Birds are feather-clad, air-breathing, warm-blooded, oviparous (egg-laying) and bipedal flying vertebrates.
They have a streamlined spindle-shaped body that helps them to fly in the air.
The bird’s body is divisible into four distinct parts- head, neck, trunk, and tail. Jawbones are prolonged into a tooth-less beak or bill. The neck is long and flexible. The tail is short and stumpy.
The entire body of birds is invested with a close covering of feathers, constituting the plumage. The feathers form the exoskeleton of birds. These unique structures are light, elastic, waterproof, and most important in flight. The different colors of feathers are due to the presence of melanin pigment of various shades and also due to iridescence (i.e. play of rainbow colors, caused by interference as on bubbles). Old feathers may be replaced by new ones such as in peacock. The shedding of old feathers is called moulting. Feathers are generally classified into two groups: flight feathers (or quill) and covering feathers. Flight feathers of wings are called remiges and those of the tail are called rectrices. Covering feathers cover the body of birds and are of three types- contour feathers, filoplume, and down feathers. A typical feather has three parts- shaft, vane, and aftershaft. The shaft contains a basal, hollow stalk called calamus and the upper part, called the rachis. The vane is made up of two unequal lateral halves. Each lateral half of the vane consists of numerous narrow, parallel, and closely spaced thread-like structures, called barbs. In a flight feather, the shaft is long and the vanes are quite broad and characterized by the presence of interlocking arrangement of barbules in the barbs. Among the covering feathers, the filoplumes are small, delicate, hair-like feathers, each containing a long shaft with a few barbs with smaller barbules at its tip. The contour feathers have a short shaft (i.e., rachis does not extend up to distal extremity of vane). The vane contains poorly developed barbules. Down feathers are small, soft, wooly feathers found all over the body of young birds and are used in the conservation of body heat. They have short calamus and are replaced by contour feathers in most birds. Each type of feather is provided with muscles at its base so that it can be moved in different directions. In flying birds, both the wings and tail feathers are well developed. But, in birds living near the ground, the tail feathers are almost absent. In flightless birds, the wing feathers are very small.
The birds have two pairs of limbs. The fore-limbs are modified into wings helping in flying. The hind-limbs or legs are large and variously adapted for walking, running, scratching, perching, food-capturing or swimming. Each foot usually bears four-clawed toes, of which the first or hallux is directed backward.
Birds have a Four chambered heart and their red blood corpuscles are nucleated.
Birds skin is dry and devoid of glands except the oil or preen glands at the root of the tail.
All the birds have a light body. This is because of the presence of air cavities inside their bones. Their bones have no marrow.
Birds are the first vertebrates to have a warm-blood. Like any other warm-blooded animal, their body temperature remains more or less constant and independent of the surrounding environment, so they are also called homeothermous animals. The body temperature of birds ranges from 40°C to 45°C.
Bird’s tongue is small. In a pigeon, the mouth opens into oesophagus which enlarges into a crop where food is stored. The crop can secrete “pigeons milk” which is brought out of the mouth to feed the young. The stomach has a thick glandular proventriculus for digestion and a muscular gizzard for grinding the food grains. There is no gall bladder in birds.
Lungs are compact, spongy but non-distensible in birds. Various balloon-like membranous sacs, called air sacs are present in association with the bronchi of the lungs. They are extra reservoirs of air and increase the breathing efficiency of the lungs.
The voice box, called syrinx is located in the chest at the base of the windpipe (trachea).
Paired kidneys excrete semisolid waste in the form of uric acid which is passed down the ureter into the cloaca. There is no urinary bladder.
Sense organs for smell (or olfactory sensory organs) are poorly developed. However, eyes are large and power of vision in birds is very well developed.
Sexes are separate in birds. Sexual dimorphism (i.e. morphological distinction between male and female birds) is often well marked.
Male birds have a pair of abdominal testes (male gonads) and often lack the penis. Each female bird has a single functional left ovary (female gonad) and oviduct (due to a flight adaptation).
Fertilization is internal, preceded by courtship and copulation. Females are oviparous (i.e. egg-laying), never viviparous (i.e giving birth to live young example- human beings). This is a flight adaptation.
Eggs of birds are large with much yolk and hard calcareous shell. They are incubated by the birds at a more or less constant temperature usually in a nest.
Newly hatched young ones of the birds are either fully formed (called precocial example- chicks of fowl) or immature (called altricial example nestlings of pigeon). Parental care is well marked in birds.