Calcium is an important part in a peafowl's diet, especially for a peahen during breeding season. During the breeding season, a peahen uses calcium to form eggs. If the peahen begins to run low on calcium, many things can happen, the worst being your peahen could die from being egg bound if left un-treated.
If you take your peahen's eggs away, causing her to keep laying more eggs than she would without you taking them, I strongly suggest that you provide calcium for her to keep up with producing the 30 or so eggs she will be producing in one breeding season.
Now that you understand why it is important to provide calcium I will explain how to provide it. What some people might not realize is that there are several ways in which you can give your birds calcium.
Egg Shells - Oh the irony! To help your peahen produce eggs you can feed her egg shells! So get into the habit of saving those egg shells from that breakfast omelet. It can be store bought eggs, egg shells from your peahen's eggs (if you eat her eggs or if you accidentally break one, etc). To serve the shell to your peahen, first crush up the egg shell. You should crush it to make it bite size, but also just to make sure that your peahen does not develop an egg eating habit (I doubt one would, but I am always very careful). When my peahens need the calcium they will come up and eat the egg shell bits out of my hand. I can sprinkle the fragments in or not in their food and they will eat it either way.
Oyster-shell - Most people use oyster-shells for calcium. You can find a bag of crushed up oyster-shell at Tractor Supply Co. and if you don't have one of those perhaps a local feed store will have some. I sprinkle the oyster-shell into my bird's food. Others say that they don't really touch it unless if it is mixed in with their food.
Hi-Calcium grit for birds - You can pick this up at a pet store in the bird section. I use the Kaytee brand and it is for small birds such as parakeets. I never use a whole lot of this stuff. I just sprinkle a little in their food. When I smell this stuff, I wonder if I am actually smelling the calcium it has such a strong smell to it. Yet another reason why I don't use too much!
Other calcium sources are certain vegetables. There is even a calcium powder you put in their water a believe. The 3 calcium sources I talked about above are just the ones I use.
Alright so how much is too much calcium? Well, generally I think the peahens are good about not eating too much of the calcium rich things you give them, but you must be super careful if you use something like powdered calcium. I really love using crushed egg shells because the peahens can see it and they can eat it when they need it. It isn't hidden and it doesn't have to be. I have watched them readily eat it. A way that I monitor calcium is by looking at the eggs I get from my peahens. I tap the egg and hold it making sure the shell seems nice and thick. If I see calcium deposits on the egg - which will be like raised areas on the egg - then I know that the peahen is definitely getting enough calcium, so much so that I might want to hold back on giving her more until after a few days. It is just something you have to use your best judgment on.
As far as calcium deficiency issues, I know that if your peahen is not getting enough calcium she might start laying soft shelled eggs or eggs without shells at all. These types of eggs can be very hard for your peahen to pass, and of course one of the worst things that can happen is for your peahen to become egg bound. I have not really dealt with an egg bound peahen and thus can't really go into detail about it, but I will say that on days when I see one of my peahens with her wings down and her tail tilted upward and fanned slightly - I know she might be having a bit of trouble laying an egg so I make sure I put out calcium and by the next day I find the hen looking fine and I find an egg.
So in conclusion it is very important to provide your peafowl with calcium especially during the breeding season. You can avoid a lot of stress and issues and you can also get a max amount of eggs and successful hatches if you keep up on feeding your peahens calcium. It is extremely important for the peafowl, and even the eggs themselves and the peachick developing inside.