My 5 month old chickens started laying eggs a few weeks ago, after laying for about 1 1/2 weeks a found an egg without a shell, a few days later I again found an egg without a shell. I have been feeding them a mix of layer feed, grower, & shell calcium, & they range the fields every day. I am not sure which kind of bird is laying the unshelled egg, it may be a Rhode Island Red. What do you think is causing this? & what would you recommend doing?