I had the same issue last week. I never did find the culprit but I only have 12 hens, not 60 to manage. I really can't help out much, I have the luxury of being able to constantly check on them. I practically took the eggs away as soon as they dropped them. I have done this for for 3 days now. I am hoping they will forget about it and move on. I was reading on an online blog about clipping the ends of the beaks of the culprits. Don't know if I could do it. I hope someone chimes in that can give you some better advice. Good luck