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