8 - From beak to cloaca

Once the food has passed through the beak, it enters the oral cavity, where the salivary glands moisten it. The tongue carries the food to the oesophagus. Digestion has not yet begun because, unlike in humans, a bird’s saliva does not contain enzymes. The food travels down the oesophagus to the stomach. It can also travel in the opposite direction, from the stomach via the oesophagus back to the beak, for example to feed the young. Indigestible remains can also be regurgitated from the stomach. This usually takes the form of a pellet. In many birds, the oesophagus has a secondary function: it is used to temporarily store food. The wall is highly elastic, allowing a considerable amount of food to be stored. This is done, for example, by gulls, ducks, owls, woodpeckers and many songbirds. Other species store food in a special pouch within their oesophagus, known as the crop. In gallinaceous birds and pigeons, the crop consists of two large pouch-like sections. It is situated where the neck meets the body. A crop acts as a passageway to the stomach. It supplies the stomach with food at a steady rate, which is important for proper digestion. Many birds experience periods during which they gather a large quantity of food in a short space of time. Afterwards, they must wait until another opportunity arises. The food can be stored in the crop for up to a day. When they eat again after a period of fasting, access to the crop is blocked so that the stomach can fill first. Only once the stomach is full is the crop used as a storage site.

Another function of the oesophagus is to store food for the young. Sometimes this is pre-digested in the crop. Many birds of prey feed their young with food that has been stored in the oesophagus for some time. The crop of pigeons also plays an important role in feeding the young. This is where pigeon milk is produced. This ‘pigeon milk’ consists of nutritious cells that are secreted through the wall of the crop. The cells are swollen and contain a lot of fat.

A bird’s stomach consists of two parts: the muscular stomach and the glandular stomach. The glandular stomach contains few muscles. In birds that eat large quantities, such as fish-eaters, it has a large capacity. The gastric glands produce two types of substance. One probably serves as protection, as it can kill all manner of invading pathogens, whilst the other contains enzymes that begin the digestion of proteins. The stomach has at least five functions:

1) It stores food. The muscles ensure that it is mixed with the digestive juices from the glandular stomach.

2) In carnivores, proteins are digested there.

3) The muscular stomach has a grinding function; that is, the food is processed so that it is suitable for passing on to the intestines.

4) It passes the food in small portions to the intestinal tract via a sphincter muscle known as the pylorus.

5) The muscular stomach acts as a filter for substances that cannot be digested.

A sturdy muscular wall is necessary for this processing (kneading). There are seed-eaters that do not first peel the seeds, such as finches. In these birds, the inside of the stomach is hard and sturdy. It contains ridges and nodules, made of keratin, which grind the food. And if this is not enough, small stones are swallowed to lend a hand. This living mill is capable of grinding even the hardest shells to a fine powder. It goes without saying that the stomach wall suffers from this mechanical labour. Every now and then, it is replaced in its entirety. The old lining is expelled from the body via the oesophagus and the beak, or via the rectum and the cloaca.

Once the food has been digested in the stomach, it passes into the intestinal tract. In a section of the intestinal tract (the duodenum and small intestine), new digestive juices are added to the food mass. Digestion continues and the digested food can be absorbed into the bloodstream through the intestinal wall. Undigested material is excreted via the rectum and the cloaca. The cloaca is a common opening for waste products from the intestinal tract, the ureter, and the reproductive organs. The waste products have had their moisture removed. The concentrated urine is often excreted at the same time. It can be recognised as a white cap of waste salts on top of the black faeces from the gut.

Birds require a great deal of energy, and their metabolism is therefore under considerable strain. Carnivorous birds digest large quantities of food at great speed. Seed-eaters have a long digestive tract and require less time to process large quantities of food. Berry-eaters break all records when it comes to rapid digestion. A Bohemian waxwing, for example, can regurgitate the stone just fifteen minutes after swallowing a berry. It almost seems as though all birds are constantly taking in food at one end whilst producing droppings at the other. Sometimes this is quite useful, for example when defending the nest or the young. Anyone who has been in a seagull colony can attest to this; intruders are treated to a massive barrage of droppings.