With the usual excitement about the approaching breeding season, it’s a good time to consider the “mechanics” of producing an egg.

Birds, reptiles, and monotreme marsupials have evolved a complex system of reproduction where eggs are laid then incubated outside the body. The eggs can be produced independently of a male, but obviously the female should be mated and inseminated if fertile eggs are required. This need only happen once before a clutch is laid as many have evolved means of storing sperm as eggs are produced singly (most clutches are laid over a period of days with eggs being laid every 1-2 days). However, some species will mate throughout the laying process.

Firstly, the female must be stimulated to come into lay.

Just as in mammals coming into season this starts with a cascade of hormones starting in the brain that stimulate hormonal activity.

One of the first things that will happen is the laying down of nutrient stores to supply the manufacture of eggs. In particular calcium deposits are laid down in the bones and fat is deposited in the liver. These stores are not replenished through the breeding season which is why birds that are induced to lay several clutches may “run out” of calcium and start laying soft-shelled eggs, become egg bound or have chicks with soft bones

The female will also be stimulated to start nest building. In some species the male may do this, may assist her or may do absolutely nothing!

During this period the male may develop breeding plumage or characteristics. He will then start displaying/ calling.

What factors set off these changes in the female? They vary with species and may include:

1. Daylight changes
2. temperature changes
3. humidity/ rainfall changes
4. flushes of available nutrients
5. presence of a nestsite/- box
6. the calling/ displaying of a male

Internally the ovaries enlarge and start producing egg follicles. These are the “yolks” of the eggs. The yolk is the main nutrient source of the chick containing proteins, fats, vitamins and calcium. The chick develops from a germinal disc on the surface of the yolk (the yolk is covered by two membranes). From the disc blood vessels are produced that radiate over the yolk and are used to absorb nutrients. This is what we look for when we “candle” an egg – where a bright light is shone through the egg so we can see these blood vessels and, eventually, the chick itself.

The follicle is ovulated from the ovary (note in birds there is generally only one ovary (the left)) and "captured" by the infundibulum – the opening of the oviduct. Here it will be fertilised and the outer egg layers will be added.

The egg proceeds down the oviduct:

1. Infundibulum. After fertilisation the first layer of albumen (egg white) is added. Albumen is a clear protein substance that forms the majority of the egg. It is not a single layer but is a complex of many layers and even includes twisted cords (the chalazae) at each end that support the yolk in the centre of the egg. Albumen acts as a shock absorber during incubation and supplies minerals water and protein to the chick as well as allowing gases to diffuse through to the chick. Enzymes in the albumen prevent infection
2. Magnum. Outer layers of albumen are added
3. Isthmus. Shell membranes are produced. These cover the albumen and determine the final shape of the egg. They are also active in incubation allowing gas exchange and also (at the end of incubation) secrete an acid that removes some calcium from the shell so the chick can develop its bones.
4. Shell gland. This is where the complicated shell is laid down. The majority is called the testa. It is calcified being 98% calcium carbonate secreted round a protein matrix. It contains pores that allow gas exchange. The cuticle is the outer layer of shell. This is shiny and consists mainly of protein and fat. It repels water and bacteria
5. It then enters the muscular vagina from which it is expelled during laying

It is hard work producing such a complicated structure!

After laying the work continues as the egg must be incubated. This, again, varies between species with variable amounts of turning required. Incubation temperatures vary as does the amount of evaporation of water from the egg.

John Chitty BVetMed CertZooMed MRCVS