Chicken reproduction involves a series of biological steps. Unlike many other animals, chickens lay eggs externally, but fertilization occurs internally. This avian reproductive strategy allows for offspring development within a protective shell, outside the hen’s body.
Reproductive Anatomy
Both male (rooster) and female (hen) chickens possess a single opening called the cloaca, which serves as a common exit point for digestive, urinary, and reproductive systems. In roosters, the reproductive system includes two testes located internally near the backbone, which produce sperm. These testes connect to deferent ducts that transport sperm to a small, pimple-like structure, or papilla, inside the cloaca. Roosters do not possess an external copulatory organ like a penis.
Hens typically have only one functional ovary and oviduct, usually on the left side, with the right side regressing during development. The ovary, resembling a cluster of grapes, contains thousands of tiny ova, which are potential yolks. Once a yolk matures, it is released from the ovary and enters the oviduct, a long, convoluted tube essential for egg formation. This oviduct is divided into distinct sections, each playing a role in the egg’s journey.
The Mating Act
Chicken mating, often referred to as the “cloacal kiss,” is a rapid and efficient process. The rooster initiates mating by mounting the hen, typically grasping her neck feathers with his beak for stability. This behavior, known as “treading,” helps him maintain position on the hen’s back. The hen, if receptive, will crouch down, spread her wings, and lower her tail, allowing the rooster to align his cloaca with hers.
During this brief moment, the cloacal openings of both birds touch, facilitating the transfer of sperm. The rooster everts his cloaca, and his papilla delivers semen directly into the hen’s everted cloaca. This contact is brief, lasting only a few seconds. Despite its brevity, this “cloacal kiss” is the primary mechanism for sperm transfer in chickens, enabling internal fertilization without penetration.
Internal Fertilization Process
The sperm then embarks on a journey through the oviduct, where it encounters the newly released ovum, or yolk. Fertilization in the hen occurs specifically in the infundibulum, the uppermost section of the oviduct, shortly after the yolk is released from the ovary.
When present, a single sperm penetrates the blastodisc, a small whitish spot on the surface of the egg yolk. This penetration forms a blastoderm. Cell division in the fertilized ovum begins almost immediately after this event, even as the egg continues its passage through the oviduct.
Sperm Storage and Egg Development
A unique aspect of hen reproduction is her ability to store sperm for extended periods within specialized structures called sperm storage tubules (SSTs). These tubules are located primarily in the utero-vaginal junction of the oviduct. This storage capacity means a hen does not require daily mating to lay fertile eggs; a single successful mating can result in fertile eggs for up to 10 days to 2 weeks.
As a fertilized yolk travels through the oviduct, various layers are added. In the magnum section, albumen (egg white) is secreted around the yolk. The egg then moves to the isthmus, where the inner and outer shell membranes form. Finally, in the shell gland (uterus), the hard, calcium-rich shell is deposited, a process that takes approximately 20 hours. The complete journey from ovulation to egg laying typically spans about 25 to 26 hours.