One thing I am doing differently than in the past has to do with how and where temperature is measured. This is my theory, and so far it has dramatically improved my results, though there is room for further improvement.
In my cabinet, air flow is excellent, and the temp on a tray applies to the whole tray, top and bottom of the eggs, so I measure at tray level and results are consistent with temp. This was not true in my forced air foam incubator, it still had varying thermal profiles and with a turner inside, the eggs closest to the element were dramatically hotter than those in the center or edges, so I had a U shaped pattern of more highly developed eggs, leading to staggered hatches and higher failure rates of both extremes. If I were reliably at home, I'd remove the turners if I was to use a foam incubator again to increase the distance between the eggs and the element, especially with turkey eggs.
In the redwood, which is a still air, conventional wisdom says to measure the temperature at the top of the eggs. The temperature differential between the top and bottom of the eggs is a couple of degrees. The eggs lie flat on their sides, and the upper surface of the incubator is essentially an electric radiator. There is a thermocline, it is a couple of degrees warmer at the top than at the bottom.
When an embryo is forming, it is on the upper surface of the yolk, at the top of the egg, which is why turning is so important to prevent it from sticking to the shell. Temps at the top of the egg, in a still air, should be 100 -101 to promote early growth. Temps at the bottom may be lower without causing harm.
By the midpoint of incubation, the embryo is encased within a protective membrane and not subject to sticking. It cannot regulate its own body temperature, and is no longer on the upper surface of the yolk. So the temperatures at the center of the egg should be conducive to embryo growth, which is 99.5-100.5. I don't believe that still air or forced air makes a difference, as long as the average internal temperature of the egg is within this range, or close to it.
While the thermostat is at the top of the incubator and is not set to the egg temp, but rather at whatever temperature is necessary to gain results (that's the beauty of the wafer over a digital, you can't see what it's set at, it's all results-driven), I place my probes not touching an egg, and set at midpoint of egg level, or center of mass of a growing embryo. At this level I want 99.5-100.5 for the main incubation period to ensure proper growth.
For lockdown, Kevin Porter and a number of other turkey specialists recommend lowering hatch temperature by 1F to reduce heat stress as a result of increased humidity. I have hatched both ways and find that lowering the temp slightly does help prevent shrink wrapping and stressed poults that cannot hatch. For chickens, which seem to have fewer struggles at hatching, the results have been about the same. Here is the reasoning:
100F, 40% humidity: heat index 109F
98F, 40% humidity: heat index 105F
100F, 65% humidity: heat index 136F
98F, 65% humidity: heat index 128F
96F, 65% humidity: heat index 121F
100F, 70% humidity: heat index off the chart
98F, 70% humidity: heat index 134F
96F, 70% humidity: heat index 126F
With a still air incubator, there is no evaporative cooling as there is in a forced air. The egg becomes a sauna, and the fact that the chick/poult is still able to hatch under heavy exertion under these conditions is amazing. If the air cell is too small or incubator humidity at the time of internal pip is too high, I believe that moisture absorption into the air cell creates respiratory distress, and when the external pip is made, conditions don't improve or even become worse.
I have another set of eggs in the incubator and I want to try something different next time. I will maintain incubator humidity around 40% at 98F (heat index 105F) and will use a damp microfiber cloth on top of the eggs to create a humid microclimate. If it doesn't work, I may lose my whole hatch...but if it does work, I may have a better hatching technique.
