Birds evolved from reptilian ancestors in the Jurassic period, 200 million years ago. At the same time, mammals were evolving from a different line of reptilians.
The most characteristic feature of birds is the possession of feathers. Because the bird-like reptiles of the Jurassic were becoming warm-blooded, the principle advantage of the feathery covering was probably the reduction of heat loss from a warm body. Modern birds have temperatures of about 40-41 degrees C which contributes to the high rate of metabolism necessary for the muscular activity involved in flight.
The upper and lower mandibles of a bird are extended to form a beak. The shape of the beak varies with the species. There are long, narrow beaks which penetrate into flowers for their nectar; short stout beaks which crack open seeds; sharp, hooked beaks which tear flesh from prey.
There are two layers of feathers covering the body. The down feathers are fluffy and form an insulating layer close to the body. The contour feathers are flatter and broader and make a waterproof layer as well as giving the bird its characteristic shape The power of flight became possible as the skeleton of the fore-limbs became modified to form wings and the flight feathers developed. The flight feathers on the wings have long shafts and flat vanes on each side of the shaft. The vanes are formed from parallel rows of fine filaments which interlock in such way that, should the feathers be damaged, they can easily be restored by preening with the beak.
The feathers are produced from pits in the skin, much as hairs are produced in mammals. Muscles in the skin can move the feathers, fluffing them out in cold weather for example. The skin is loose and dry with few glands except for an oil-secreting gland which carries the tail feathers. The birds spread the oil from this gland over their feathers when they preen, thus increasing the water repellent properties.
Flight is of three kinds, soaring, gliding and flapping. For gliding and soaring the bird extends its wings, keeping them still except for small adjustments. In soaring, the bird is carried upwards on currents of warm air (thermals) rising from the ground. In gliding the bird slowly loses height while gaining forward momentum. These forms of flying may be combined, e.g. when sea-birds use the currents of wind blowing up a cliff face.
In flapping flight, powerful muscles depress and raise the wing rhythmically, forcing air down and backwards which gives the bird lift and forward movement. During the up-stroke the wings are flexed at the wrist and offer less air resistance than in the down-stroke where they are fully extended. The way the flight feathers overlap also helps; air pressure forces the vanes apart on the upstroke but closes them on the down-stroke.
Apart from the wings, other features help to make flight possible. The contour feathers give the bird a streamlined shape; the bones are hollow and therefore very light; parts of the skeleton are fused together making a rigid ‘box’ which resists the tendency to be squashed when the flight muscles contract; the flight muscles are very powerful and are attached to a keel-like extension of the breastbone; the bird’s raised temperature enhances the high metabolic rate which is needed to supply the energy for flapping flight.
All birds reproduce by eggs which are fertilised before laying. The male bird, after a successful courtship display, mounts the female, applies his cloaca to hers and passes sperms into her reproductive tract. As the eggs travel down the oviduct, a layer of albumen is added and finally a hard shell. The eggs are laid in a nest which may be carefully constructed from vegetation or be simply a scrape in the ground or a ledge on a cliff. The eggs are kept warm by incubation. That is, the bird covers them with her body where they are kept close to the skin. The young finally hatch out by pecking their way out of the shell.
The chicks of ground-nesting birds, including waterfowl, hatch out with a downy covering of feathers and can run about or swim in a very short time. They stay close to the parent bird who, in the case of waterfowl, may feed them or they may simply forage for food, learning what is suitable or unsuitable to eat.
The chicks which hatch in nests above ground are often without feathers and are kept warm by the adult brooding them, that is, covering them with the body, which also keeps off the rain. Both adults bring food to the nest and feed the chicks until they are old enough to leave the nest, and continue to feed them for some time afterwards.