The digestive system of birds is complex for the size of most birds. A small bird can eat up to twenty percent of its body weight daily. Birds have an extremely high metabolism so to keep up with the requirements bird must eat a large amount of food.
The digestive system of the bird begins with the beak and tongue. Evolution has eliminated the teeth in birds. Birds have a very strong beak. The beak and the tongue are modified according to the diet and the environment of the bird. The curved beak and thick tongue of a macaw are well adapted to breaking large nuts and climbing. The long curved beak of a hummingbird is suitable for drinking nectar. The beak grows constantly and must be worn down by climbing and cracking nuts. When a bird eats it uses its beak and tongue to gather food. Since a bird has no teeth, chewing time is eliminated. Since a bird does not have to chew its food they can often eat a large quantity of food at one time. Saliva lubricates the food much like in a human so that it can pass to the esophagus. The oesophagusis a tube-like structure which passes food in waves better known as peristalsis, to an organ known as the crop.
The crop separates birds from many animals because it is an organ that is exclusive to these animals. A crop’s main function is to store food. It is like an extra fuel tank for the bird. The crop is located at the base of the neck and can be easily seen after a bird has just eaten. If you have a pet bird a sure way to tell if a bird is full is to look at the size of the crop. If the crop is large in size the bird is probably full. If the crop is deflated and has a flat appearance the bird has not eaten yet. The crop continuously supplies small amounts of the food to the stomach.
The food passes from the crop to the stomach, the most active part of the digestive system of a bird. There are two parts to the stomach of a bird. The first part is known as the proventriculus. This is the glandular portion of the stomach. This portion secretes digestive juices which break down the food. In a popular breed of pet bird, the budgie stomach can produce what is known as crop milk which the budgies can feed to their young. The proventriculus joins a large muscular portion of the stomach known as the ventriculus, or more commonly known as the gizzard.
The gizzard grinds up food even more. The gizzard contains gravel, or girt, which works alongside with muscles in grinding up food. Some birds have gizzards that are so powerful they could grind up needles of steel in a matter of hours. In birds that ingest whole seeds a gizzard is very important. In these birds digestive enzymes alone cannot effectively break the seed hull. This is where the gizzard comes in to help. These birds require a large amount of grit in their diet. Larger birds such as the parrot that tend to de-hull their seeds do not require such a continuous supply of grit in their diets.
After leaving the gizzard the food is passed on to the small intestine where it mixes with bile and enzymes. The enzymes help with the breakdown of sugars, fats and proteins. Bile from the liver breaks down the larger fat molecules. The nutrients are then absorbed and passed on to the blood stream.
The liver of the bird has two equal lobes and is nestled next to the heart under the rib cage. Like in humans, the liver acts as a detoxifier, purifying toxins that enter the bloodstream and it recycles red blood cells some which are used to create bile. In some birds the bile is stored in the gallbladder. In birds such as the budgie, and some members of the parrot family lack a gallbladder.
The pancreas in birds is located near the small intestine. The first function of the pancreas is to neutralize acids that are found in the mixture passed on from the stomach. If this does not occur serious damage could occur to the intestine of the bird. Another major function of the pancreas is to produce insulin so that all the bird’s cells are supplied with glucose.
Some birds have an appendix or cecum which helps in the digestion of grains and fibers. This is not present is some members of the parrot family.
Whatever does not get digested is passed through the single opening in the urogenital system known as the cloaca or vent. The waste is excreted in the form of bird droppings. Birds tend to make droppings often because they have such a high metabolism and eat often. If your bird is a seed eater its droppings should have a firm greenish or brown part, this is the feces, and the dropping should have a white pasty part, this is the urine. A normal healthy bird should have anywhere from 25 to 50 eliminations a day. There are several reasons why a bird can have loose droppings. It is recommended that you should never drastically change your bird’s diet. If this has occurred it might cause diarrhea in your bird and should be taken to the vet immediately. Stress can also cause this condition in birds. It is rare for a bird to have constipation and if you find this happening in your pet bird you should consult a veterinarian.
A bird’s digestive system is extremely efficient because it has to be to keep up with the metabolic reactions the bird has. Birds that are fruit eaters can digest berries in thirty minutes, and seed eaters usually digest their food within three hours.
In my next post i will be posting about the digestive system of the insects.