Birds do not have teeth. Without teeth, a bird cannot chew its food down to bits in its mouth like humans do. As detailed in the textbook Ornithology by Frank B. Gill, birds must instead rely on the muscular stomach-like pouch called the gizzard to crush down their food. Many species swallow stones and grit to aid in digestion. These stones remain in the gizzard and crush the food as the gizzard contracts. From a functional perspective, gizzard stones in birds are the equivalent of teeth in humans. When a gizzard stone becomes too smooth to do its job, the bird regurgitates the stone or passes the stone down and out through its digestive tract. Species that do not swallow stones are able to use the sheer muscle power of their gizzards to grind down their food. Birds do not have teeth because the teeth and the jaw bone to support them are too heavy for efficient flight. Many birds have a series of notches in their beak or spikes on the inside of their beak or tongue. These notches and spikes are not true teeth as they are not used to crush down their food. Instead, these features are used to get a good grip on the prey so that it does not escape. In ancient evolutionary history, there were birds with true teeth. Known as odontornithes, these animals are no longer alive today.