A rooster fertilizes chicken eggs by mating with a hen before the formation of an egg. If a hen lays an egg without first mating with a rooster, the egg is not fertilized. However, if she lays an egg after mating with a rooster, it is fertilized, according to Chow.
Hens do not need a rooster to lay an egg. Eggs sold in supermarkets and at farmers markets are all unfertilized eggs. Hens lay eggs whether or not they mate with a rooster based on natural light patterns, reports Chow. When a hen mates and then lays an egg, under suitable incubation conditions, that egg is likely to bear a chick.