Why Eggs Lose Weight During Incubation — And Why It Matters

The Science of Incubation — Part 2

If you've ever weighed your eggs during incubation, you might have noticed something surprising: they're getting lighter. Not dramatically lighter, but measurably so. By day 18, a healthy chicken egg should have lost about 13% of its starting weight. This isn't a sign something's gone wrong—it's actually essential for the chick's survival.

In Part 1 of this series, we explored how oxygen enters the egg through the shell's tiny pores. But those same pores work both ways. Water vapour exits through them constantly, and managing that moisture loss is one of the most critical factors in successful incubation. Get it right, and you'll hatch strong, healthy chicks. Get it wrong, and you'll face dehydrated chicks, sticky membranes, or chicks that never make it out of the shell.

Let's explore why water loss matters so much, how to measure it, and what happens when it goes off track.

The Essential Mystery: Why Eggs Must Lose Moisture

Here's something that seems counterintuitive: eggs need to lose moisture to hatch successfully. But why would nature design such a seemingly wasteful process?

The answer lies in what happens inside the egg as the chick develops. As the embryo grows, it consumes nutrients, uses oxygen, and produces metabolic waste—including carbon dioxide and water vapour. At the same time, the yolk sac shrinks as the chick absorbs it for energy. All of this activity creates a need for space.

But there's something else happening, too. At the wide end of the egg, an air cell begins to form and gradually expand. This air cell is vital. In the final days before hatching, the chick's lungs develop fully, and it must be able to breathe air rather than relying solely on gas exchange through the shell. This process is called the internal pip—when the chick's beak penetrates the membrane surrounding the air cell and takes its first breath of air.

Without a properly sized air cell, that internal pip never happens. The chick runs out of oxygen before it can break through the shell, or it emerges too wet and exhausted to survive. The air cell grows specifically because water is leaving the egg.

The 13% Target: The Magic Weight Loss Number

For chicken eggs, the target is clear: 13% weight loss over the first 18 days of incubation. Some sources cite a range of 11–15%, but 13% is the sweet spot for most breeds and conditions.

Why 13%? Because that's the amount of moisture loss that, in combination with the chick's growth and yolk absorption, produces an air cell of exactly the right size. An air cell that's too small, and the chick can't breathe before pipping the shell. An air cell that's too large, and the shell becomes too fragile—or the chick exhausts itself trying to hatch.

This seems like a precise target, and it is—but it's not about hitting a number on a scale with laboratory accuracy. It's about achieving a range that works. Your eggs don't need to lose exactly 13%; they need to be in the ballpark. But understanding the target helps you troubleshoot when things go wrong.

How Water Escapes: The Path Through the Shell

Remember those pores we discussed in Part 1? They're the same doorway through which oxygen enters and water leaves. Every egg shell has thousands of pores—between 7,000 and 17,000, depending on the breed and individual egg.

The rate at which water escapes depends on several factors:

• Humidity inside the incubator: Water naturally moves from a wetter environment to a drier one. High humidity slows evaporation; low humidity speeds it up.
• Shell porosity: Different breeds have different pore characteristics. Marans, for example, are famous for their thick, dark shells with smaller, tighter pores. They lose moisture more slowly than lighter-shelled breeds. Sussex, Leghorns, and many other breeds lose moisture more readily.
• Ventilation: Air movement in the incubator removes water-saturated air from around the eggs, allowing more water to evaporate. Better ventilation = faster moisture loss.
• Altitude: At higher altitudes, the air pressure is lower, which means water evaporates more readily. If you're incubating above sea level, you may need slightly higher humidity to achieve the same moisture loss as someone at coastal altitudes.
• Egg size: Larger eggs have more surface area and slightly different pore distributions. They may lose moisture at slightly different rates than medium eggs.

All of these variables explain why there's no single humidity setting that works perfectly for everyone. A setting that's ideal at sea level in a well-ventilated cabinet incubator might not work at 1,500 metres altitude in a still-air styrofoam incubator.

Too Much Water Loss: The Dehydrated Chick

When eggs lose weight too quickly, the air cell becomes too large. The membranes surrounding the air cell dry out and become thick, papery, and inflexible. When the chick hatches, several things go wrong:

• The chick is dehydrated. Its tissues are too dry; its skin might be wrinkled.
• The membranes are too tough to break through. The chick's beak is strong, but it can't cut through leather-like tissue efficiently.
• The chick becomes exhausted before it can pip out of the shell entirely.
• Even chicks that manage to emerge are weak, sticky (because they can't absorb the remaining albumen), and often don't survive their first 24 hours.

Dehydration also affects the chick's ability to absorb the yolk sac. The yolk sac is supposed to be reabsorbed completely just before hatching, providing the chick with energy reserves for its first few days of life. If the chick is dehydrated, that process is compromised.

Too Little Water Loss: The Waterlogged Chick

The opposite problem—too little moisture loss—is equally problematic, though it often presents differently.

When eggs lose moisture too slowly, the air cell stays too small. The chick grows normally and reaches hatching day, but there isn't enough air space for the internal pip. Or, the air cell is just barely large enough, leaving virtually no margin for error.

The membranes stay soft and pliable—which sounds good, but it's not. The chick absorbs excess moisture from the white of the egg (the albumen). Inside the chick, tissues are swollen with fluid. When the chick finally does pip the shell and the air cell, it's exhausted from the effort and soaking wet. These chicks are weak, prone to not fully absorbing the yolk sac, and vulnerable to infection.

Additionally, a small air cell means the chick may pip the shell but fail to pip internally. It's one of the most heartbreaking hatching failures: a chick that has the strength to break out of the shell but lacks the air supply to complete the job.

Measuring Water Loss: Candling and Weighing

So how do you know if your eggs are on track? The honest answer is: you measure them.

Weighing eggs is the most reliable method. Here's the protocol:

1. Weigh each egg (or a representative sample of eggs) on day 0, before incubation begins. Record the starting weight.
2. Weigh again on day 7. Eggs should have lost about 6% of their starting weight.
3. Weigh again on day 14. Eggs should have lost about 10%.
4. Weigh a final time on day 18. Eggs should have lost about 13%.

If your eggs are tracking at about 6.5% on day 7, 10.5% on day 14, and 13% on day 18, you're in excellent territory. If they're at 5% on day 7, they're losing moisture too slowly. If they're at 7.5%, they're losing it too quickly.

A digital scale that measures to 0.1 gram is ideal. Many incubating enthusiasts use cheap kitchen scales (available from supermarkets) and find them perfectly adequate.

Candling the air cell is a complementary visual method. Candling involves holding the egg up to a bright light and observing the interior. Around day 14, you can see the air cell as a dark space at the wide end of the egg. If you've candled before, you can judge whether the air cell is the right size relative to the egg.

For someone familiar with the process, candling becomes intuitive. For beginners, it can be harder to judge. This is why weighing is more objective and more reliable—a number doesn't lie the way an eye can be deceived.

Beyond the Numbers: Why Humidity Settings Aren't Everything

Many incubator manuals recommend specific humidity levels: perhaps 40% relative humidity for the first 18 days, then 65–75% for the final three days. These guidelines are useful starting points, but they're not gospel.

The problem is that humidity settings are influenced by temperature, altitude, and the accuracy of your hygrometer. A hygrometer that's 5% off in either direction can throw off your entire moisture loss profile. Moreover, two different incubators running at the same humidity setting might achieve different moisture loss rates because of differences in ventilation, design, and air circulation.

This is why the best incubators focus on outcomes, not just sensor readings. Weigh your eggs and adjust your humidity settings based on the results. If your eggs are losing moisture too quickly, raise humidity. If they're losing it too slowly, lower humidity or improve ventilation. Let the eggs tell you what they need.

Over time, you'll develop a feel for what works in your particular incubator, in your climate, with your breeds. A hygrometer is a tool, not a rulebook.

Lockdown: The Humidity Shift

Here's something that confuses many newcomers: on day 18, when you move eggs to the hatching phase (called lockdown), humidity is supposed to increase significantly—often to 65–75%, or even higher. If moisture loss is so important, why are we suddenly trying to prevent it?

The answer is about changing priorities. For the first 18 days, the embryo needs air circulation and a properly sized air cell. But from day 18 onwards, the chick is ready to pip. The membranes need to be soft and flexible so the chick can break through them. If membranes become too dry and tough during the final three days, the chick won't be able to pip internally, even if it successfully breaks the shell.

Additionally, in the final stages of incubation, the chick is absorbing the albumen (egg white) into its body. That fluid is crucial. High humidity keeps the membranes from drying out and becoming leathery, which allows the chick to absorb this fluid properly.

The shift at lockdown, then, isn't a contradiction—it's a deliberate change to meet the chick's changing needs.

What's Next?

We've explored why eggs lose moisture, how to measure that loss, and what goes wrong when the balance isn't right. But moisture is only one piece of the incubation puzzle.

In Part 3, we'll explore another critical factor: temperature. A chick's development depends on steady, consistent heat. Too hot and the chick develops too quickly; too cold and development lags. We'll discuss why the 37.5°C figure matters, where in the incubator temperature is most stable, and how to troubleshoot temperature problems.

Questions?

Moisture loss is one of those topics where a little knowledge goes a long way. If you've got questions—about your specific incubator, your breed of choice, your local climate, or anything else about managing humidity and moisture during incubation—please get in touch. Reach out in the comments below, and let's chat about what's working (and what isn't) in your hatching setup.

Happy incubating!