Hens do engage in sexual activity, but the process is highly specialized compared to mammals. Reproduction requires a rooster and a hen to transmit genetic material, producing fertile eggs that can develop into chicks. This biological mechanism is unique to birds and involves distinct behaviors and anatomy.
The Mechanics of Avian Mating
Chicken mating is a rapid event that hinges on a momentary physical connection. Courtship often begins with the rooster performing a “waltzing” display, circling the hen while lowering one wing. If the hen is receptive, she signals compliance by crouching down in a submissive squat, lowering her body and spreading her wings slightly. The rooster then mounts the hen’s back, grasping her neck feathers with his beak to maintain balance.
Unlike mammals, roosters lack external male genitalia. Sperm is transferred through the cloaca, a single opening that serves as the exit point for the digestive, urinary, and reproductive tracts in both sexes. The transfer occurs in a quick maneuver called the “cloacal kiss,” where the rooster briefly presses his cloaca against the hen’s. During this contact, which lasts only a few seconds, sperm is transferred into the hen’s reproductive tract. A single, successful mating can provide the hen with enough sperm to fertilize eggs for up to several weeks because she possesses internal sperm storage tubules.
Fertilized vs. Unfertilized Eggs
A hen produces eggs regardless of the presence of a rooster. The difference lies in whether the egg contains the genetic material necessary to develop into a chick. Eggs laid for consumption, such as those bought in a supermarket, are almost always unfertilized because the hens are kept separate from roosters.
The rooster provides the sperm that fertilizes the hen’s ovum, which is the yolk. Fertilization takes place high in the oviduct, shortly after the yolk is released from the ovary. The key indicator of an egg’s potential is the germinal disc, a small white spot found on the yolk’s surface.
In an unfertilized egg, this spot, called a blastodisc, is small, dense, and solid white. If the egg has been fertilized, the spot is called a blastoderm and appears larger, often resembling a tiny white ring or bullseye with a more translucent center. This difference is due to cell division, as the fertilized embryo begins development immediately, even before the egg is laid.
The Hen’s Reproductive Anatomy
The hen’s reproductive system consists of a single functioning ovary and the oviduct. Although a hen is born with two ovaries, only the left one typically develops and matures to produce ova, which are released as yolks. The ovary contains thousands of potential ova, which develop into mature yolks over time.
The Oviduct Stages
Once a mature yolk is released, it is captured by the infundibulum, the funnel-like entrance of the oviduct. Fertilization occurs here if sperm is present. The oviduct is a long, muscular tube responsible for adding the rest of the egg’s components. The yolk then travels through the magnum, where the thick egg white, or albumen, is added, a process that takes about three hours.
Next, the egg moves to the isthmus, where the inner and outer shell membranes are formed. The developing egg spends the longest time, approximately 20 hours, in the shell gland or uterus. Here, the hard shell made of calcium carbonate is formed and pigment is applied. Finally, the egg passes through the vagina and is expelled through the cloaca, which acts as the single exit for the egg, urine, and feces.