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