Digestive system of birds
Compared to terrestrial animals (read more), birds have a different digestive system. Throughout evolution, their bodies have adapted to be as light as possible as this helps with flying. An explanation of the general digestive system of birds is given below.
First of all, birds take in food using their beaks. The beak is adapted to the type of food that is mainly eaten (see picture). Inside the beak, the food is partially ground or swallowed whole. There are also birds that drop their food from the air or smash it against a hard surface to make it easier to swallow. The tongue ensures that the food moves towards the esophagus, after which the food enters the crop.
Adapted from: https://en.wikipedia.org/wiki/Beak (not to scale)
The crop has several functions. As many birds naturally eat as much as possible per feeding moment, the food is stored in the crop. The food is stored and gradually sent to the stomach. In addition, the liquids in the crop ensure soaking of the food and a 'pre-fermentation', which improves the digestibility. Adult birds feed their young with food that has soaked in the crop, so that it is easier to digest. After the crop, the food goes to the stomach.
Birds have two stomach compartments: the proventriculus and gizzard. First the food enters the proventriculus, which is similar to the stomach of many mammals. In the proventriculus, mucus, hydrochloric acid and pepsinogen are released into the food mash. These substances aid in the digestion of food. In carnivorous birds, this part of the stomach is the most developed because animal material such as bone has to be broken down here. The food slurry then ends up in the gizzard. The gizzard consists of a thick muscle wall that slides past each other. By means of sliding and using grit, food is ground (<1mm) so that the nutrients can be absorbed in the intestines. Grit can be classified into insoluble (stomach grit) and soluble. Insoluble grit is not digestible and therefore has a grinding function in the gizzard. In contrast, soluble grit is digestible and does not function as a grinding component but as a mineral (mainly calcium). Soluble grit is especially important for birds that have a higher calcium requirement, such as laying birds and growing birds. Read here how grit is processed in the diet of animals in captivity.
When comparing the small intestine, there are little differences compared to terrestrial animals. The functions are also the same: hydrolysing and absorbing nutrients. The large intestine is where the difference really shows again; in birds this consists of three parts: caeca, colon (rectum) and cloaca (see picture). The caeca plays a role in fermenting fiber and absorbing nutrients and water. Only dissolved fibers can end up in the caeca and thus be fermented. Insoluble fiber is largely excreted. The colon is mainly for resorption and transport to the cloaca. Finally, the cloaca provides storage of faeces and extra resorption.
Then there is another important principle in the digestion of birds: reflux. This is a mechanism that enables food material to be 'pushed' back to a previous compartment of the digestive system. The function of reflux is that nutrients are better absorbed. This is because the time in the digestive system is longer. This leaves more time for e.g. absorption, mixing and grinding. In addition, fibers can be pushed back into the caeca to still be fermented. Reflux can occur in the following places:
Gizzard > proventriculus
End of small intestine > beginning of small intestine
Colon > ceaca
Compared to land animals, birds have a different type of digestive system. Next to this, also between birds significant differences can be seen in digestive system as they also adapted to their environment and diet. For this reason, birds can also be classified into a category such as herbivore, carnivore or omnivore (read more here).