How a shell is supposed to form
A hen builds the shell in the last stretch of the egg's journey through her reproductive tract, depositing calcium carbonate over many hours, mostly overnight. Anything that shortens that process or starves it of calcium produces an egg that arrives thin, rubbery, or with no real shell at all — sometimes just a membrane that tears as it is laid. Understanding that timing explains most soft shells: the egg was rushed, the calcium was not there, or the shell gland itself is not working as it should.
The everyday causes keepers see most
The commonest cause is calcium — either the diet does not supply enough, or the hen cannot take it up properly. Hens need calcium available in a form they can eat in quantity at the times they need it, which is why experienced keepers offer crushed oyster shell free-choice in a separate dish beside a balanced layer ration rather than relying on the feed alone. Young pullets just coming into lay produce soft and odd eggs while their systems find a rhythm, and older hens nearing the end of lay often do the same from the other direction. Heat stress, a fright in the night, a move, a new flock member, or a disrupted routine can also push an egg through too fast. Illness matters too: some respiratory and reproductive infections damage the shell gland and leave a hen laying persistently poor eggs even on a perfect diet.
Reading the pattern in your own flock
One soft egg in the nest box is information, not an emergency. A pattern is different. Egg quality across the rest of the flock is a useful control: if every hen's shells thin at once, look to the shared feed, water and heat before blaming an individual bird. Try to identify which hen is responsible — membrane eggs in the same spot, a hen visiting the nest without producing, or soft eggs under one bird's roost tell you where to look. Count the frequency: a pullet settling in should improve week by week, while a mature hen producing soft eggs several times a week on a corrected diet deserves investigation. Watch the hen herself as well as the eggs: appetite, weight, droppings, breathing and whether she strains, hunches or has a soiled, swollen vent all change what the soft shells mean.
When soft shells stop being a diet problem
Contact a vet promptly if the hen seems unwell alongside the soft eggs — lethargic, off her feed, penguin-like in stance, straining with nothing produced, or with discharge around the vent. Those signs can point to an egg stuck inside her or an infection of the reproductive tract, both of which can become emergencies, and neither is fixed by extra calcium. A vet can also check a persistently affected hen for shell-gland damage and advise whether she should stay in the laying flock. Keep a simple log of which hen laid what, and when, plus the diet changes you made in response: KinsLe's Pet Passport stores those laying notes with each hen's record, so the pattern you hand to a vet is documented fact rather than a guess.