The gizzards of birds act like a gastric mill, grinding up food particles. The muscular walls of the gizzard are lined with a tough cuticle that helps to break down the food. Birds swallow small stones, called gastroliths or grit, which help to grind up the food in the gizzard. As the gizzard contracts, the stones rub against each other and against the food particles, breaking them down into smaller pieces.
Proventriculus
The proventriculus is a glandular stomach that secretes digestive enzymes and acids to begin the process of breaking down food. The enzymes help to break down the proteins and carbohydrates in the food, while the acids help to kill any bacteria that may be present.
Crop
The crop is a storage pouch that is located at the base of the neck. Birds store food in the crop until it is ready to be digested. The crop also helps to moisten the food and break it down into smaller pieces.
Esophagus
The esophagus is a muscular tube that carries food from the mouth to the crop. The esophagus also helps to move food from the crop to the proventriculus and gizzard.
The process of digestion in birds
1. Birds swallow food whole.
2. The food is stored in the crop.
3. The food is moistened and broken down into smaller pieces in the crop.
4. The food moves from the crop to the proventriculus.
5. The proventriculus secretes digestive enzymes and acids to begin the process of breaking down food.
6. The food moves from the proventriculus to the gizzard.
7. The gizzard contracts and the stones rub against each other and against the food particles, breaking them down into smaller pieces.
8. The digested food moves from the gizzard to the intestines.
9. The nutrients in the food are absorbed into the bloodstream through the walls of the intestines.
10. The undigested food is eliminated as waste through the cloaca.