Chickens do not menstruate in the same way that mammals do. While both processes involve reproductive cycles, the biological mechanisms and outcomes are fundamentally different. Understanding these distinctions requires examining what menstruation entails and how the avian egg-laying process functions.
What Menstruation Is
Menstruation is a regular biological process that occurs in sexually mature females of certain mammalian species, including humans and some primates. It is the monthly shedding of the uterine lining, the endometrium. This shedding happens when a fertilized egg does not implant in the uterus.
Hormones like estrogen and progesterone regulate this cycle. Estrogen causes the uterine lining to thicken and become enriched with blood vessels, preparing it to nourish a potential embryo. If pregnancy does not occur, hormone levels drop, signaling the body to expel the thickened lining, blood, and tissue through the vagina. This expulsion is the menstrual period, typically lasting a few days.
The Avian Egg-Laying Process
The egg-laying process in chickens is distinct from mammalian menstruation, involving a specialized reproductive tract. A hen has a single functional ovary, usually on the left side, holding thousands of tiny ova or potential yolks. When a yolk matures, it is released from the ovary via ovulation, typically 30 to 75 minutes after the previous egg is laid.
After ovulation, the yolk enters the infundibulum, the first segment of the oviduct, where fertilization can occur if sperm are present. The yolk then travels into the magnum, the longest section of the oviduct, where the albumen, or egg white, is added around the yolk. Subsequently, the egg moves to the isthmus, where the inner and outer shell membranes are formed around the albumen.
The developing egg then enters the shell gland, also known as the uterus, where it spends the majority of the egg formation time, roughly 18 to 21 hours. Here, the hard outer shell, composed primarily of calcium carbonate, is deposited. Before the egg is laid, a protective outer coating called the bloom or cuticle is applied in the vagina.
Why Chickens Don’t Menstruate
Chickens do not menstruate because their reproductive biology differs significantly from that of menstruating mammals. The primary distinction is the absence of a uterine lining that builds up and sheds if pregnancy does not occur. In mammals, the uterus prepares for embryo implantation by thickening its lining, and if implantation fails, this lining is expelled.
Birds, including chickens, have a different reproductive strategy: the egg, containing the developing embryo (if fertilized) and all necessary nutrients, is laid externally. The hen’s reproductive system continuously produces and expels these self-contained packages. There is no internal uterine environment needing regular shedding or regeneration in the absence of pregnancy, as reproductive output is externalized.
Instead of a shedding uterine lining, a chicken’s reproductive tract focuses on forming the various components of the egg around the yolk as it travels through the oviduct. Each part of the oviduct contributes specific elements, such as albumen, membranes, and shell, ultimately resulting in a fully formed egg ready for laying. This continuous, efficient process means there is no physiological need for a menstrual cycle involving the breakdown and expulsion of uterine tissue.