I only started marking mine when I found hens sitting on my broodies to lay their eggs. When my egg collection rate went down by 1/2 I knew I had to do something or the broodies would end up falling out of the nest due to the growing pile of eggs under them. So, marking was in order. This also helped to assure that there weren't any 'special'
surprises when opening an egg that had actually been slated for a hatch. Good thing the person with that egg was a good ole farm girl. Ah well, live an learn.

