Everyone knows, birds lay eggs -- specifically, they lay very typical amniote eggs with calcified shells. Birds brood their eggs until hatching. Some bird species are naked and helpless at birth, and must be fed by their parents; these birds are said to be altricial. Many common songbirds, such as the American robin, are altricial species.
Precocial birds, on the other hand, are born feathered, and are able to walk and to feed on their own shortly after hatching -- ducks and chickens are well-known precocial birds. Whether precocial or altricial, birds grow rapidly, reaching adult size within one year.
Feathers are one of nature's finest achievements. They can be extremely strong for flight but must be extremely light and very flexible. If feathers were too heavy, a bird would never become airborne. Feathers are used by birds to fly, keep warm, provide camouflage from predators and for identification and attraction of mates.
Looking at a bird you can see that the various types of feathers are arranged over the bird's body in an extremely ordered fashion. For flying, the feathers on the front leading edge of the wing are very streamlined. The outer trailing edge of the wing may be fringed to break up the flow of the air, as well as to silence the flight. Feathers overlap each other, usually covering most of a bird's skin. This overlapping curved and streamlined shape is perfect for flight. And, the shape of each type of feather is different. The down is fluffy while the flight feather is smooth and straight, perhaps with just a slight arc. A flight feather will have a narrow leading edge that the wind hits first with a wider trailing edge. Flightless birds will have almost symmetrical sides on corresponding feathers.
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