I have been looking for the answers to two questions:
1. How much oyster shells should I give to 34 birds? I am giving two cups once a week on top of their food, but several times a week I find a mess in one of the nests, and one or more eggs have been eaten.  They are Buff Orpingtons, and with 31 hens I am getting just around a dozen eggs a day.  I do not provide artificial light,  so I get more and more each week.  They are fed organic laying mash which has a calcium supplement.  They also get kitchen scraps and their own egg shells back.  I live alone, so that's not much.  
2. I sometimes get an otherwise clean egg with a bloody smear on it.   Is this something to worry about?  The hens hatched Apr 20, 2008, so they are not novices.