Birds have often been called "glorified reptiles". Although the birds are grouped as a separate vertebrate class "Aves", apart from the power of flight and features which are connected with it, they are structurally similar to reptiles. In many regards they are no feather removed from the general reptilian stock, than are some of the ruling reptiles from which they evolved from.
The defining characteristics of birds, that people think of first, are their feathers and beak. All modern birds have beaks and feathers.
The function of feathers are:
body temperature control
attracting their mates
The tail of a bird is a bony structure, which
is very short compared to many reptiles.On
the tail is a spreading fan of stout feathers, which forms a very efficient
insulation for the control of birds' body heat. As you can see, feathers
are a very important structure for the bird, weather the bird flies or
not. Feathers are, in reality almost the only distinctive feature of the
class, for almost every other characteristic can be matched in some archosaurian
group. Furthermore, the feathery covering of birds is, in contrast, not
so different than the horny scales which normally cover a reptile body.
Feathers are comparable to such scales, with the exception of the complexity
of the structure of barbs and barbells instead of the simple scale shape.
The reason for any change in bodily structure and function in evolutionary
terms for the birds, has been directly related to the ability to fly and
fly more effectively. We must make note; that during any evolutionary adaptation,
the organism uses what it had to adapt. By that rationale, feathers were
not originally evolved for the function of flying, but rather to control
body temperature and/or the animal's adornment for attracting mates.