thin egg shells

You know, I'd try eliminating everything from their diet except the layer pellets, oyster shell and some greens.  They may be filling up un treats and not getting enough of the good stuff. 

Also how old are they?  If they just started laying, it may take a few days for them to produce regular shells.  In my experience the first week or so of laying is a mess, and then everything works out.

I agree with Poppycat,  mine are "regular" now, but in the beginning I would get good eggs and then thin shelled ones, it would come and go.  Are they beginners?  It only took a week or so then they get "regular".

Try putting the oystershell in a dish rather than scattering it - they may not be picking up enough for their needs

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.

& to our 2 newbies - the OP & Barb!

I keep this bookmarked for easy reference http://www.thepoultrysite.com/ourbooks/1/egg-quality-handbook/ - all the problems and remedies are listed. Barb I get the occasional one with blood - no prblems yet. But the egg eating is a concern, put some golf balls in there so that they can get out of that habit! Oyster shell should be free choice & available all the time. Few scraps are fine, Good Luck!

ETA:the link

Don't measure out the oyster shells.

Offer it free choice in a sheltered area.  They will eat it as they need it.

I agree. It looks like you two have been lurking around BYC for quite awhile.

I agree about free choice calcium. In addition to the oyster, I really recommend fedeing back their own eggshells.  (That's the only calcium we use around here and it works great).  Just let shells dry in a bowl for a little bit and every week or so throw the whole mess of them into the blender/food processor.  Grind it all up fine in a whirr or three, and feed em right back to the source, also free choice.  That should take care of it.

Barb's Organic Garden :

They also get kitchen scraps and their own egg shells back.  .

This is an old thread that I searched, but doesn't giving their eggshells encourage eating their eggs? This posted had a problem with them eating eggs.