Chickens mate when a rooster and a hen each brings an external orifice called a cloaca into contact with one another, according to Real Clear Science. The cloaca is found on both roosters and hens. When a rooster and a hen place their respective cloaca in contact with one another, sperm passes from the rooster into the hen.
Unlike many other animals, no penetration is involved in the mating of rooster and hens, as chickens lack external sex organs. The process typically is referred to as a cloacal kiss. Once sperm from a rooster enters into a hen's reproductive tract, it flows into sperm nests, located near her oviduct. Rooster sperm remains viable within these nests for up to 30 days.
Interestingly, egg production is not tied to this process. The presence of sperm does not trigger egg production in a hen. Rather, egg production is stimulated in a hen because of light, and fertilization is incidental when sperm is present. A hen begins producing eggs at approximately two months of age and produces an egg roughly once every 26 hours. A rooster begins producing sperm at about the same age, as well.
The cloaca is the primary reproductive organ in chickens but is also used to pass waste from a chicken's body. A hen's uterus actually turns inside out when an egg passes, covering the interior of the cloaca to protect against any waste residue that may remain in the passageway.