Science of Incubation


Incubation means maintaining conditions favorable for developing and hatching fertile eggs.

Still-air incubators do not provide mechanical circulation of air. Forced-air incubators are equipped with electric fans. Optimum operating temperatures differ slightly.

Four factors are of major importance in incubating eggs artificially: temperature, humidity, ventilation and turning. Of these factors, temperature is the most critical. However, humidity tends to be overlooked and causes many of the hatching problems encountered by teachers. Extensive research has shown that the optimum incubator temperature is 100°F when relative humidity is 60 percent, concentrations of oxygen 21 percent, carbon dioxide 0.5 percent, and air movement past the egg is at 12 cubic feet per minute.

Incubator Temperature

During the warm-up period the temperature should be adjusted to hold a constant 100°F for still air, 99° to 100°F for forced air. To obtain reliable readings, the bulb of the thermometer should be at the same height as the tops of the eggs and away from the source of heat. Using two thermometers is a good idea to ensure you are getting an accurate reading.

Incubator temperature should be maintained between 99° and 100°F. The acceptable range is 97° to 102°F. High mortality is seen if the temperature drops below 96°F or rises above 103°F for a number of hours. If the temperature stays at either extreme for several days, the egg may not hatch. Overheating is more critical than underheating. Running the incubator at 105°F for 15 minutes will seriously affect the embryos, but running it at 95°F for 3 or 4 hours will only slow their metabolic rate.

Do not make the mistake of overheating the eggs. Many times, when the eggs remain clear and show no development, it is due to excessive heat during the first 48-72 hours. Do not adjust the heat upward during the first 48 hours. This practice cooks many eggs. The eggs will take time to warm to incubator temperature and many times the incubator temperature will drop below 98°F for the first 6 to 8 hours or until the egg warms to 99° to 100°F.

Humidity and Ventilation

Too much moisture in the incubator prevents normal evaporation and results in a decreased hatch, but excessive moisture is seldom a problem in small incubators. Too little moisture results in excessive evaporation, causing chicks to stick to the shell sometimes and hatch crippled at hatching time.

Table 3 (Relative Humidity) will enable you to calculate relative humidity using readings from a wet-bulb thermometer and the incubator thermometer.

During the hatching period, using an atomizer to spray a small amount of water into the ventilating holes may increase the humidity in the incubator. (This is especially helpful when duck or goose eggs are being hatched.)

An 8-inch pie tin or petri dish containing water and placed under the tray of eggs should provide adequate moisture. The relative humidity in the incubator can also be varied by changing the size of the water pan or by putting a sponge in the pan to increase the evaporating surface. The pan should be checked regularly while the incubator is in use to be sure that there is always an adequate amount of water.

Whenever you add water to an incubator, it should be about the same temperature as the incubator so you do not stress the eggs or the incubator. A good test is to add water just warm to the touch.

In the latter stages of incubation (from the 19th day on), condensation on the glass indicates the presence of sufficient moisture. However, the condensation is also related to the temperature of the room where the incubator is being operated. There will be more condensation on the glass if the room is cold, so be sure the temperature in the incubator remains steady.

Using a wet-bulb thermometer is a good learning experience for determining relative humidity. The wet-bulb thermometer measures the evaporative cooling event. If the wet and dry bulb read the same temperature, you would have 100 percent humidity. The greater the evaporation taking place, the lower the temperature reading on the wet-bulb thermometer and the larger the spread will be between the wet- and dry-bulb reading.

To make a wet-bulb thermometer, just add a cotton wick to the end of a thermometer. Then place the tail of the wick in water. The cotton then absorbs the water. As the water evaporates from the cotton it causes a cooling effect on the thermometer.

It is also possible to determine whether there is too much or too little humidity in the incubator by candling the eggs and observing the size of the air cells.

Table 3. Relative Humidity
100°F 81.3 83.3 85.3 87.3 89.0 90.7
101°F 82.2 84.2 86.2 88.2 90.0 91.7
102°F 83.0 85.0 87.0 89.0 91.0 92.7
Percent Relative Humidity 45% 50% 55% 60% 65% 70%

Incubator Temperature Wet Bulb Readings


The best hatching results are obtained with normal atmospheric air, which usually contains 21 percent oxygen. It is difficult to provide too much oxygen, but a deficiency is possible. Make sure that the ventilation holes are open to allow a normal exchange of air.

This is critical on homemade incubators. It is possible to suffocate the eggs and chicks in air-tight container


Chicken eggs should be turned three to five times daily from the 2nd to the 18th day. Do not turn the eggs during the last 3 days!

To insure proper turning, mark each side of the egg with a pencil. Put an “x” on one side and an “o” on the opposite side.

Place the eggs on the welded wire platform horizontally, in a single layer, with the end marked “x” on top. When the eggs are turned, all the “x”s will be on the bottom and the “o”s on top. At the next turning, the “x”s will be in view, and so on.

When incubators are used in schools, it may be difficult to turn the eggs on weekends. If the eggs are not turned, the hatch may be somewhat slower, so it is recommended that the eggs be turned at least once daily on weekends. In some schools, the temperature is reduced on weekends and holidays, and it may be advisable to make an insulation cover for your incubator by placing a large cardboard box over the incubator.

Except for the 19th through the 21st day, it is safe to move the incubator with the eggs in it. Some teachers take the incubator with its eggs home on weekends. Rolling and cracking of the eggs can be prevented during the move by packing the eggs in a carton. The incubator should be wrapped in a heavy blanket and placed in a warm vehicle to maintain the temperature of the eggs, and the trip should not take more than half an hour.

After the 18th day, do not open or move the incubator until the hatch is completed because the chicks are in a hatching position in the eggs and because a desirable hatching humidity must be maintained.


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