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. Researchers have found that birds that fend for themselves shortly after hatching emerge from eggs that contained a high percentage of yolk. Megapode eggs contain about 67 percent yolk, while mothers of needy young birds like the house sparrow invest much less in egg yolk, about 25 percent.
Owl eggs can be round as golf balls.
Hummingbird and swift eggs tend to be shaped more like a capsule, long and narrow. Shorebirds and birds that nest on cliffs often lay eggs pointed on one end and rounded on the other, a teardrop shape when viewed from above. Some biologists think this shape allows the mother bird to cover eggs more efficiently when the pointed ends face inward, making the eggs fit together snugly. The shape also makes the eggs roll in a circle, which may be an advantage to birds nesting on flat, narrow ledges. Eggs vary in color from the pale white of tree-cavity nesters to the camouflaged speckles of species that nest on the ground.