Features of Class Aves (Birds):

• Body covered with feathers made largely of keratin (they are the only animals which have feathers)
• powerful bony endoskeleton
• bones having large air spaces
• forelimbs altered as wings for flight (some have lost ability to fly)
• bipedal- two legs for locomotion (lower part of legs has scales)
• toothless horny beak; use gizzard to grind food
• warm-blooded animals (body temperature is internally regulated; endothermic/ homeothermic)
• Effective lungs having pouches for gas exchange
• Hard-calcareous shelled eggs with large yolk.

Birds are said to have come from a common ancestor (monophyletic lineage) and are therefore associated through common origin. Modern birds contain traits associated to high metabolism, skill to fly. Some birds, particularly parrots, are among the cleverest animal species; number of bird species has ability to tools, and several social species show cultural transmission of knowledge across generations.

Adaptation to flight in birds is facilitated by light body weight (a consequence of the absence of teeth and ultra-light bones and air sacs), high body temperature, improved blood circulation, high metabolism, and acute vision (that enables avoidance of danger such as tree branches at high speed).

The class Aves is made of two subclasses like Archaeonithes (extinct ancient birds) and Neornithes (recent birds). So it is correct to say that all living birds belong to subclass Neornithes, that is further separated in three Superorders namely Odontognathae (extinct), Paleognathae and Neognathae.

Features of Subclass Archaeonithes:

Members of this subclass are extinct ancient birds. They were characterized by comprising clawed wings, reptilian style ribcage without large carina and presence of long, bony tail. Known members of group were Archaeopteryx and Archaeornis.

Characteristics of the Class Subclass Neornithes:

In this subclass, there are 3 superorders namely Odontognathae (extinct), Paleognathae and Neognathae.

Superorder Odontognathae:

Members of this superorder considered as modern birds, on account of anatomy are though extinct. As name signifies, they included teeth-like structures.

Unlike Archaeornithes, Odontognathae had short tails with well- developed carina for flight muscle attachment and wings are usually without claws (although few modern birds like kiwis have claws). Brain of Odontognathae seems to be fairly simpler than those of modern birds.

Superoder: Palaeognathae

The superorder Palaeognathae derived its name from paleognath, ancient Greek word for old jaws in reference to skeletal anatomy of palate of bird that is described as more primitive and reptilian than of other birds.

Orders under this superorder are given below:

Order: Struthioniformes (Ostriches, Emus, Kiwis)

Order Struthioniformes included birds known as ratites that are large flightless birds; these are birds which have lost their skill to fly and contain flat breastbone. Ratites have simplified wing bone structure, strong legs, and no feather vanes, making it unnecessary to oil feathers. Therefore, they contain no preen gland which has preening oil. This group is made up of of ostriches, rhea, cassowaries and emus.

Kiwis (Apteryx) are flightless birds endemic to New Zealand. Kiwi is national symbol of New Zealand. Kiwi is by far the smallest living ratites and lay largest eggs in relation to body size of any species of bird in world. Ostrich (Struthio) is one of two species of large flightless birds inhabitant to Africa, the only living member(s) of genus Struthio. Ostrich is the largest living species of bird and lays largest egg of any living bird.

Emu is the largest bird native to Australia and only extant member of genus Dromaius. It is second-largest existing bird in world by height, after its ratite relative, the ostrich. They contain long thin necks and legs. Emus can travel great distances at fast, economical trot and, if essential, can sprint at 50 km/h (31 mph) for specific distance at a time.

Order: Tinamiformes (Tinamous)

The Tinamous are found in Central and South America. They are one of the most ancient living groups of bird, they are associated to ratites. Usually ground dwelling, they are found in the range of habitats and can fly and contain keeled breastbone (shaped like wishbone). Though they look like other ground-dwelling birds like quail and grouse.

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