Egg-eating

Hey, Everyone!
I have a flock of about sixty (60) hens and two roosters. I used to get at least thirty (30) eggs a day, but within the last month production has really dropped. At first I thought it was because of the colder temperatures (I live in Wisconsin), but then I started finding partially eaten eggs or yoke on other eggs. I put golf balls in the nesting boxes as well as mustard-filled eggs, but so far there have been no results. I have never actually seen any hens eating the eggs, and even if I knew who was eating the eggs, I really would rather not kill the egg-eater(s). Does anyone have any ideas on how to stop this? Proven methods (other than culling)? I'm starting to get really desperate because I read that the longer egg-eating goes on, the harder it is to break.