Candling — What You're Actually Looking At and What It Tells You

The Science of Incubation — Part 6

By day 7 of incubation, you might start feeling a bit of curiosity—or perhaps anxiety—wondering what's actually happening inside those eggs sitting quietly in your incubator. Are they developing? Which ones are viable? This is where candling comes in. It's one of the most useful and revealing techniques in poultry keeping, and it doesn't require any fancy equipment or specialised knowledge. Just a bright light, a dark room, and a few minutes of your time.

In this article, we're diving deep into candling: what it is, how to do it properly, and most importantly, what you're actually looking at when you hold that glowing egg up to the light.

What Is Candling?

Candling is simply the process of shining a bright light through an egg shell in a dark room to observe what's happening inside. The light passes through the semi-transparent shell and the clear albumen (egg white), allowing you to see the developing embryo, blood vessels, air cell, and other internal structures.

The name comes from a centuries-old practice: farmers literally used candles to shine light through eggs during the incubation period. The technique has barely changed since then—we've just upgraded from candles to LED lights, which are brighter, safer, and more consistent.

What You'll Need

The beauty of candling is that it requires minimal equipment:

• A bright light source: A dedicated LED candler (available from most poultry suppliers) is ideal, but even your phone's torch works in a pinch. Look for something bright—at least 5 watts if using an LED.
• A dark room: A cupboard, shed, or room with blackout conditions works perfectly. The darker the environment, the more detail you'll see.
• Clean, warm hands: Your hands transfer heat to the eggs, so keep them as warm and clean as possible before handling.
• A gentle touch: Eggs in development are delicate, especially as they progress through incubation.

When to Candle: The Three Checkpoints

For chicken eggs, there are three key times to candle during the incubation period. These checkpoints allow you to track development and remove non-viable eggs early, improving your hatch rate.

Day 7 Candling

By day 7, the embryo has begun its first major growth phase. This is your first real look inside, and it's remarkably informative.

What a fertile, developing egg looks like: You'll see a small dark spot—this is the embryo and the eye beginning to form. Around this spot, you should see a network of fine red blood vessels radiating outward like a delicate spider web. These are the embryonic blood vessels forming to nourish the growing chick. You might even notice slight movement when you gently adjust the light. The rest of the egg will appear relatively clear, with the yolk visible as a shadow.

Clear eggs (infertile): An egg that never got fertilised will look almost identical to the day you put it in—just a yolk shadow with no veins, no dark spot, and no development whatsoever. These should be removed immediately, as they'll never hatch and they're taking up valuable space in your incubator. A clear egg is simply a clear egg; there's nothing subtle about it.

Early deaths (blood ring): Occasionally, you'll see a distinct reddish-brown ring inside the egg with no other visible development. This is called a blood ring, and it means the embryo started developing but died within the first few days. These should also be removed. A blood ring is one of the clearest indicators of early embryonic failure.

A note on dark-shelled eggs: If you're incubating dark-shelled varieties like Marans or Barnevelders, candling can be genuinely challenging. The dark pigment in the shell absorbs much of your light, and you may only see vague shadows rather than clear details. For these breeds, you'll need a very bright candler, and you might not see as much detail. Don't assume they're infertile if you can't see much—dark eggs simply require more patience and a brighter light. When in doubt, leave them in and check again at day 14.

Day 14 Candling

By day 14, the embryo has grown substantially. This is the point where most of the egg's contents have been dedicated to supporting the developing chick.

What you'll see: The embryo is now much larger, and most of the egg will appear dark—almost opaque. The most visible feature will be the air cell at the blunt (wider) end of the egg, which appears as a clear, distinct pocket. You should clearly see movement when you adjust the light. The blood vessel network is now extensive and less distinct because the embryo is so large.

Checking air cell development: Day 14 is an excellent time to assess your humidity settings. The air cell should have grown to roughly the size of a small grape or chickpea. Compare the air cell size across several eggs—they should be relatively consistent. If air cells are too large, your humidity was too low, and moisture loss was excessive. If they're too small, your humidity was too high. This feedback ties directly back to the humidity principles we covered in Part 2 of this series. If you're seeing inconsistent air cell sizes, you now know to adjust your humidity for the remaining days.

Removing problem eggs: At day 14, remove any eggs that show no development or no movement. Remove any with unusual odours (though you won't smell these through a normal egg, occasionally a really bad one becomes apparent). If an egg was infertile and you somehow missed it at day 7, it will still look clear at day 14—remove it now.

Day 18 Candling (Pre-Lockdown)

Day 18 is your last checkpoint before lockdown—the point at which you stop turning eggs and keep the incubator completely sealed. After lockdown, you won't open the incubator again until pips appear.

What you should see: The egg should be almost entirely dark, with only the air cell visible at the blunt end. The air cell should now occupy roughly one-third of the egg's volume. There's very little clear space—the chick has grown to fill nearly all available room.

Your last chance: This is your final opportunity to remove any eggs that have died. Look for eggs with no movement, unusual odours, or anything that looks obviously wrong. After lockdown, opening the incubator can disrupt temperature and humidity at a critical stage, so you want to be confident before you seal it up.

After day 18, no more opening: Once lockdown begins, resist the urge to candle again. The incubator needs to remain closed and undisturbed until you see pips (the first breaks in the shell as chicks prepare to hatch).

Candling Tips for Success

• Be quick: Don't leave eggs out of the incubator for more than 15–20 minutes. They cool rapidly, and temperature drops can stress developing embryos, especially after day 10.
• Handle gently: Support eggs carefully, particularly from day 10 onwards. The embryo is increasingly fragile as it develops.
• Candle in order: If you're tracking individual eggs, candle them in the same sequence each time. This helps you monitor progress consistently and makes it easier to spot problem eggs.
• Don't panic if you're unsure: Candling takes practice. If an egg is borderline and you're genuinely uncertain, leave it in and check again at the next checkpoint. It's better to keep a slightly questionable egg than to accidentally discard a viable one.
• Keep records: A simple notebook or spreadsheet noting which eggs showed what at each checkpoint is incredibly useful. Over time, you'll develop a feel for what normal looks like in your own setup.

What Candling Cannot Tell You

Candling is a powerful tool, but it has limits. It won't give you an exact fertility percentage on day 1 (many eggs look very similar early on). It can't diagnose specific genetic or developmental problems—it can only show you whether an embryo is developing or not. And with dark-shelled eggs, candling can be inconclusive, requiring you to rely more on experience and intuition.

Candling also won't tell you whether a chick will hatch successfully, even if development looks normal throughout. A normally developing embryo can still fail to hatch due to positioning problems, inadequate internal pip, or other factors that only become apparent during the actual hatch.

Looking Ahead

Once you've completed your day 18 candling and locked down your incubator, you're entering the final and most exciting phase of incubation. In Part 7, we'll explore what happens during lockdown, how to recognise the signs that hatching is imminent, and what to expect when those first pips appear.

For now, remember: candling is one of your most useful tools for managing a successful hatch. It demystifies what's happening inside the eggs, gives you confidence in your setup, and helps you make informed decisions about which eggs to keep.

Questions?

Have you had interesting experiences with candling? Spotted something unusual? Struggled with dark-shelled eggs? We'd love to hear from you. Drop your questions and observations in the comments below—every hatch teaches us something new.