Now, you can buy bags of oyster grit at your local feed store; my chickens don't like it because the lumps are so big- and maybe they don't taste good either. ( I don't know - I haven't licked any). Either way, my hens ignore them.
Did you know you had a free source of calcium for them right in your own house? This!
That's right, your egg shells! Find a container to put them in, ad save your shells! Now I live in what is considered a high mountain desert area; shells dry very quickly, so I don't tend to rinse them out. But of you live in a more humid climate, you will want to rinse your shells clean and let them dry on the counter before putting them in the container. As you can see, I leave the container open to the air. When the container gets full - like this one obviously is - it's time to bake the shells.
Set your oven at 240 degrees and let it warm to temp. Get a cookie sheet with a lip and spread the eggshells out on the sheet - like this:
Place the cookie sheet in the oven for 7-10 minutes - you will be able to smell them baking when they are pretty much done. Your shells, coming out of the oven, should look like this:
Not a huge amount of difference, but your shells should look brownish where membrane of moisture remained. If your white or blue eggshells start looking brown, you've kept them in too long. Since each oven is different, experiment with yours until you get them looking just right.
Once the eggshells have cooled, crush them up into small bits - they don't have to be a fine powder, but get them into small pieces. This can be hard on your hands, so what we do is throw them in a container and use a meat mallet to break them up.
Then just scatter them on the ground or in the feed dish - they chickens will eat them up.
Simple, free and easy calcium.
Since free ranging chickens also need more protein in the winter time - fewer or no bugs to eat, no matter how they scratch in the yard - eggs are also your solution to get them that extra protein. And you can kill two birds, metaphorically, with one pan.
Find your oldest frying pan, and make them scrambled eggs. However, instead of breaking the eggs and putting the shells aside, toss the whole thing in. As you cook it, you will break the shells up more and more. Cook the eggs thoroughly, let them cool til just warm, and put them in a pie tin or other dish you don't mind being out with the chickens, and let them at it. They will eat every bite!
We do the same things with hard boiled eggs - smash the whole egg down on a plate, shell and all, and then chop it up to about 1/4 inch sized bites. Put it in a dish, set it out for the chickens and stand back - you will get run over.
As simple and frugal solution to helping your chickens get what they need.