After the incubator has been running for a few days, it's possible to scan the eggs with a bright light to see inside the shells. This will give you a good idea of whether or not an egg is progressing according to plan, and it will also allow you to follow the process of embryo development at first hand.
Traditionally, a candle was used to scan eggs, and the process became known as candling. Obviously, complications could arise when eggs were held up to hot candles, so thanks to recent developments in technology, brighter and cooler light sources are available which do not carry the risk of harming the development of the embryos. By shining a high intensity LED candling torch through the shell, you can often make out signs of progress as and when they take place.
After four days, a living embryo will reveal a complicated system of blood vessels which appear to be growing around a dark blob of matter, as shown in diagram 1. This blob is the embryo itself, and the network of blood vessels will develop as the days go by. A conspicuous air space at the the fat end of the egg will also be clearly visible, and this space will expand as the process continues.
Any problems early in the incubation process could well kill the vulnerable embryo, and this will become clear during candling. Early death is often recognised by the presence of a narrow smear of blood inside the egg shell, which will develop in time into an unbroken loop of rotten blood, as shown in diagram 2. The loop may swill back and forth as you move the egg, and this is a fairly certain sign that the embryo is dead.
It may be that the egg is infertile, in which case you will see no sign of development whatsoever. If you see no progress, don't despair. Put the egg back into the incubator and scan it again in a few days. All embryos develop differently, and it could be that a second scan will reveal obvious signs of progress.
Incubators are used to develop the eggs of a wide variety of different bird species, and all eggs are different. Pheasant eggs can be any colour from blue to brown, and guineafowl are renowned for having very thick eggshells. A number of factors play a part in the candling process, and you will have different results not only between different species, but also between different individual eggs. While the High Intensity LED Candler is one of the brightest torches on the market, don't be disappointed if you can't get clear results every time.
If you are unsure as to the results of any scan, always put the egg back into the incubator and leave it for a few days. Scanning eggs with a candler is fascinating and it helps you to learn more about eggs, chicks and birds. However, don't be tempted to scan them too often, because an unnecessary amount of tampering could harm the development of the embryos.