CLASS AVES

What are birds ?

Birds(class Aves) are feathered, winged, bipedal, endothermic (warm-blooded), egg-laying, vertebrate animals. With around 10,000 living species, they are the most speciose class of tetrapod vertebrates. All present species belong to the subclass Neornithes, and inhabit ecosystems across the globe, from the Arctic to the Antarctic

Modern birds are characterised by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. All living species of birds have wings; the most recent species without wings was the moa, which is generally considered to have become extinct in the 16th century. Wings are evolved forelimbs, and most bird species can fly.

Origin of Birds

The origin of birds has historically been a contentious topic within evolutionary biology. However, most researchers now support the view that birds are a group of theropod dinosaurs that evolved during the Mesozoic Era.

A close relationship between birds and dinosaurs was first proposed in the nineteenth century after the discovery of the primitive bird Archaeopteryx in Germany. Birds share many unique skeletal features with dinosaurs.[1] Moreover, fossils of more than twenty species of dinosaur have been

collected which preserve feathers. There are even very small dinosaurs, such as Microraptor and Anchiornis, which have long, vaned, arm and leg feathers forming wings. The Jurassic basal avialan Pedopenna also shows these long foot feathers. Witmer (2009) has concluded that this evidence is sufficient to demonstrate that avian evolution went through a four-winged stage.[2]

Characteristic of Birds

Body is stream-lined and spindle shaped,viz; head,neck,trunk and tail. These are warm-blooded (homoeothermic)

Limbs are adapted for flying. The fore-limbs are modified into wings and hind limbs fro perching and in some birds for running as in ostrich.

There is epidermal exoskeleton of feathers, legs bear scales

The skeleton is light due to air spaces which is an adaption for flying.

The skull had large sockets, jaws extend into horny beak, teeth are absent.

The circulatory has 4-chambered heart and there is only right aorta which curves to the right side and then bend backwards.

•The lungs have extensions known as air-sacs which extend into the bones also.• The organ of voice is called syrinx,it is called at the lower end of the trachea near the origin of two bronchi.

Excretory system does not have a bladder, urine is semi solid. Birds excrete their nitrogenous wastes, derived mostly from the

breakdown of proteins, in the form of uric acid rather than urea as mammals do. Unlike urea, uric acid is almost insoluble in water, and is excreted in the form of crystals that form a semisolid white paste.

Sexes are separate. Fertilization is internal and eggs are of large size with much yolk. Only one ovary and oviduct is functional.

Since birds do not have teeth they have developed a thick muscular structure (gizzard) which is used for crushing food.

Some birds have secondarily lost the power of flight and are called running birds e.g. Ostrich, kiwi,etc.

Presentation made by : Narjis Malik

Class : XI – A

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