Unlike mammals, birds use internal fertilization but package the developing embryo within a hard shell for external development. This means the fertilized cell, the earliest stage of the future chick, must be sealed inside the egg before it is laid. This process requires specialized biological timing to ensure the ovum is fertilized precisely when it is released from the ovary, before the protective layers of the egg are added.
Mapping the Avian Reproductive Tract
The female bird’s reproductive system typically uses only the left ovary and oviduct. The ovary produces the yolk, which is the egg cell surrounded by a massive nutrient supply. The yolk matures and is released from the ovary via ovulation, starting its swift journey through the oviduct.
The oviduct is a long tube divided into five regions, each assembling a specific part of the egg. The initial segment is the infundibulum, a funnel-shaped structure that engulfs the ovum after its release. Fertilization must occur here, at the very beginning of the assembly process.
The magnum secretes the majority of the albumen (egg white). Next, the isthmus adds the inner and outer shell membranes. The final section is the uterus, or shell gland, where the hard, calcareous shell is deposited before the egg is laid.
The Critical Window of Fertilization
Fertilization occurs immediately after the ovum is released from the ovary, before the yolk travels far down the oviduct. The window for successful fertilization is narrow, occurring within the first 15 to 18 minutes of the ovum entering the infundibulum. This timing is essential because once the ovum moves into the magnum, the dense layers of albumen create a barrier that sperm cannot penetrate.
This rapid timing is possible due to a specialized system for sperm storage within the female tract. Sperm are stored in microscopic structures called Sperm Storage Tubules (SSTs), located primarily near the junction of the uterus and the vagina. These tubules protect the sperm and maintain their viability at the bird’s high body temperature.
A single successful mating can lead to the fertilization of multiple eggs over an extended period. Depending on the species, sperm can remain viable in these SSTs for two to three weeks, and sometimes up to 10 to 15 weeks, such as in turkeys. When an ovum is released, stored sperm are periodically released from the SSTs, migrating up the oviduct to the infundibulum to await the yolk.
The fertilized ovum is now a zygote, and cell division begins almost immediately, even as the egg continues its journey through the oviduct.
Building the Egg Around the Embryo
After fertilization in the infundibulum, the ovum proceeds down the oviduct, where the remaining components are added around the developing embryo. This journey typically takes about 25 to 26 hours for a single egg. The next stop is the magnum, the largest section of the oviduct, where the egg spends about three hours.
The magnum secretes the thick albumen (egg white), which provides cushioning and nutrients for the future embryo. As the developing egg moves into the isthmus, it receives the shell membranes. These two keratinous membranes are laid down in about an hour and serve as the foundation for the shell.
The final and longest stage occurs in the uterus, or shell gland, where the egg spends approximately 19 to 21 hours. Here, the hard, porous shell, composed mainly of calcium carbonate, is deposited onto the membranes. This process requires the bird to mobilize a significant amount of calcium rapidly from its diet or bones.
By the time the egg is fully formed and laid, the embryo is safely encased. The developing cells then become temporarily dormant until the parent bird begins incubation, which provides the necessary warmth to resume growth.