Digestion in birds

The anatomy of the digestive system of birds is inevitably quite varied depending on the species, as different species have different diets (for example, they can be carnivores, seed-eaters or insectivores). However, the main structures are similar for most species, so this article is about the general structure of a bird’s digestive system. Some parts of the system are similar to parts of the human digestive system, but there are also other structures such as a crop and gizzard in most birds which humans don’t have. Whereas humans usually only convert 10% of ingested food into energy, birds are able to convert around 30%, as they require a large amount of energy, especially to sustain flight.

Mouth

Digestion in birds begins at the mouth, where birds do not have teeth but have jaws covered by a beak (made mainly of keratin). A few species of birds (specifically sawbill ducks such as Goosanders) have teeth-like serrations along the edges of their beak, but these are mainly used for holding onto their prey (fish), rather than for helping with digestion. Whereas in mammals the tongue is made up of muscle layers, a bird’s tongue contains 5 bones which collectively make up the Hyoid Apparatus. This could be due to the fact that, while mammals use their tongues for vocalisations, birds only use theirs for moving food around. Like humans, most birds have salivary glands which secrete enzymes to start the process of digestion. The extent to which the salivary glands are developed depends on the species’ diet: for example, they are more developed in grain-eating birds than fish- or meat-eaters, as grains may be harder to digest.

Oesophagus/crop

From the mouth, the food (which has now formed a bolus when mixed with saliva) moves down the oesophagus by peristalsis (contraction and relaxation of muscles). This is also aided by the secretion of mucus from glands in the oesophagus, which helps with lubrication. Many birds (but not all) have a crop - a pouch which is an enlargement of the oesophagus. Food can be diverted into the crop to be temporarily stored, depending on how much food is in the bird’s stomach at that point. Often, the crop also contains liquids which enable ‘pre-fermentation’ of food to occur, which makes it easier to digest later. This enables some species to make the most of feeding opportunities when they arise, and gradually send food to the stomach afterwards. Adult birds also feed their offspring from food in their crop, because it is easier for the young birds to digest.

Proventriculus/gizzard

The stomach of birds has two parts: a glandular stomach, known as the proventriculus, and a muscular stomach, known as the gizzard. The bolus reaches the proventriculus first, but it is then moved between the proventriculus and the gizzard. The proventriculus is fairly similar to the human stomach, by having a low pH and secreting mucus, hydrochloric acid and pepsinogen to break down proteins (when it is converted to pepsin by the acid). In some species, the acid is very strong, such as in Bearded Vultures, where it is needed to dissolve bones.

The gizzard, however, is very different, and is used to physically grind and break down the food. It has muscular walls lined with a thick protein cuticle (made of koilin) to protect it from acid or enzymes from the proventriculus, and from injury from the food itself or grit. Many birds swallow small insoluble stones or grit (called gastroliths), which help to grind the food in the gizzard, essentially acting like teeth chewing food. Over time, these may be worn down until they are smooth and rounded, at which point they may be replaced. Some species, such as owls (and even much smaller birds such as robins!), also produce pellets in the gizzard, which are made up of undigested food (e.g. fur, bones or parts of insects). These are then regurgitated through the beak.

Small and large intestines

Following digestion in the stomach, the bolus passes into the small intestine. Like a mammalian small intestine, this has three parts: the duodenum, jejunum and ileum. This is where further digestion and absorption of food occurs, as bile and digestive enzymes are secreted into the small intestine from the liver and pancreas.

The large intestine of birds also has three parts: the caeca, colon and cloaca. There are two caeca, which are the site of fermentation of soluble fibre, so complex carbohydrates can be broken down. Insoluble fibre is usually excreted. Any food left is then transported through the colon to the cloaca, which is where waste products are excreted. The cloaca is the opening for the digestive, reproductive and urinary systems, which is different to the human digestive system, where there are separate openings for different organ systems.

Summary

A significant difference between the digestive system of humans and birds is that, in birds, food material can be pushed back to a section of the digestive system by ‘reflux’, meaning that it can spend more time in a specific compartment, allowing for more absorption of nutrients. For example, food can be pushed back into the proventriculus from the gizzard, allowing for further chemical breakdown. Other than this, the main differences are the presence of the crop, gizzard and cloaca, which are not present in humans, and the absence of teeth in birds. The functions of the other sections, however, are relatively similar in humans and birds.

To summarise, the diagram below shows the pathway of food through the digestive system of a bird.

Mouth (beak)

Colon

Oesophagus

Caeca

Cloaca