‘Candling’ means shining a directly beam of light through the eggshell of a developing egg to look for signs of the embryo growing inside the egg. Originally a candle was used to shine the light, hence the term ‘candling’.

The purpose of candling is to monitor whether or not the egg is fertile and see how well it’s developing through the incubating period.

First of all we researched… a lot! We wanted to know exactly what to look for inside the egg and what it meant.

Useful websites

Differences between incubating chicken and duck eggs

Having previously enjoyed hatching chicken eggs, we knew that hatching duck eggs would be similar, with a few noticeable differences, one being the humidity levels required in the incubator. 

The biggest difference, is that duck eggs take an extra week to hatch. Our chicken eggs took 3 weeks (21 days) to hatch and duck eggs take an 4 weeks (28 days). That extra 7 days can really test the patience of little ‘hatch observers’!

Some steps to follow:

  1. Always wash your hands before candling, to avoid transmitting any bacteria onto the egg. Everybody coming to watch the candle washed their hands too. This was important because as the egg gets further into incubation it becomes more porous, and we didn’t want a warm, humid incubator full of bacteria.
  2. Try to candle when it’s quiet. We chose to do this without Cian to avoid any sudden movements and somebody accidentally dropping an egg.
  3. Find a dark room and have a torch available before you remove the egg from the incubator.
  4. Remove the eggs from the incubator one at a time and only keep them out for as long as it takes to candle them.

Candling at Day 7

Then we took an egg and a torch into the cupboard under the stairs, for complete darkness. We shone the torch light through

Unfertilised eggs

The unfertilised eggs were the easiest to spot as the light shone through them and lit the egg up. Ioan found our first unfertilised egg.

Finny, having spent hours watching videos on candling, was up on his ‘egg handling lingo’ and knew that an unfertilised egg was commonly called a ‘yolker‘.

You can see the difference between an unfertilised ‘yolker’ and a fertilised egg here. The left hand egg is unfertilised and appears clear and appears to glow. In contrast, the fertilised egg on the right has a clear air cell at the bottom, spidery veins and you can see the shadow of the developing duckling at the top.

Fertilised – Developing embryo

With the fertilised eggs we were looking for three main things:

  • 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.

Again, Finny was keen to call it by the term, ‘winner‘ that he’d learnt during his research. I keep having to remind Finny of the phrase, “Don’t count your chickens before they hatch…” as he is convinced that all the embryos that seem to be developing well will hatch in to ducklings.

Fertilised – Blood ring

A blood ring occurs during the incubation of duck eggs when the duck begins to develop but dies. 

The blood vessels which had begun to form begin to decompose and, rather than remaining attached to the embryo, they float in the yolk and form a circle which spans the circumference of the egg.

Why does this happen? It may simply be nature’s way of not growing a duckling which would be weak or in some way not properly developed.

In the picture on the right below, you can see the red ring right around the edge of the egg.

Once again, Finny was keen to use the right term, calling it a ‘quitter’.

It is important to remove the eggs showing signs of non-development from the incubator. Those which have not developed, or started but failed to continue, can 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.

Candling at Day 21

This was the last time candling them as we didn’t want to keep taking them out of the incubator after this point. You could see the shadow of the duckling moving around inside the egg.

Candling a cracked egg at Day 25

When the incubator turned the eggs, we saw one of the eggs hit the inside of the incubator and crack. We candled the egg to check that the embryo was still moving about inside the egg, then attempted to seal up the crack quickly to prevent bacteria getting inside. I used some nail varnish over the hairline crack, then topped it with a piece of sticky tape. There is a limit as to how much damage you can repair when it comes to egg shells and if the membranes are damaged the chance of failure is much higher. We’re keeping our fingers tightly crossed for this duckling.

DfES Outcomes for EYFS and National Curriculum (2013)

Science Year 1 programme of study

Animals, including humans

  • identify and name a variety of common animals including fish, amphibians, reptiles, birds and mammals

Science Year 2 programme of study

Living things and their habitats

  • identify that most living things live in habitats to which they are suited and describe how different habitats provide for the basic needs of different kinds of animals and plants, and how they depend on each other
  • identify and name a variety of plants and animals in their habitats, including micro-habitats