Birds sit on their eggs to make sure the young ones develop properly inside the eggs due to the warmth provided by their mother. The young one cannot develop well when the temperature is less than about 80°F (27°C). Parent birds normally keep the eggs warm with their body heat. This process of providing warmth to the eggs is called incubation.
Feathers actually block the body heat transfer to the eggs. So, the parent bird drops off some of its belly feathers (molt) at incubation time. Thus a ‘brood patch’ as it is called develops on the female birds. The brood patch causes the warm skin on the belly touch the eggs. In addition to it, a dense network of blood vessels brings additional heat to the skin.
There is one group of birds that rear its young one in a much different way. These are pheasant like birds called megapodes (meaning ‘large-footed’) found near in Australia. Megapodes include scrub fowl, mallee fowl and brush turkeys. These birds bury their eggs in a mound (big piles of earth and leaves) and nature does the job of incubation. Heat from sun and fermenting vegetation helps in development of eggs. When the young ones hatch, they dig their way out and then fend for themselves