Why Hot Weather Causes Soft Eggshells in Chickens (It's Not Always a Calcium Deficiency)
- Jul 12
- 5 min read
Finding a soft-shelled or thin-shelled egg in the nest box during a summer heat wave can be alarming. Many backyard chicken keepers immediately assume their hens need more calcium, but that isn't always the case.
While calcium is essential for building strong eggshells, hens also require another important ingredient that is often overlooked—carbon dioxide. During periods of extreme heat, laying hens cool themselves by panting. Although panting is necessary to prevent overheating, it changes the chemistry of the hen's body in a way that can interfere with normal eggshell formation.
Understanding why this happens can help you address the real cause of soft shells and better support your flock during hot weather.
How Is an Eggshell Made?
A common misconception is that eggshells are made from calcium alone. In reality, a strong eggshell is made primarily of calcium carbonate, which accounts for approximately 95% of the shell.
To produce calcium carbonate, the hen needs two essential components:
Calcium from her diet and stored bone reserves
Carbonate, which is produced from carbon dioxide circulating in her bloodstream
These ingredients are combined in the shell gland (also called the uterus), where the eggshell is formed over approximately 20 hours before the egg is laid.
If either component is lacking, shell quality can suffer.
Why Do Chickens Pant in Hot Weather?
Unlike humans, chickens cannot sweat. Instead, they rely on several natural cooling methods to regulate their body temperature, including:
Holding their wings away from their body
Seeking shade
Drinking more water
Reducing their activity
Panting with an open beak
Panting is a normal and necessary response to heat stress. By rapidly moving air through their respiratory system, chickens release excess body heat and lower their body temperature.
However, this cooling mechanism comes with an unintended consequence.
How Panting Affects Eggshell Formation
When a hen pants, she exhales much larger amounts of carbon dioxide (CO₂) than she would under normal conditions.
This creates a chain reaction inside her body.
Step 1: Carbon dioxide levels decrease
Rapid breathing removes carbon dioxide from the bloodstream faster than the body can replace it.
Step 2: Blood chemistry changes
As carbon dioxide levels fall, the hen develops a condition called respiratory alkalosis, meaning her blood becomes more alkaline than normal.
Step 3: Less bicarbonate is available
Carbon dioxide combines with water in the body to produce bicarbonate. Bicarbonate supplies the carbonate needed to build calcium carbonate—the primary mineral found in an eggshell.
When carbon dioxide levels drop, less bicarbonate is produced.
Step 4: Shell quality declines
Even if the hen has adequate calcium available, there may not be enough carbonate to efficiently produce calcium carbonate. As a result, the shell gland cannot build a normal, strong eggshell.
The outcome may be:
Thin-shelled eggs
Soft-shelled eggs
Shell-less eggs
Rough or uneven shells
Smaller eggs
This is one of the reasons eggshell quality often declines during prolonged periods of extreme heat.
Heat Stress Reduces Feed Intake
Heat affects more than a hen's breathing.
As temperatures rise, hens naturally reduce their feed intake because digestion produces body heat. Eating less helps them stay cooler, but it also means they consume fewer nutrients needed for egg production.
When hens eat less feed, they also consume less:
Calcium
Phosphorus
Vitamin D
Protein
Energy
Because these nutrients are all essential for producing strong eggs, reduced feed intake can further contribute to poor shell quality.
This is also why offering treats—including popular "cooling treats" such as watermelon, frozen corn, peas, fruits, or other snacks—is not the best nutritional choice during a heat wave.
While these treats may seem refreshing, they take up valuable space in a hen's crop without providing the balanced nutrition found in a nutritionally complete layer feed. Since hens are already eating less than normal, every bite matters. If a significant portion of those bites consists of treats rather than their complete feed, they consume even fewer of the protein, vitamins, minerals, calcium, and other nutrients needed to support egg production.
The best way to nutritionally support laying hens during hot weather is surprisingly simple: encourage them to eat as much of their nutritionally complete layer feed as possible. Offer their feed and ensure they have constant access to cool, clean drinking water, and avoid filling them up with treats. Their balanced layer feed is specifically formulated to meet their nutritional requirements—even during periods of heat stress.
💡 Tip: Place a frozen water bottle in your chickens' drinker to help keep their drinking water cool throughout the day. As hens drink the cool water, it absorbs heat from their body as it passes through the digestive tract, helping lower their core body temperature. The absorbed heat is then released through their droppings—a natural cooling process known as excretory heat transfer. You may notice their droppings become looser during hot weather, which is a normal part of this cooling mechanism.
Soft Egg Shells in Chickens - Why More Calcium Isn't Always the Answer
Many flock owners respond to soft-shelled eggs in chickens by offering additional calcium supplements.
While laying hens should always have access to free-choice oyster shell, simply increasing calcium intake may not solve the problem if heat stress is the underlying cause.
If panting has reduced the amount of carbon dioxide available for shell formation, the hen may be unable to efficiently convert available calcium into calcium carbonate, regardless of how much calcium she consumes.
In other words, the problem isn't always a calcium deficiency—it may be a limitation in the shell-building process itself.
Signs Heat Stress May Be Affecting Egg Production
Heat stress can affect more than just eggshell quality. You may also notice:
Excessive open-mouth panting
Wings held away from the body
Reduced activity
Increased water consumption
Reduced feed intake
Smaller eggs
Fewer eggs
Thin or soft eggshells
Occasional shell-less eggs
As temperatures return to normal and hens resume eating their usual amount of feed, shell quality often improves.
Helping Your Hens During Hot Weather
While you cannot eliminate summer heat, you can help reduce its impact on your flock.
Provide:
Plenty of cool, clean drinking water at all times
Shade throughout the day
Excellent coop circulation and airflow (meaning open windows on all four sides of the coop if possible, along with fans)
Reduced overcrowding
Free-choice oyster shell for laying hens
A nutritionally complete layer feed as the primary diet
Minimal handling during the hottest part of the day
These management practices help reduce heat stress and support better egg production throughout the summer.
Frequently Asked Questions
Can heat cause shell-less eggs?
Yes. Severe heat stress can interfere with normal shell formation, occasionally resulting in shell-less or very soft-shelled eggs.
Should I give extra calcium during a heat wave?
Continue providing free-choice oyster shell, but remember that soft shells during hot weather are often caused by changes in blood chemistry and reduced feed intake rather than a simple calcium deficiency.
Will eggshell quality improve when temperatures cool down?
In many cases, yes. Once hens begin eating normally again and panting decreases, shell quality typically improves.
Final Thoughts
Soft-shelled eggs during hot weather are often the result of normal physiological changes that occur when hens are trying to stay cool—not necessarily because they are lacking calcium.
Heavy panting reduces the carbon dioxide needed to build calcium carbonate, while heat stress also decreases feed intake, reducing the nutrients required for healthy egg production.
Rather than automatically reaching for calcium supplements, focus on helping your hens cope with the heat by providing excellent circulation, constant access to cool water, and a nutritionally complete layer feed. Supporting the hen as a whole—not just the eggshell—is the best way to maintain egg quality throughout the hottest days of summer.
Recommended Reading:
If you're wondering whether your laying hens need a calcium supplement, read our blog
Excellent info