When a hen reaches 18 to 20 weeks of age, she begins to lay eggs. In ideal conditions, she will generally produce about one per day, but may occasionally release two in the same 24-hour period. In a natural environment, the bird will continue to lay until she has several in the nest, at which point she will sit on them until they hatch. In a farm, however, the eggs are removed, and she will continue laying. During the course of her lifetime, a hen may produce as much as thirty times her body weight in eggs; at one per day, for almost two and a half years, that is about 900.
The Reproductive Cycle
The reproductive cycle is set by day length and generally lasts about twenty-four hours during the summer months. As the days shorten, and winter approaches, a hen will lay fewer eggs, skipping some days. Some will stop laying altogether until the spring arrives. The reason behind this is that winter is a bad time to raise chicks as the cold weather decreases their chances of survival.
A hen does not need to be fertilized to lay an egg, and most birds in a laying facility have never even been in contact with a rooster. The egg-laying passage, or oviduct, opens into the cloaca, the passage through which droppings and urine are expelled; however, during laying a flap of skin stretches down to separate these areas so that the eggs are not contaminated by droppings. For this reason, the newly laid egg is clean, although it can pick up dirt later, for example, from the hen’s feet.
After laying, the hen will leave the nest, allowing the egg to cool. This prevents it from hatching, but the embryo remains viable for up to two weeks in these conditions. The bird will continue laying each day until she has several eggs in her nest, at which point she becomes “broody.” A broody hen will sit on her nest all day and night, her wings spread slightly to help keep the eggs warm. Since the embryos’ growth was paused while the nest was being laid, they will all develop at the same time.
A broody hen will leave the nest, briefly, once a day to defecate, eat, and drink, and anyone who gets too close to her eggs will be pecked at. After three weeks of brooding, her chicks will emerge. Any that don not hatch will be left behind as she brings the new chicks out into the world for the first time, and the nest will be abandoned.
In farms or egg-producing facilities, this natural process is broken up. Lighting can be adjusted to make the hen think that the day length has not changed, so that production continues at summer rates throughout the year. Eggs are removed after they are laid, leading the bird to think that there are still not enough in her nest, and causing her to continue laying. A hen that is resistant to laying in a nest can be encouraged to do so by placing a few fake eggs there.
After several months of laying, a hen may go through a molting cycle, during which old feathers are shed and new ones grown. Due to the energy required to molt, the hen’s body will not have the resources to produce eggs. After molting, however, production tends to reach a new peak. When birds in a modern facility have been laying for some time, production is reduced and the quality of the eggs declines, and because of this, many facilities will use various methods to induce molting, in order to improve production and quality after the molt. One controversial method is by withholding feed for 7-14 days; however, this is not allowed in some countries, and an alternative method is changing to lower density food.
Factors Affecting Laying
Hens will normally continue to lay until they reach two or three years of age, but there are a number of reasons, apart from lighting, molting and age, that can reduce or stop egg production. One of these is poor nutrition. If a hen is not receiving proper amounts of feed, or if there is an imbalance, for example too much or too little salt, she will be unable to produce eggs. The bird must receive adequate amounts of vitamins, and minerals such as sodium and calcium.
If feed is not stored properly, molds can grow on it. These can release toxins that affect the health of hens and may prevent laying. Birds raised in backyards may also eat objects that are not part of their feed, and may be harmful, such as the seeds of toxic plants. External parasites — such as fleas, lice and mites — and external parasites — such as roundworms and tapeworms — may affect laying. A drop in egg production can also be caused by various diseases, and by stress.