What Causes Thin-Shelled Eggs?

cracked chicken eggs

Backyard poultry keepers have sure become popular lately! With the demand for our eggs increasing and spring right around the corner, it’s only logical that we are looking forward to increased egg production very soon. If you have had backyard chickens or ducks for a few years now, you may have occasionally had problems with thin-shelled eggs. Let’s talk about the potential causes for thin-shelled eggs and some tricks to help prevent this issue in the future.

How Is an Egg Made?

An egg is made from the inside out. This means in the process of producing an egg, the first thing the hen makes is the yolk and the last thing she does is complete the shell. All of the shell formation happens in the Shell Gland which is located just above the cloaca and vent in the chicken’s reproductive tract. Shell formation is also the most time-consuming part of the egg-making process. It takes about 15-20 hours for a hen to make the shell for a single egg.

Causes of Thin-Shelled Eggs #1 – Calcium Deficiency

A calcium deficiency is arguably the most common reason that your birds may have thin-shelled eggs. An eggshell is comprised of over 90% calcium. Calcium is found in complete layer feeds and supplemental calcium sources, like oyster shells or limestone chips. However, calcium does not travel alone! Have you ever wondered why Vitamin D is added to whole milk for humans? Vitamin D is a vital nutrient that is required to help calcium be absorbed during digestion and vice versa. Without sufficient levels of Vitamin D, hens cannot absorb enough calcium to continuously produce high quality eggshells. Other trace minerals, like manganese and zinc, also play very important roles in the formation of eggshells. For these reasons, it is important to make sure your egg-laying poultry are receiving a complete layer feed that contains all of these nutrients in the correct proportions to help maximize egg quality.

Causes of Thin-Shelled Eggs #2 – Age of Your Hens

Older hens are more likely to lay thin-shelled eggs. An older hen is a bird that is 3+ years old. This happens for a couple of reasons – 1. Older hens usually lay larger eggs and; 2. As a hen ages, her reproductive efficiency decreases. Just like any other animal, as a hen ages, her reproductive tract starts to show signs of wear and tear. She will start to produce eggs that are of lesser quality, which can include meat spots, blood spots, and thinner shells. Allowing your hens to molt can help alleviate some of these challenges, but cannot eliminate them entirely. This is the primary reason that some egg producers choose to cull their older hens.

Causes of Thin-Shelled Eggs #3 – Egg Size

Egg size can have a negative effect on the quality of your eggshell. The amount of calcium in a large eggshell is the same as the amount of calcium in a jumbo eggshell. This means that a hen has to cover more surface area with the same amount of “building material” (e.g., calcium). The only way to do that is by making a thinner shell. Controlling the amount of protein that your hens consume can help control the size of your eggs. Birds fed very high protein feeds (20%+) or lots of high protein treats (think mealworms or cat food) are more likely to lay extra large or jumbo eggs and, thus, have thinner eggshells. The age of your hens is also a contributing factor. Older hens tend to lay larger eggs.

Causes of Thin-Shelled Eggs #4 – Poorly Functioning Shell Gland

As mentioned above, the entirety of the shell-making process happens in the shell gland. Since this organ is located directly above the cloaca, it is vulnerable to infection. A hen with a poorly-functioning shell gland can lay thin-shelled eggs. A hen with an infected shell gland is likely to bypass the shell altogether and lay a shell-less egg that is only held together with the soft membranes. If you see a sudden or significant increase in shell-less eggs in your flock, consult your veterinarian to determine if you may have a disease outbreak that is having a negative impact on your egg quality.

Causes of Thin-Shelled Eggs #5 – Stress

It takes about 15-20 hours for a hen to produce the shell of a single egg. If during those 15-20 hours, a hen is exposed to a significant stress, she is more likely to lay a thin-shelled or a cracked egg. Stress can happen from a number of factors, but common sources would include heat stress and overcrowding. A small amount of heat stress is unavoidable during the summer. However, on very hot days, heat stress can have a significant impact on the quality of your eggs. Make sure your hens have access to fresh, cool water and plenty of shade during those hot days to help prevent these stressors. Overcrowding can also have a negative effect on the quality of your eggs. When hens are competing for roost space or nesting box space, they are pulling energy and resources away from egg production. Additionally, overcrowding can exacerbate poor egg quality because thin-shelled eggs are more likely to break as birds compete for nesting-box space.

Causes of Thin-Shelled Eggs #6 - Mycotoxins

Mycotoxins are toxic compounds that are naturally produced by some types of molds or fungi. These mycotoxins start to develop on crops during the growing season if we have very wet, hot, or humid conditions. All grain crops can be susceptible to these types of fungi and would include wheat, barley, oats, corn, rye, etc. Mycotoxins are invisible and have no smell. Even if you do not see mold or fungus growth, that doesn’t mean that mycotoxins are not present. Hens exposed to mycotoxins are more likely to lay thin-shelled or soft-shelled eggs. Look for a feed manufacturing company (like Kalmbach Feeds®!) that tests incoming grains for mycotoxins to help prevent these exposures.


We hope these guidelines are helpful as you continue your chicken-keeping journey. At Kalmbach Feeds®, we love our chickens, and we know you do too. Stay tuned! We are so excited to continue writing about all of the topics that are important to you. Keeping poultry is such a wonderful experience and the rewards are many! If you have any questions about the nutrient needs of your birds, feed options, or general poultry keeping, please let us know.

Nancy Jefferson, Ph.D.

Dr. Nancy Jefferson has been a member of the Nutrition and Technical Services team at Kalmbach Feeds since 2013. She received her Ph.D. from West Virginia University in 2008 and has worked in the feed industry for over 15 years. She lives on a farm in Crown City, OH with her husband, John, and their children. Dr. Jefferson is a passionate poultry enthusiast and loves her chickens! Together, she and her family raise beef cattle and she keeps an ever-growing flock of backyard chickens.