As it turns out, chicken procreation is a lot weirder than I thought. Here are just a few of the finer points.
1) Asymmetrical gonads
Here's how gonads are usually arranged: males have two testes, and females have two ovaries. Both genders usually have one gonad on the right side of their body and one on the left. It's pretty simple.
In contrast, hens only have one functional ovary. In fact, most birds have this lopsided anatomy probably because it's more practical for flight. Birds need to be light and compact in order to fly, so they only develop one of their gonads.
For most male birds, the right testes is also smaller than the left. This trend does not carry over into roosters, however, probably because chickens are land-dwelling birds. In some bird species, the smaller right testes can even compensate if (God forbid) the left one gets damaged.
2) No copulatory organs
As with most bird species, roosters and hens don't have external genitalia. Instead both partners procreate using an external orifice called a cloaca. When the cloacae are touched together, sperm is transferred into the female reproductive tract. Since no penetration is involved, the act is simply called a "cloacal kiss."
Both genders also use their cloacae for defecation. But don't worry--chicken eggs do not get covered with feces on their way out. When a hen lays an egg, her uterus turns inside out beyond the cloaca that the egg never touches the nasty stuff.
3) Most are virgins
After a rooster inseminates a hen, her eggs will be fertilized for up to four weeks. This is because the sperm remains viable for about 30 days, stored in "sperm nests" along the hen's oviduct.
However, hens don't actually need need roosters in order to lay eggs. In fact, most hens raised in commercial farms have never even set eyes on a rooster.
The only thing hens need in order to stimulate egg-laying is light. Hens are programmed to lay eggs in the spring and summer, which they judge by the amount of daylight. Of course, commercial farmers tap into this tendency by simulating summer days in their chicken coops all year around.
4) Very productive
Chickens are egg-laying machines. A hen hits puberty only 18-24 weeks after hatching out of an egg herself. It only takes about 26 hours for a hen to make an egg, and she can start producing another one 40-60 minutes later. What's more, hens lay a lot eggs--up to 300 a year.
Comparatively, turkeys, are lazy slobs. They start laying eggs later than chickens and lay 100 eggs a year at the most. Because of these and other factors, turkey eggs are a lot harder to come by than chicken eggs. One farmer valued a turkey egg at $3.50 per egg. So that's why we don't use them for omelets.
5) Unique egg shell pigment
So get this: out of the 26 hours it takes to make a chicken egg, 20 of those hours are required to make the shell. Which I guess shows you it's pretty important. Probably the most aesthetically important step of making the shell--adding the pigment--occurs in the last few hours of shell formation.
Egg color is useful because it's is an expression of the bird's fitness. It also makes them look pretty. It seems that this quality is especially important to private chicken farmers.
Differing types of pigment make some eggs a lovely brown or a mysterious blue. The pigment that makes blue eggs is called biliverdin, which is a precursor to the bilirubin pigment that is found in blood. After some dispute, one study showed that biliverdin doesn't come directly from the chicken's blood. Instead, it is produced separately in the chicken's uterus.
Interestingly, recent studies have shown that the enzyme that converts biliverdin to bilirubin in humans is an important regulator of the innate immune response.
6) Can't be twins
Chicken "twins" occur when the ovary releases two yolks at the same time. The yolks are processed together down the chicken's oviduct and a single shell forms around them. Thus two chicken embryos are encapsulated by the same egg.
Once the chicks are ready to hatch, they encounter a problem. In order to get out of their egg, they have the peck at an air space at the top of their egg. But there is simply not enough room for both of them to crane their heads around and peck, so they fight each other instead.
Usually, both twins end up dying--unless they are rescued by a chicken "C-section" performed by a skilled human.
Photo by Muhammad Mahdi Karim via Wikimedia Commons