If your chicks are getting to hatch day and then dying without pipping through the shell, and if they are large, I'm going to put my money on too high humidity during the main incubation period (first 18 days). Here's why:
A day or two before a chick hatches, it pokes its beak through an inner membrane into an air cell at the top of the egg. This is its first breath of air, and once it has done that, there is no going back--it must breathe air. If the air cell is the right size (1/4-1/3 of the volume of the egg), all is well, and 24 hours or so later, the chick will poke a little hole through the shell and start hatching. If the air cell is too large, the chick won't be able to finish growing because it will run out of room, and it will die small and underdeveloped. If, on the other hand, the air cell is too small, then the chick may poke its little nose into the air cell but be unable to keep its nose above the membrane, and it drowns. Sometimes, it can't even get its nose into the air cell to begin with. Often, the chick will be large because it's had more room inside the egg.
Whether that air cell is the right size or not depends almost entirely on the humidity inside the incubator during the first 18 days of incubation, and that is why it is important. But the part that makes it difficult, is that conditions outside the incubator change what humidity is correct inside the incubator. Here's how it works:
In a healthy, fresh egg at the time of setting, the air cell will be very small--almost invisible in some cases. During the course of incubation, the egg must lose a certain amount of moisture in order for the air cell to develop to the proper size. In a dry part of the country, the air entering the incubator contains very little moisture. In order to maintain, for instance, a 50% relative humidity, the air must pick up moisture from inside the incubator. It can do so from water wells if they are present, but if they are not, it will take the moisture from the eggs themselves. The drier the air coming into the incubator, the more moisture it will take from the wells and/or eggs. If your water wells are inadequate to provide the moisture, too much will come out of the eggs and the air cells will be underdeveloped.
On the other hand, in a moist part of the country, air coming into the incubator already contains a great deal of moisture. In order to maintain that same 50% relative humidity, it may not need to pick up much or any moisture from inside the incubator. If there are water wells filled in the incubator, the air cells may not develop at all. A lower humidity may require less moisture picked up from inside the incubator, but at the same time, the lack or minimal number of water wells filled mean that any moisture that does evaporate has to come from the eggs, which allows the air cells to develop better.
Other factors, such as altitude, can affect air cell development because of differences in evaporation temperatures and air pressure. Likewise, the presence or lack of an air conditioning unit, the time of year, the size of the eggs, the porousness of the eggs, the number of eggs, the type of eggs, the presence of bad eggs in the incubator, and many other factors come into play. The type of incubator matters too because of the way the air flows around the eggs, how many eggs it holds, and so much more.
So, to put it in practical terms, here is my recommendation:
* Put in another batch of eggs.
* Incubate at 20-30% LESS humidity than you've previously tried.
* At 7, 14, and 18 days, candle the eggs. Make a mental (or written) note of where the air cell is at each juncture. It can be informative to draw a circle on the outside of the shell indicating the size of the air cell at each candling.
* At day 18, make a particular note of how well the air cells are developed. If they are 1/4 or more of the egg, congrats, sit back and enjoy a good hatch (hopefully!)
* If they are too large, make a note and raise your humidity for the next hatch.
* If they are too small, make a note and lower your humidity for the next hatch.
* When you've perfected it, make a note and keep your humidity there for future hatches.
Note: If your air cells are having trouble developing, one other technique can be used to help them grow. Although it's counter-intuitive, spritzing the eggs daily or more often with lukewarm water causes the moisture to evaporate more quickly from the eggs thanks to the polarity of water molecules. If you get close to lockdown and your air cells are small, remove all water wells and start spritzing the eggs regularly. Sometimes those measures will help air cells develop very quickly to their proper size, even if they are behind in development.
As you get skilled at candling and judging air cell development, you'll be able to make adjustments earlier in the hatch cycle and hopefully save batches that might otherwise have air cell problems if not caught early.
And P.S. I hatch in a Hova-bator and I swear by it. Very good unit for the price. I've had outstanding success in it. And I do a dry hatch, because I live in a humid area and a completely dry hatch (i.e., I add NO water during the first portion of incubation) is what works for me. It's not what works for everyone.
