Temperature fluctuations for short periods of time usually do not severely affect hatchability or chick quality because the temperature inside the egg changes more slowly than the air inside the incubator. However, a consistently low temperature will result in a late hatch and decreased hatchability. The chicks may be large, soft bodied, and weak.
A consistently high temperature will result in an early hatch and decreased hatchability. The chicks may have short down (same results with low humidity) and have rough navels (not necessarily infected -- just abnormal closure). More chicks will be malformed, spraddled, weak, and small.
You do not want either, but if you have to choose one or the other, remember that high temperature is more harmful than low temperature. You can incubate eggs for three or four hours at 90oF. without killing many embryos, but a temperature of 105oF. for 30 minutes will kill many embryos. In general, the older the embryo at the time of the high temperature mishap, the greater the death loss.
Incubators can easily overheat when kept where the sun can hit them, such as in a hot, room on the west of the house or in a small building that is subject to heating up considerably during hot summer afternoons. Machines in such conditions, when set near full capacity and with improper ventilation will almost surely overheat. This statement does not imply that the incubator should not be set to full capacity; on the contrary, other factors must be considered and corrected before you can take full advantage of the incubator's capabilities.
HUMIDITY IN THE INCUBATOR AND HATCHER
Most people think the wet bulb reading in a hatcher or incubator is percent relative humidity. This is, of course, not true. Percent relative humidity is determined by using both dry bulb and wet bulb readings. For example, if the dry bulb reading is 100oF. and the wet bulb reading is 87.3oF., the relative humidity is 60 percent. Under normal conditions the relative humidity in an incubator or hatcher should always be 57 to 60 percent. The following table gives the percent relative humidity figures for various dry and wet bulb readings.
Incubator and hatcher manufacturers offer various suggestions for dry and wet bulb settings. However, you may find by experimenting with various settings that the best way is to simply run the dry bulb at 100oF. and the wet bulb at 85 to 87oF. (Keep as near to 86oF. as possible.) Use these settings from the first day of incubation until hatching is complete.
There will be no need to vary the humidity level from 86oF. if the hatching eggs were gathered and stored properly to prevent excessive moisture loss before setting, if the temperature in the machines was maintained at 100oF., if eggs were turned frequently, if sanitation was good, and if your ventilation was properly adjusted during incubating and hatching. Attempting to increase the wet bulb reading to 90 or 92oF. may decrease hatch if vents on the incubators and hatcher are closed too much. Closing the vents may increase the wet bulb reading and humidity inside the machines, but the developing embryos suffer from poor ventilation.
Old, dirty, too short, and wrong-sized wicks on wet bulb thermometers can cause erroneous readings. It is essential that wicks be kept in the best condition. You should thoroughly clean the wicks weekly and replace them with new ones after four to eight washings. Regular changing of wicks is often thought to be unnecessary; it may not be, but if the relatively small cost of new wicks is compared to the cost of low hatchability caused by incorrect wet bulb readings, the new wicks are justified every time.
Inferior wicks tend to give higher readings than are actually present. In other words, the wet bulb tends to act more like the dry bulb. This is because the flow of water through the wick has been slowed. Therefore, if attempting to maintain an 86oF. wet bulb reading with faulty wicks, you may actually have an 84oF. wet bulb environment in the machine. The two degrees difference for an entire incubation and hatch period can noticeably reduce hatchability. Where possible and practical, use a dual set of wet and dry bulb instruments in each machine.
Excessive moisture loss from the eggs during storage before setting can produce the same symptoms that low humidity in the machines produces. A sign of low humidity is sticky embryos during pipping and hatching that results in embryos not being able to turn themselves in the shell and complete the act of pipping and detaching themselves from the shell. Low humidity also results in short down on the chicks, malformed, malpositioned, weak, and small chicks. Low humidity contributes to (but is not wholly responsible for) spraddlers, star gazers, and those that cannot stand, walk, or orient themselves well enough to reach food and water.
If several large, soft bodied, mushy chicks are observed that make it through pipping and hatching but are dead in the tray, it is a sign of high humidity. A bad odor usually accompanies this condition. The condition normally occurs only in incubators and hatchers that have forced spray humidity systems that force too much moisture into the machines. Rarely does humidity run too high in a machine that relies on evaporation from pans if you are using the recommended evaporative pans, if the temperature is correct, and if the machines are properly and amply ventilated with fresh air.
If by restricting ventilation the humidity is made too high (92o to 94oF.) during the final stages of incubation, the embryos are moist and develop to the 19th, 20th, or 21st day of incubation, but die in the shell from suffocation. This suffocation results from improper ventilation rather than high humidity.