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Birds evolved over millions of years from reptiles, with
which they share many common features to this day. Birds have
two pairs of limbs, the front pair being in the form of wings
which are usually used for flying. The breastbone is well
developed and anchors the powerful muscles, which raise and
lower the wing.
The bird skeleton is not only strong but also light, because
most of the bones are hollow and filled with air. Birds' feathers
are also very light and arranged to form an impermeable surface
to trap warm air. Birds see far better than other animals.
The eye is large and focuses not only by means of muscles
squeezing the lens as in the case of mammals, but also by
flattening or bulging. They have little sense of taste, and
the sense of smell is underdeveloped as well.
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