Digestive System
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 esophagus is 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.
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