The hard shell keeps an egg from drying out and allows parents to sit on the eggs during incubation. Even though bird eggs are hard-shelled, they have microscopic pores which allow oxygen to pass into and carbon dioxide to exit the shell. Eggs come in a variety of colors, patterns, shapes and textures. Colors and patterns on eggs vary depending on the need for camouflage. The shape of the egg depends on where the bird nests.
Most eggs are oval.
Birds that lay their eggs on ledges need eggs with a pointed end so they will not roll off the ledge. Simply having feathers does not permit birds to be creatures of the sky. Extremely lightweight bones are also necessary for flight. Bird bones are strong and hollow with inside supports.