The fertilization of a chicken egg is a unique biological event that takes place entirely inside the hen, well before the egg is laid. Avian reproduction incorporates specialized anatomy and a remarkable ability for sperm storage. Understanding this process requires tracing the path of the rooster’s genetic material from the moment of mating to the precise location where it meets the hen’s reproductive cell. This sequence of events ensures that a single encounter with a rooster can lead to the fertilization of multiple eggs over an extended period.
The Mating Process
The physical act of copulation in chickens is a swift maneuver often referred to as the “cloacal kiss.” Chickens lack external genitalia, so sperm transfer occurs through the brief, direct contact of the male and female cloacas. The cloaca is a single posterior opening that serves as the exit point for the digestive, urinary, and reproductive tracts.
During courtship, the rooster may perform a waltzing display or a “tidbitting” ritual to attract the hen’s attention. If receptive, the hen signals her readiness by crouching low, allowing the rooster to mount her back. The rooster then balances himself using his feet in a process called “treading” and briefly presses his cloaca against the hen’s. This fleeting contact allows for the transfer of a large volume of semen.
Sperm Storage and Viability
Immediately following the cloacal kiss, the transferred sperm travel up the hen’s oviduct. The hen possesses a remarkable adaptation: the sperm storage tubules (SSTs), which are microscopic glands located primarily at the uterovaginal junction (UVJ) of the oviduct. These SSTs function as a biological reservoir, allowing the hen to store viable sperm for days or even weeks after a single mating.
The stored sperm remain in a quiescent, or inactive, state within the tubules. This prolonged survival enables continuous fertilization of subsequent eggs without the need for daily mating. In chickens, sperm can remain viable and capable of fertilization for approximately two to three weeks. Only a small percentage of the initial sperm population is successfully selected to enter the SSTs.
The Actual Moment of Fertilization
The precise location and timing of fertilization are highly specific within the hen’s reproductive tract. The ovum, which is the yolk released from the hen’s ovary, is fertilized in the infundibulum, the funnel-shaped upper section of the oviduct. Fertilization must occur here because this is the only part of the oviduct the sperm can reach before the egg acquires its protective layers.
The ovum enters the infundibulum, where it remains for approximately 15 minutes. During this window, sperm released from the SSTs travel up the oviduct to meet the ovum. Fertilization takes place on the germinal disc, a small white spot on the yolk surface. Once fertilized, the ovum begins its journey down the oviduct, where the albumen, shell membranes, and hard shell are added over the next 24 to 26 hours. By the time the hen lays the egg, the fertilization process is complete, and the egg is already an early-stage embryo.
The Difference Between Eggs
The presence of a rooster determines the difference between a fertile and an infertile egg. Hens lay eggs based on their reproductive cycle, regardless of whether a male is present, meaning the egg is simply the hen’s reproductive cell. An infertile egg has the potential only to be food, while a fertilized egg holds the potential to develop into a chick if incubated.
The vast majority of eggs sold commercially are infertile, as laying hens are typically kept without roosters. The distinction between the two types is found on the yolk’s surface: an unfertilized egg has a small, solid white spot called the blastodisc. A fertilized egg, having undergone initial cell division, will have a larger, ring-shaped spot called the blastoderm.