All those that fly are not birds. Some creatures can fly though they are not actual birds. They have backbones. True birds do not have them; they are invertebrates. But among the flying animals, actual birds vastly outnumber those with spines. Bats are not real birds.
We must also note that some actual birds cannot fly. Such birds had existed long before the appearance of man on earth. Ostrich is a bird which cannot fly.
With such exceptions, all birds can fly to some extent. Some may fly only a little distance, while others fly far; some may fly faster than others. The Robin can fly about 30 miles an hour, and the Chimney Swift attains a rate of over 60 miles!
We know that wings enable birds to fly. The wings of birds have strong muscles. These muscles are fastened to the keel, a projection on the breast bone. While passing, the bird beats the air and propels itself forward.
The size for size birds is lighter than animals. One reason is that the bird’s bones are hollow and air-filled. Besides, the bird’s body contains innumerable air sacks connected to the lungs. For these two reasons, a bird is lighter than an animal of its size. Being light helps the bird to fly quickly, and the bodies of birds are streamlined-another fact which allows them to fly in comfort.
Bodies of birds are covered with feathers. These feathers cover the bird’s body. The feathers are slightly oily and can keep the rain off the bird’s body.
Feathers serve another purpose. Birds are warm-blooded creatures, and the heat of their bodies should be preserved. Feathers help the birds to maintain their warmth.
All birds come out of eggs. The shell of a bird’s egg is hard. The size of the egg varies from one kind of bird to another. While the egg of an ostrich is 6 inches long, that of the sparrow is only as big as a pea.
Birds lay their eggs in nests. Each kind of bird builds a different type of nest. Some of the nests are very cleverly made. But birds do not learn the art of making nests; they inherit the capacity as they inherit the colours of their feathers. An experienced man can see a nest and tell which bird has built it. Though birds generally build their nests in trees, some build them in human dwellings and other places.
Warmth is required to hatch the eggs. To supply this warmth, the mother bird sits on her eggs. Though in most cases, this work is done by the mother, there are instances where the father takes turns and relieves the mother.
As a rule, the male bird is more colourful and attractive than the female. Perhaps it is a provision of nature. The less bright female attracts the enemy less while she sits over the eggs in the nest. If the more colourful male were to sit over the eggs, the danger from enemies would be more significant.
Probably for the same reason, male and female baby birds are colourless until they grow up; the male bird acquires the colours when he can care for himself.
There are about 14,000 kinds of birds in all. Scientists have divided them into more than 20 orders. One of these orders is called “the perching birds.” Most singing birds, like the cuckoo, the nightingale and others, belong to this order.
Birds vary significantly in the weight of their bodies. The smallest bird, the tiny hummingbird, weighs less than a penny, while the ostrich may weigh 300 pounds.
Such differences can be noticed in the birds’ legs, beaks, feathers, and other parts. These vary according to the food of the bird and the nature of the place where it is found. That is why some birds have long beaks while others have short ones. Some have very long legs, others short, while some have webbed feet. Some birds have sharp claws. In each case, the limb is best suited to get at the food which the bird lives upon.