Incubation

Incubation or brooding, is when oviparous (egg-laying) animals sit on their eggs to hatch them. It also refers to the development of the embryo in the egg under good environmental conditions. 
Since birds are warm-blooded, they sit on the eggs to keep them warm. Reptiles and invertebrates are cold-blooded, and so need to keep their eggs in warm places. Sometimes even birds need the help of the Sun.
Many factors may be vital to the incubation. In some reptiles for example, no fixed temperature is necessary. However, the actual temperature determines the sex ratio of the offspring. In birds, by contrast, the sex of offspring is genetically determined, but a constant and particular temperature may be necessary for successful incubation.
In poultry, the act of sitting on eggs to incubate them is called 'brooding'. The action or tendency to sit on a clutch of eggs is also called "broodiness".