Chickens are oviparious animals, meaning their offspring develop outside of the mother's body in an egg. When the chicks hatch, they require little paternal care other than protection from predators. The egg contains the chicken embryo and everything needed to nourish it until hatching, which takes about 21 days.
A hen's reproductive system has two main parts: The ovaries and the oviducts. Hens are born with two ovaries and oviducts, but only the left one develops and is functional in the adult hen. This is true of most bird species.
Ovary and Eggs
The ovary produces ova, or egg cells. The tiny ova develop in the chick's ovary while she is still in the egg. Chicks hatch with more than 10,000 ova that could develop into chicken eggs.
Oviduct and Vent
The oviduct is a 2-foot-long tube that extends from the ovary to the vent, the hen's one opening for her urinary, intestinal and reproductive tracts. Developing eggs travel from the ovary to the vent through the oviduct. The oviduct has four parts: the infundibulum, magnum, isthmus and shell gland.
Ovulation, Egg Formation and Laying
When hens reach sexual maturity at 4 or 5 months of age, the tiny ova in the ovary begin maturing a few at a time. As they mature, yolk forms in the ova next to the blastodisc, or egg cell. The yolk and blastodisc are enveloped by a membrane.
The blastodisc, if fertilized by a rooster's sperm, will develop into a chick. The yolk provides all the nutrients the developing chick needs until hatching. Hens lay eggs whether or not they are fertilized. Only fertilized eggs develop into chicks, although occasionally unfertilized eggs develop into embryos. These embryos usually die in the egg within a few days.
Ovulation and Fertilization
The egg-laying process beings with ovulation, when the ovary releases a mature ovum. The infundibulum, or top part of the oviduct, surrounds the released ovum and removes it from the ovary. Sperm is stored in the infundibulum for up to 30 days after mating. The egg is fertilized here by the stored sperm, if present.
Travel through the Oviduct
The ovum moves down to the next part of the oviduct, called the magnum. The albumen, or egg white, is deposited around the membrane containing the yolk and blastodisc in the magnum.
The ovum then moves down to the isthmus. The isthmus forms two tough white membranes around the ovum, called the shell membranes. Next, the ovum descends to the shell gland. It stays in the shell gland for about 20 hours while the egg shell forms around the shell membranes.
Finally, the hen lays the egg through the vent. The entire process takes about 25 hours.
Frequency of Egg Laying
Hens can lay an egg about once a day. After a hen lays an egg, she can ovulate again 30 to 75 minutes later. But ovulation is controlled by exposure to light, or photoperiod, and rarely occurs after 3 p.m. So if a hen lays an egg later in the day, she will skip laying the next day. The average hen lays 230 to 300 eggs a year.
Kimm Hunt has been writing professionally since 1990. She has written for businesses, government agencies and nonprofit organizations, and previously served as the editor of a weekly suburban Chicago newspaper. Hunt holds a B.S. in agriculture from the University of Illinois. She is also a professional dog trainer.