In Animal Science, there is a lot of enthusiasm to learn as much as possible about the animal or types of animals you like the most. In my case, I found a passion for chickens. As a graduate student, I've had the opportunity of being a teaching assistant and lab instructor for the Poultry Science course. Today, I share some of the most commonly asked questions from my experience as a poultry science TA inside and outside the classroom.
How are eggs formed?
First of all, what are eggs? We all have that image pop up, right? An oval object laid by a hen, rich in nutrients that could very well develop into life and sustain it outside of the hen. We are aware that eggs are a means of reproduction, but when comparing against other species like mammals, what would the egg be equivalent to? To be able to answer that, we have to look at the structures of the egg.
The yolk is the first thing formed inside the hen. It grows in the ovaries until it is mature and ovulation occurs. Yes, the yolk is the ovule (ovum). Fertilization--if it were to occur--will happen before other structures begin to form and impede the sperm cells from reaching the ovum.
The following structure that starts to form is the albumen or the egg white, which contains the primary source of water and proteins (Willems et al., 2014). Since the embryo won't be growing inside the chicken, she makes sure that the egg has plenty of the nutrients needed for development. The chalaza, a structure that keeps the yolk suspended in the middle of the egg, starts to form. It prevents it from touching and thereby sticking to the wall of the shell during development. Before getting to the shell gland (uterus), the hen adds two membranes--an internal and external layer--that help protect the embryo from any harmful pathogen that might get past the shell.
Finally, in the shell gland (uterus), we see the formation of the shell. The calcium carbonate (CaCO₃) shell has tiny pores on the surface that allow the embryo to breathe during its development--similar to how we exchange gases inside our mother's womb through the placenta--. The final product is the egg as we know it. Now we understand how complex and well-calculated this process can be so that the egg may sustain an embryo during its development. Their nutritional content is why humans use them as a food source. A single egg will contain all six nutrient sources (water, proteins, carbohydrates, lipids, vitamins, and minerals).
What conditions do chickens need to lay an egg?
A chicken will hit sexual maturity around 24 weeks of age. She'll start laying eggs if the appropriate conditions are met. But what does a chicken need in order to lay an egg? An important factor is the number of light hours they receive. You see, chickens lay their eggs when the days are getting longer. A hen would need at least 12 hours of light to lay an egg. In the wild, this usually starts after the winter solstice and reaches its peak during early spring. This period also represents an optimal time for reproduction because it's when food is plentiful for chicks to eat in nature (Sharp, P.J. 1993). Nutrition is also a key component. Chickens will not lay as many eggs or any eggs at all if nutritional requirements are not met. Along with a good layer diet, water, and 16 hours of supplemented light to achieve the maximum potential, hens are able to meet the appropriate conditions to lay eggs (Brand H. V. et al., 2014).
How many eggs does a hen lay in a day?
Short answer? Technically one.
Hens lay an egg every 25-26 hours, meaning that a single hen won't lay an egg at the same time every day. Once ovulation occurs, the yolk will spend around 15-30 minutes in the infundibulum, where it can be fertilized by sperm cells. After that, it will spend three hours in the magnum where the egg whites will form and about one hour and 15 minutes in the isthmus where the membranes will form. Once it arrives at the shell gland (uterus), the shell formation process will take between 18-22 hours until oviposition (Coutts J. & Wilson G. 2007).
Does the chicken need a rooster to lay an egg?
No, the chicken lays her egg regardless of having the presence of a male. Now, if the question is: "Does the chicken need a rooster to be able to have baby chicks?" then the answer is yes. The egg needs to be fertile in order to have chicks, yet the hen would gladly lay an egg if the right conditions are met. Most store-bought eggs are not fertile with a few exceptions; most hens that are part of the table egg industry have not even seen a rooster in their lifetime.
Do chickens pee?
Chickens do pee. They just have a different way of doing it. Despite not having a bladder, chickens still need to get rid of ammonia from their body. They excrete it as uric acid instead of urea as mammals do. The thing is that uric acid is not water-soluble like urea, so it ends up as small crystals on top of the chicken poop. Since both urinary and digestive systems connect through the cloaca, you will find uric acid crystals as the white coating in bird poop. The cloaca in birds and reptiles connects three different systems, the urinary, digestive and reproductive systems.
What gives the eggshell its color?
Genes, yes genes. Many people believe that eggshell color is due to feed additives or even nationality. In Puerto Rico, for example, there is a cultural belief that locally produced eggs are brown. In contrast, white eggs are thought of as imported eggs--even I was taught this as a child--. Local markets even produce brown shell eggs to satisfy the public's belief. Eggshell color depends on the breed of the chicken and thus its genes. Eggshell color results from the pigments the chicken is available to produce. An absence of pigment production results in white eggshells, while pigments such as protoporphyrin-IX, biliverdin-IX, and Zinc chelate result in a brown eggshell (Cavero D. et al., 2012). There are also blue eggshells given by oocyanin pigment.
Is the eggshell soft or hard when it's laid?
For some reason, when eggs are laid, there is this common belief that they have a soft shell, and by some act of magic, when in contact with air, the shell hardens. Considering the oxygen in the blood flowing through the hen's body, including her reproductive system, this is a strange belief. In reality, when the egg arrives at the uterus or shell gland, tiny calcium carbonate crystals start to form on the surface of the egg. Once the shell is completely formed, it will be as hard as the shell outside of the uterus. One could easily verify this by touching the area between her pelvic bones. If there is an egg with a fully formed shell, you will be able to feel it.
Chickens genuinely are remarkable creatures, and this article only scratches the surface of the wonderful world of poultry science. I hope this small overview can clear up any misconceptions surrounding chickens, especially egg production and reproduction. Feel free to reach out if you have any more questions!
Works cited
Brand, H. V., Parmentier, H. K., & Kemp, B. (2004). Effects of housing system (outdoor vs cages) and age of laying hens on egg characteristics. British Poultry Science, 45(6), 745-752. doi:10.1080/00071660400014283
Cavero, D., Schmutz, M., Icken, W., & Preisinger, R. (2012). Attractive Eggshell Color as a Breeding Goal. Lohmann Information, 47(2), 15. Retrieved May 1, 2021, from https://www.lohmann-information.com/content/l_i_47_artikel12.pdf.
Coutts, J. A., & Wilson, G.(2007.). Formation of the egg - Optimum Egg Quality: A Practical Approach - The Poultry Site: The Poultry Site. Retrieved April 22, 2021, from https://www.thepoultrysite.com/publications/egg-quality-handbook/2/formation-of-the-egg
Sharp, P. J. (1993). Photoperiodic Control of Reproduction in the Domestic Hen. Poultry Science, 72(5), 897-905. doi:10.3382/ps.0720897
Willems, E., Decuypere, E., Buyse, J., & Everaert, N. (2014). Importance of albumen during embryonic development in avian species, with emphasis on domestic chicken. Worlds Poultry Science Journal, 70(3), 503-518. doi:10.1017/s0043933914000567