On January 5th, I put my first ever batch of eggs into my incubator, after each one was weighed, numbered, and recorded. The instructions I’ve read all recommended candling after 7 days to check for evidence of development, which would have been this past Thursday.

Unfortunately, at short notice, I had to go away for a few days on Wednesday morning. Not knowing quite when I’d be back, I couldn’t resist sneaking a quick peek with the candling lamp on Tuesday night, at only 5 days incubation. I quickly picked four eggs at random from the 24 in the incubator and was delighted to see clear evidence of developing blood vessels in every one of them. Now, four was a small sample of the total, but it was a reassuring result!

I asked my husband to check all the eggs on Thursday, and let me know his findings. He emailed me his list, with ‘question marks’ over six eggs and definitely nothing in two. For a batch of eggs that had been through the post, these weren’t too bad, amounting to a failure to develop of 1/3rd of the total batch. But it was his first time candling and he wasn’t that confident, so we decided to leave all the eggs in the incubator, and I would double-check when I got back, whenever that happened to be.

A word or two about egg candling – this is a really simple technique which quite literally allows you to see inside the egg, by shining a very bright light through the shell while sitting in a darkened room. I’m afraid I haven’t got any photos from my candling session – I was far more concerned about getting it done carefully and quickly than about documenting the process – but there are plenty of good guides online, and any book about the hatching process should also be a good resource.

This photo gives an idea of what the developing embryo looks like within the egg at around 7 days. You can see the developing blood vessels inside the shell, the dark shadow of the embryo, and the bright air cell at the pointy end of the egg. Unfertilised eggs just look bright, with a bit of a shadow from the yolk. Darker-shelled eggs are harder to see detail in.

When I got home yesterday, I couldn’t wait to double check the eggs. To my delight, while I agreed with Hubby on his two ‘definite’ infertile eggs, only one of his six ‘question mark’ eggs appeared not to be developing. That means that, as of yesterday, I have 21 of my original 24 eggs still in play.

While I had them out for candling, I also weighed all my eggs. Tracking the weight loss of the eggs through incubation is really important because eggs need to loose around 14% of their total weight by evaporation during incubation. That way, the air cell is the right size at hatching so the chick is able to breathe without drowning during the hatch process. If the incubator has been too humid, there will be too little evaporation; if it has been too dry, then too much moisture can be lost, resulting in weak, undersized chicks. There are reported ‘target’ humidity ranges for hatching eggs of different species, but only really expensive incubators directly control and monitor humidity. What’s more, cheap electronic humidity meters (hygrometers – not the same thing as a hydrometer, which measures the density of a fluid) are notoriously unreliable, particularly outside normal room temperature ranges.

In most cases, then, we’re making a best guess at the humidity levels in the incubator. My incubator has two water channels in the base, and the instructions recommend filling one of them during incubation. (The second is filled later, at the 18 day mark, when we want to ramp the humidity of the incubator right up to help with hatching.) But the resulting humidity in the incubator has as much to do with the ambient, climatic temperature and humidity levels. The colder the outside air, the less moisture it can hold within it, resulting in lower humidity levels inside the warmed incubator. So how do we know we’re getting it right? Well, if we can compare the weight loss from the eggs with the target of 14% over 21days, by graphing, we can get a good idea if we’re in the right area.

This is what my weight loss data looks like.

The long lines are the upper, middle, and lower bound of my expected weighs and weight losses. The shorter lines each represent one of the eggs in my incubator. As you can see, they aren’t quite all neatly parallel but all of them are within a reasonable variation of the target weight loss. One or two are more steeply angled down than the target. When I looked to see which eggs these were, they tended to be the ones located more centrally in the incubator. This makes a certain sense, as my incubator is a ‘forced air’ style incubator with a fan which circulates the air around. The fan is in the centre of the incubator lid, so while all the eggs in the incubator should be broadly at the same temperature, the ones in the centre are getting more of a draft, and that air movement will increase the evaporation from those eggs.

Bearing this in mind, I have ‘reshuffled’ the eggs in the incubator today to bring the eggs from the corners and sides in to the middle, and from the middle out to the edges, making sure to keep them pointy-end down at all times. This should hopefully equalise the water loss from the eggs during the second half of the incubation process.

At 10 days, now, we’re approximately half way there. A lot of things can still go wrong, and I’m certainly not counting my chickens just yet! But the excitement is definitely increasing…

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