Biology - Birds, An Introduction

Birds evolved from reptilian ancestors in thebe combined, e.g. when sea-birds use the currents
Jurassic period, 200 million years ago. At the sameof wind blowing up a cliff face.
time, mammals were evolving from a differentIn flapping flight, powerful muscles depress and
line of reptilians.raise the wing rhythmically, forcing air down and
The most characteristic feature of birds is thebackwards which gives the bird lift and forward
possession of feathers. Because the bird-likemovement. During the up-stroke the wings are
reptiles of the Jurassic were becomingflexed at the wrist and offer less air resistance
warm-blooded, the principle advantage of thethan in the down-stroke where they are fully
feathery covering was probably the reduction ofextended. The way the flight feathers overlap
heat loss from a warm body. Modern birds havealso helps; air pressure forces the vanes apart on
temperatures of about 40-41 degrees C whichthe upstroke but closes them on the
contributes to the high rate of metabolismdown-stroke.
necessary for the muscular activity involved inApart from the wings, other features help to
flight.make flight possible. The contour feathers give
The upper and lower mandibles of a bird arethe bird a streamlined shape; the bones are hollow
extended to form a beak. The shape of the beakand therefore very light; parts of the skeleton
varies with the species. There are long, narroware fused together making a rigid 'box' which
beaks which penetrate into flowers for theirresists the tendency to be squashed when the
nectar; short stout beaks which crack openflight muscles contract; the flight muscles are
seeds; sharp, hooked beaks which tear flesh fromvery powerful and are attached to a keel-like
prey.extension of the breastbone; the bird's raised
There are two layers of feathers covering thetemperature enhances the high metabolic rate
body. The down feathers are fluffy and form anwhich is needed to supply the energy for flapping
insulating layer close to the body. The contourflight.
feathers are flatter and broader and make aAll birds reproduce by eggs which are fertilised
waterproof layer as well as giving the bird itsbefore laying. The male bird, after a successful
characteristic shape The power of flight becamecourtship display, mounts the female, applies his
possible as the skeleton of the fore-limbs becamecloaca to hers and passes sperms into her
modified to form wings and the flight feathersreproductive tract. As the eggs travel down the
developed. The flight feathers on the wings haveoviduct, a layer of albumen is added and finally a
long shafts and flat vanes on each side of thehard shell. The eggs are laid in a nest which may
shaft. The vanes are formed from parallel rowsbe 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 easilyThe 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. Musclesout by pecking their way out of the shell.
in the skin can move the feathers, fluffing themThe chicks of ground-nesting birds, including
out in cold weather for example. The skin is loosewaterfowl, hatch out with a downy covering of
and dry with few glands except for anfeathers 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 theirwho, in the case of waterfowl, may feed them
feathers when they preen, thus increasing theor 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. Inthe adult brooding them, that is, covering them
soaring, the bird is carried upwards on currents ofwith the body, which also keeps off the rain. Both
warm air (thermals) rising from the ground. Inadults bring food to the nest and feed the chicks
gliding the bird slowly loses height while gaininguntil they are old enough to leave the nest, and
forward momentum. These forms of flying maycontinue to feed them for some time afterwards.