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