As promised, I am starting a description of the different systems inside the modern chicken. I thought I’d start with a description of the intestinal tract of a hen. From beak to butt, there are a lot of specializations that allow chickens to magically convert grubs, worms, corn, grains and calcium into eggs…one of the most nutritionally dense foodstuffs in nature.
First off, it is always amazing to me that people are surprised to find that chickens have triangular tongues. What other shape could it be? There is also a slit (called a choanal slit) in the roof of the top beak. It is here that many substances that are eaten and drank are exposed to the immune system. The triangular tongue is very poor at sensing taste. The human tongue has about 10,000 taste buds, cows have about 20,000, and chickens have 25. Not 25,000….25. Chickens, like most birds, rely heavily on their eyes to locate food, and don’t invest a lot of brain space to taste. It explains some of the things they are willing to eat!! You will also notice the scarcity of teeth. (Yes, your grandmother was right….hens teeth ARE rare). More on this later.
After the food passes through the beak mostly untasted, it passes through the esophagus, a 3″ muscular tube, until it is deposited into the crop. The crop is basically a holding tank for food and water. It is muscular, but weak. If a
chicken eats things that are difficult to pass through the crop (like long blades of grass or hay), or the stuff in the crop gets too dried out, you get impacted crops. If the contents stay there too long and start to rot, sour crop can result. To help with either of these problems, it is important first to get the offending material out of the crop, and allow the muscles to relax, and the interior of the crop to become normal.
The the food is now the consistency of gruel and enters the proventriculus. This stomach is similar to ours. It secretes acids and enzymes and starts actively breaking down the food. The food now enters the gizzard, which is a very muscular organ that is lined with a tough substance called koilin. Inside the gizzard, the hen stores small pebbles. These pebbles get rolled around inside the gizzard through the muscular contractions, and the action is like a grist mill. Remember the part about the lack of teeth? This is the compensation. Stones are lighter than teeth, because you don’t need too many of them, since the feed is already pre-softened by soaking in the crop. In professional farms, the stones are provided as pieces of oyster shell or limestone, both of which also act as a source of calcium for egg-shell and bone strength.
Once through the gizzard, the gut contents are like a very moist paste, very uniform, and still smelling like chicken feed. It enters the small intestine, where the pancreas and the gall bladder add their contributions of enzymes and bile acids respectively. The food is now basically reduced into its components. For the next 30 cm or so, the protein, fats, sugars and other nutrients are absorbed into the bloodstream. This is the function of the small intestine. You can imagine, if there is some problem with the intestine, absorption will be reduced, and the volume of manure will increase…..this is the root cause of many cases of diarrhea. The gut contents are now a darker brown, and more dense and pasty…pretty much half-way between feed and poo.
The food now enters the two cecums (called cecae when plura….great word to remember for Scrabble). The primary functions of the cecae are to absorb
water and to act as an immune organ. The contents of these are fairly light….lighter than normal chicken poo. Every couple of days, the cecae empty themselves, resulting in a cecal dropping, which is sticky, somewhat foamy, and lighter in colour. These droppings are very distinct, and completely normal. It also explains why anything that stimulates the immune system can result in diarrhea, since the same organ is responsible for water resorption from the gut contents.The final step in this process is the ejection of the manure (sometimes with a fair bit of velocity!). The interesting part of this is that birds have “vents”. That means that the urinary tract, reproductive tract and gastrointestinal tract all leave the body through the same orifice. Meaning, everything comes out of the same hole…..sometimes at the same time. That is why the normal manure has the white, uric acid cap on it…that is the bird’s version of urine. Importantly, it is also a source of manure contamination of eggs, especially if the hens are sick and have loose manure. I have talked about safe egg handling previously, but I would like to re-emphasize the importance of this.
All in all, it takes around 3 hours for a piece of chicken feed to become chicken manure. For humans, gut passage time is around 2 days. This speedy transit time is important for….wait for it…..flight. Birds will actually spend a fair bit of time with almost nothing in their guts….thus lighter and more able to fly. But, this short passage time means that diets that are high in indigestible components will seriously increase the fecal volume, since there isn’t time to break down the more fibrous bits.
So, that is the story of chicken feed becoming raw materials for eggs and the necessary body processes that support the chicken in everything she does. Remember that bird digestion is quite different from mammalian digestion, and some of the gut problems you might see in a small (or large) flock will show certain symptoms because of the way the gut works.
Mike the Chicken Vet