Birds often swallow gravel or other hard particles that sit in the gizzard and aid the grinding process. The gizzard can be thought of as the bird equivalent of 'teeth'. Food then passes to the small intestine and rectum, and between these two regions are two structures called caeca. The caeca hold bacteria which further aid digestion. The rectum leads to the cloaca, the common exit for waste materials and sperm or eggs.
Birds handle excretory material in a similar way to reptiles.
This paste is largely made of a chemical called uric acid. The skin of birds is covered in feathers - all birds have feathers. Feathers are formed in special cells in the bird's skin from a chemical called keratin, a chemical which also produces the scales on the skin of reptiles. Feathers provide a variety of functions. They help maintain the body temperature, they provide an aerodynamic, streamlined surface for flying, and movement of the feathers provides control for movements in the air, as well as for braking and landing.