Believe it or not, the egg laying process for a chicken begins in its eye. Chickens lay eggs only after receiving a light cue, either from natural sunlight entering a coop or artificial light illuminating a commercial egg hatchery. The light stimulates a photo-receptive gland near the chicken's eye, which in turn triggers the release of an egg cell from the chicken's ovary.
Most chickens lay eggs on an almost daily basis, unlike some other egg-laying animals which only release an egg every thirty days or so. The chicken also releases a small disk of material which surrounds the egg cell and provides nutrition. The chicken's uterus also fills up with albumen, the viscous substance we know better as egg white.
Meanwhile, a membrane forms around the inside uterine wall, which seals in the egg cell, yolk and albumen. Eventually a mixture of water, salt and calcium surround this membrane and form a thin but structurally sturdy outer shell. This shell is molded in the shape of the uterine wall, thus giving a chicken's egg its distinctive shape. While in the uterus, the egg's narrower end points downward, but it will later turn and be ejected wider end first.
Once the egg has fully formed, the chicken's uterus begins to contract in an effort to expel it. The egg moves down a vaginal canal towards an external opening known as a vent. The vent is a common opening for both egg laying and waste elimination, but a chicken cannot perform both functions at the same time. An internal flap known as a cloaca keeps the vaginal canal and the intestinal track separate until either an egg or excrement reach the vent. When a chicken is laying an egg, the cloaca descends and blocks the intestinal track.
Once the egg passes the cloaca, it is carefully expelled through a series of muscular contractions which essentially turn the vaginal canal and cloaca inside out at one point. Eventually the egg is pushed out through the vent and ideally lands intact on the ground. Many chickens let out an audible cluck at this point, but designated egg layers rarely display any other maternal concern. These chickens lay eggs every 24 to 36 hours at the height of their productive years, so individual eggs rarely attract their attention.
Once a chicken lays a certain number of eggs, it may stop producing more eggs and go into a "brooding" stage, a maternal stage in which the mother will sit on her eggs until they hatch. In order to prevent egg layers from brooding, freshly laid eggs must be removed on a daily basis. Chickens lay eggs as long as they believe they need more to form a proper brood. The egg laying process continues whether or not a rooster provides enough sperm for fertilization. A chicken cannot tell if an individual egg has been successfully fertilized or not.
