Birds have bones that are very lightweight in comparison to mammals. The largest bones, such as the thigh bone, are for the most part hollow. The light bones make it easier for birds to stay in the air, but at the same time it makes the bones more brittle and easier to break. Broken bones are therefore much more common in birds than in mammals.

The breastbone of birds is very large and has a keel that sticks out quite far. The surface on both sides of the keel is covered entirely by large pectoral muscles. These are needed for the wings to move up and down and require enough power for the bird to fly. When a bird is too thin, the keel will protrude further and feel slightly sharp.

One thing all birds have in common, unlike mammals, is that they have no diaphragm. A bird's abdominal cavity is therefore one big space. It houses all the abdominal organs, such as the gastrointestinal tract, reproductive organs, kidneys and liver. This is also where the lungs are, up against the bird's back in the upper part of the torso. In addition to having lungs, birds also have air sacs. The air sacs have transparent membranes, which divide the abdominal cavity into separate compartments. These ensure that the air the bird inhales flows in one direction through the body. This is different in mammals, where the air flows in different directions when inhaling and exhaling. The air that birds breathe in travels in a circle through the different air sacs, the lungs and then back out of the body. The oxygen-rich air enters the lungs both during inhalation and exhalation, thus making the oxygen intake very efficient.

A bird inhales by moving its breastbone outwards. This provides more space in the abdominal cavity and the air sacs for fresh air to be sucked in. If a bird is held in such a way that the breastbone cannot move outwards (for instance if the body is held too tightly in the hand), the bird can suffocate. So please take extra care and make sure the breastbone can move outwards.

The first part of the gastrointestinal tract is different to that of mammals. After the food is swallowed, it passes through the gullet or oesophagus, which is located to the right of the windpipe. Many bird species have a dilatation of the oesophagus in the lower part of the neck, called a crop. Crops come in different shapes and sizes, depending on the bird species and can increase greatly in size when food enters. The function of the crop is to store and moisturise the food if the stomachs are full. When a stomach is empty, the food can also pass directly through the crop without any need for it to be stored.

After the crop is the glandular stomach. This is an organ that has many glands, which secrete digestive mucus and stomach acid. When the food has been pre-processed by the glandular stomach, it enters the gizzard or muscular stomach. This is a disk-shaped, thick-walled organ with very powerful muscles. The inside of the gizzard is covered with a hard inner lining that has a rough surface. The combination of muscle contractions and the rough inner lining helps the bird to grind the food. In addition, grit can be added to the feed. This facilitates the grinding process. The gizzard of seed-eating birds can be felt from the outside. It is located to the left of the abdominal cavity, just under the breastbone.

Unlike mammals, whose young develop inside the mother, the development of a bird's offspring takes place inside an egg.

Laying an egg is quite an achievement for a bird. Most birds only have one functional ovary and oviduct, the left one. The right ovary is inactive and underdeveloped. The first step of egg development is the ovulation. An egg cell is released from the ovary. This later turns into the embryo and yolk. The egg cell travels through the female reproductive tract where substances are added, such as the yolk membrane, egg white, egg membrane and shell membrane (this process takes 24 hours in chickens). The shell is then calcified inside the uterus (this also takes about 24 hours in chickens). The bird lays the egg by contracting the muscles of the uterus and the vagina.

The two testes of male birds are located inside the abdominal cavity against the back and cannot be seen from the outside.

Birds are very light animals for their size. This is mainly due to them having hollow bones. They therefore weigh very little. A large part of the abdominal cavity is made up of air (in the air sacs). Naturally, the feathers also weigh next to nothing. Because birds weigh so little, it has consequences for the dosage of medicine. This must be very precise to prevent the very real risk of an overdose.

List of weights of some well-known birds: