudal vertebrae.

 A few tail vertebrae are compressed laterally and the last three or four are fused to form a plough-shaped bone called pygostyle which furnishes attachment for muscles that control the tail fan.

10. Sternum :

The sternum is largely expanded and bears a large vertical mid-ventral ridge or keel for the attachment of flight muscles as the pectoralis major and pectoralis minor ( except in flightless birds e.g. ostrich).

11. Ribs :

Ribs are double-headed ( bicephalous) and bear posteriorly directed uncinate processes that hook together when in flight.

Both the clavicles and single interclavicle fuse to form a V-shaped bone called furcula, wishbone or merrythought_ bone.

12. Pelvic girdle :

The pelvic girdle is large strong and fused with synsacrum throughout its length. Pubic and ischiatic symphyses lacking. Acetabulum perforated.

Proximal carpals free. Distal carpals fuse with three metacarpals to form carpometacarpus.

Proximal tarsals and tibia fused to form tibiotarsus. Distal tarsals fuse with IInd, IIIrd and IVth metatarsals to form tarso metatarsals. Ist metatarsal remains free.

The ankle joint is inter-tarsal.

13. Alimentary canal :

The Alimentary Canal is complete and has an additional chamber for storage and quick feeding called crop ( modified oesophagus ). The stomach is divided into a glandular proventriculus and a muscular gizzard.

A pair of rectal caeca is present in the junction of the small intestine and rectum. A three-chambered cloaca is present to conserve water.

 The tongue is pointed in front and bifid behind.  It lacks the muscles for movement which is done by the altering movement of the hyoid apparatus.  Gallbladder is absent.

14. Heart :

The heart is four-chambered ( two auricles and two ventricles ). The hepatic portal system is well-developed and has a special coccygeus mesenteric vein. The renal portal system is vestigial.

RBCs are oval biconvex and nucleated.

Birds are the first vertebrates to have warm blood. Body temperature is regulated (homeothermic).

15. Respiration:

Respiration is pulmonary. Lungs are spongy and Elastic with air sacs. 

The trachea has complete bony rings and the bronchi are covered with cartilaginous rings and are slightly expanded to form a sound-producing organ the syrinx near the junction of the trachea and bronchi.

 Air sacs ( cervical, anterior thoracic, posterior thoracic and abdominal ) are the expansion of mucus membrane of secondary bronchi. These air sacs act as internal cooling agents.

 The larynx is without vocal cords.

Bursa fabricii/cloacal thymus ( lymphatic tissue ) is attached to the cloaca in some young birds and takes part in the formation of lymphocytes.

16. Kidneys :

Kidneys are metanephric and trilobed. Ureters open into the cloaca. Urinary bladder absent except in ostrich, American rhae ).

 Birds are uricotelic ( uric acid ). The excretory substance of urates is eliminated with semisolid faeces. Sea birds deposit it as guano (phosphate-rich).

17. Brain:

The brain is well developed. The cerebrum, cerebellum and optic lobes are large-sized. 

Olfactory lobes are poorly developed. Birds have 12 pairs of cranial nerves.

18. Sense organs :

Sense organs include large eyes with nictitating membranes, sclerotic plates, comb-like structures of vascular pecten ( accept kiwi ) and double accommodation power. Ciliary muscles - Crompton muscle + Bruke muscles are well developed. 

Olfactory organs are poor. Internal ear with single ear ossicle ( stapes ).

Cochlea as columella auris is present. Lagaena is also a part of the membrane labyrinth associated with hearing. Grandy's corpuscles are touch receptors in the beak of birds and Herb's corpuscles are heat and vibration receptors in the dermis of the birds.

19. Sexes :

Sexes are separate with well-marked sexual dimorphism. 

The male has a pair of abdominal testes and a pair of sperm ducts. A copulatory organ is absent except in ratites, ducks, geese, etc. 

Female with single functional left ovary and oviduct.

Internal fertilization is preceded by copulation and courtship. Females are oviparous. Eggs large with much yolk and hard calcareous shell.

External incubation of eggs. Cleavage is discoidal and meroblastic. Extra embryonic membranes  ( amnion, chorion, allantois and yolk sac ) are present. Development is direct.

Newly hatched young are fully formed ( precocial ) or immature ( altricial ).

Show courtship, nest building ( nidicolous), parental care ( altricial ), migration and territory behaviour.

Also, check out for related important question 

Flight Adaptation in Birds