A chicken produces an egg through an assembly-line process inside a tube called the oviduct, and the whole journey from ovary to nest box takes roughly 25 to 26 hours. That’s why most hens lay about one egg per day, with each egg arriving slightly later than the one before it.
Where the Egg Starts
A hen is born with thousands of tiny immature yolks (ova) clustered on her single working ovary, which sits near her left kidney. As a yolk matures over about ten days, it accumulates fat, protein, and the yellow pigments that give it its color. When it’s ready, it’s released from the ovary in a process identical in principle to ovulation in mammals. From this point on, the yolk travels through five distinct sections of the oviduct, each one adding a new layer to the finished egg.
Five Stops Along the Oviduct
The oviduct is roughly 25 to 27 inches long in a laying hen, and each section has a specific job.
Infundibulum (15 to 17 minutes). This funnel-shaped opening catches the released yolk. If a rooster has mated with the hen, fertilization happens here. Whether the egg is fertilized or not makes no difference to the rest of the process: the egg forms the same way either way.
Magnum (about 3 hours). The longest section of the oviduct, at around 13 inches. Here, glands secrete the thick egg white (albumen) that surrounds the yolk. The albumen is mostly water and protein, and it cushions the yolk while also supplying nutrients a developing chick would need.
Isthmus (about 75 minutes). This shorter, four-inch section wraps the egg in two thin, flexible shell membranes. You’ve seen these if you’ve ever peeled a hard-boiled egg and noticed the papery layer clinging to the inside of the shell. These membranes give the egg its basic shape and provide a scaffold for the hard shell to build on.
Shell gland (about 20 hours). This is where the egg spends the vast majority of its formation time. Calcium carbonate is deposited onto the shell membranes in three stages, building up a rigid shell that contains roughly 10,000 microscopic pores. These pores are small enough to keep bacteria out but large enough to let gases pass through, which would be critical for a developing embryo. Any pigment the hen produces is also laid down here. Brown eggs get their color from a pigment applied in the shell gland during the final hours of shell formation, which is why brown eggs are white on the inside.
Vagina (final minutes). The last section adds the “bloom” or cuticle, a thin, waxy coating made of proteins, lipids, and fat. The bloom seals those thousands of shell pores, slowing moisture loss and acting as an antimicrobial barrier against bacteria and fungi. It also lubricates the egg for a smoother exit. This is why unwashed farm eggs can safely sit at room temperature for a time: the bloom keeps them naturally protected. Commercial washing removes this coating, which is why store-bought eggs need refrigeration.
How the Egg Comes Out
Chickens have a single opening called the cloaca (or vent) that serves the digestive, urinary, and reproductive systems. This sounds unhygienic, but the hen’s body has a built-in solution. As the egg moves into the cloaca, the lower part of the oviduct actually pushes outward through the vent, turning partially inside out. At the same time, a muscular fold seals off the digestive chamber, preventing any fecal material from contacting the egg. The egg slides out on the everted tissue, which then retracts back inside. The whole process takes only a minute or two, and it’s why freshly laid eggs are usually clean.
Hens typically lay in the morning or early afternoon. You might notice a hen becoming restless, visiting the nest box repeatedly, or squatting low before laying. Some hens vocalize loudly right after laying, a behavior sometimes called the “egg song.”
Why Hens Lay One Egg per Day
Because shell formation alone takes about 20 hours, a hen physically cannot produce more than one egg in a 24-hour period. Most hens ovulate a new yolk shortly after laying, usually within 30 to 60 minutes, but the timing shifts a little later each day. After several consecutive days of laying (a “clutch”), the hen’s ovulation cycle eventually pushes too late in the day for her body to trigger a new release, and she takes a day off before starting the cycle again.
Light exposure plays a major role. A hen needs about 14 to 16 hours of light per day to maintain consistent laying, because light stimulates the hormones that trigger ovulation. This is why egg production naturally drops in winter when daylight hours shorten, and why commercial operations use artificial lighting to keep production steady year-round.
What Affects Egg Size and Color
Young hens (pullets) just starting to lay produce smaller eggs. Egg size increases as the hen ages and her oviduct grows, typically reaching full size by about 40 weeks of age. Breed, nutrition, and body size also influence egg dimensions, but age is the strongest factor in an individual hen’s production.
Shell color is entirely genetic. White-feathered breeds with white earlobes generally lay white eggs, while reddish-brown breeds tend to lay brown eggs. Breeds like the Ameraucana produce blue shells, and olive-colored eggs come from crosses that layer brown pigment over a blue base shell. None of these color differences affect the egg’s nutritional content or flavor. The only thing that meaningfully changes how an egg tastes is what the hen eats.