I just started getting eggs from 3 of my 4 girls who free-range part of the day when I'm around, and are in the coop for the rest of the day. I was prepared for the fact that the first eggs might come out with weak or non-existent shells. Instead, all three are laying what seem to be indestructable eggs! I have to really smack the heck out of them to get them to crack. Now, I have a little feeder with oystershell and was told that they would eat it if they need it (self--regulating). They are plowing through it like crazy (I have to refill often). Do you think they could be overdoing the calcium, or is this normal?