Birds do not have teeth or maxillary bones of the jaw like other vertebrates. Instead, they have a pair of mandibles covered by a horny sheath of keratin.

The absence of teeth and heavy jaw bones helps to reduce a birds’ anterior weight. If birds had teeth, they would not be able to maintain an efficient posture during flight. The digestive system of a bird is adapted to this lack of teeth. They have a second stomach known as the gizzard, which grinds food. The position and weight of the gizzard also contributes to flight, as it lowers the bird's center of gravity.