Oxygen enters the egg the same way carbon dioxide and water get out, through tiny pores in the eggshell. An average chicken egg has about 7,500 of these microscopic holes. To develop into a bird, an egg needs to stay as warm as its mother, roughly the same as human body temperature. Mother birds provide this heat by plucking themselves bare in a spot, known as a brood patch, that will touch the eggs when mothers sit on the nest. Perhaps to ensure the eggs receive an even warming, mother birds often turn their eggs in the nest.
Some birds emerge from the egg more ready to take on the world than others.
Megapodes, birds that live in Australia and on other Pacific islands, hatch with almost everything they need to fend for themselves. Rather than sit on eggs, mother megapodes bury the eggs in rotting vegetation. The heat of decomposition incubates the eggs. When fledgling megapodes peck their way free from their eggshells and dig out of the muck, the birds are feathered, almost ready to fly, and quite alone. The anti-megapode is the house sparrow, which is born naked, blind, and helpless.