Chickens are oviparous animals. This means that they lay fertilized eggs externally and then incubate them until they are ready to hatch. This is called “brooding”. The chicken egg incubation period lasts 21 days. However this does not necessarily mean 21 days from when it is laid. The fertilized egg does not start to develop until the hen starts brooding. This is why chicks can hatch all at one time even when the eggs were laid days apart. In hatcheries this is simulated with a mechanical process. Controlling temperature and humidity of the egg during the brooding period is vital.
The optimum temperature range is 100⁰F to 102⁰F depending on the type of incubator. Forced air incubators should be as close to 100F as possible, where a still air incubator should be maintained at 102⁰F. Humidity should be at 58-60% until about 3 days prior to hatching. When hatching starts on day 19 the humidity should increase to 65%. Humidity is important to keep in mind because egg shells are porous. By keeping the humidity in the proper range eggs will not lose excess water to evaporation. This causes improper growth of the embryo and can reduce the number of hatchlings. Ventilation and turning the eggs is also important. The egg allows for oxygen and carbon dioxide to exchange through the shell wall. Which is why proper ventilation of the incubator is necessary. Eggs must also be turned 4-6 times throughout the first 18 days. This allows the embryo to maintain proper orientation within the egg to allow for optimum hatching conditions.