Candling is the process of using light to check an egg to see if it is fertile. Candling is also helpful in monitoring the development of the growing embryo. Historically, eggs were checked by holding them above a candle flame to light up the interior. The process is still called candling even though we now use modern technology instead of an actual candle.

Any light source that produces a focused beam of bright light can be used as a candler. The light beam should be no wider than the egg, since it needs to shine into the egg and not around it. You want to light up the interior of the egg as much as possible, and stray light that passes in front of the egg will make it harder to see details. A bulb that stays cool is highly desirable since it allows longer views; a hot bulb can only be used for a quick look since too much exposure to heat can damage the contents of the egg.

Commercial candlers are easily available in a variety of styles and a variety of prices. Some candlers let you candle inside the nestbox without touching the eggs, and other candlers require you to take the eggs out of the nest. You can use a small flashlight as a simple homemade candler.

It is often possible to determine the status of the egg just by looking at the color of the eggshell, without actually candling the egg. An egg that is infertile or recently laid has an eggshell with a slightly translucent pinkish look to it. A fertile egg that has been incubated for several days will have an opaque white shell. A DIS (dead in shell) egg that has started to decompose will have a dark tint.

Candling is simple. It’s standard to place the light source against the wide (air cell) end of the egg to light up the interior, although other angles can be used if desired. You will get better illumination of the egg if the light is very bright and the room is very dark, and you will see a lot less detail with a relatively dim light. Signs of development will not be visible in the earliest stage of incubation. The standard recommendation is to candle eggs after they have been incubated for at least five days, although it may be possible to see signs of development earlier than this.

Whether you are candling inside the nest or taking the eggs out for candling, it is highly desirable to remove the parents from the breeding cage first, or at least block them out of the nestbox so they won’t be too alarmed by the intrusion or by the sight of an empty nestbox. If you’re blocking the nestbox door, use a lightweight material that won’t cause harm or damage if the parents manage to push their way past it. A hand towel stuffed into the nestbox door is fairly effective at keeping the parent birds out.

If you are going to touch the eggs with your hands, wash your hands first to minimize the amount of bacteria deposited on the egg shell.

What to look for. The yolk should be centered in the egg. An infertile or undeveloped egg will have a yellow glow with no sign of red blood vessels or a dark semi-opaque mass (the embryo). In the early stages of development, a fertile egg will have visible bright red blood vessels. As the egg continues to develop, the embryo will become visible as a dark mass that the light doesn't penetrate. If you have a good candling light, you will be able to see the heartbeat, the eye color, and the movements of the baby. The embryo will increase in size until it fills the whole interior of the egg except for the air cell. However you will still be able to see a red glow caused by the light shining through living blood cells.

If there is a blood ring (a more or less round marking inside the egg) in an egg that is otherwise clear yellow, it means that the embryo died at an early stage of development. An egg that has a dark mass inside but is dull brown instead of having a red glow may have died at a late stage of development. Inexperienced candlers often misread the signs however, so don't assume that the egg is dead and dispose of it unless it has a blackish tint that is visible on the outside under normal light conditions.  Bubbles in the egg are a sign that the air cell has ruptured, which is fatal to the embryo.

Candling can reveal a great many other details about the egg. The size of the air cell is supposed to slowly increase as the egg develops due to water evaporating through the egg shell. An expert egg candler can use the  size of the air cell at different stages of development to determine whether the humidity is too high (air cell hasn't grown enough) or too low (air cell has grown too much). This is an advanced topic that will not be discussed in this article, but these articles by the Brinsea incubator company are very helpful (they're aimed primarily at artificial incubation of poultry):

Humidity in incubation - detailed information on humidity issues
What is egg candling - a general overview including color pictures of candled eggs.
Incubation handbook - mostly focused on artificial incubators, but it includes drawings of air cell changes over time.

There are more pictures of candled eggs on Avian Web and on International Cockatiel Resource.

As hatch time approaches, the air cell will tilt at an angle. This is called draw down, and it is a normal part of the development process.  When it's time to hatch, the baby should pip at the lowest point of the air cell. 

Copyright 2014 Carolyn Tielfan all rights reserved