Chickens are not born in the traditional sense. They hatch from eggs produced by a hen, or female chicken, after she mates with a rooster. This process is called fertilization.
A hen produces eggs without a rooster. He is only needed for fertilization. If a chicken and a rooster mate before the formation of an egg, she will lay a fertilized egg.
Hens lay their eggs in nests. After she drops about 12 eggs, called a clutch, she sits on the eggs for about 21 days to incubate them. During this time, she maintains the nest at a steady temperature and turns the eggs regularly during the first part of the incubation period. She seldom leaves the nest to eat or for any other reason and will peck and squawk at anything that threatens her nest.
At the end of the 21 days, the fertilized eggs begin to hatch. Regardless of when the hen laid the eggs, development of the embryos only begin with fertilization. The entire nest of fertilized eggs will hatch within a day or two of each other.
If the eggs are placed in the refrigerator before the incubation period is complete, the growth of the embryo ceases.