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The proventriculus is one of two parts of a bird’s stomach. This organ has a very acidic pH and is used to soften food so that it is easier for the bird to digest. Food that cannot be digested in the proventriculus is moved to the gizzard, where gravel and debris work to break food down.
There are a number of differences between how a bird eats and how mammals often eat. As birds have beaks rather than teeth, they cannot chew their food but swallow it in bite-sized pieces. Birds also eat very quickly, consuming food faster than their stomachs can handle. A bird’s digestive system is equipped to accommodate these eating habits, the proventriculus being just one of the organs in this process.
For many birds, the first organ in their digestive system is called the crop. Many, yet not all, birds have this organ, which is located in the esophagus. The crop acts as a type of food storage area. When a bird consumes more food than its stomach can digest at one time, some of this food sits in the crop. Some food is moved from the crop to the stomach when the stomach is ready to digest more food. When food leaves the crop, it enters the proventriculus.
A bird's stomach consists of two parts, the proventriculus and the gizzard. Stomach acid in the proventriculus, especially hydrochloric acid, along with mucus and the pepsin enzyme, are used to break down and soften whole pieces of food. This part of the stomach can be extremely acidic, sometimes reaching 0.2 on the pH scale. The high acidity level helps food to be digested very quickly.
After the food is softened, it moves into the second part of the stomach, called the gizzard. The gizzard works to digest any especially hard food that the proventriculus failed to sufficiently soften. This may include seeds, or in carnivorous birds, bones. The gizzard contains sand and small rocks that the bird has consumed, and these particles help to further break apart these undigested food particles.
While all birds are alike in having a proventriculus and a gizzard, these organs may vary greatly between species. If a bird’s diet consists mostly of fruit or soft flesh, the work of the proventriculus may be sufficient to break down food, and the gizzard may be very small. Birds that eat a lot of seeds or bony animals, however, may have an especially large and strong gizzard.
Very interesting! There's a lot of information here about the eating and digestion habits of birds. I've had a good vantage point to watch baby birds eating in the nest. It's really true, they eat like there's no tomorrow.
The robins, which are a very common bird in our area, eat a lot of worms and bugs. Their digestion in the proventriculus must be easily done, so their gizzard is probably quite small.
It's surprising that some birds eat sand, and small rocks that are used in the gizzard to help digest the undigested food. I don't know what happens to the sand and rocks after that. Do they sit there for the next meal or are they eliminated?