Beak, Mouth, Tongue and Pharynx

The avian gastrointestinal tract (GIT) has undergone a multitude of changes during evolution to become a unique anatomical and physiological structure when compared to other animal orders. On the one hand it has evolved to take advantage of the physical and chemical characteristics of a wide variety of food types. On the other hand, it has had to do so within the limitations of the requirements for flight. To this end, birds have evolved a lightweight beak and muscular ventriculus, which replaces the heavy bone, muscular and dental structure characteristic of reptiles and mammals. The ventriculus and small intestine are the heaviest structures within the gastrointestinal tract and are located near the bird’s centre of gravity within the abdomen. The overall length of the GIT is also less than that of a comparable mammal, another weight-saving flight adaptation. Interestingly, these characteristics are still shared with the flightless species such as ratites and penguins. In addition, the actual digestive process needs to be rapid to support the high metabolic rate typical of flighted birds.