Candling

Candling is the process of holding a fertile egg up to a bright, concentrated light source in a dark room so the inside of the egg becomes visible through the shell. It is used during incubation to check whether an embryo is developing normally, and it gets its name from the days before dedicated candlers, when a simple candle flame was the light source.

Most keepers candle for the first time around day 7 to 10 of incubation, looking for visible veins branching out and a dark spot that shows movement, both signs of active development. An egg that looks clear with no visible structure at that point is usually infertile or stopped developing early and can be removed, while a later check closer to hatch mainly confirms the air cell has grown properly and the chick has moved into hatching position.

A dedicated egg candler gives a brighter, more focused beam than a household flashlight, which makes darker or thicker-shelled eggs much easier to read. A common mistake is candling too early, before any visible development has had time to show, or handling eggs roughly during the process, which can disturb the temperature and membranes an embryo depends on.