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In-Depth Information
undergrowth. There are birds that have webbed feet to assist in swimming; others
have large, splayed feet that enable them to spread their weight over a large area
so that they can walk over soft ground or across beds of delicate lily pads.
The Bird Skeleton
Although the size and shape of different species of birds may vary a great deal
from each other, birds, unlike mammals, are in a very general sense all very
similar to each other in their basic skeletal structure. Weight is a major factor in
most birds, and in most cases the bird skeleton reflects the need to keep weight
as low as possible. The bone structure of birds differs from bone structure of
mammals; birds have far fewer bones and although, as in mammals, the bones
are very strong, enabling them to deal with the stresses and rigors of flight, they
are much lighter than mammalian bones. Birds' bones are honeycombed and
filled with air cavities that make them less dense while retaining great strength.
The demands of flight have resulted in the development of large muscles in the
bird's breast that provide power to the wings. This in turn has resulted in the
development of a very pronounced keel on the breastbone that anchors these
muscles to the skeletal frame. The feathers that cover the bird are also very light,
making birds much lighter than mammals of a comparative size.
FIG 4.36
Bird skeletal structure.
There are exceptions to all cases, however. A kiwi's bones, for example, have
adapted to the bird's flightless lifestyle. No longer restricted by weight,
the kiwi's skeleton has lost the keel on the breastbone that in other birds
