Did you ever figure out what this is? I had 1 girl with this in her poop about 3 days ago. I thought maybe oyster shell? Now I have 3 piles with it in them. I checked all girls and 2 have mites. I will treat right away. Could this be the cause?
They're all very even, rectangular and white. Like miniture grains of rice
I am in the process of dealing with tapeworms and everything I read says "looks like grains of rice..." so I would treat for tapeworms. Or a least google tapeworms and chickens and see if you see anything familiar.