Candling incubated chicken eggs: the fascination of watching a speck grow into a chick.

Candling your eggs as the embryo develops is one of the most exciting parts of incubating and hatching your own chicks.

If you're not sure what candling means, how to do it, what to look for, or when is the best time to candle, you're in the right place.

What does candling chicken eggs mean?

Put simply, candling is the name given to looking at the inside of a developing egg by shining a bright light through it. 

It's used by people who are incubating chicken eggs (or any other kind of egg) for two reasons:

• to test whether an egg is good enough quality to go into the incubator, and

• at different stages during incubation to see whether the embryo is developing as it should.

Candling gives valuable clues as to whether you should expect problems during incubation – whether the embryo has died, for example – or after hatch. It's a critical skill to develop if you want to incubate your own eggs.

In this article, you'll learn all you need to know to candle eggs effectively.

Candling eggs: how to do it.

It's really very simple.

• Always wash your hands before candling, preferably with a good quality medicated hand wash. 

• This avoids transmitting any bacteria onto the egg. As the egg gets further into incubation it becomes more porous, and the last thing you want is a warm, humid incubator full of bacteria.

• Candle on a soft surface. I usually use a towel or a blanket. If the egg accidentally drops – which can happen easily – it has less chance of smashing than if it fell onto, for example, a wooden table.

• Try to candle eggs when it's quiet and you'll be undisturbed. If you have children who want to watch, make sure they know to avoid sudden movements.

• You'll need to turn the room light off, so it's a good idea to have a torch available if the switch is any distance from the candling table.

• Take the eggs from the incubator one at a time. Try to do this when your incubator is having its "cool down" time. 

• Always candle with the pointed end facing downwards. Avoid jolting the egg - be very gentle.

• Keep the eggs outside the incubator only for as long as it takes to candle them. The heart rate cools quickly the longer the eggs are outside its warmth.

Candling eggs at day 5 of incubation - a video.

In this video, I use Brinsea's candler and OvaScope to candle some eggs at day 5 of incubation. Look for...

• the "spider" of embryo plus veins which tell us the egg is fertile
• the air cell, which tells us the chick is growing well
• any cracks in the shell which might become a problem.

When to candle.

• Candling eggs before they go into the incubator is always a good idea. To find out why, have a look at this page.

• Once they are set in the incubator it's not a good idea to candle every day, even though it's very tempting. If you've got children they will probably want to because it's an exciting process. But accidents can happen, no matter how careful you are, so try to limit it. 

• Candling eggs somewhere between Day 5 and Day 7 is a good idea to see which are developing. At that stage, mark those that don't seem to have developed.

• At Day 10 re-candle those you've marked as possibly not being viable and, if they still show no signs of developing take them out of the incubator. It's hard, but it has to be done.

• Egg candling at Day 18, just before you 'lock down' the incubator, is critical. Candle all your eggs to make sure none have died in the meantime. If any are showing signs of non-development take them out of the incubator now.

• Leaving those which have not developed, or started but failed to continue, can cause them to explode and scatter bacteria over the rest of the incubator. That's a real no-no if you want a successful hatch because bacteria can enter the other shells and kill the embryos.

Different types of egg candle lights (candlers).

Using a candle to candle eggs.

Using a flashlight as an egg candler.

DIY egg candlers.

• A second option for this hand-held method is a flashlight app on a smartphone.

• I occasionally use this if I want to candle an egg quickly and don't have time to set up my ovascope. As long as you have a strong light, it works reasonably well.

• I've also used an inexpensive LED light I bought at a supermarket, with a round cardboard ring which I taped on to concentrate light into the egg.

• Although the light was bright enough, I couldn't prevent it from spilling outside the cardboard ring even using duct tape. 
• I do use this little light sometimes, but need to use my hand to prevent the light spill.

• The LED lights are strong and don't overheat, so from that perspective it's reasonable. But a properly made candler is better! 

The home made box candler.

If you're not sure whether you're going to hatch again (although I guarantee you will!), you may not want to invest in a commercially-made candler. 

A home-made, table-top candler isn't difficult to achieve and can be very effective. Have a look at this short video which shows how it can be done.

• Advantages of a home made candler: it's cheap to make and very effective.

• Disadvantages: you need to be very sure the heat doesn't build up. You don't want to cook the eggs...

• Because it doesn't enclose the egg, dark shells such as the chocolate brown Marans are very difficult to candle, even in a darkened room.
  

Commercially made candlers.

This is the modern equivalent of both the candle and the flashlight, and much easier to use.

Powered by batteries, this high intensity candler's light is not hot, but is strong enough to give an excellent view through the shell, especially if used in a darkened room.

• Advantages: Very easy to use and, when combined with the ovascope (the black contraption above which looks like a microscope), they're particularly good for candling dark shells like the Marans.

• Wonderful for children to be able to see the developing chick - brings alive the wonder of birth as if by magic.

• Disadvantages: Quite expensive to buy, particularly if you're not sure you'll hatch chicks frequently.

• For my detailed review of the Brinsea candlers, including pictures showing how they work, see this page.

Frequently Asked Questions.

Why is candling important?

For four reasons:

• To make sure the eggs are the best possible quality before they go into the incubator.

• To watch the embryo develop during the 21 days of incubation.

• To help know to discard eggs which are not developing before they explode and spread bacteria through the incubator.

• To analyse problems if you have a poor hatch rate.

Will candling tell me whether an egg is fertile or not before incubation?

No. 

The only way of knowing whether it's fertile, short of breaking the shell open, is by incubating it for six or seven days. If you candle then, a fertile egg will have a small embryo forming together with some spider-like veins.  

This is one of my Wyandotte eggs at Day 5 of incubation. You can just begin to see the tiny dot of the embryo in the middle, with veins running off to the side.

How do I know what I'm looking for?

Does it matter which way up the egg is when candling?

It's wise to keep the more pointed end of the egg facing down. I very occasionally candle with the blunt end down to be able to see the embryo more clearly, but it does run the risk of damaging the embryo, particularly in the first couple of weeks.

Please note: fertile eggs should always be stored and incubated with their more pointed end downwards.

Does moving the egg during candling damage it?

No, as long as you're careful and as long as you don't candle after day 18 of incubation. You will generally need to turn the egg to see what's going on inside, particularly as you come to the later stages of incubation. Always take it slowly.

Some commercially-made ovascopes like the Brinsea have a turning wheel to help with this. If you don't have an Ovascope, use your hand to turn but be careful – eggs slip easily. 

How long should the egg be out of the incubator?

As little time as possible.

If you're just checking whether it's damaged or developing, candling should only take a few minutes. If you've got children who are keen to see what's going on inside, it may take longer.

As a 'rule of thumb', you shouldn't keep it out of the incubator for longer than thirty minutes absolute maximum. I try to candle when the incubator is cooling down for an hour each day, as both the Brinsea incubators I use do.

Is it safe to mark eggs during candling?

Yes, it's perfectly safe to mark eggs as you candle them, and it helps keep an eye on progress. 

Like to know more about incubating and raising your own chicks?

Here are some of my most popular incubating articles. Click the buttons to go to that page.