CAUSES OF THIN EGG SHELLS

The following should be considered as general information; however, you should always seek professional advice, specific to your individual circumstances.

Causes of thin egg shells include:

NUTRITIONAL DEFICIENCIES

Calcium Deficiency – the most common cause. A hen’s egg contains approximately 2 grams of calcium, which is primarily found in the eggshell. The shell itself is made up of about 94–95% calcium carbonate, and typically weighs around 5–6 grams, with at least 2 grams being pure calcium regardless of egg size. The hen will prioritize maintaining vital functions and bone structure over eggshell production, so in cases of deficiency, calcium is diverted away from the shell and used to support the hen’s bones and metabolism.

The recommended calcium requirement for laying hens is typically 3.5-4.5% of the total feed, depending on hen age and production phase. Higher calcium levels are critical for older hens to offset age-related declines in calcium absorption, and their eggs tend to become larger, requiring more calcium for shell formation.

Vitamin D Deficiency – Vitamin D is essential for calcium absorption in the gut. Without adequate vitamin D, even if calcium intake is sufficient, hens cannot utilize it effectively, resulting in thin shells.

Phosphorus Imbalance– both deficiency and excess phosphorus can interfere with calcium metabolism and shell formation. A total calcium-to-non-phytate phosphorus ratio of 8:1-12:1 is recommended.

Most phosphorus in plant-based feeds is bound as phytic acid and is not readily available to hens unless the feed contains phytase.

Trace Minerals (Zinc, Manganese, Copper) – are necessary for the activity of enzymes involved in eggshell formation. Deficiencies can impair shell membrane synthesis and the calcification process, causing thinner and weaker shells.

AGE OF HENS

Older Hens – as hens grow older, they lay progressively larger eggs. However, the total amount of shell material deposited remains relatively constant, so the shell is spread over a larger surface area, making it thinner.

While larger eggs contribute to thinner shells, studies show that even when egg size is accounted for, hen age itself is the dominant factor in declining shell thickness and strength. The uterus (shell gland) of older hens undergoes deformation, fibrosis, atrophy, and loss of ciliated cells, impairing its ability to efficiently deposit shell material.

Young Hens– pullets just starting to lay often produce thin-shelled eggs until their reproductive system matures and bone calcium reserves build up.

DISEASE AND HEALTH ISSUES

Infectious Bronchitis – a viral disease attacks the shell gland, leading to thin, misshapen, or shell-less eggs.

Other Diseases– Newcastle disease, avian flu, egg drop syndrome, and other infections can also impair shell formation.

Shell Gland Defects– some hens may have a physiological issue with the shell gland, making them chronically unable to produce normal shells.

ENVIRONMENTAL AND MANAGEMENT FACTORS

Heat stress – reduces feed intake and blood flow to the oviduct, impairs calcium metabolism, and alters the properties of the shell membrane, collectively reducing shell thickness.

Heat stress also causes hens to pant, which lowers blood carbon dioxide levels and raises blood pH (respiratory alkalosis). This disrupts the acid-base balance and reduces the availability of bicarbonate ions, which are crucial for calcium carbonate formation in the shell gland. The result is impaired shell mineralization and thinner shells.

Lighting Programs – increasing the photoperiod generally leads to larger egg size, which can also result in thinner eggshells. Longer photoperiods accelerate egg production cycles, which may reduce the time available for shell deposition and mineralization, leading to thinner shells.

The recommended light intensity for optimal eggshell quality in laying hens is around 60 lux. Research indicates that eggshell thickness and shell percentage are highest at this moderate light intensity, while both lower (15 lux) and higher (240 lux) intensities result in reduced shell quality.

Poor Ventilation/High Ammonia- poor air quality in the house can contribute to health issues and stress, indirectly affecting shell quality.

Inadequate Water – insufficient water intake can reduce egg production and shell quality.

FEED AND FEEDING PRACTICES

Feeding Time – eggshell calcification happens mostly during the night. Hens fed later in the day (afternoon or evening) increases the availability of dietary calcium during the period of eggshell formation. Studies show that hens fed in the afternoon or late evening produce eggs with significantly thicker shells compared to those fed in the morning.

Midnight feeding ensures hens consume calcium-rich feed closer to this active shell-building phase, providing a direct dietary calcium source instead of relying on bone reserves.

Calcium Particle Size – diets containing only finely ground calcium sources (such as ground limestone or oyster shell <1 mm) are absorbed quickly and may not provide sustained calcium release during the critical period of shell calcification. Substituting part of the fine calcium with coarse particles (2–4 mm) allows for slower, more prolonged calcium release in the digestive tract, improving calcium availability for shell formation. Diets lacking coarse calcium or containing only fine particles can result in thinner eggshells.

Mycotoxins – cause thin eggshells in layer hens through multiple mechanisms that disrupt nutrient absorption, metabolism, and reproductive physiology. Examples include:

Aflatoxins and ochratoxins impair kidney function, increasing calcium excretion and reducing its availability for eggshell formation.

Mycotoxins like aflatoxins damage the liver, where vitamin D3 is metabolized into its active form. This reduces calcium absorption from the gut, weakening shell deposition.

Deoxynivalenol, T2, and fumonisins – compromise intestinal integrity, reducing the absorption of calcium, phosphorus, and other critical nutrients needed for shell formation.

OTHER FACTORS

Molting – hens in molt may temporarily lay eggs with thinner shells as their bodies redirect nutrients to feather regrowth.