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.
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:
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).
First of all, birds take in food using their beaks. The beak is tailored to the primary food consumed (see picture). The food is either fully swallowed or partially ground inside the beak. Some birds also smash their food against a hard surface or drop it from the air to make it easier to swallow. The food enters the crop after passing through the esophagus, which is guided by the tongue.
The crop has several functions. Food is stored in the crop because many birds instinctively consume as much as they can during a feeding moment. The food is stored and gradually sent to the stomach. Furthermore, the liquids in the crop guarantee that the food is soaked and undergoes a pre-fermentation, thereby enhancing its digestibility. To make their food easier to digest, adult birds soak it in the crop before feeding it to their young. After the crop, the food goes to the stomach.
Birds have two stomach compartments: the proventriculus and gizzard. The proventriculus, which resembles the stomach of many mammals, is where food first enters. Mucus, hydrochloric acid, and pepsinogen are released into the food mash in the proventriculus. These substances aid in the digestion of food. This is the most developed area of the stomach in carnivorous birds because it is where animal material, like bone, must be broken down. The food slurry then ends up in the gizzard. Thick muscle walls that slide past one another make up the gizzard. Food is ground using grit and sliding techniques. Grit can be classified into insoluble (stomach grit) and soluble. Because insoluble grit cannot be digested, it serves as a grinding agent in the gizzard. Soluble grit, on the other hand, is soluble and acts more like a mineral (primarily calcium) than a grinding element. For birds with a higher calcium requirement, like laying and growing birds, soluble grit is especially crucial. Check out this article to learn how animals kept in captivity metabolize grit.
There aren’t many differences between terrestrial animals and their small intestines. The functions are also the same: hydrolysing and absorbing nutrients. The real distinction is found in the large intestine, which is divided into three sections in birds: the caeca, colon (rectum), and cloaca (see illustration). The caeca is involved in the fermentation of fiber and the absorption of water and nutrients. Only dissolved fibers have the ability to enter the caeca and undergo fermentation. Insoluble fiber is largely excreted. For resorption and transport to the cloaca, the colon is primarily used. Finally, the cloaca provides storage of faeces and extra resorption.
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The two stomachs contract in a complicated cycle that grinds food and increases exposure to digestive enzymes as it passes between them. Periodically, the contents of the duodenum are also retropelled back into the stomachs, which is thought to help in the mixing of food with enzymes. Raptors regurgitate pellets of hair, feathers, and bones from their stomachs as a last form of motility.
Especially in birds that swallow large meals, the esophagus has a large diameter. Esophageal peristalsis, which is responsible for swallowing, appears to be facilitated by neck extension in the majority of birds. The majority of birds, though not all of them, have a crop, which can range from one or two esophageal pouches to just an esophageal expansion. Food is pushed into the crop by waves of peristalsis in the crop, which subsequently propels it into the stomach based on the degree of stomach contraction.
The small intestines of birds and mammals appear to be quite similar. There is a duodenum, jejunum, and ileum, but they are not as histologically distinct as the segments found in mammals. The liver provides bile to the proximal small intestine, while the pancreas supplies digestive enzymes. The absorptive epithelial cells in this intestine are embellished with a similar array of enzymes and transporters found in mammals.
There are usually two ceca and a short colon that make up the large intestine. Unlike those found in mammals, the colon’s lumen is filled with short villi. The common entrance of the digestive, reproductive, and urinary systems is the enlarged, tubular cloaca, which opens as the vent to the exterior of the bird.
The large intestine’s main job is to absorb water and electrolytes, just like in mammals. The avian colon exhibits a notable motility pattern called antiperistalsis, which is thought to help empty the ceca and move urine from the cloaca into the large intestine for water absorption. In certain birds, the ceca seem unnecessary and can be removed without causing any harm. The ceca are crucial locations for fermentation in other species, and the energy requirements are greatly increased by the volatile fatty acids produced by the microbial digestion of cellulose.