You can ponder the age-old question about whether the chicken or the egg came first, but it is all part of the life cycle story. Whether managed for commercial use by farmers or kept in small flocks to provide eggs for a family, laying hens are raised all over the world.
A hen is female chicken that has reached reproductive age. Immature female chickens are called pullets. Most pullets reach maturity at about 20 weeks. During the time a pullet matures, she grows in size, becomes covered with adult feathers and takes on the characteristics of her breed, including colour, fleshy comb and wattles on her head. Her breed determines the colour of the eggs she lays; there is no nutritional difference among eggs of different colours. Eggs can range in colour from deep chocolate brown to blues and greens to white.
Every female is born with all the potential eggs she will ever lay stored inside her ovaries. She will lay approximately 270 eggs a year during her peak production period, whether or not the eggs are fertilised. If a hen does mate with a rooster, she can store spermatozoa inside her body for up to 30 days and produce fertile eggs that are capable of developing into chicks.
Each egg begins as a single-celled ovum. It takes about 25 hours for the ovum to pass through and out of the hen's reproductive tract. The immature ova are attached to yolk cells. Through hormonal influence, one ova and attached yolk begin to mature and is released into the infundibulum. If sperm are present, this is where the egg is fertilised. If not, the egg will be produced in its infertile form. As described by Judy Nielsen, DVM, at Purdue University, the egg white, or albumin, is added in the next part of the reproductive tract, called the magnum. A tough membrane is formed in the isthmus. In the uterus, the hard, calcium-rich shell is applied. Then, the egg passes through the vagina to the cloaca, which is a common area for the elimination of waste as well as eggs.
A broody hen is one who sits on her eggs to hatch them. She can lay as many as 8 to 10 eggs over the same number of days before stopping to incubate the clutch. Once a fertile egg is laid it takes about 21 days to hatch. Development occurs rapidly. According to an informational report from the Mississippi Agricultural and Forestry Experiment Station/Mississippi State University Extension Service, by the end of the first 24 hours after fertilisation the embryo has the beginnings of a spinal column, digestive system, nervous system, head and eye. By the end of the second day its heart starts beating. Legs and wings start to grow on the third day. Feathers begin to appear on the eighth day. By the 14th day, the chick begins to get ready to hatch. While inside the shell the chick gets its nutrition from the yolk. Over the final week the yolk becomes depleted and the chick begins to breathe air inside the shell.
The mother hen does not begin to incubate her clutch until all of the eggs have been laid; the eggs will not begin to develop until the temperature is correct and consistent. This ensures that they will all hatch on the same day. A day or two before the chicks hatch, they may begin to peep, signalling to the mother that they will soon be born. When ready, each chick uses a special hard growth on its beak called an egg tooth to break through the shell and emerge. Newly hatched chicks need to dry and fluff up. If they are being raised by their mother, they will create a visual connection with her called imprinting; they will follow her and stay with her for warmth and protection.
Once a hen begins laying, she will produce the most eggs during her first year. Production declines significantly after one year, so most commercial egg producers have the hens slaughtered, according to the British Columbia Society for the Prevention of Cruelty to Animals. Otherwise, the lifespan of a healthy hen is between 5 and 11 years.