Take temperature for example. Many of the still air incubator manuals still state that the incubator should maintain 99.5F. Ask any veteran hatcher and they will tell you that still air incubators run better hatches at 101-102F measured near the top of the egg. A newbie follows this instruction and ends up with a late hatch. I've seen it. It doesn't mean the hatch will be “late”, but it certainly can contribute to it.
The biggest thing with these [incubator] manuals that bother me though is the humidity recommendations. How many people know why you regulate humidity as a newbie? I know I didn't. I read in a book it should be between this number and that number. I didn't know why, I just went with it. These manuals either throw out a number, (that in my opinion is usually too high) or tell you how much water to put in the wells (regardless of how much humidity that causes.)
Ask what your humidity should be on a forum and you will undoubtedly get at least a dozen different opinions. The only thing that is widely agreed upon is that at lockdown and hatch it needs to be higher.
Why is this?? Because different things work for different people because of various factors that these books and manuals do not take into consideration. The habits of the hatcher, the area that they are in and whether they have a dry or humid atmosphere. The quality of eggs also can play a role. Getting a definite answer is impossible and the issue of humidity can be very confusing. No one is wrong. They have just found what works for them. Some very seasoned hatchers don't even bother with monitoring humidity because they've done it so much they just know what works for them.
So how do you make the confusing understandable? In my opinion the first step is to understand why we control the humidity. An egg needs to loose 13/14% of it's weight during incubation. This weight that it is loosing is actually moisture. Moisture leaves the eggs through the pores of the shell. As the moisture leaves, the air cell in the egg grows. This is very important because when your little chick decides it's time to hatch, he/she is going to pip into the area where the air cell should be. If that air cell is not big enough and there is too much moisture there he/she can drown. On the flip side of that if your air cells grow too big the membrane can “shrink wrap” your chick. This can suffocate them if they have not pipped, if they manage to pip they will be stuck and not be able to move to finish the job.
That's the why of it. Now, the how of it. So how do we know how big the air cells should be? There are many egg pictorials or air cell charts out there. This is the one I use: (I'd give credit to the creator if I knew who that was.)
If the air cells are too big then you need to higher the humidity. This will slow down/stop air cell growth and let the development catch up with the air cell size. (Again, how much is going to depend on the discrepancy. Borderline big, raise it 10%. Significantly large, I'd say raise it TO 60% (not raise it 60% more...just up to 60% total,) for a couple days and check to see progress. If they are still growing raise it a bit more.
By keeping track of what your percentages are, you'll have a better idea of what percentage of humidity works for you.
The second way: start with a dry incubation if your incubator holds at least 25% when completely dry. Dry incubation is becoming more and more popular among chicken hatchers. Many people that have had not so great hatches (especially with the cheaper styrofoam incubators) have switched to the dry method and have had better results. I myself run dry when I can. (Seasons have a big impact on humidity levels and running dry. Being in Northern New York with regular below 0 temps and running a pellet stove for heat dries the ambient humidity in my home making it impossible to go completely dry in the winter.)
If your incubator holds at least 25% dry start your incubation &..... monitor your air cells! As long as your air cells are growing at the proper rate, you don't have anything to worry about. If you find that they are growing to fast, higher it, I'd say in increments of 5-10%. Rarely should you find too small air cells doing a dry incubation, providing you aren't in a tropical region.
What about lockdown and hatch??
And there's another question that you are going to get a dozen different answers for. I shoot for 75% many people do prefer a 70-75% range. Many people are happy with the recommended 65% and still there are others that insist 55-60% is perfectly fine for hatching.
More confusion.
Here are my thoughts: Are you a meddler? If you have a chick that you feel needs assisting, (There is an awesome thread on BYC on assisted hatching and why it should only be done if you feel it's absolutely necessary and the what happens if you assist too soon.) are you willing to open the incubator to help?
Many people have a hands off philosophy after lockdown. They will not, for any reason open that incubator until the hatch is complete. If a chick is stuck..so be it. If there are 15 chicks running around and it takes 2 days for the rest to hatch, then those chicks are in there for two days. (There is nothing wrong with their philosophy, but....)
If you are a hands off hatcher, then you can probably successfully hatch out chicks with 60/65% humidity in your bator.
If you are anything like me, then a higher humidity is better for you. I like to move my chicks to the brooder once they are active and bouncing off my incubator walls, thermometers the other eggs and each other. I do not leave my chicks in the bator until hatch is over. If I feel it is absolutely necessary I will assist a hatch. To properly assist a hatch you have to take things slow, help a little and replace the chick in the egg for rest and to give them a chance to finish. This constitutes opening the bator periodically. Every time you open the bator humidity slips out. Chicks need that humidity to hatch. If you are a “meddler” or someone who feels it necessary to open the bator, then naturally a higher humidity level is going to help keep adequate humidity in your bator. So take into consideration your actions and you should be able to judge a good humidity range for hatching. I personally believe you can't go wrong having extra humidity at hatch, but you most certainly can by having it too low
These are my thoughts and theories of humidity based on research and experience. I by no means am an expert, but I have hatched some adorable little fuzzy butts with this knowledge.
Feel free to check out my hatching guide from the hands on perspective, here: https://hatching411.weebly.com/
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Thank you,
Amy