In the old days milk products used to be given a lot for protein and calcium. Now, I wouldn't use it FOR the calcium as the source. I'd use your feed and oyster shell for that. But I'd use it for the protein and the living bacteria that, since unpasteurized, are still in the milk.
If you google it, you'll find a lot of excerpts from older books where milk was given to birds because back then people had cows. sometimes they even used it as the water source, not that I'd recommend that now. But certainly using it to make wet mashes is wonderful and healthful.
I personally wouldn't recommend store-bought milk this way at all because of its lack of the bacteria in it that digest the lactose that you're giving.
If you see changes in their droppings, excess runniness, discontinue. And of course always give it safely - don't allow it to set particularly when mixed with feeds.