A chicken egg will hatch in roughly 21 days, regardless of whether it was placed into an incubator or under a broody hen. The process of hatching takes place over the last couple of days, from the chick turning itself in the shell, to the moment that the first chip appears in the shell, until the chick finally emerges into the open air. The process of hatching is quite difficult for a chick and, unless the conditions are right, a hatching chick is at risk of death before the process is complete.
Several days before hatching, the chick will begin to rotate in the shell, positioning herself so that her head is near the air cell at the large end of the egg. Her neck will curve in such a way that she is better able to strike at the membrane of the air cell. Striking at the air cell membrane enables the chick to start breathing air, setting her up for the pipping process.
Pipping and Zipping
Pipping is the process of the chick breaking through the egg. It begins with the first strike of the chick's egg tooth against the eggshell, creating the first crack. Pipping usually starts on Day 20, but may begin earlier or later depending on environmental conditions surrounding the egg, such as temperature and humidity. The chick will take a several-hour break after breaking a hole through which she can breathe. Resting gives her the opportunity to fill her lungs with air and to finish absorbing the yolk into her body. Once she's rested, the chick will continue to break away portions of the shell in a continuous line around its circumference as the chick continues to turn in her shell, a process called zipping. After she's zipped her way about 75 percent of the way around the shell's circumference, the chick stops breaking the shell and begins to push.
After the chick has unzipped her shell, she will push against the large end so that she can escape. She will push for about 40 minutes or more, until the large end opens. She will be exhausted and take a long rest after finally pushing her way out of the shell -- your chick will be wet and appear almost naked before her down dries out. Chicks can take up to six hours to dry out and start moving.
Chicks can live up to 24 hours on the nutrition provided by the yolk they have absorbed. Take your time before moving any hatched chicks from the incubator to give all of the eggs enough time to hatch.
Temperature and Humidity
The ideal temperature for hatching chicken eggs is 99.5 degrees F. This temperature should be maintained from the day before placing eggs into the incubator until after the last chick has been placed into the brooder. Short periods of fluctuating temperature are fine; however, eggs that have spent more than three hours above 103 degrees or under 96 degrees are unlikely to hatch.
Humidity does not need to be controlled as rigidly in an incubator as temperature does. Eggs should be kept between 50 percent and 55 percent relative humidity during the first 18 days that the eggs are in the incubator. The humidity should not go significantly higher than this level during the first 18 days or the chick can drown in the moisture retained in the shell when it tries to hatch. On Day 19, the humidity should be increased to between 65 and 70 percent. The higher humidity is needed to keep the exposed membranes moist during the pipping and zipping processes, allowing the chick to hatch easily. The lid to the incubator should remain closed during the final three days of incubation, to prevent humidity from dropping.
Humidity can drop within 1 to 2 minutes if the incubator lid is opened, even to remove a hatched chick. Humidity that drops too low causes egg membranes to dry out, trapping any remaining chicks in their shells.