Candling eggs, day by day

Candling means shining a bright light through the shell to see what is happening inside. Here is what a developing chicken egg looks like at each check, what a problem looks like, and when it is safe to remove an egg.

How to candle

Wash your hands and darken the room completely.
Hold the egg blunt end up against a bright LED torch or candler, so no light leaks around the shell.
Tilt and turn it slowly. Look for veins, the dark spot of the embryo, movement and the air cell at the blunt end.
Work quickly and put each egg back the same way up. A few minutes out of the incubator does no harm; avoid long sessions.
Mark doubtful eggs with a pencil and check them again 2–3 days later. Never throw out an egg you are unsure about.

Dark or speckled shells (Marans, Welsummers, quail) are hard to read before about day 10. Use the brightest light you have and rely more on the later checks. White and duck eggs are the easiest.

What a chicken egg looks like (21-day hatch)

Day 3–4A small dark dot with thin red veins spreading out like a spider.

Day 7A clear network of veins and a dark spot: the eye. You may see it move.

Day 10The embryo is a larger dark shape. Veins reach most of the shell.

Day 14Most of the egg is dark. The air cell at the top is larger.

Day 17–18Dark except the air cell, which is big and often tilted. Lockdown.

Illustrations, not photographs. Development speed varies a little between eggs. For ducks and geese the same stages come later in proportion to the longer incubation; for quail, earlier.

Eggs to remove

Clear eggJust the yolk shadow, no veins at day 7–10. Infertile (or stopped in the first day or two).

Blood ringA red ring inside the shell. The embryo died early. Remove it.

QuitterA blurry dark blob with no veins and no movement, often stuck to the shell.

Remove clear eggs and blood rings at the day 7–10 check. Infertile eggs can rot in the warmth.
Remove any egg that smells, weeps or shows a greenish-black colour straight away. A rotten egg can burst and contaminate the others.
Cracked shells: a hairline crack can be sealed with a dab of candle wax or glue early on. Remove badly cracked eggs.
If in doubt, mark it and check again later.

When to candle

Bird	Candle on day	Lockdown
Chicken	7, 14	18
Duck	7, 14, 21	25
Muscovy duck	7, 14, 21, 28	32
Goose	7, 14, 21	27
Turkey	7, 14, 21	25
Coturnix quail	7	14
Bobwhite quail	7, 14	20
Guinea fowl	7, 14, 21	24
Pheasant	7, 14	21
Partridge (chukar)	7, 14	20
Peafowl	7, 14, 21	25

Use the hatch date calculator to turn these into dates for your eggs.

Reading the air cell

The air cell at the blunt end tells you whether the humidity is right. It should grow steadily: small on day 7, about a quarter of the egg by day 14 and roughly a third by lockdown. Draw its outline on the shell with a pencil at each check so you can compare. Too small means the humidity is too high; too large means it is too low. The humidity guide explains how to correct it.