H is for Humidity
During incubation, a waterfowl egg should lose about 15% of its weight. In natural incubation, this is affected by nest selection and structure, egg surface, attentiveness of the sittiing bird and ambient conditions.
Too much evaporation means a dry chick, at risk of getting stuck to the membrane in the egg. Not enough and the embryo may be big and strong, but does not have enough room to turn in the egg to get out.
This is why we candle eggs to see how they are doing, and adjust moisture in the incubator if necessary. There's more about incubation in our husbandry section.
