Incubation is simply the creation of a controlled environment that is manipulated to the ideal conditions for hatching an egg. With that in mind, there are incubators which are more effective at controlling the environment than others and there is a slight bit of controversy over the specifics of ideal conditions for hatching eggs. The following information is not derived from my experience although following it has led me to successful hatches; it is rather derived from collaborative and comparative information sourced from college and university publications.
There are three components to the environment we are trying to create for hatching eggs; temperature, humidity, and oxygen exchange. Temperature and humidity are most commonly measured while oxygen exchange is often assumed. Measuring temperature and humidity can be done in many ways but I prefer the wet/dry bulb method as opposed to a digital or analog hygrometer/thermometer. In addition to the three components, an unspoken component is a clean environment. You want to be sure your incubator and eggs are as clean as possible before starting incubation to minimize bacteria exposure into the environment and prevent a bacterial explosion which could spell disaster for your eggs.
A significant amount of research time has been devoted towards determining the ideal temperature for egg incubation and the results of those findings indicate that 100F for an incubator with forced air (a fan) or 101F for a still air incubator (no fan) is optimum.
Humidity seems to be the condition that has the most controversy surrounding it in the hobbyist world. You will find recommendations all across the spectrum and there is very little research available to help clear the waters. What research that does exist found that incubation which results in 13% weight loss through water loss over the 21 day period yields the highest hatch rates. Water is lost during incubation as a direct relationship to the humidity. Higher humidity results in less water loss and and lower humidity results in more water loss. The study (E mbryonic osmoregulation: Consequences of high and low water loss during incubation of the chicken egg) found that incubation using the extremes of high and low humidity resulted in slightly lower hatch rates using extremely high humidity and drastically lower hatch rates using extremely low humidity. I interpret this to mean that I would be better served to error on the side of too much humidity when I am incubating eggs. As a result I use a wet bulb reading of 89F or 65% radiant humidity (RH) for incubation from start to finish. This humidity level is increased during hatch, but not by me. It is increased naturally by the chicks hatching and releasing additional moisture into our incubator. For this reason you will know the hatch is complete even without windows by your wet bulb (humidity) reading returning to its set point.
Oxygen exchange is not something that is typically measured in hobbyist hatching but it is accomplished by ventilation holes in the incubator. I recommend always keeping any vent plugs out for a table top incubator and opening the ventilation holes on a cabinet as much as possible while still maintaining appropriate humidity levels. Normal atmospheric air (20-21% oxygen) is the goal but It is difficult to provide too much oxygen, however easy to provide not enough. For cabinet incubators humidity is often adjusted by opening and closing ventilation holes. The ideal for a cabinet incubator is that the more oxygen exchange you are able to provide while still maintain appropriate conditions, the less you will need to worry about your exact oxygen level.
Turning of the eggs is done to prevent the developing chick from sticking to one side of the internal shell, preventing it from properly positioning for hatching. Eggs should be either laid flat on their side or positioned pointed end down if using an automatic turner. The crucial period for turning is the first two thirds of incubation or 14 days (E gg Incubation: Its Effects on Embryonic Development in Birds and Reptiles) . Following that turning is not required, but is often carried out until day 17 or 18 of incubation. On day 17 or 18 the eggs should be candled to discard any which are not developed and the developing eggs should be returned to the incubator and laid on their side for hatching. From this point on, eggs should no longer be turned. Candling during the incubation process is not necessary and only serves to feed your own curiosity at the cost of subjecting the eggs to unnecessary temperature and humidity drops.
“Lockdown” is a term in which has been coined to signify the period of time from day 17 or 18 to the completion of hatch in which the incubator is not to be opened. The purpose of this practice is for emphasis and concern of maintaining the controlled conditions inside of the incubator at arguably the most crucial point in the incubation process. A normally incubating egg will hatch between day 19 and 21 depending on the age of the egg at the start of incubation. The hatch process begins with the chick breaking through the internal membrane which is often called an internal pip. An internal pip is not visible from outside the shell, but the shell is porous and subject to the humidity conditions inside the incubator. This is the time in which opening the incubator can become a death sentence for a chick. Opening the incubator causes a drastic drop in temperature and humidity as the controlled environment is breached by the external environment. This change is less dramatic in a larger incubator like a cabinet but still exists especially for those eggs closest to the incubator opening. This drop in humidity can cause the membrane to constrict around the chick and it will become shrink wrapped and unable to hatch successfully.
Under proper incubation conditions, an egg will hatch in 504 hours after incubation plus an additional hour for each day over 5 days old at the time of setting. At the end of day 22, it is time to finally open the incubator for the first time since lockdown and remove the dry fluffy chicks. You may have eggs that did not hatch or failed to hatch. These should be considered cull chicks and should be humanely dispatched. The goal of incubation is to produce healthy viable chicks. Chicks which are unable to hatch on their own do not make good breeding stock and would only serve to weaken the bloodline often resulting in crippled and unhealthy birds. After removing fluffy chicks, unhatched eggs and shells from the incubator, and before sitting down to enjoy your success, clean the incubator to prepare for the next hatch.