Understanding the Shell

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The eggshell is an impressive structure, designed to provide protection to the growing chick: strong enough to be sat on by mother hen, yet pliable enough to provide calcium for bones, exchange gases and break through to hatch. Shell ‘defects’ are mostly thought of as a financial loss, with consumers wanting a uniform, aesthetically pleasing egg; but the shell should be looked at as a window to the health, management and productivity of your flock. A sudden or early increase in seconds should always be looked at as a significant issue, and the type of second can help enable the farmer to identify and address issues before they have further implications on the production and welfare of the hens.

Ensure a positive energy balance

Nutrition obviously plays a key part in shell makeup. The shell largely consists of calcium carbonate and takes 20 of the 25hrs needed to produce one egg. Up to half of dietary calcium is needed for shell production, with additional coming from bone storage when required, but this should be prevented where possible. Birds who are in negative energy balance due to poor feathering, red-mite infestations, poor intestinal health or over-sized eggs; or when the diet is deficient in calcium, vitamin D3 and phosphorus; will need more support or thinner shells, leading to hairline cracks are inevitable. Heat stress can also affect calcium absorption due to metabolic pathways in the bird, particularly when panting; this summer we have had many reports of weak shell or even shell-less eggs following the hot weather. In these instances, supplementation is important and research has shown providing this; either via feed or water will improve production, FCR, hormone and immune function as well as the egg mass and shell quality. It is important to remember, if birds are laying a larger egg, the shell will naturally be weaker and thinner. As birds age, the bones also deplete in calcium and supplementation is very important in both cases before shells weaken.

A good proportion of the shell is formed during the night, so it is important that the bird has eaten enough to ensure enough food is available within the intestine before lights out. Physical disturbances, such as heavy red mite infestation or mucking out, can significantly interrupt rest periods and cause shell deformity.

Hairline cracks and shell damage can also be exacerbated by packing machinery and efforts should be made to make transitions as smooth as possible and keep belts moving consistently rather than stopping-and-starting. Mechanical eggs can be a useful tool to check for risk areas.

Eggs as a diagnostic aid

As a vet, the eggs are an important diagnostic aid before we even look at the hens. For example, we get clients calling up to say they are having more pale eggs. The colour is mostly genetically determined, but an increase in ‘pale’ eggs in brown hens can be caused by infectious bronchitis (IB), poor gut health, medication, nutrition, age, parasites or stress causing premature or delayed laying. Shell pimples and ‘sandpaper’ shells can also be caused by IB due to damage to the oviduct. Extra calcium layers may also indicate the egg was just laid too late, particularly when nest box availability is limited to the hen.

Certain Viral and bacterial diseases can cause quite typical seconds. Certain IB strains commonly cause crinkled shells and others pale eggs; Newcastle disease can cause pointy or misshapen eggs and Mycoplasma synoviae typically causes a translucency, or ‘halo’ at the point of the egg. Egg drop syndrome (EDS) can also lead to pale, misshapen and weak eggs, but usually comes along with a significant production drop.

Dirty eggs and blood on eggs

Another common issue is ‘normal’ but dirty eggs, these can’t be sold as first grade eggs and washing is prohibited, so can account for significant financial losses. Preventing system and floor eggs is a given for preventing shell contamination. Most of this comes down managing nest boxes, system use and litter in these corners when birds are very early in lay; particularly through removing eggs frequently and promptly. However we can also see dirty eggs when the intestinal health is sub-optimal – if faeces are loose, this can stain the egg when laid or make nest boxes dirty. Gut health can easily be challenged in laying hens due to the short and efficient digestive tract and high demands on the bird to produce each egg. It is commonly assumed when eggs become dirty, the hens have a worm challenge; although this is important to manage, it is not the only cause. General gut microbiome can be impacted by dirty water, inappropriate ventilation/environment, temperature, disease, worms, feed management, stress and generalised immunosuppression from any number of causes. Poor water hygiene would be a very common issue we see causing an enteritis. Frequently testing water and ensuring correct sanitisation and drinker management is very achievable. Note if the gut health is not optimal, the feed conversion and, subsequently profits will also be dented, so always take early action on dirty eggs.

Blood on eggs usually indicates either a significant red mite infestation, injurious pecking or prolapse/damage to the oviduct due to large eggs. We have excellent solutions on the market including Exzolt and Dergall, which can help to keep red mite at bay. Care should always be taken when trying to manipulate egg size, it is important in involve nutritionists to prevent deficiency or we can see serious impacts on the health of the birds.


In summary, there are many factors influencing the shell, most of which are preventable through good planning of management, biosecurity, disease control and nutritional supplementation. Vaccines are available to aid with IB symptoms, good solutions for worm and red mite control and plenty of advice is easily available from our specialist poultry vets.

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