The reproductive process in chickens is an internal biological event, which is often misunderstood because the final product—the egg—is laid externally. Unlike mammals, the hen possesses a single functional ovary and a long, complex oviduct where the entire process of fertilization and egg formation takes place. The female reproductive tract is designed for rapid, sequential egg production. This specialized system allows the hen to produce an egg almost every day, regardless of whether a male is present. Fertilization, when it occurs, must happen very early in this journey, before the structural components of the egg are added.
Mating and Sperm Storage
Fertilization requires the presence of a rooster, but the act of copulation itself is brief and distinct from mammalian mating. The male chicken lacks an external copulatory organ, so reproduction is accomplished through a maneuver known as the “cloacal kiss.” This involves the rooster mounting the hen, and both birds briefly everting and touching their cloacae, which are the common exit for the digestive, urinary, and reproductive tracts. Semen is transferred directly from the rooster’s papilla into the hen’s cloaca.
Once the sperm is inside the hen’s reproductive tract, it travels upward toward the oviduct. The hen possesses specialized sperm storage tubules (SSTs) located near the junction of the uterus and the vagina. These microscopic glands can house viable sperm for an extended period, typically around ten days to two weeks, and sometimes longer.
The SSTs are a remarkable adaptation that ensures continuous fertility without the need for daily mating. Sperm is released from these tubules in small numbers over time, migrating up the oviduct to the site of fertilization. This long-term storage mechanism allows the hen to lay a sequence of fertile eggs from a single copulation event.
Fertilization: Where the Sperm Meets the Yolk
The ovum, or yolk, is released from the ovary approximately every 25 to 26 hours, a process called ovulation. This ovum, which is the actual female reproductive cell, enters the oviduct and is immediately engulfed by the infundibulum, the funnel-like top section of the oviduct.
Fertilization must occur in the infundibulum, as this is the only part of the oviduct where the sperm can reach the ovum before protective layers are added. The window for this event is extremely short, lasting only about 15 to 17 minutes after the yolk is released. If sperm is present, one sperm penetrates the germinal disc—a small, whitish spot on the yolk’s surface—and fertilization is complete, creating a blastoderm.
If the ovum is successfully fertilized, cell division begins almost immediately, even before the egg is laid. The blastoderm starts to divide into a multi-celled structure that will eventually develop into the embryo if incubated. If no sperm is present in the infundibulum during this brief period, the ovum remains unfertilized, and the journey down the oviduct continues.
Layering the Egg Structure
Whether fertilized or not, the ovum continues its approximately 25-hour journey through the rest of the oviduct, where the remaining egg structures are added sequentially. The first major stop after the infundibulum is the magnum, the longest section of the oviduct, where the egg spends about three hours. Here, glandular cells secrete the thick, protein-rich albumen, or egg white, which surrounds the yolk and provides water and protection for the developing embryo.
Next, the egg moves into the isthmus, where it remains for about 75 minutes. In this section, the inner and outer shell membranes are formed, creating a fibrous casing that provides the first structural boundary for the egg contents.
Finally, the egg enters the shell gland, also known as the uterus, for the longest part of the process, lasting 20 or more hours. In the shell gland, the hard outer shell, composed mainly of calcium carbonate, is deposited around the egg. The hen mobilizes a significant amount of calcium from her diet and skeletal reserves to complete this calcification process. Before the egg is laid, a protective outer layer called the bloom, or cuticle, is applied in the vagina, sealing the shell pores.
The Difference Between Table Eggs and Hatching Eggs
The eggs purchased in grocery stores, referred to as table eggs, are universally infertile and will never develop into a chick. Commercial egg production farms do not house roosters with their laying hens, meaning the initial step of sperm transfer and the possibility of fertilization never occurs. The egg laid is simply the hen’s unfertilized ovum, packaged for consumption.
Hatching eggs, conversely, come from a flock that includes roosters and have therefore been fertilized. A fertilized egg can be eaten just like an infertile egg, as no significant embryonic development happens at room temperature. The difference is that a hatching egg, if subjected to the correct temperature and humidity for approximately 21 days, contains a living blastoderm capable of developing into a chick.