Healthy female chickens, known as hens, are able to lay eggs, whether or not a rooster is present. Eggs will be unfertilized if the hen has no access to a rooster, which means the egg will never develop into a chick.
In general, hens become mature enough to lay eggs around six months of age, though this varies by breed. The first eggs laid may have soft shells or abnormal shapes.
Healthy hens are able to lay an egg about once a day, but may occasionally skip a day. Some hens will never lay eggs. This is often due to a genetic defect but may have other causes, such as poor diet. Hens must have enough calcium in their diets in order to produce the hard shells of eggs. Contact a veterinarian if one of your hens seems to be having trouble laying eggs.
How Hens Produce Eggs
The reproductive system of hens consists of an ovary and an oviduct. Although hens are born with two ovaries, one will shrink and become non-functional. The remaining healthy ovary is where the development of the egg begins. It takes about 24 to 28 hours for a hen to develop an egg.
The yolk is created in the ovary and, when ready, gets ejected into the first part of the oviduct, called the infundibulum. This is where fertilization takes place if the hen has mated. After mating, the sperm of the rooster travels to the infundibulum, where it fertilizers the newly released yolk from the ovary.
After about 30 minutes, the egg travels further along the oviduct to the magnum where the albumen, or egg white, begins to form. After about three hours, the thick, white albumen is about halfway developed, and the egg moves further along the oviduct.
The next stop is the isthmus, where the inner and outer shell membranes form. The albumen continues to develop, and the shell membrane forms over about an hour or two.
After leaving the isthmus, the egg enters the uterus where it will remain for 18 to 24 hours while the hard outer shell develops. Once the shell is formed, the egg is pushed through the vagina and exits the hen's body via the cloaca, the posterior orifice.
Fertilized Eggs vs. Non-Fertilized Eggs
If a hen is able to mate with a rooster, then its eggs may be fertilized. Fresh eggs can be consumed whether or not they are fertilized. If you are collecting the eggs every day or two, then there is no chance of finding a developed chick in an egg when you crack it open.
Although there are minor differences between fertilized and unfertilized eggs, there is not necessarily a difference in the flavor, consistency, or nutrient content of the egg.
In order for the chicken embryo to develop into a chick, the egg must be incubated for 21 days. The hen can do this by sitting on the eggs; this is called "going broody." If the hen does not go broody, then the eggs can be placed in an incubator for 21 days.
If fertilized eggs are collected shortly after laying and are kept in a cool place, then the embryo will not mature. Some breeds of chickens are more likely to go broody than others. In addition, if one hen becomes broody, the other hens around her may also become broody.
If you want to know whether or not a freshly laid egg is fertilized, this may be accomplished through a process called candling. Hold the egg is held up against a bright light, and examine the contents for shape and opacity. You may be able to see the early stages of the embryo and sometimes blood spots as well. However, it typically takes a trained eye to see the differences between fertilized and unfertilized eggs at this stage.