Candling Eggs: How and When to Check for Life

A woman who runs a 200-egg incubator in Kasoa Ofaakor called us in a panic in the second week of a batch. An egg had burst inside the machine overnight and the smell was, in her words, like something died in the wall. Something had, in a sense: an infertile egg that had been sitting at 37.5 °C for twelve days, quietly rotting, until the gas pressure cracked the shell and sprayed bacteria over every other egg on the tray. She had never candled. She did not know she was supposed to.

Candling is the simple act of shining a bright light through an egg in a dark room to see what is inside. It costs almost nothing, takes a few seconds an egg, and is the only window you have into an incubator batch before the hatch. Done at the right time it tells you which eggs were never fertile, which embryos have died, whether your humidity is right, and, over several batches, whether the problem in your hatch is the breeder flock, the storage, or the machine.

In our hatchery we candle every batch, and we treat the numbers from candling as seriously as the hatch count itself. This guide explains the equipment, the days to candle, what you will see, what to do about it and how to turn a candling session into information that improves the next batch.

Why candling matters more than people think

There are four reasons to candle, and only one of them is curiosity.

First, removing infertile and dead eggs protects the rest of the batch. A dead egg does not just take up space; it warms up, decomposes, and becomes a bomb of bacteria. Rotten eggs that burst or weep contaminate neighbouring shells and are a common source of the yolk-sac infections and "mushy chick" losses that show up in the brooder a few days after hatch. Clearing them out at day 7 and again at day 14 is basic hatchery hygiene, on the same level as the fumigation we describe in our hatchery hygiene guide.

Second, candling measures fertility, and fertility is a breeder-flock question, not an incubator question. If 30 percent of your eggs are clear at day 7, no adjustment to the machine will fix it; you need to look at the ratio of cocks to hens, their age, their feed and their health. Third, candling measures early embryo death, which is mostly a storage and first-week-temperature question. Fourth, candling at day 14 and 18 shows the size of the air cell, which is the most direct indicator of whether humidity has been right.

Put those together and candling separates the three big causes of a poor hatch into three separate numbers. That separation is what makes troubleshooting possible.

Equipment: a bright torch and a dark room

You do not need a purpose-built candler, though they are cheap and useful. What you need is a light source that is bright, focused and not hot. In practice:

• A powerful LED torch with a narrow beam, 500 lumens or more, is the best general tool. Many farmers use a phone torch for white eggs; it struggles with brown ones.
• A cardboard or foam collar around the torch head that fits snugly against the egg stops light leaking around the shell and makes the picture much clearer. A toilet roll tube works.
• A dark room. Candling in daylight is nearly useless. Do it at night or in a windowless room with the door closed.
• Purpose-made LED candlers with a rubber cup and a high-intensity lamp cost modest amounts and make brown eggs far easier. Commercial hatcheries use candling tables that light a whole tray at once.

Avoid incandescent bulbs held against the shell for long; the heat is not good for the embryo. Keep each egg out of the incubator for as short a time as you can; a few seconds per egg is plenty once you know what to look for, and the whole tray should be back in the machine within 15 to 20 minutes. Warm the room if you can. Chicken embryos tolerate brief cooling well, but a tray of eggs left on a table under a fan for half an hour is not brief.

Technique: how to hold and turn the egg

Hold the egg with the large end, where the air cell sits, against the light, tilted at a slight angle so you can look in from the side. Rotate the egg slowly around its long axis. The contents move as you turn, and a live embryo often reacts to the light with a twitch or a slow drift away from it. Look for three things in sequence: the air cell at the top, the network of blood vessels, and the dark shadow of the embryo itself.

Work over a soft surface or an egg tray so that a fumbled egg does not fall on concrete. Mark each egg with a soft pencil, never a marker with a solvent smell, as you go: a small tick for a live embryo, a cross for a clear, a "B" for a blood ring. Keep a tally sheet. If you candle 100 eggs and remove 12 clears and 5 blood rings, you have measured fertility (88 percent) and early death (5 of 88, about 6 percent) in ten minutes.

Day 7: the fertility candle

The first candling is on day 7 for chicken eggs, and for good reason. Before day 5 the embryo is too small to see reliably, and inexperienced eyes discard live eggs as clear. By day 7 a healthy embryo has dark, pigmented eyes and a spreading web of vessels and cannot be mistaken for anything else. Candling later than day 8 is fine, but it leaves infertile eggs in the machine longer than necessary.

What you will see falls into four categories.

A live embryo shows as a dark spot, usually the eye and head, with fine red vessels spreading out from it across the yolk like the branches of a tree or the legs of a spider. In a white egg it is obvious. In a brown egg you may only see the vessels as faint lines and the embryo as a shadow; rotate the egg and look for movement. The air cell is small and clearly defined at the large end.

An infertile egg (a "clear") glows evenly and uniformly, with the yolk a faint shadow that moves freely as you rotate, and no vessels at all. Some people mistake a slight yolk shadow for an embryo; the difference is the total absence of vessels. Clears are almost always infertile from the start, though an embryo that died in the first 24 to 48 hours looks the same. Either way, remove them.

A blood ring is a dark, well-defined ring or streak of blood around the inside of the shell, often with a smudged dark spot in the middle and no living vessel network. This is an embryo that developed for two to four days and then died. The vessels collapsed and the blood pooled at the edge. Blood rings are the signature of early embryonic death, and if you see many, the questions are storage, first-week temperature, turning and breeder health. Remove them.

Doubtful eggs where you cannot see clearly, usually dark brown or heavily speckled shells, are left in and re-checked at day 14. Never discard on doubt at day 7.

Learn on white or tinted eggs first; brown eggs from Lohmann or Isa layers and dark speckled guinea fowl eggs take practice.

Day 14: the second look

By day 14 the embryo fills a large part of the egg. When candled, the lower two-thirds are dark and opaque, and the air cell at the top should have grown to about a quarter to a third of the egg. Vessels are still visible around the edge of the dark mass, and a live chick will often kick or shift when the light hits it. This is a satisfying candle, because live eggs are unmistakable.

What you are looking for at day 14 is anything that has changed since day 7. An egg that was alive at day 7 but now shows no vessels, a dark mass that does not move, and often a cloudy or blotchy look has died in the middle stage. Middle deaths (days 7 to 14) are the least common in a well-run machine and usually point to a specific event: a temperature spike, a power cut, a day of missed turning, or contamination. Note the date of any such event in your records.

Day 14 is also the first serious check of the air cell. Compare its size to a reference: the air cell should be a shallow dome at the large end at day 7, a deeper cap by day 14, and about a third of the egg at day 18. Draw the outline of the air cell on the shell with a pencil at each candle. If the outlines are stacking up tightly, humidity is too high and the eggs are not losing enough water; open vents or reduce the water surface. If they are far apart, humidity is too low; add water. Weighing the tray is more precise, as explained in our temperature and humidity guide, but the pencil method works on any farm.

Day 18: the lockdown candle

The final candle happens on day 18, when eggs are moved to the hatcher or the turner is removed. This is not the time for a long inspection; it is a quick pass to pull out any obvious late deaths and to confirm the air cell has reached about a third of the egg and has usually begun to tilt to one side, the "saddle" shape that shows the chick has started pushing its beak toward it. A live egg at day 18 is almost completely dark except for the air cell, and you may see the shadow of the beak moving in the air cell in some eggs.

Anything still clear at day 18 was missed at day 7 and must go. Eggs that show a dark mass with no vessel activity and a sharply defined, stationary shadow are late deaths and should also go; they will not hatch and they will contaminate the hatcher if they rot or are broken by hatching chicks. Do the day-18 candle fast, with the hatcher already warmed and humid, and get the eggs in.

Candling other eggs: quail, guinea fowl, duck and turkey

The principles are identical; the days shift with the incubation period. Quail eggs (17 to 18 days) candle at about day 5 to 6, day 10 and day 14, and their dark speckled shells make them the hardest of all; a strong LED candler with a cup is nearly essential, and many farmers accept a few misjudged eggs. Guinea fowl eggs (26 to 28 days) have very thick, hard shells; candle at day 10 and day 24, and expect to see less detail. Duck eggs (28 days, 35 for Muscovy) candle beautifully because the shells are pale and thin; check at day 7, 14 and 25 (32 for Muscovy), and watch the air cell closely, since duck eggs need to lose slightly more water than chicken eggs. Turkey eggs (28 days) follow the duck schedule. The quail and guinea fowl guides give the specific incubation settings.

Turning candling into hatchery records

The value of candling multiplies when you write it down. For every batch, record the number set, the number of clears removed at day 7, the number of blood rings, the deaths at day 14, the deaths at day 18 and the number hatched. From these you calculate:

• Fertility = (set minus clears) ÷ set. Target 90 percent or better from a well-managed breeder flock; below 80 percent is a breeder problem.
• Hatch of fertile = hatched ÷ (set minus clears). Target 85 to 90 percent; below 75 percent is an incubation, storage or egg-handling problem.
• Hatch of set = hatched ÷ set. The number that matters for money, and the product of the other two.

The point of separating the first two is that they have different owners. Low fertility is fixed in the breeder pen; low hatch of fertile is fixed in the egg store and the incubator. A farmer who only counts chicks cannot tell which one is failing and often changes the wrong thing. In our own records at Kasoa, a batch that hatched poorly in 2024 turned out to have a fertility of 93 percent and a hatch of fertile of 71 percent, with most deaths at day 7 as blood rings. That pointed at storage, and we traced it to a delivery of eggs that had sat in a hot vehicle for a day before reaching us. Nothing about the machine needed changing. The record-keeping habits in our poultry record-keeping guide apply to the hatchery as much as to the broiler shed.

Common candling mistakes we see on Ghanaian farms

The mistakes are few and repeatable.

Candling too early and discarding live eggs. At day 3 or 4, especially in brown eggs, a live embryo is nearly invisible to an untrained eye. Farmers throw out a third of the batch as "clear" and then wonder at the tiny hatch. Wait until day 7.

Candling in a bright room. The eye cannot pick out vessels against daylight. If you have no dark room, candle at night or under a thick blanket.

Too long out of the machine. A 200-egg tray candled one egg at a time by a beginner can be out for an hour. Work in small groups of 20 to 30 eggs, returning each group before taking the next, and keep a lid or cloth over the eggs waiting their turn.

Confusing the yolk shadow with an embryo. A clear egg often shows a faint, mobile shadow as the yolk swings. The test is vessels: no vessels, no embryo.

Frequently asked questions