My son recently combined eggs from two broody hens under one hen. The first hen's eggs were at least a week ahead of those that were taken from the other hen. He knew start date for incubation with first hen although have only a rough estimate for the other. First hen has hatched off and switched from incubation to chick brood mode causing son to move eggs to an incubator where he disabled turner so he can hand turn eggs and inspect for development by candling. Since in third week now we can only see that volume of eggs that are not air sack on large end is black. The problem is that occurs starting about two weeks into incubation cycle. We wondering if there is some way we can determine if a given egg is viable in the last week when we don't know when pipping and actual hatch are to occur? My son thinks he saw an egg rock, but that could be his sister's sheep bumping bookshelf supporting incubator. Don't ask about the sheep in the house.