What is Candling?
Candling is a way of checking the fertility of an egg and the development of the embryo, with the use of a light source in a darkened room. In a darkened room, carefully hold the egg up to the light to observe the contents of the egg.
The embryo is located at the large end of the egg, where blood vessels will be present under the surface if the egg is fertile. The embryo appears as a dark spot which becomes larger as the incubation period continues.
The egg will appear to have a black spot which as the embryo grows and incubation continues will grow larger until light will only pass through the air cell end of the egg.
Eggs appear clear.
If the egg was fertile but the embryo has died then you will see a blood ring around the yolk or possibly a dark spot dried to the inside of the shell depending on when the embryo stopped growing.
Note that dark or brown shelled eggs are more difficult to candle than white or pale shelled eggs.
When To Candle
Candling can be done at any time, although day 8 onwards is usually when the embryo is more easily identified.
Day 3 of incubation (usually pale shelled eggs).
Day 5/6 of incubation (usually dark shelled eggs).
Between day 8 - 12 of incubation (embryo more easily identified).
3 days prior to hatching.