Precocial chic"s leave the nest after hatching you re more developed than altricial birds. Incubation is also a part of parental investment. Incubation of eggs is also a large investment, besides laying the eggs, the birds also have to keep them warm and have special adaptations for doing so. Birds that incubate eggs form heating pads on the underside of their bodies. They develop, on the parts of their breasts special patches called brood patches. Brood patches are simply areas where the feathers fall out, or in some casses like with ducks the female pulls the feathers out to create a bare patch on the belly. The belly skin underneath where these patches form is very wrinkled, is tougher skin and can inflate with blood or water having numerous folds in it and therefore more surface area contact with the eggs.