Candling chicken eggs: when to check and how to interpret what you see

Candling means shining light through an egg without breaking it so that some internal features can be seen. It can help identify likely development, an egg that remains clear, some early embryo losses and changes in the air cell. It does not always provide a certain answer at the first check.

The FAO notes that fertility can be checked after about one week. Its small-scale poultry guidance also describes checks around days 5 and 13 in some systems. In a small incubator, the aim is to obtain useful information without chilling, shaking or contaminating the eggs.

Useful checks during a chicken cycle

Time	What to look for	Cautious decision
Before setting	Cracks, abnormal shell or displaced air cell	Exclude cracked or clearly damaged eggs
Around days 5–7	Vessel network, small dark area, difference from a clear egg	Check again after 48 hours if the view is uncertain
Around days 13–14	Larger dark mass, vessels and air cell	Compare eggs and record abnormalities
Before turning stops	General position and air cell, only if the manual recommends it	Work quickly, then prepare for hatch

After turning has stopped, avoid candling from curiosity. During hatch, stable humidity and temperature take priority.

Prepare a simple candling station

Use a purpose-made candler or a focused LED torch in a dark room. A soft opening or suitable adaptor should direct light into the egg without putting pressure on the shell.

Before opening the incubator, prepare:

• a charged or safely powered light;
• a clean tray that prevents eggs from rolling;
• a pencil for marks;
• a record sheet with egg number, date and observation;
• clean, dry hands.

Avoid a lamp that heats the shell strongly. Do not hold the egg against a flame or leave it over a hot source. Keep water away from the light, incubator and electrical connections.

How to hold and observe the egg

Handle each egg gently and for as little time as practical.

1. Pick up the egg without shaking it.
2. Place the broad end near the light; this is where the air cell is normally located.
3. Use only a small, slow rotation if needed to see the structures.
4. Compare the view with the previous check and with other eggs of the same age.
5. Return the egg to its previous position and orientation.

Candling is harder through dark, thick or stained shells. A poor view proves neither infertility nor embryo death.

What common signs may mean

A defined network of vessels

Near the end of the first week, branching lines around a small dark area are consistent with a developing embryo. Visibility depends on shell colour and the light source.

A completely clear egg

An egg that looks clear on day 5 may simply be too early or difficult to read. By day 7, persistent absence of visible vessels suggests infertility or a very early loss more strongly. Mark the observation and check again after about 48 hours if uncertain.

A red ring or disrupted vessels

A blood ring or visible breakdown of the vessel network can be consistent with early embryo loss. Compare it with a developing egg from the same batch before deciding.

An egg becoming mostly dark

As the chick grows, more of the egg becomes opaque. The air cell and some areas near the shell remain easier to see. Strong late-stage opacity is not a problem by itself.

Leakage, bad smell or a crack

These are no longer only candling observations. Isolate the egg carefully, do not break it in the incubation room and clean any contamination using an appropriate hygiene procedure.

Use the air cell as a trend

The air cell changes during incubation because the egg gradually loses water. Observe its size and shape from the same position, at the same angle and on the same days from one cycle to another.

An unusual-looking air cell can guide a humidity review, but one egg does not justify a sudden adjustment. Compare a representative group, check the sensor and consult the manual before changing settings. Our temperature and humidity guide explains this trend-based approach.

Keep a useful record

Give eggs a number or tray position and use simple terms:

• clear;
• vessels visible;
• development uncertain;
• air cell observed;
• abnormality or damage;
• removed, with the reason.

These notes help separate flock fertility, early embryo loss and incubator-management problems. To assess the full cycle, use them with the complete 21-day guide and the low hatch-rate diagnosis.

Common mistakes

• candling every day without a clear purpose;
• shaking an egg to look for movement;
• declaring infertility too early;
• leaving a leaking or foul-smelling egg in the incubator;
• failing to record the exact day of the check;
• immediately changing humidity because of one egg;
• opening the incubator during hatch for another check.

Candling is a comparison and cautious decision tool. Interpret each observation using the egg’s true age, shell colour and cycle history.