Chicken eggs become fertilized when chickens mate and the rooster's sperm successfully comes into contact with the ovum released by the hen's ovary. After the ovum's release, the sperm have around a half an hour to fertilize it before the shell begins to form.
A hen has one ovary, which produces the egg yolk, or ovum. The ovary releases the ovum into the oviduct once it matures. The first section of the oviduct, the infundibulum, pulls forward and encompasses the yolk. This process takes around 20 minutes. The hen keeps sperm in specialized storage glands located in the infundibulum after she mates in order to facilitate quick fertilization after she ovulates. The sperm don't have much time to reach the germinal disc, a tiny superficial white area on the yolk where the egg is fertilized and the embryo begins to develop. Only 15-17 minutes after being released into the infundibulum, the egg begins to move down the oviduct regardless of fertilization. Once it reaches the magnum section of the oviduct, the albumen, or egg white, begins to form. Once the first layer of albumen has formed, the sperm can no longer reach the ovum, and any chance for fertilization is gone.