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Below the breast lies the belly, extending, as in quadrupeds and
in man, to the vent and to the place where the legs are jointed to the
trunk.
Such, then, are the parts which lie between the wings and the
legs. Birds like all other animals, whether produced viviparously or
from eggs, have an umbilicus during their development, but, when the
bird has attained to fuller growth, no signs of this remain visible.
The cause of this is plainly to be seen during the process of
development; for in birds the umbilical cord unites with the
intestine, and is not a portion of the vascular system, as is the case
in viviparous animals.
Some birds, again, are well adapted for flight, their wings being
large and strong. Such, for instance, are those that have talons and
live on flesh. For their mode of life renders the power of flight a
necessity, and it is on this account that their feathers are so
abundant and their wings so large. Besides these, however, there are
also other genera of birds that can fly well; all those, namely,
that depend on speed for security, or that are of migratory habits. On
the other hand, some kinds of birds have heavy bodies and are not
constructed for flight. These are birds that are frugivorous and
live on the ground, or that are able to swim and get their living in
watery places. In those that have talons the body, without the
wings, is small; for the nutriment is consumed in the production of
these wings, and of the weapons and defensive appliances; whereas in
birds that are not made for flight the contrary obtains, and the
body is bulky and so of heavy weight
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