Aves — built for air

This is the class of birds, very well adapted to flight.

Ostrich (largest bird) Hummingbird (smallest bird) Jungle fowl Penguin

Figure to be added

Figure 13.31 — several species of Aves: jungle fowl, ostrich and penguins.

They bear a very light bony internal skeleton.
The body takes a streamlined shape so as to move easily through the air.
They bear a skin covered with feathers. Scales are present but confined to the hind limbs.
Pentadactyl limbs are present for movement. The fore limbs have become wings.
They have eyes with eyelids and keen sight.
No teeth are present in the mouth. There is a beak shaped to the pattern of feeding.
The heart has four chambers — two atria and two ventricles.
They are homoiothermic — body temperature does not change with the environment.

All of it is about weight

Light bones, a beak instead of teeth, a streamlined shape — three characteristics solving one problem. Teeth are heavy; a beak is not.