A mixed flock has a quiet nutritional problem: laying hens need considerably more calcium than growers, older non-layers or males, yet everyone eats from the same feeders. Solving it by enriching the shared feed forces calcium on birds who do not need it. Solving it by ignoring it leaves hens drawing calcium from their own bones and laying thin, fragile shells. Free-choice feeding is the way out, because it lets each hen dose herself.
The Separate Dish Does the Work
Offer crushed oyster shell in a separate dish, fixed where it stays dry and near the feeders, and keep it available at all times. A hen approaching lay, or in full lay, eats shell from that dish in the amounts her body asks for, typically taking more as a shell is forming. A bird not in lay largely ignores it. The mechanism is appetite, not arithmetic: hens regulate their own intake with surprising accuracy when the source is separate, palatable and always there.
Oyster shell suits the role because it dissolves slowly, releasing calcium over hours rather than in one quick pass, which matches how a shell is built, largely overnight. Keep the dish topped up and dry, because damp, caked shell is eaten reluctantly, and site it under cover so rain does not turn it into paste. Place a second dish if the flock is spread across a large run, so a hen low in the order is never bullied away from the only source during the hours she needs it. No chart sets the amount: the entire point of free-choice is that the hen herself decides her intake.
Layer Feed Stays the Base
The shell dish is a top-up, not a diet. The flock's base remains a proper layer feed for birds in lay, which carries the protein, energy and baseline calcium the day requires, and the dish covers the extra demand that varies hen by hen and day by day. Mixing extra shell or limestone through the whole ration undoes the system: the calcium is no longer a choice, every bird eats it whether she is laying or not, and the birds the arrangement was meant to protect are the ones carrying the surplus.
Keep the groups honest too. Growing birds and males do better on their own appropriate feed without a layer ration's calcium load, so where a genuinely mixed flock shares one space, the separate dish matters even more, because the shared base can stay moderate while layers help themselves to what they need.
Shell Is Not Grit
Keep shell and grit distinct in your mind and in the yard. Grit is insoluble stone a hen holds in her gizzard to grind feed; it contributes no calcium and does a mechanical job. Oyster shell is soluble, is consumed as a mineral, and does a nutritional job. A flock needs access to both, in separate containers, and confusing the two leaves one of the jobs undone while appearing to have covered it.
Feeding duties are assigned through the Care Teams feeding list for the flock: who checks and refills the shell dish on the morning round, who confirms the layer feed bin, and who notes soft or thin shells in the laying record, because a run of poor shells after the dish runs empty is information, not coincidence. A relief keeper reading the Care Teams notes on KinsLe learns in one glance why the white dish stays apart from the feed and the grit tray.