Biology Lesson: the Chicken Vent

When we first started to raise hens we did a ton of research to figure out exactly how these little egg producing creatures work. I can remember sitting at the kitchen table with Adam (a friend of the family who was staying with us while on a business trip) searching Google for things like "Where does an egg come from?" and "How does a chicken lay an egg?"

While most of us are happy to eat eggs, it remains a mystery to many people how exactly a chicken produces them.  Below are five interesting facts about how chickens lay eggs.

Female chicken.

1) A hen (aka female chicken) does not have to have sex with a rooster (aka male chicken) in order to lay an egg.  Hens are similar to human females in that they ovulate (i.e. produce an egg) regularly regardless of whether a male is present or not.  However, unlike human females who ovulate monthly, most reproductively mature hens ovulate daily.

Completely unnecessary.

2) A hen only has one opening (called the vent) where the intestine and oviduct meet in a chamber called the cloaca.  This opening serves as the exit point for both digestive waste (aka poop and pee) and eggs.  In case you were unsure, human females have three separate openings.  An interesting fact is that chickens don't have a bladder because they don't actually pee liquid like humans do, their "pee" comes out in a white solid form called uric acid.

Anatomical diagram of the chicken oviduct.

Chicken vent.

3) Hens never lay more than one egg a day.  However, sometimes an egg will be a "double yolker" which means that there are two yolks inside one shell.  This type of egg could have produced identical twin chicks if it had been fertilized, incubated and the chicks had been allowed to hatch.

Double yolker.

4) To lay an egg, a hen turns part of the cloaca and the last segment of the oviduct inside out, like a glove. The egg emerges from these inverted membranes so it cannot contact the walls of the cloaca and get contaminated by digestive waste. Moreover, the intestine and inner part of the cloaca are kept shut by the emerging egg and their contents cannot leave when the hen strains to deliver the egg. Therefore, eggs are always clean when they are laid.

5) Most of the egg is formed in the early portion of a hen's oviduct; however, the shell material is added towards the end in the shell gland portion of the oviduct (aka the uterus).  The shell is composed of calcium carbonate and takes about 20 hours to form.  If a hen lays brown eggs, the brown pigments are added to the shell in the last few hours of shell formation.

Amazing, isn't it?