Picture a hen asleep in a dark barn. She looks perfectly still, but inside her body a small construction project is running, and part of the building material is her own skeleton. By morning she will lay an egg wrapped in a hard, smooth shell. To make it, she has spent the night borrowing calcium from her bones.
An eggshell is a heavy bill
An eggshell is almost pure mineral, about 95 percent calcium carbonate. A single shell contains around 2.2 grams of calcium, which is more than a tenth of all the calcium in the hen’s body.
Now multiply that by an egg nearly every day for a year or more. A modern laying hen moves more calcium through her body in a laying cycle than her entire skeleton contains, many times over.
The night shift worker
The shell is built inside the shell-gland, the part of the oviduct sometimes called the uterus. Shell formation runs from roughly 7 to 20 hours after the yolk is released, and the fastest building happens late in that window.
That schedule puts the heaviest work in the dark hours, when the hen is roosting and not eating. Her gut supplies only about half of the calcium a shell needs, and by the early-morning hours the calcium from her last meal is running low.
A bone that exists to be spent
Nature’s answer is medullary bone; a special spongy bone found only in female birds. As a pullet approaches her first egg, rising oestrogen switches off growth of her main skeleton and tells her body to lay down this new tissue inside the hollow spaces of her leg bones.
Medullary bone works like a rechargeable battery. It is filled with calcium during the day and drained at night as the shell is built, supplying around a third of the calcium in each shell. Scientists have watched its mineral content shift measurably within a single daily laying cycle.
When the battery runs flat
The system works beautifully when supply and demand stay in balance. If the hen cannot get enough calcium, or cannot get it at the right time, she starts resorbing her structural bone as well.
Over a long laying cycle this can leave a hen with thin, fragile bones, a condition called osteoporosis in laying hens. It is linked to fractures, including damage to the keel bone, and remains one of the most important welfare issues in egg production.
Feeding the night shift
This is where nutrition science meets daily farm practice. Fine limestone powder dissolves quickly and is absorbed fast, while coarser particles linger in the gizzard and release calcium more slowly.
Many producers therefore offer coarse limestone, often in the afternoon, so hens go to roost with a slow-release supply on board. The aim is to cover more of the overnight demand from feed rather than from bone, although trials of split calcium feeding have not all shown clear gains in shell quality, so the details are still being refined.
The Byte to Remember
Every eggshell is partly made of the hen herself, and good layer nutrition is really about feeding her night shift so she never has to spend more bone than she can rebuild.
