Fertilization in chickens is an internal process, taking place within the hen’s reproductive tract, specifically in the infundibulum of the oviduct. Unlike mammals, where fertilization leads to internal gestation, chickens lay eggs that are already fertilized, and embryonic development begins even before the egg is laid.

The process starts during mating when the rooster deposits sperm into the hen’s cloaca. From there, sperm swim up the reproductive tract to the sperm storage tubules (SSTs) located at the uterovaginal junction. These SSTs are specialized structures that can store viable sperm for several weeks, allowing a hen to lay fertile eggs for an extended period after a single mating.

When an ovum (yolk) is released from the ovary during ovulation, it enters the infundibulum, the site of fertilization. Here, multiple sperm may attach to the perivitelline membrane surrounding the yolk, but typically only one sperm nucleus fuses with the ovum’s nucleus, forming a diploid zygote. This marks the precise moment of fertilization, initiating embryonic development.

After fertilization, the zygote travels down the oviduct. As it moves, protective layers are sequentially added: the albumen (egg white), shell membranes, and finally the hard calcareous shell, which shields the developing embryo from mechanical damage and microbial invasion. Meanwhile, cell division begins almost immediately. By the time the egg is laid, the embryo has usually progressed to the multicellular blastoderm stage, ready for further development upon incubation.

This internal fertilization strategy, combined with SSTs, ensures efficient reproduction, allowing hens to lay fertile eggs over weeks without repeated mating. It highlights the remarkable adaptation of avian reproductive biology.

Here is a diagram representing the stages of fertilization in a chick: