Egg Candling Information

Author: Aussie Chook Supplies  

Getting a sneak peek on your developing chicks before they hatch! 


Doesn’t matter whether you’re using an incubator or a broody Mama hen to hatch your chicks, candling your eggs is an important part of the process. In this article, we’re hoping to answer some of the more common questions and help explain what is actually involved.


So, what is candling?

Candling is a term that dates back centuries when people used to use actual candles to shine light through an egg, allowing them to visualise its contents. A candle would be placed inside a wooden box with a hole in the top, then the egg was placed over the hole and the light would shine through it. Thankfully, these days, the process is much simpler – we actually sell Brinsea Candlers made specifically for the purpose! Shining light through the egg helps us to visualise the developing embryo and see the air sac and other features as everything grows in preparation for hatching.


Do I need to candle?

Absolutely! Not sure if you’re familiar with the smell of putrid, rotting eggs, but even if you’re not, it only takes one experience and you’ll remember it for life! Candling allows us to check the viability of our eggs – so whether or not a chick is actually growing, whether the egg is empty, or worse – if the egg is turning rotten.


Is it bad if there is a rotten egg?

Continuing to incubate a rotten egg in an incubator risks harmful gas building up inside it and then risking the egg exploding! In addition to making you want to vomit from the smell, it can also spread harmful bacteria across the other developing eggs (not to mention it takes A LOT to get the smell and all the gak out of your incubator! *yuck*). As for leaving it under your broody hen, well it’s basically a similar deal, only it covers the underside of the mother-to-be as well as all the remaining eggs. It’s all bad, all round, really.


Why else should I candle?

Candling is really interesting! Shining a light through the eggshell makes it kind of see-through and you can actually watch the embryo moving around when it is really small. More importantly though, you can check for signs that the chick may not be developing properly, or that it may have stopped completely.


For keen poultry breeders who keep records of their matings, reproductive history of different birds and success rates for hatching, candling enables them to see any patterns of issues from particular birds. Early embryonic death of the chick can be genetic, as can low fertility rates. Candling is an accurate way to collect data at various stages of development and incubation.


When should I candle my eggs?

Candling eggs much before Day 7 of development often leads to confusion when features to look for may not be clear or easily seen by the less experienced eye. The developing embryo is also really fragile during that first week, so rather than risk handling your eggs too much, it is better to leave them until around Day 7 to Day 10. You can do it again around Day 16 – Day 17 just to make sure all is ok before you lock your incubator down or before your broody hen flat out refuses to leave the nest and sits tight knowing her chicks will hatch soon.  Resist the urge to open the incubator or lift your hen to check in those crucial last few days prior to hatching – the humidity needs to be bang on for high hatch rates!


How do I candle and what will I see?

Your Brinsea Candler is your new best friend – safely holding your egg upright while illuminating all the internal structures so you can take a sneak peek!  There are lots of DIY versions, but many risk damaging the shell or its precious contents. Often people suggest using a phone or other device, but these lights can be harmful to the developing chick due to the brightness and/or heat they emit, so be cautious of any ‘helpful’ suggestions when it comes to candling!


For successful candling, find a dark room (or do your candling at night) and carefully place your egg on the device with the air cell at the top – this should be the more rounded end - and watch the magic as the light shines through the shell and reveals what is hidden within.


But what am I looking for?

An infertile, empty egg on a candling device looks like this when we shine the light through:

We can clearly see the pores in the eggshell that allow the transfer of oxygen and release of moisture and carbon dioxide. Very porous eggs (such as this one) often have a reduced hatch rate simply due to the increased risk of bacteria entering the egg through all those pores - they are literally little openings in the surface of the shell. Ideally, we want our eggs to have some light mottling from the pores, but not too much.


In a fertile egg around Day 7 or Day 8, we can see a blob (developing embryo) and a spider web of veins and membranes connecting it to the yolk and holding it in place.


This great image from Wikihow gives a good impression of what we should be looking for in a fertile, developing chicken egg:



I had some eggs stop developing just prior to this age, so once I was certain they were non-viable (ie the embryo was no longer developing), I gently cracked them open to see if I could find out why. Seeing the embryo like this did help me better understand what I saw when I used my candling device and was looking at the same image through the eggshell. This embryo is around 4-5 days old and clearly shows the spidery veins we can visualise through the shell when we candle.


Here we can see the red dot (the embryo – see its little dark spot? That’s its eye!) and those same veins radiating out as the light from the candling device shines through:

Image thanks to

As the chick grows and fills up more and more of the egg, it becomes harder to visualise any features, so when we candle at Day 17 - Day 18 before lockdown, we may only see blackness and the clear air cell at the top of the shell:

Image thanks to

The chick now takes up most of the space in the egg and is working into the position ready to hatch (head over shoulder, beak ready to ‘pip’ or tap through the shell). The yolk is still clearly visible and will continue to be absorbed by the chick right up until hatching.



In the image on the left, the chick is still surrounded by the membrane that sits just inside the eggshell, but on the right, the membrane has been removed to show all the features, right down to the tiny nails on its feet. Unfortunately, this chick was one of a couple of eggs pushed out of the nest by a very experienced Mama hen. Mother does know best as the chicks never matured beyond this stage (despite our attempts to artificially incubate) and were never able to hatch, so while we were thankful for the learning opportunity, we were very sad that they didn’t make it.




What is the ‘red ring of death’?

When the embryo dies and stops developing in the first 10 or so days, the blood vessels detach from the chick and float around the edges, forming a dark ring that is easily visualised when the egg is candled. It is important these eggs are discarded before they start to rot (remember the gak from earlier?! Get rid of any non-viable eggs as soon as they are discovered).

Image thanks to

Candling is an important process for both the backyard enthusiast with a broody hen or small incubator, or the experienced poultry fancier with dozens of chicks due to hatch. Either way, it is important to remove infertile or non-viable eggs and it is so amazing to watch the tiny chick embryos bouncing around inside the egg well before they hatch!


Incubation is an exciting time – don’t forget to check back here for our next instalment about what to expect when all your hard work pays off and your chicks arrive safe and sound.

What’s better than one broody hen sitting on eggs? Two that want to share the mothering duties!  This odd broody duo of an Australorp and a Silky made a fantastic parenting combo and the chicks happily followed both hens, who really didn’t mind either way.