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Humidity during chicken incubation

Humidity readingHumidity is one of the most important factors determining your hatch rate, but, contrary to popular belief, high humidity is more troublesome than low humidity during most of the incubation period.  During hatch, of course, you want high humidity in the range of 65% or more so that the chicks won't get stuck in the shell, but the goal is 40 to 50% for the first 19 days.

In order to hatch correctly, a chicken egg should lose 13% of its weight during incubation, and that weight is lost in the form of water evaporating out of the egg.  Over time, the air pocket in the egg will get larger as water evaporates out, creating a safe spot for your chick to breath in between around day 19 and the time the chick hatches.  If the humidity in your incubator is too high, then your chick won't have the appropriate air pocket and will die soon before pipping.

There are a few different ways to get the right humidity in your incubator.  The simplest is to follow the instructions and fill a certain number of wells with water, but this is a very hit or miss approach --- humidity in your incubator is determined by the humidity outside the incubator as well as by the amount of water in the wells.  We live in a very damp climate, and I suspect that following the instructions last time around is part of what resulted in such a low hatch rate.

Egg air sac sizeThe second method is to pencil the size of the air pocket on the outside of each egg at intervals while candling.  A chart like the one shown here can be used to see if the egg's air pocket is growing at the right speed.  However, this technique requires a lot of judgement calls, and would be time consuming if you're hatching more than a few eggs.

Another easy method to get the proper level of humidity is to buy a fancy incubator with a humidity readout.  Our new Brinsea Octagon 20 incubator will definitely help us in that regard, but there's a big difference between 40 and 50% humidity and I'd like to know whether my eggs are losing weight at the proper rate.

Weigh chicken eggsWhich brings us to the final method of determining egg weight loss --- weighing your eggs.  This is the method I've chosen, so I'll go over the specifics of the calculations in a later post.

No matter which method you choose, you should be aware that it's the average humidity over time that's important to your eggs, not the humidity at any given moment.  So it's okay to let the incubator wells completely dry out for a day if you need to in order to get the average humidity down lower.  In fact, some incubation experts practice dry incubation where they seldom or never fill the wells at the bottom of the incubator.  I plan to use a hybrid approach, adding water as needed to keep our eggs' weight loss on track.

Our chicken waterer makes daily chores so easy that you have plenty of time to learn incubation.

Incubating chicken eggs

After several rounds of trial and error, I figured out the best way to incubate chicks.  You can read the blow by blow experimentation here, or splurge 99 cents on my ebook for the more refined solutions.

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How do chicks not raised in incubators deal with this? Does the broody hen help regulate moisture somehow? Or do they just lay enough eggs that it doesn't matter if most of them die, from a species survival point of view.
Comment by irilyth [livejournal.com] early Wednesday morning, May 4th, 2011
I've been pondering this myself and don't have any really good answers. I'm guessing that the hen's feathers may trap in enough moisture that the eggs stay at the right humidity level during the majority of incubation. Then, after the first chick hatches, the humidity under there goes up drastically as all of that wet fluff dries off. Maybe it works because it's such a small, contained environment? I really don't know, and don't even know what percentage of chicks survive under a mother hen --- clearly I've got more research to do.
Comment by anna late Wednesday evening, May 4th, 2011
And, I should add, chickens evolved in southeast Asia, where I believe conditions are pretty damp. So it's possible that has a lot to do with it (although chickens clearly manage to hatch out living chicks all over the world.)
Comment by anna late Wednesday evening, May 4th, 2011
This question has been nagging at me for weeks, and I think I may have finally found the answer in an ancient book on incubation! The book mentions that eggs under broody hens lose less moisture than those in an incubator because oils from the hen's feathers rub off on her eggs and make less moisture escape. The author says that if you take an egg out of an incubator and a similarly aged egg from under a broody hen and drop them in a pot of water, the former has air bubbles escaping into the water while the latter doesn't. So, perhaps that's the difference?
Comment by anna late Tuesday morning, May 17th, 2011
Hens due better than incubators at getting humidity right, because their clutch of eggs sits on the ground and the moisture needed for humidifying the eggs comes from the earth. Let your hens set on their eggs on the good old firma terra and you'll see how mother nature improves her hatch rate. ~ old farmer
Comment by Anonymous late Wednesday evening, May 8th, 2013

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