Birds with "false teeth"

Birds don't have any teeth in their mouth, so how to chew their food.?

Well, birds have their own mechanism for grinding the food for them. It is a sort of a set of false teeth (dentures) that birds have. On regular intervals of time, birds swallow some pebbles and gravels that go and settle in their stomach. It's these pebbles that help the flying creatures grind their food with a vigorous agitation of the stomach. This agitation leads to grinding of the food particles in the bird's stomach as the food passes through the digestive system.