The digestive organs of birds are composed of two stomachs, so as to quickly convert food into energy and lose weight. Forestomach (glandular stomach) is a gland that crushes food with chemical enzymes, which can dissolve the bones of prey. Muscle stomach, commonly known as sand sac, contains a lot of coarse particles such as quartz sand, which can actually grind walnut shells into paste. The digestive system of birds includes beak, mouth, esophagus, crop, stomach, small intestine, large intestine and cloaca. Others, such as liver and pancreas, also belong to digestive organs. Birds have no lips and teeth, instead, they have horny hard beaks. The shape of the beak varies from species to species. The bird's mouth is just a passage for food and has no chewing function. The esophagus is elastic, and the lower end obviously expands to form crop, which becomes smaller or disappears for staple insects or carnivorous birds. Crops can secrete some liquids, which can soften food and promote digestion. During the incubation period, some birds can secrete milk from crops to feed their young. The stomach of birds consists of glandular stomach (forestomach) and muscular stomach (sand sac). The stomach wall of glandular stomach is very thick and can secrete a lot of digestive juice. The stomach wall of muscular stomach is composed of solid muscles, and the inner layer is attached with yellow horny membrane, which contains a certain amount of sand and stone, and has the function of grinding food. Small intestine is the main organ for birds to digest food and absorb nutrients. Carnivorous birds and insect-eating birds have short intestines, and their feces can be excreted from cloaca at any time. Birds that eat seeds and other plant feeds have long intestines and can fully digest and absorb food.