Fertile eggs will be produced by breeders if cocks are always kept with the hens. This is because fertilization occurs when a rooster mates with a hen, and the fertilized egg can develop into a chick. Keeping the hens and roosters together allows for this natural process to occur, and results in the production of fertile eggs.
While illumination of laying units, prevention of brooding, and proper storage of eggs can all impact the quality and viability of the eggs, they are not necessary for the production of fertile eggs. It is the presence of a rooster and his ability to fertilize the eggs that is the critical factor in the production of fertile eggs.