Incubation refers to the process by which certain oviparous (egg-laying) animals hatch their eggs, and to the development of the embryo within the egg. The most vital factor of incubation is the constant temperature required for its development over a specific period. Especially in domestic fowl, the act of sitting on eggs to incubate them is called brooding. The action or behavioral tendency to sit on a clutch of eggs is also called broodiness, and most egg-laying breeds of poultry have had this behavior selectively bred out of them to increase production.
A wide range of incubation habits is displayed among birds. In warm-blooded species such as bird species generally, body heat from the brooding parent provides the constant temperature, though several groups, notably the Megapodes, instead use heat generated from rotting vegetable material, effectively creating a giant compost heap while Crab Plovers make partial use of heat from the sun. The Namaqua Sandgrouse of the deserts of southern Africa, needing to keep its eggs cool during the heat of the day, stands over them drooping its wings to shade them. The humidity is also critical, and if the air is too dry the egg will lose too much water to the atmosphere, which can make hatching difficult or impossible. As incubation proceeds, an egg will normally become lighter, and the air space within the egg will normally become larger, owing to evaporation from the egg.