Egg production
Have you ever wondered how an egg is produced? The production of an egg in birds generally takes about 24-26 hours, in which time yolk is released from an ovary, the egg is fertilised, egg white is added, a shell is formed and pigment is added to create the colour of the shell! In this article, I will explain in more detail how each of these parts of the process occur, illustrated with diagrams to make it easier to visualise. This diagram shows the female reproductive system of a bird, labelled with the key sections.
Ovary - yolk
Whereas mammals have two ovaries, there is only one functional ovary (on the left side) in adult female birds. Interestingly, the right ovary is present as an embryonic structure when a bird is developing, but this degenerates after hatching. In the left ovary, there are many follicles where immature ova (eggs), also known as oocytes, develop. The raw materials of these oocytes - proteins and lipids - are transported in the blood from the liver, where they are produced, to the ovary. They are then formed into spheres of yolk in the developing ovum, and the nucleus containing all the female genetic material is condensed into a disc on the surface of the yolk, called a germinal disc.
The main part of a female bird’s reproductive system is the oviduct, which is a long, coiled tube connecting the ovary to the vagina. It is divided into five sections: the infundibulum, magnum, isthmus, shell gland (part of the uterus) and the vagina; each of these sections has a different function so that the egg develops as it passes through the oviduct.
Fertilisation and addition of albumen
Fertilisation of the egg takes place when the oocyte enters the infundibulum. This only occurs once the yolk is large enough to support a developing chick, and often requires more than one sperm to penetrate the egg, so that whilst one sperm fertilises the egg, the others release chemicals which stimulate the egg’s development. Interestingly, in birds, the sex of the chick is determined before ovulation and fertilisation because the sex chromosomes are reversed compared to humans: in humans, males have XY chromosomes and females have a repeated X chromosome (XX), whereas in birds, males have a repeated Z chromosome (ZZ) and females have ZW chromosomes. This means that the sex of the bird depends on whether the ovum released during ovulation contains the Z chromosome or the W chromosome from the female parent.
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As the fertilised egg (zygote) continues to pass through the infundibulum, a thin layer of dense albumen is added. Chalazae, twisted strands of the dense albumen which suspend the yolk between the two ends of the egg, also form. Albumen, the white of the egg, is a protein, which provides nutrients for the developing embryo, has antibacterial properties and also helps to provide an aqueous environment for the embryo to develop in. The majority of the albumen is then added in the magnum, along with ions such as calcium, magnesium and sodium.
Formation of shell membranes and hard shell
In the isthmus, the membranes of the shell begin to form, which help to hold the shape of the egg. Not all species of birds have an isthmus - in some species the magnum leads directly to the uterus. Once the shell membranes have formed, the egg moves into the uterus where the hard shell forms. This is a slow process and the egg spends about 80% of its time in the oviduct in the uterus. Cells lining the uterus secrete solutions including calcium carbonate, which are added to the egg to form the shell. The shells of most species’ eggs have many pores to enable the exchange of respiratory gases during the chick’s development, but the surface of the shell varies between species depending on their habitat. For example, the shells of some species living in particularly humid environments may be textured so that they are more hydrophobic, preventing water from blocking the pores. This also has the additional benefit of helping to remove bacteria, as dirt is trapped in the water droplets and subsequently falls off the shell. However, regardless of the species’ habitat, every shell has some waterproofing and protection against bacteria due to the thin cuticle which covers the outside.
Pigmentation
Some cells in the uterus also release pigment to give the shell its distinctive markings or colour. Although there is a lot of variation in the colour of egg shells (even within a particular species), only two different pigments are used to produce this range: protoporphyrin, which produces a red-brown colour, and biliverdin, which produces a blue-green colour. Streaks and spots of pigment are created as the shell slowly revolves in the uterus whilst cells surrounding it release pigment. The amount of pigment, like the shell surface, partly depends on the species’ habitat, as pigmentation can help to regulate the temperature inside the egg by controlling the amount of light which can penetrate the shell. There are also other factors which affect how much pigment is added to the shell, for example, the shells of ground-nesting birds’ eggs may be more patterned to make them more camouflaged to avoid predation.
Incubation
Once the shell has fully formed, the egg can leave the uterus and pass through the vagina out of the bird’s body. During the period of incubation, the egg changes slightly as the embryo develops inside it. Firstly, water is lost from the egg, forming an air sac at one end. The shell also gradually thins as calcium is leached from it by the embryo to help form bones and muscles. Additionally, calcium is used to create an egg tooth in the developing chick, which enables it to break the shell from the inside when it is ready to hatch. Thinning of the shell is also helpful in making it more permeable as the rate of gas exchange increases.
After hatching, the egg shell has no further use so it is usually removed from the nest by the parents.
References
All diagrams were drawn by me!
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​https://www.bbc.co.uk/news/science-environment-40016817
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https://www.birdguides.com/articles/understanding-eggshells/
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https://ohiostate.pressbooks.pub/vethisto/chapter/13-avian-female-reproductive-tract/
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http://www.scottemcdonald.com/pdfs/Article%20Ana%20Phy%20Repro%20Systems.pdf
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https://www.youtube.com/watch?v=PcI72ldPvx8
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'Animal Biology' (Grove and Newell), Sixth Edition. Published 1942, revised 1961.
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All websites accessed July 2024.