1. Length of time in lay: the longer the bird is in lay, the weaker the shells will become because of her inability to obtain daily enough calcium from her diet to supply all of her needs for one egg. As a consequence, the better layers will deplete their skeleton.
2. Increased environmental temperature: this results in reduced food consumption (and calcium) and the reduction of carbonate ions because of panting.
3. Egg laying time: Eggs laid early in the morning are more likely to have thinner shells than those laid by the same bird later in the day. This is because in the case of those eggs laid early the shells have been deposited during the hours of darkness when the bird doesn’t eat i.e. consume calcium for the shell.
4. Stress: Stressed birds lay thinner shelled eggs.
5. Body checked and misshapen eggs: Most of these defects are caused by the birds being startled shortly after the egg has entered the uterus and the first layers of calcium carbonate have been deposited. At this stage the shell is very fragile and weak and when startled the hen’s muscles contract (including those in the wall of the uterus) and thus crack the newly forming shell. These are covered by subsequent depositions of shell but the damage remains in the form of body checks and/or misshapen eggs.
6. Disease: Certain diseases e.g. infectious bronchitis cause weak shell and misshapen eggs.
7. Drugs: Certain drugs influence eggshell formation and deposition.

The shell of an egg contains openings or pores. There are approximately 8000 such pores in the shell of a normal hen’s egg. The function of these pores is to provide for the gaseous exchange during incubation and embryonic development. The developing embryo requires oxygen and gives off carbon dioxide. When the egg is first laid most of the pores are closed. However, as the egg ages more and more pores open up. The cuticle deposited on the outer shell is composed of organic material and water and blocks the pores. During the laying process the cuticle acts as a lubricant, but once laid, the egg’s surface soon dries and the residue - mainly protein closes off most of the pores as a barrier to the invasion of bacteria and fungi.

Vagina

The vagina is about 12 centimetres in length. While not known for sure, it may have the function of adding pigment to the outer shell to provide the egg with its colour.

Cloaca

The egg is held in the cloaca immediately prior to being laid. It may be in the cloaca for several hours, but usually is held there for a much shorter time. Although the egg usually enters this organ small end first, it usually rotates there to be laid large end first. However, if the bird should be startled at this time the egg may be forcibly expelled small end first.

Summary of the reproductive system

The domestic fowl’s reproductive system, while functioning in some ways in a manner similar to that of mammals, is very different in many aspects. Evolution has given the birds a reproductive system where the offspring are separated from the mother to enhance her ability to fly and reproduce at the same time. The formation of an egg is a very complex activity during which much can go wrong. The quality of the final product, the egg as it is laid is influenced by both genetic and management factors. The efficient manager, to be able to produce eggs efficiently whether for incubation or for table use, should have a good knowledge of the reproductive system and the formation of the egg.