The structure of a bird is very similar to that of a mammal. Birds differ from mammals in that their front limbs are modified as wings, and they possess feathers instead of hair. Birds do not have teeth but use their horny beak to cut, crush and tear their food, although much of it is swallowed whole. They use a muscular organ called the gizzard to grind their food after swallowing it. Many birds swallow stones which lodge in their gizzard and help this grinding process. Birds’ wings are based on heavily adapted forelimbs. On each wing a very small thumb is hidden beneath the feathers. The other fingers are fused together to provide a firm support for the wing.
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