The fertilization of a hen’s egg by a rooster is a precise biological process. Understanding the rooster’s role in this reproductive cycle reveals how a chick can develop.
The Rooster’s Role in Reproduction
A rooster’s reproductive role begins with sperm production. Roosters have two internal testes, located near their backbone, which produce sperm and male hormones. Unlike mammals, avian sperm remains viable at the bird’s body temperature, allowing the testes to be housed inside the body cavity. A mature rooster can produce up to 35,000 sperm per second.
Roosters attract hens through courtship behaviors. These include a “wing-dragging dance,” where the rooster lowers one wing and circles the hen. Another behavior is “tidbitting,” where the rooster picks up and drops food while clucking, signaling a potential food source. If receptive, the hen crouches, indicating readiness to mate.
The Hen’s Reproductive System and Egg Formation
The hen’s reproductive system consists of a single functional ovary and a long, coiled oviduct. The ovary develops yolks. When a yolk matures, it is released into the oviduct, known as ovulation. The oviduct is a specialized tube with sections that add different components to the egg. The infundibulum captures the ovulated yolk, the magnum adds the albumen (egg white), the isthmus contributes shell membranes, and the uterus forms the hard outer shell. The egg then passes through the vagina before being laid through the cloaca, a common opening for reproductive, digestive, and excretory systems.
The hen’s reproductive tract contains specialized sperm storage tubules (SSTs), located in the uterovaginal junction of the oviduct. These tubules can store viable sperm for up to three weeks after a single mating. This allows subsequent eggs to be fertilized without immediate re-mating.
The Fertilization Process
Mating in chickens is called the “cloacal kiss.” During this brief interaction, which lasts only a few seconds, the rooster mounts the hen, holding her neck feathers and treading on her back. Both birds evert their cloacas, bringing their openings into contact. Sperm transfers from the rooster’s cloaca to the hen’s.
The sperm then travels up the hen’s oviduct towards the infundibulum. Fertilization occurs in the infundibulum, the first section of the oviduct, shortly after the yolk is released. This process must happen quickly, within 15 minutes of the yolk entering the infundibulum, before egg components like albumen and shell are added. The sperm penetrates the germinal disc, a small white spot on the yolk’s surface, which contains the hen’s genetic material.
The Path from Fertilized Egg to Chick
After fertilization, the single cell begins to divide, forming a cluster of cells called the blastoderm. This division starts within five hours of fertilization while the egg is still traveling through the hen’s oviduct. The developing embryo continues its early growth as the egg progresses, receiving albumen, shell membranes, and the hard shell.
Eggs sold for consumption are unfertilized and cannot develop into a chick. A fertilized egg, however, contains a blastoderm that can develop into a chick if provided with warmth and humidity. This development requires an incubation period of about 21 days, during which the embryo grows within the shell. If not incubated, a fertilized egg remains biologically similar to an unfertilized one and can be consumed without difference in taste or nutritional value.