I don't think I've ever had a broken egg. Some of the things that occur to me are: how many nestboxes do you have? If they are all crowding into one box, that could explain some breakage. Also, do they have oyster shell available. If the eggs are breaking that easily, it sounds like the shells are too thin. Heck, I dropped one onto hard ground once after collecting it and even that egg didn't break! Last, is it possible you have an egg eater who is deliberately breaking the eggs in order to get into them?