Birds

Bird are flaying animals. That it the single most important fact about them. Bird are not the only animals with the power of flight insect and bats fly; so did certain ancient reptiles. But birds are the only really numerous flaying animals that have backbones. There are about 8,600 species of bird, but fewer than 1,000 of bats. The flying reptiles have long been extinct. And insects do not have backbones.
Flight is a very difficult activity for a living thing. The strains and streses on the bones and muscles of a flying animal are tremendous. Its senses, such as those of balance and sight, must be of a special sort. Flying demands a much greater supply of energy than do most forms of getting about. Therefore birds have very special features and abilities.
For instance, only a very light animal can fly, but its supporting framework must be very strong. A bird skeleton is a simple, stiff frame work that is both light and strong.
It is light because most of the bones are thin plates or hollow, air-filled tubes and struts.
It is strong because many of these bones have special shapes; the shapes give the bones their strength, rather than the weight or thickness of the bones.
A flying animals also needs strong but flexible surfaces for wings. Feather fit this need perfectly. Overlapped like shingles, they also serve as the warmest yet lightest of insulators for these warm-blooded, flying animals. Fetahers are found only in the group of animals we call birds the class Aves.
The forelimbs (or arms) of bird serve as wings. This means that they are of little use for anything except flying. So birds must walk on two legs. They must have very flexible necks and strong beaks to care for yopung, as weapons of defense, and for many other jobs tha non flying animals do with the paws, claws, or hands on their forelimbs.